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CORRELATION 
 
 OF 
 
 PHYSICAL FORCES 
 
 &c. 
 
LONDON : PRINTED BY 
 
 SPOTTISWOODE AND CO., NEW-STREET SQUARE 
 AND PARLIAMENT STREET 
 
THE CORRELATION OF 
 
 PHYSICAL FORCES. 
 
 SIXTH EDITION. 
 
 WITH OTHER CONTRIBUTIONS TO SCIENCE. 
 
 BY THE HON. 
 
 SIR W. R. GROVE, M.A., F.R.S. 
 
 ONE OF THE JUDGES OF THE COURT OF 
 COMMON PLEAS. 
 
 LONDON : 
 LONGMANS, GREEN, AND CO. 
 
 1874. 
 
 J// rights reserved. 
 
PREFACE 
 
 TO 
 
 THE SIXTH EDITION. 
 
 THE fifth edition of the * Correlation of Physical Forces ' 
 having been exhausted, I have done my best during 
 the few spare hours at my disposition to prepare a 
 sixth. Partly induced by the suggestions of others, 
 partly, perhaps, by egotism, I have added a selection 
 of papers on scientific subjects, which I have from 
 time to time previously published. Physical science 
 has moved on far too quickly for me to follow it, and 
 no one is more convinced than I am that 
 
 To have done, is to hang 
 Quite out of fashion, like a rusty mail, 
 In monumental mockery. 
 
 Still, having gone through my past contributions to 
 science, forgotten by all others, half-forgotten by my- 
 self, I find something of suggestion in most of them 
 which may not be altogether useless ; and as the papers 
 are scattered about in various periodicals, a collection 
 of them in one volume may be accepted as a last 
 legacy. 
 
vi PREFACE TO THE SIXTH EDITION. 
 
 I have read all with care. I have added a few 
 paragraphs here and there to the Essays ; the other 
 papers I have not altered, farther than by correcting 
 some mere verbal errors, and, in two or three instances, 
 incorporating with the text, paragraphs printed at the 
 time as footnotes. 
 
 I have prefixed the dates and names of the volumes 
 in which they were published ; and though in their 
 present arrangement I have not adhered strictly to 
 chronological order, I have done so as far as is consis- 
 tent with papers on the same subjects following each 
 other. 
 
 I have entitled them ' Experimental Investigations/ 
 to avoid confusion with the better title, appropriated 
 by Faraday, of ' Experimental Researches.' 
 
 Towards the end I have given a few observations 
 of little value in themselves, but made on phenomena 
 of rare occurrence. While this edition was going 
 through the press Sir C. Wheatstone has called my 
 attention to a book entitled ' Elemens de Physique 
 Gen^rale, par Jules Guyot, Paris, 183 2,' in which, though 
 the style of argument is a priori and somewhat dog- 
 matic, the author attributes natural phenomena to 
 motion either of translation, as he terms it, or molecular, 
 and attacks the theories of fluids, ethers, &c. It is a 
 work of considerable merit, and shows, when conjoined 
 with the references I have given, that many minds are 
 independently converging to the same theory, the which 
 I believe will ultimately prevail. 
 
PREFACE 
 
 TO 
 
 THE FIFTH EDITION. 
 
 THE PHRASE * Correlation of Physical Forces/ in the 
 sense in which I have used it, having become re- 
 cognised by a large number of scientific writers, it 
 would produce confusion were I now to adopt another 
 title. It would, perhaps, have been better if I had in 
 the first instance used the term Co-relation, as the 
 words ' correlate/ ' correlative/ had acquired a peculiar 
 metaphysical sense somewhat differing from that which 
 I attached to the substantive correlation. The passage 
 in the text (p. 165) explains the meaning I have given 
 to the term. 
 
 Twenty-five years having elapsed since I promul- 
 gated the views contained in this Essay, which were 
 first advanced in a lecture at the London Institution, 
 in January 1842, printed by the Proprietors, and subse- 
 quently more fully developed in a course of lectures in 
 1843, published in abstract in the 'Literary Gazette/ 
 I think it advisable to add a little to the Preface with 
 reference to other labourers in the same field. 
 
 It has happened with this subject, as with many 
 
viii PREFACE TO THE FIFTH EDITION. 
 
 others, that similar ideas have independently presented 
 themselves to different minds about the same period. 
 In * Liebig and Wohler's Annalen ' for May 1842,* a 
 paper appeared by M. Mayer, which I had not read 
 when my third edition was published, but which I 
 have now read in the translation by Mr. Youmans, of 
 New York. It deduces very much the same conclu- 
 sions to which I had been led, the author starting 
 partly from d priori reasoning and partly from an ex- 
 periment by which water was heated by agitation ; and 
 from another, which had, however, previously been 
 made by Davy, viz. that ice can be melted by friction, 
 though kept in a medium which is below the freezing- 
 point of water. 
 
 In 1843 a paper by Mr. Joule on the mechanical 
 equivalent of heat appeared, which, though not in 
 terms touching on the mutual and necessary depen- 
 dence of alt the Physical Forces, yet bears most im- 
 portantly upon the doctrine. 
 
 While my third edition was going through the press 
 I had the good fortune to make the acquaintance of 
 M. Seguin, who informed me that his uncle, the emi- 
 nent Montgolfier, had long entertained the idea that 
 force was indestructible, though, with the exception of 
 one sentence in his paper on the hydraulic ram, and 
 where he is apparently speaking of mechanical force, 
 he has left nothing in print on the subject. Not so, 
 however, M. Seguin himself, who in 1839, in a work 
 on the ' Influence of Railroads/ has distinctly expressed 
 
 * I am informed that these papers are, in fact, published some time after the 
 date which they bear in the magazine. 
 
PREFACE TO THE FIFTH EDITION. ix 
 
 his uncle's and his own views on the identity of heat 
 and mechanical force, and has given a calculation of 
 their equivalent relation, which is not far from the 
 more recent numerical results of Mayer, Joule, and 
 others. 
 
 Several of the great mathematicians of a much 
 earlier period advocated the idea of what they termed 
 the Conservation of Force ; but although they consi- 
 dered that a body in motion would so continue for ever, 
 unless arrested by the impact of another body and, 
 indeed, in the latter case, would, if elastic, still continue 
 to move (though deflected from its course) with a force 
 proportionate to its elasticity yet with inelastic bodies 
 the general and, as far as I am aware, the universal 
 belief was, that the motion was arrested on impact and 
 the force annihilated. Montgolfier went a step farther, 
 and his hydraulic ram was to him a proof of the truth 
 of his preconceived idea, that the shock or impact 
 of bodies left the mechanical force undestroyed.* 
 
 Previously, however, to the discoveries of the voltaic 
 battery, electro-magnetism, thermo-electricity, and pho- 
 tography, it was impossible for any mind to perceive 
 what, in the greater number of cases, became of the 
 force which was apparently lost. The phenomena of 
 heat, known from the earliest times, would have been 
 a mode of accounting for the resulting force in many 
 cases where motion was arrested, and we find Bacon 
 enouncing a theory that motion was the form, as he 
 quaintly termed it, of heat. Rumford and Davy 
 
 * See also a paper by Mr. Rankinc, On the Dynamical Principles of JVeu'fon, 
 in the Engineer, Oct. 26, 1866. 
 
x PREFACE TO THE FIFTH EDITION. 
 
 adopted this view, the former with a fair approximative 
 attempt at numerical calculation, but no one of these 
 philosophers seems to have connected it with the in- 
 destructibility of force. A passage in the writings of 
 Dr. Roget, combating the theory that mere contact of 
 dissimilar bodies was the source of voltaic electricity, 
 philosophically supports his argument by the idea of 
 non-creation of force. 
 
 As I have introduced into the later editions of my 
 Essay abstracts of the different discoveries which I 
 have found, since my first lectures, to bear upon the 
 subject, I have been regarded by many rather as the 
 historian of the progress made in this branch of thought 
 than as one who has had anything to do with its initia- 
 tion. Every one is but a poor judge where he is him- 
 self interested, and I therefore write with diffidence ; 
 but it would be affecting an indifference which I do not 
 feel if I did not state that I believe myself to have 
 been the first who introduced this subject as a gene- 
 ralised system of philosophy, and continued to enforce 
 it in my lectures and writings for many years, during 
 which it met with the opposition usual and proper to 
 novel ideas. 
 
 Avocations necessary to the well-being of others 
 have prevented my following it up experimentally, to 
 the extent that I once hoped ; but I trust and believe 
 that this Essay, imperfect though it be, has helped 
 materially to impress on that portion of the public 
 which devotes its attention rather to the philosophy of 
 science than to what is now termed science, the truth 
 of the thesis advocated. 
 
PREFACE TO THE FIFTH EDI1ION. xi 
 
 To show that the work of to-day is not substantially 
 different from the thoughts I first published on the 
 subject, at a period when I knew little or nothing of 
 what had been thought before, I venture to give a few 
 extracts from the printed copy of my lecture of 1842: 
 
 Physical Science treats of Matter, and what I shall to-night 
 term its Affections ; namely, Attraction, Motion, Heat, Light, 
 Electricity, Magnetism, Chemical-Affinity. When these re-act 
 upon matter, they constitute Forces. The present tendency of 
 theory seems to lead to the opinion that all these Affections 
 are resolvable into one, namely, Motion : however, should 
 the theories on these subjects be ultimately so effectually 
 generalised as to become laws, they cannot avoid the 
 necessity for retaining different names for these different 
 Affections ; or, as they would then be called, different Modes 
 of Motion 
 
 QErsted proved that Electricity and Magnetism are two 
 forces which act upon each other ; not in straight lines, as all 
 other known forces do, but in a rectangular direction : that is, 
 that bodies invested with electricity, or the conduits of an 
 electric-current, tend to place magnets at right angles to 
 them ; and, conversely, that magnets tend to place bodies 
 conducting electricity at right angles to them 
 
 The discovery of CErsted, by which electricity was made 
 a source of Magnetism, soon led philosophers to seek the con- 
 verse effect ; that is, to educe Electricity from a permanent 
 magnet. Had these experirhentalists succeeded in their ex- 
 pectations of making a stationary magnet a source of electric- 
 currents, they would have realised the ancient dreams of 
 perpetual motion, they would have converted statics into 
 dynamics, they would have produced power without expen- 
 diture ; in other words, they would have become creators. 
 They failed, and Faraday saw their error : he proved that to 
 
xii PREFACE TO THE FIFTH EDITION. 
 
 obtain Electricity from Magnetism it was necessary to super- 
 add to this latter, motion ; that magnets while in motion 
 induced electricity in contiguous conductors ; and that the 
 direction of such electric-currents was tangential to the polar 
 direction of the magnet ; that as Dynamic-electricity may be 
 made the source of Magnetism and Motion, so Magnetism 
 conjoined with Motion may be made the source of Electricity. 
 Here originates the Science of Magneto-electricity, the true 
 converse of Electro-magnetism ; and thus between Electricity 
 and Magnetism is shown to exist a reciprocity of force such 
 that, considering either as the primary agent, the other be- 
 comes the re-agent ; viewing one in the relation of cause, the 
 other is the effect 
 
 The Science of Thermo-electricity connected heat with 
 electricity, and proved these, like all other natural forces, to 
 be capable of mutual reaction 
 
 Voltaic action is Chemical action taking place at a dis- 
 tance, or transferred through a chain of media ; and the 
 Daltonian equivalent numbers are the exponents of the 
 amount of voltaic action for corresponding chemical sub- 
 stances 
 
 By regarding the quantity of electrical, as directly pro- 
 portional to the efficient chemical action, and by experimen- 
 tally tracing this principle, I have been fortunate enough to 
 increase the power of the Voltaic-pile more than sixteen 
 times, as compared with any combination previously 
 known 
 
 I am strongly disposed to consider that the facts of 
 Catalysis depend upon voltaic action, to generate which three 
 heterogeneous substances are always necessary. Induced by 
 this belief, I made some experiments on the subject, and suc- 
 ceeded in forming a voltaic combination by gaseous-oxygen, 
 gaseous-hydrogen, and platinum, by which a galvanometer 
 was deflected and water decomposed 
 
PREFACE TO THE FIFTH EDITION, xiii 
 
 It appears to me that heat and light may be considered 
 as affections ; or, according to the Undulatory theory, vibra- 
 tions of matter itself, and not of a distinct ethereal fluid 
 permeating it. These vibrations would be propagated, just 
 as sound is propagated by vibrations of wood or as waves by 
 water. To my mind, all the consequences of the Undulatory 
 theory flow as easily from this as from the hypothesis of a. 
 specific ether ; to suppose which namely, to suppose a fluid 
 sui generis, and of extreme tenuity, penetrating solid bodies 
 we must assume, first, the existence of the fluid itself; 
 secondly, that bodies are, without exception, porous ; thirdly, 
 that these pores communicate ; fourthly, that matter is 
 limited in expansibility. None of these difficulties apply to 
 the modification of this theory which I venture to propose ; 
 and no other difficulty applies to it which does not equally 
 apply to the received hypothesis. With regard to the plane- 
 tary spaces, the diminishing periods of comets is a strong 
 argument for the existence of an universally-diffused matter: 
 this has the function of resistance, and there appears to be no 
 reason to divest it of the functions common to all matter, or 
 specifically to appropriate it to certain affections. Again, the 
 phenomena of transparency and opacity are, to my mind, 
 more easily explicable by the former than by the latter 
 theory, as resulting from a difference in the molecular 
 arrangement of the matter affected. In regard to the effects 
 of double-refraction and polarisation, the molecular structure 
 gives at once a reason for the effects upon the one theory, 
 while upon the other we must, in addition to previous 
 assumptions, farther assume a different elasticity of the ether 
 in different directions within the doubly-refracting medium. 
 The same theory is applicable to Electricity and Magnetism ; 
 my own experiments on the influence of the elastic inter- 
 medium on the voltaic-arc, and those of Faraday on electrical 
 induction, furnish strong arguments in support of it. My 
 
xiv PREFACE TO THE FIFTH EDITION. 
 
 inclination would lead me to detain you on this subject much 
 longer than my judgment deems advisable ; I therefore con- 
 tent myself with offering it to your consideration, and, should 
 my avocations permit, I may at a future period more fully 
 
 develop it 
 
 Light, Heat, Electricity, Magnetism, Motion, and Chemical- 
 affinity, are all convertible material affections ; assuming either 
 as the cause, one of the others will be the effect : thus heat 
 may be said to produce electricity, electricity to produce 
 heat; magnetism to produce electricity, electricity magnetism; 
 and so of the rest. Cause and effect, therefore, in their 
 abstract relation to these forces, are words solely of conve- 
 nience. We are totally unacquainted with the ultimate 
 generating power of each and all of them, and probably shall 
 ever remain so; we can only ascertain the normse of their 
 action : we must humbly refer their causation to one omni- 
 present influence, and content ourselves with studying their 
 effects and developing, by experiment, their mutual relations. 
 
 I have transposed the passages relating to voltaic 
 action and catalysis, but I have not added a word to 
 the above quotations ; and, as far as I am now aware, 
 the theory that the so-called imponderables are affec- 
 tions of ordinary matter, that they are resolvable into 
 motion, that they are to be regarded in their action on 
 matter as forces, and not as specific entities, and that 
 they are capable of mutual reaction, thence alternately 
 acting as cause and effect, had not at that time been 
 publicly advanced.* 
 
 My original Essay being a record of lectures, and 
 being published by the Managers of the Institution, I 
 
 * See also an experiment shown at the London Institution, p. 196, post. 
 
PREFACE TO THE FIFTH EDITION. xv 
 
 necessarily adhered to the form and matter which I 
 had orally communicated. In preparing subsequent 
 editions I found that, without destroying the identity 
 of the work, I could not alter the style ; although it 
 would have been less difficult and more satisfactory to 
 me to have done so, the work would not then have been 
 a republication ; and I was for obvious reasons anxious 
 to preserve as far as I could the original text, which, 
 though added to, is but little altered. 
 
 The form of lectures has necessarily continued the 
 use of the first person, and I would beg my readers 
 not to attribute to me> from the modes of expression 
 used, a dogmatism which is far from my thought. If 
 my opinions are expressed broadly, the reason is that 
 when opinions are always hedged in by qualifications, 
 the style becomes embarrassed and the meaning 
 frequently unintelligible. 
 
 As the main object of a course of lectures is to 
 induce the auditor to think, and to consult works on 
 the subject he hears treated, so the object of this Essay 
 is more to induce a particular train of thought on the 
 known facts of physical science than to enter with 
 minute criticism into each separate branch. 
 
 In one or two of the reviews of previous editions 
 the general idea of the work was objected to. I believe, 
 however, .that will not now be the case ; the mathema- 
 tical labours of Sir W. Thompson, Clausius, and others, 
 though not suitable for insertion in an Essay such as 
 this, have awakened an interest for many portions of 
 the subject, which promises much for its future progress. 
 
 The short and irregular intervals which my profes- 
 
xvi PREFACE TO THE FIFTH EDITION. 
 
 sion permits me to devote to science so prevent the 
 continuity of attention necessary for the proper evolu- 
 tion of a train of thought, that I certainly should not 
 now have courage to publish for the first time such an 
 Essay ; and it is only the favour it has received from 
 those whose opinions I highly value, and the, I trust 
 pardonable, wish not to let some favourite thoughts of 
 my youth lose all connection with my name, that have 
 induced me to reprint it. 
 
 My scientific readers will, I hope, excuse the very 
 short notices of certain branches of science which are 
 introduced, as without them the work would be unin- 
 telligible to many for whom it is intended. I have 
 endeavoured so to arrange the subjects that each 
 division should form an introduction to those which 
 follow, and to assume no more preliminary knowledge 
 to be possessed by my readers than would be expected 
 from persons acquainted with the elements of physical 
 science. 
 
 The notes contain references to the original memoirs 
 in which the branches of science alluded to are to be 
 found, as well as to those which bear on the main 
 arguments; where these memoirs are numerous, or not 
 easy of access, I have referred to treatises in which 
 they are collated. To prevent the reader's attention 
 being interrupted, I have in the notes referred to the 
 pages of the text, instead of to interpolated letters. 
 
 For this fifth edition I have revised the whole of 
 the text ; and several correspondents having suggested 
 that I should add to it my address as President of the 
 British Association, 1866, it seemed to me on consi- 
 
PREFACE TO THE FIFTH EDITION. xvii 
 
 deration not inappropriate. In it are noticed several 
 new discoveries relating to the subjects treated of in 
 this Essay, which, had I not decided on adding the 
 discourse, I should have incorporated with the text. 
 The doctrine of Continuity, however, though closely 
 allied with that of Correlation, could not well have been 
 interwoven with the Essay, and is more suitable as a 
 sequel. 
 
 I cannot but feel gratified with the reception that 
 address has met with ; and although the portions re- 
 ferring to the continuity of succession of organised 
 beings have, as I rather expected, been the subject of 
 some adverse comment, yet, as I have seen no argu- 
 ment against them, I have nothing at present to answer 
 or to alter. 
 
 Several of the arguments adduced by me were 
 written some years ago, and before the appearance of 
 Darwin's now celebrated work. I had then no notion 
 of the effects of natural selection in modifying organisms, 
 but in order to test as fairly as I could the reasons for 
 and against continuity, as opposed to special creations, I 
 wrote down at different times in the form of a dialogue 
 everything that occurred to me as bearing most strongly 
 on each side of the question, and showed it to several 
 friends with whom I discussed the subject. My result- 
 ing opinion is given in the text, 
 
 I thought I had sufficiently guarded myself in the 
 passage at p. 221, from being supposed to deny that 
 there are or have been catastrophes or cataclysms ; but 
 it appears from some comments that I have not done 
 so. If sea or river undermine a cliff and the cliff fall, 
 
xviii PREFACE TO THE FIFTH EDITION. 
 
 it is undoubtedly a cataclysm ; if I tread on a beetle, 
 it is a catastrophe to the beetle ; but formation and 
 destruction are very different things, and the tenour of 
 my discourse applies to genesis, not extinction. Even 
 in phenomena such as those of Geology, though there 
 were doubtless cataclysms, and sometimes on a much 
 larger scale than at other times, yet the evidence seems 
 to me to point to their being limited in extent at any 
 one period, when compared with the whole terrestrial 
 surface. 
 
CONTENTS. 
 
 PAGE 
 
 CORRELATION OF PHYSICAL FORCES i 
 
 CONTINUITY . 18 1 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 NITRIC ACID VOLTAIC BATTERY 231 
 
 VOLTAIC POLARISATION 234 
 
 CHEMICAL INACTION OF AMALGAMATED ZINC . . . 236 
 A VOLTAIC PROCESS FOR ETCHING DAGUERREOTYPES . .241 
 
 ELECTRO -NITRURETS 247 
 
 VOLTAIC SYNTHESIS OF WATER 249 
 
 VOLTAIC REACTION 250 
 
 GAS VOLTAIC BATTERY 253 
 
 VOLTAIC ACTION OF PHOSPHORUS, SULPHUR, AND HYDRO- 
 CARBONS . 285 
 
 THERMOGRAPHY AND VOLTAISM . . . . . .301 
 
 MOLECULAR VOLTAIC PHENOMENA . . . . . 302 
 
 VOLTAIC IGNITION AND THE DECOMPOSITION OF WATER INTO 
 
 ITS CONSTITUENT GASES BY HEAT 306 
 
 SUPPLEMENTARY PAPER ON THE SAME SUBJECT. . . 330 
 
 EFFECT OF SURROUNDING MEDIA ON VOLTAIC IGNITION . 336 
 
 ELECTRICITY AS A MOTIVE POWER . . . . . 350 
 
 MOLECULAR MOTION BY MAGNETISM 354 
 
 PRODUCTION OF HEAT BY MAGNETISM . . . . 355 
 
xx CONTENTS. 
 
 PAGE 
 
 ELECTRO-CHEMICAL POLARITY OF GASES . . . .356 
 STRIDE IN THE ELECTRICAL DISCHARGE . . . 376, 379 
 ANOMALOUS CASES OF ELECTRICAL DECOMPOSITION . .384 
 ELECTRICITY OF THE BLOWPIPE FLAME . . . . 392 
 METHOD OF INCREASING EFFECTS OF INDUCED ELECTRICITY 396 
 APPARENT CONVERSION OF ELECTRICITY INTO MECHANICAL 
 
 FORCE 400 
 
 NEW METHODS OF PRODUCING AND FIXING ELECTRICAL 
 
 FIGURES 402 
 
 INFLUENCE OF LIGHT ON THE POLARISED ELECTRODE . . 406 
 TRANSMISSION OF ELECTROLYSIS ACROSS GLASS . . 415 
 EXPERIMENT IN MAGNETO-ELECTRIC INDUCTION . . .418 
 
 SOME EFFECTS OF HEAT ON FLUIDS 419 
 
 NEW CLASS OF APLANATIC TELESCOPES . . . .431 
 
 MODE OF REVIVING DORMANT IMPRESSIONS ON THE RETINA 435 
 NATURAL PHOTOGRAPHY . . . . . . -436 
 
 REFLEXION AND INFLEXION OF LIGHT BY INCANDESCENT 
 
 SURFACES 437 
 
 NOTES ON OCCULTATIONS OF JUPITER, 1856 AND 1857 . 442 
 
 STAR ARCTURUS AND COMET, 1858 444 
 
 OCCULTATION OF SATURN, 1859 445 
 
 MARS WHEN NEAR THE EARTH, 1862 .... 446 
 
 AURORA BOREALIS, 1870 446 
 
 ARTIFICIAL ROCKING-STONES 449 
 
 NOTES AND REFERENCES 453 
 
Library. ) 
 
 CORRELATION 
 
 OF 
 
 PHYSICAL FORCES. 
 
 WHEN natural phenomena are for the first time observed, a 
 tendency immediately developes itself to refer them to some- 
 thing previously known to bring them within the range of 
 acknowledged sequences. The mode of regarding new facts, 
 which is most favourably received by the public, is that which 
 refers them to recognised views stamps them into the mould 
 in which the mind has been already shaped. The new fact 
 may be far removed from those to which it is referred, and 
 may belong to a different order of analogies, but this cannot 
 then be known, as its co-ordinates are wanting. It may be 
 questionable whether the mind is not so moulded by past 
 events that it is impossible to advance an entirely new view ; 
 but, admitting such possibility, the new view, necessarily 
 founded on insufficient data, is likely to be more incorrect 
 and prejudicial than even a strained attempt to reconcile the 
 new discovery with known facts. 
 
 The theory consequent upon new facts, whether it be a 
 co-ordination of them with known ones, or the more difficult 
 and dangerous attempt at remodelling the public ideas, is 
 generally enunciated by the discoverers themselves of the 
 facts, or by those to whose authority the world at the period 
 of the discovery defers ; others are not bold enough, or if 
 they be so, are unheeded. The earliest theories thus enun- 
 ciated obtain the firmest hold upon the public mind, for at 
 
2 CORRELATION OF PHYSICAL FORCES. 
 
 such a time there is no power of testing, by a sufficient range 
 of experience, the truth of the theory ; it is accepted solely 
 or mainly upon authority : there being no means of con- 
 tradiction, its reception is, in the first instance, attended with 
 some degree of doubt, but as the time in which it can fairly 
 be investigated far exceeds that of any lives then in being, 
 and as neither the individual nor the public mind will long 
 tolerate a state of abeyance, a theory shortly becomes, for 
 want of a better, admitted as an established truth : it is 
 handed from father to son, and gradually takes its place in 
 education. Succeeding generations, whose minds are thus 
 formed to an established view, are much less likely to aban- 
 don it. They have adopted it, in the first instance, upon 
 authority to them unquestionable, and subsequently to yield 
 up their faith would involve a laborious remodelling of ideas, 
 a task which the public as a body will and can rarely under- 
 take, the frequent occurrence of which is indeed inconsistent 
 with the very existence of man in a social state, as it would 
 induce an anarchy of thought a perpetuity of mental revolu- 
 tions. 
 
 This necessity has its good; but the prejudicial effect 
 upon the advance of science is, that by this means theories 
 the most immature frequently become the most permanent ; 
 for no theory can be more immature, none is likely to be so 
 incorrect, as that which is formed at the first flush of a new 
 discovery; and though time exalts the authority of those 
 from whom it emanated, time can never give to the illustrious 
 dead the means of analysing and correcting erroneous views 
 which subsequent discoveries confer. 
 
 Take, for instance, the Ptolemaic System, which we may 
 almost literally explain by the expression of Shakspeare : 
 ' He that is giddy thinks the world turns round.' We now 
 see the error of this system, because we have all an imme- 
 diate opportunity of refuting it ; but this identical error was 
 received as a truth for centuries, because, when first pro- 
 mulgated, the means of refuting it were not at hand; and 
 when the means of its refutation became attainable, mankind 
 had been so educated to the supposed truth, that they rejected 
 the proof of its fallacy. 
 
INTRODUCTORY REMARKS. 3 
 
 I have premised the above for two reasons : first, to obtain 
 a fair hearing, by requesting as far as possible a dismissal 
 from the minds of my readers of preconceived views by and 
 in favour of which all are liable to be prejudiced ; and, 
 secondly, to defend myself from the charge of undervaluing 
 authority, or treating lightly the opinions of those to whom 
 and to whose memory mankind looks with reverence. Pro- 
 perly to value authority we should estimate it together with 
 its means of information : if ' a dwarf on the shoulders of a 
 giant can see farther than the giant,' he is no less a dwarf in 
 comparison with the giant. 
 
 The subject on which I am about to treat viz. the 
 relation of the affections of matter to each other and to 
 matter peculiarly demands an unprejudiced regard. The 
 different aspects under which these agencies have been con- 
 templated ; the different views which have been taken of 
 matter itself; the metaphysical subtleties to which these 
 views unavoidably lead, if pursued beyond fair inductions 
 from existing experience, present difficulties almost insur- 
 mountable. 
 
 The extent of claim which my views on this subject may 
 have to originality has been stated in the Preface ; they 
 became strongly impressed upon my mind at a period when 
 I was much engaged in experimental research, and were, as 
 I then believed, and still believe, regarding them as a system, 
 new : expressions in the works of different authors, bearing 
 more or less on the subject, have subsequently been pointed 
 out to me, some of which go back to a distant period. An 
 attempt to analyse these in detail, and to trace how far I 
 have been anticipated by others, would probably but little 
 interest the reader, and in the course of it I should constantly 
 have to make distinctions showing wherein I differed, and 
 wherein I agreed with others. I might cite authorities which 
 appear to me to oppose, and others which appear to coincide, 
 with certain of the views I have put forth ; but this would 
 interrupt the consecutive development of my own ideas, and 
 might render me liable to the charge of misconstruing those of 
 others ; I therefore think it better to avoid such discussion in 
 the text ; and in addition to the sketch given in the Preface, 
 
 B2 
 
4 CORRELATION OF PHYSICAL FORCES. 
 
 to furnish in the notes at the conclusion such references to dif- 
 ferent authors as bear upon the subjects treated of, which I 
 have discovered, or which have been pointed out to me since 
 the delivery of the lectures of which this essay is a record. 
 
 The more extended our research becomes, the more we 
 find that knowledge is a thing of slow progression, that the 
 very notions which appear to ourselves new, have arisen, 
 though perhaps in a very indirect manner, from successive mo- 
 difications of traditional opinions. Each word we utter, each 
 thought we think, has in it the vestiges, is in itself the impress, 
 of antecedent words and thoughts. As each material form, 
 could we rightly read it, is a book, containing in itself the 
 past history of the world ; so, different though our philosophy 
 may now appear to be from that of our progenitors, it is but 
 theirs added to or subtracted from, transmitted drop by drop 
 through the filter of antecedent, as ours will be through that 
 of subsequent, ages. The relic is to the past as is the germ 
 to the future. 
 
 Though many valuable facts, and correct deductions from 
 them, are to be found scattered amongst the voluminous 
 works of the ancient philosophers ; yet, giving them the 
 credit which they pre-eminently deserve for having devoted 
 their lives to purely intellectual pursuits, and for having 
 thought, seldom frivolously, often profoundly, nothing can be 
 more difficult than to seize and apprehend the ideas of those 
 who reasoned from abstraction to abstraction who, although, 
 as we now believe, they must have depended upon observa- 
 tion for their first inductions, afterwards raised upon them 
 such a complex superstructure of syllogistic deductions, 
 that, without following the same paths, and tracing the same 
 sinuosities which led them to their conclusions, such conclu- 
 sions are to us unintelligible. To think as another thought, 
 we must be placed in the same situation as he was placed : 
 the errors of commentators generally arise from their reason- 
 ing upon the arguments of their text, either in blind obedi- 
 ence to its dicta, without considering the circumstances under 
 which they were uttered, or in viewing the images presented 
 to the original writer from a different point to that from which 
 he viewed them. Experimental philosophy keeps in check 
 
INTROD UCTOR Y REMARKS. 5 
 
 the errors both of a priori reasoning and of commentators, 
 and, at all events, prevents their becoming cumulative ; 
 though the theories or explanations of a fact be different, the 
 fact remains the same. It is, moreover, itself the exponent of 
 its discoverer's thought : the observation of known pheno- 
 mena has led him to elicit from nature the new phenomenon : 
 and though he may be wrong in his deductions from this after 
 its discovery, the reasonings which conducted him to it are 
 themselves valuable, and having led from known to unknown 
 truths, can seldom be uninstructive. 
 
 Very different views existed amongst the ancients as to 
 the aims to be pursued by physical investigation, and as to 
 the objects likely to be attained by it. I do not here mean 
 the moral objects, such as the attainment of the summum bo- 
 num, &c., but the acquisitions in knowledge which such 
 investigations were likely to confer. Utility was one object 
 in view, and this was to some extent attained by the progress 
 made in astronomy and mechanics. Archimedes, for instance, 
 seems to have constantly had this end in view ; but, while 
 pursuing . natural knowledge for the sake of knowledge, and 
 the power which it brings with it, the greater number seemed 
 to entertain an expectation of arriving at some ultimate goal, 
 some point of knowledge, which would give them a mastery 
 over the mysteries of nature, and would enable them to ascer- 
 tain what was the most intimate structure of matter, and the 
 causes of the changes it exhibits. Where they could not dis- 
 cover, they speculated. Leucippus, Democritus, and others 
 have given us their notions of the ultimate atoms of which 
 matter was formed, and of the modus agendi of nature in 
 the various transformations which matter undergoes. 
 
 The expectation of arriving at ultimate causes or essences 
 continued long after the speculations of the ancients had been 
 abandoned, and continues even to the present day to be a 
 very general notion of the objects to be ultimately attained 
 by physical science. Francis Bacon, the great remodeller of 
 science, entertained this notion, and thought that, by experi- 
 mentally testing natural phenomena, we should be enabled 
 to trace them to certain primary essences or causes whence 
 the various phenomena flow. These he speaks of under the 
 
6 CORRELATION OF PHYSICAL FORCES. 
 
 scholastic name of ' forms ' a term derived from the ancient 
 philosophy, but differently applied. He appears to have un- 
 derstood by 'form' the essence of quality that in which, 
 abstracting everything extraneous, a given quality consists, or 
 that which, superinduced on any body, would give it its 
 peculiar quality : thus, the form of transparency is that which 
 constitutes transparency, or that by which, when discovered, 
 transparency could be produced or superinduced. To take a 
 specific example of what I may term the synthetic applica- 
 tion of his philosophy : ' In gold there meet together yellow- 
 ness, gravity, malleability, fixedness in the fire, a determinate 
 way of solution, which are the simple natures in gold ; for he 
 who understands form, and the manner of superinducing this 
 yellowness, gravity, ductility, fixedness, faculty of fusion, 
 solution, &c., with their particular degrees and proportions, 
 will consider how to join them together in some body, so that 
 a transmutation into gold shall follow.' 
 
 On the other hand, the analytic method, or, ' the enquiry 
 from what origin gold or any other metal or stone is gene- 
 rated from its first fluid matter or rudiments, up to a perfect 
 mineral,' is to be perceived by what Bacon calls the latent 
 process, or a search for ' what in every generation or trans- 
 formation of bodies, flies off, what remains behind, what 
 is added, what separated, &c. ; also, in other alterations 
 and motions, what gives motion, what governs it, and the like.' 
 Bacon appears to have thought that qualities separate from 
 the substances themselves were attainable, and if not capable 
 of physical isolation, were at all events capable of physical 
 transference and superinduction. 
 
 Subsequently to Bacon a belief has generally existed, and 
 now to a great extent exists, in what are called secondary 
 causes, or consequential steps, wherein one phenomenon is 
 supposed necessarily to hang on another, and that on another, 
 until at last we arrive at an essential cause, subject immedi- 
 ately to the First Cause. This [notion is generally prevalent 
 both on the Continent and in this country : nothing is more 
 familiar than the expression ' study the effects in order to 
 arrive at the causes.' 
 
 Instead of regarding the proper object of physical science 
 
INTROD UCTOR Y REMARKS. 7 
 
 as a search after essential causes, I believe it ought to be, and 
 must be, a search after facts and relations that although the 
 word Cause may be used in a secondary and concrete sense, as 
 meaning antecedent forces, yet in an abstract sense it is to- 
 tally inapplicable : we cannot predicate of any physical agency 
 that it is abstractedly the cause of another ; and if, for the sake 
 of convenience, the language of secondary causation be per- 
 missible, it should be with reference only to the special phe- 
 nomenon referred to, as it can never be generalised, 
 
 The misuse, or rather varied use, of the term Cause, has 
 been a source of great confusion in physical theories, and phi- 
 losophers are even now by no means agreed as to their con- 
 ception of causation. The most generally received view of 
 causation, that of Hume, refers it to invariable antecedence, 
 i.e. we call that a cause which invariably precedes, that an 
 effect which invariably succeeds. Many instances of invariable 
 sequence might, however, be selected, which do not present 
 the relation of cause and effect : thus, as Reid observes, and 
 Brown does not satisfactorily answer, day invariably precedes 
 night, and yet day is not the cause of night The seed, again, 
 precedes the plant, but is not the cause of it ; so that when 
 we study physical phenomena it becomes difficult to separate 
 the idea of causation from that of force, and these have been 
 regarded as identical by some philosophers. To take an ex- 
 ample which will contrast these two views : if a floodgate be 
 raised, the water flows out ; in ordinary parlance, the water 
 is said to flow because the floodgate is raised : the sequence 
 is invariable ; no floodgate, properly so called, can be raised 
 without the water flowing out, and yet in another, and perhaps 
 more strict, sense, it is the gravitation of the water which 
 causes it to flow. But though we may truly say that, in this 
 instance, gravitation causes the water to flow, we cannot in 
 truth abstract the proposition, and say, generally, that gravi- 
 tation is the cause of water flowing, as water may flow from 
 other causes, gaseous elasticity for instance, which will cause 
 water to flow from a receiver full of air into one that is ex- 
 hausted ; gravitation may also, under certain circumstances, 
 arrest instead of cause the flow of water. 
 
 Upon neither view, however, can we get at anything like 
 
8 CORRELATION OF PHYSICAL FORCES. 
 
 abstract causation. If we regard causation as invariable 
 sequence, we can find no case in which a given antecedent is 
 the only antecedent to a given sequent : thus, if water could 
 flow from no other cause than the withdrawal of a floodgate, 
 we might say abstractedly that this was the cause of water 
 flowing. If, again, adopting the view which looks to causa- 
 tion as a force, we could say that water could be caused to 
 flow only by gravitation, we might say abstractedly that gra- 
 vitation was the cause of water flowing but this we cannot 
 say ; and if we seek and examine any other example, we shall 
 find that causation is only predicable of it in the particular 
 case, and cannot be supported as an abstract proposition ; yet 
 this is constantly attempted. Nevertheless, in each particular 
 case where we speak of Cause, we habitually refer to some 
 antecedent power or force : we never see motion or any 
 change in matter take effect without regarding it as produced 
 by some previous change ; and, when we cannot trace it to its 
 antecedent, we mentally refer it to one ; but whether this habit 
 be philosophically correct may be disputed. In other words, 
 it seems questionable, not only whether cause and effect are 
 convertible terms with antecedence and sequence, but whether 
 in fact cause does precede effect, whether force does precede 
 the change in matter of which it is said to be the cause. 
 
 The actual priority of cause to effect has been doubted, 
 and their simultaneity argued with much ability. As an 
 instance of this argument it may be said, the attraction which 
 causes iron to approach the magnet is simultaneous with and 
 ever accompanies the movement of the iron ; the movement 
 is evidence of the co-existing cause or force, but there is no 
 evidence of any interval in time between the one and the 
 other. On this view time would cease to be a necessary ele- 
 ment in causation ; the idea of cause, except perhaps as re- 
 ferred to a primeval creation, would cease to exist ; and the 
 same arguments which apply to the simultaneity of cause with 
 effect would apply to the simultaneity of Force with Motion. 
 We could not, however, even if we adopted this view, dispense 
 with the element of time in the sequence of phenomena ; the 
 effect being thus regarded as ever accompanied simultane- 
 ously by its appropriate cause, we should still refer to some 
 
INTRODUCTORY REMARKS. 9 
 
 antecedent effect ; and our reasoning as applied to the suc- 
 cessive production of all natural changes would be the same. 
 
 Habit and the identification of thoughts with phenomena 
 so compel the use of recognised terms, that we cannot avoid 
 using the word ' cause ' even in the sense in which objection is 
 taken ; and if we struck it out of our vocabulary, our lan- 
 guage, in speaking of successive changes, would be unintelli- 
 gible to the present generation. The common error, if I am 
 right in supposing it to be such, consists in the abstraction of 
 cause, and in supposing in each case a general secondary 
 cause a something which is not the first cause, but which, if 
 we examine it carefully, must have all the attributes of a 
 first cause, and an existence independent of, and dominant 
 over, matter. 
 
 The relations of electricity and magnetism afford us a very 
 instructive example of the belief in secondary causation. Sub- 
 sequent to the discovery by Oersted of electro-magnetism, and 
 prior to that by Faraday of magneto-electricity, electricity 
 and magnetism were believed by the highest authorities to 
 stand in the relation of cause and effect i.e. electricity was 
 regarded as the cause, and magnetism as the effect; and where 
 magnets existed without any apparent electrical currents to 
 cause their magnetism, hypothetical currents were supposed, 
 for the purpose of carrying out the causative view ; but mag- 
 netism may now be said with equal truth to be the cause of 
 electricity, and electrical currents may be referred to hypothe- 
 tical magnetic lines : if therefore electricity cause magnetism, 
 and magnetism cause electricity, why then electricity causes 
 electricity, which becomes, so as to speak, a rcditctio ad 
 absurdum of the doctrine. 
 
 To take another instance, which may render these posi- 
 tions more intelligible. By heating bars of bismuth and anti- 
 mony in contact, a current of electricity is produced ; and if 
 their extremities be united by a fine wire, the wire is heated. 
 Now, here the electricity in the metals is said to be caused by 
 heat, and the heat in the wire to be caused by electricity, and 
 in a concrete sense this is true ; but can we thence say ab- 
 stractedly that heat is the cause of electricity, or that elec- 
 tricity is the cause of heat ? Certainly not ; for if either be 
 
io CORRELATION OF PHYSICAL FORCES. 
 
 true, both must be so, and the effect then becomes the cause 
 of the cause, or, in other words, a thing causes itself. Any 
 other proposition on this subject will be found to involve 
 similar difficulties, until, at length, the mind will become con- 
 vinced that abstract secondary causation does not exist, and 
 that a search after essential causes is vain. 
 
 The position which I seek to establish in this Essay is, 
 that the various affections of matter which constitute the main 
 objects of experimental physics, viz. heat, light, electricity, 
 magnetism, chemical affinity, and motion, are all correlative, or 
 have a reciprocal dependence; that neither, taken abstractedly, 
 can be said to be the essential cause of the others, but that 
 either may produce or be convertible into, any of the others : 
 thus heat may mediately or immediately produce electricity, 
 electricity may produce heat ; and so of the rest, each merging 
 itself as the force it produces becomes developed : and that 
 the same must hold good of other forces, it being an irresis- 
 tible inference from observed phenomena that a force cannot 
 originate otherwise than by devolution from some pre-existing 
 force or forces. 
 
 The term force, although used in very different senses by 
 different authors, in its limited sense may be defined as that 
 which produces or resists motion. Although strongly inclined 
 to believe that the other affections of matter, which I have 
 above named, are and will ultimately be resolved into modes 
 of motion, many arguments for which view will be given in 
 subsequent parts of this Essay, it would be going too far, at 
 present, to assume their identity with it ; I therefore use the 
 term force in reference to them, as meaning that active prin- 
 ciple inseparable from matter which is supposed to induce its 
 various changes. 
 
 The word force and the idea it aims at expressing, might be 
 objected to by the purely physical philosopher on similar 
 grounds to those which apply to the word cause, as it repre- 
 sents a subtle mental conception, and not a sensuous per- 
 ception or phenomenon. The objection would take something 
 of this form. If the string of a bent bow be cut, the bow will 
 straighten itself; we thence say there is an elastic force in the 
 bow which straightens it ; but if we applied our expressions to 
 
INTRODUCTORY REMARKS, n 
 
 this experiment alone, the use of the term force would be 
 superfluous, and would not add to our knowledge on the sub- 
 ject. All the information which our minds could get would 
 be as sufficiently obtained from the expression, when the 
 string is cut the bow becomes straight, as from the expression, 
 the bow becomes straight by its elastic force. Do we know 
 more of the phenomenon, viewed without reference to other 
 phenomena, by saying it is produced by force ? Certainly 
 not. All we know or see is the effect ; we do not see force 
 we see motion or moving matter. 
 
 If now we take a piece of caoutchouc and stretch it, when 
 released it returns to its original length. Here, though the 
 subject-matter is very different, we see some analogy in the 
 effect or phenomenon to that of the strung bow. If, again, we 
 suspend an apple by a string, cut the string, the apple falls. 
 Here, though it is less striking, there is still an analogy to the 
 strung bow and the caoutchouc. 
 
 Now, when the word force is employed as comprehending 
 these three different phenomena, we find some use in the term, 
 not by its explaining or rendering more intelligible the modus 
 agendi of matter, but as conveying to the mind something 
 which is alike in the three phenomena, however distinct they 
 may be in other respects : the word becomes an abstract or 
 generalised expression, and regarded in this light is of high 
 utility. Although I have given only three examples, it is 
 obvious that the term would equally apply to 300 or 3,000 
 cases. 
 
 But it will be said, the term force is used not as expressing 
 the effect, but as that which produces the effect. This is true, 
 and in this its ordinary sense I shall use it in these pages. 
 But though the term has thus a potential meaning, to depart 
 from which would render language unintelligible, we must 
 guard against supposing that we know essentially more of the 
 phenomena by saying they are produced by something, which 
 something is only a word derived from the constancy and 
 similarity of the phenomena we seek to explain by it. The 
 relations of the phenomena to which the terms force or forces 
 are applied give us real knowledge ; these relations may be 
 called relations of forces ; our knowledge of them is not 
 
12 CORRELATION OF PHYSICAL FORCES. 
 
 thereby lessened, and the convenience of expression is greatly 
 increased, but the separate phenomena are not more intimately 
 known ; no further insight into why the apple falls is acquired 
 by saying it is forced to fall, or it falls by the force of gravi- 
 tation ; by the latter expression we are enabled to relate it 
 most usefully to other phenomena, but we still know no more 
 of the particular phenomenon than that under certain circum- 
 stances the apple does fall. 
 
 In the above illustrations force has been treated as the 
 producer of motion, in which case the evidence of the force is 
 the motion produced ; thus we estimate the force used to 
 project a cannon-ball in terms of the mass of matter and the 
 velocity with which it is projected. The evidence of force when 
 the term is applied to resistance to motion is of a somewhat 
 different character; the matter resisting is molecularly affected, 
 and has its structure more or less changed ; thus a strip of 
 caoutchouc to which a weight is suspended is elongated, and 
 its molecules are displaced as compared with their position 
 when unaffected by the gravitating force. So a piece of glass 
 bent by an appended weight has its whole structure changed ; 
 this internal change is made evident by transmitting through 
 it a beam of polarised light ; a relation thus becomes 
 established between the molecular state of bodies and the 
 external forces or motion of masses. Every particle of the 
 caoutchouc or glass must be acting and contributing to resist 
 or arrest the motion of the mass of matter appended to it. 
 
 We need some word to express this state of tension ; we 
 know that it produces an effect, though the effect be nega- 
 tive in character : although in this effort of inanimate matter 
 we can no more trace the mode of action to its ultimate 
 elements than we can follow out the connection of our own 
 muscles with the volition which calls them into action, we 
 are experimentally convinced that matter changes its state 
 by the agency of other matter, and this agency we call 
 force. 
 
 In placing the weight on the glass, we have moved the 
 former to an extent equivalent to that which it would again 
 describe if the resistance were removed, and this motion of the 
 mass becomes an exponent or measure of the force exerted 
 
INTR OD UCTOR Y REMARKS. 1 3 
 
 on the glass ; while this is in the state of tension, the force is 
 ever existing, capable of reproducing the original motion, and 
 while in a state of abeyance as to actual motion, it is really 
 acting on the glass. The motion is suspended, but the force 
 is not annihilated. 
 
 But it may be objected, if tension or static force be thus 
 motion in abeyance, there is at all times a large amount of 
 dynamical action subtracted from the universe. Every stone 
 raised and left upon a hill, every spring that is bent, and has 
 required force to upraise or bend it, has for a time, and pos- 
 sibly for ever, withdrawn this force, and annihilated it. Not 
 so ; when we raise a weight and leave it at the point to which 
 it has been elevated we have changed the centre of gravity of 
 the earth, and consequently the earth's position with reference 
 to the sun, planets, and stars ; the effort we have made per- 
 vades and shakes the universe ; nor can we present to the 
 mind any exercise of force, which is not thus permanent in its 
 dynamical effects. If, instead of one weight being raised, we 
 raise two weights, each placed at points of the earth diame- 
 trically opposite each other, it would be said, here you have 
 compensation, a balance, no change in the centre of gravity of 
 the earth ; but we have increased the mean diameter of the 
 earth, and a perturbation of our planet, and of all other celes- 
 tial bodies, necessarily ensues. 
 
 The force may be said to be in abeyance with reference to 
 the effect it would have produced, if not arrested, or placed in 
 a state of tension ; but in the act of imposing this state, the 
 relations of equilibrium with other bodies have been changed, 
 and these move in their turn, so that motion of the same 
 amount would seem to be ever affecting matter conceived in 
 its totality. 
 
 Press the hands violently together ; the first notion may 
 be that this is power locked up, and that no change ensues. 
 Not so ; the blood courses more quickly, respiration is acce- 
 lerated, changes, which we may not be able to trace, take 
 place in the muscles and nerves, transpiration is increased ; we 
 have given off force in various ways, and must, if such efforts 
 be prolonged, replenish our sources of power, by fresh chemical 
 action in the stomach. 
 
i 4 CORRELATION OF PHYSICAL FORCES. 
 
 In books which treat of statics and dynamics, it is common 
 and perhaps necessary to isolate the subjects of consideration ; 
 to suppose, for instance, two bodies gravitating, and to ignore 
 the rest of the universe. But no such isolation exists in 
 reality, nor could we predict the result if it did exist. Would 
 two bodies gravitate towards each other in empty space, if 
 space can be empty ? The notion that they would is founded 
 on the theory of attraction, which Newton himself repudiated, 
 further than as a convenient means of regarding the subject. 
 For purposes of instruction or argument it may be convenient 
 to assume isolated matter: many conclusions so arrived at 
 may be true, but many will be erroneous. 
 
 If, in producing effects of tension or of static force, the 
 effort made pervades the universe, it may be said, when the 
 bent spring is freed, when the raised weight falls, a converse 
 series of motions must be effected, and this theory would 
 lead to a mere reciprocation, which would be equally unpro- 
 ductive of permanent change with the annihilation of force. 
 If raising the weight has changed the centre of gravity of 
 the earth, and thence of the universe, the fall of the weight, 
 it will be said, restores the original centre of gravity, and 
 everything comes back to its original status. In this argument 
 we again, in thought, isolate our experiment ; we neglect 
 surrounding circumstances. Between the time of the raising 
 and falling of the weight, be the interval never so small, nay 
 more, during the rising and during the fall, the earth has been 
 going on revolving round its axis and round the sun, to say 
 nothing of other changes, such as temperature, cosmical mag- 
 netism, &c., which we may call accidental, but which, if we 
 knew all, would probably be found to be as necessary and as 
 reducible to law as the motion of the earth. A change having 
 taken place, the fall of the weight does not bring back the 
 status quo, but other changes supervene, and so on. Nothing 
 repeats itself, because nothing can be placed again in the 
 same condition : the past is irrevocable. 
 
MOTION. 
 
 MOTION which has been taken as the main exponent of 
 force in the above examples is the most obvious, the most 
 distinctly conceived of all the affections of matter. Visible 
 motion, or relative change of position in space, is a phenomenon 
 so obvious to simple apprehension, that to attempt to define 
 it would be to render it more obscure ; but with motion, as 
 with all physical appearances, there are certain vanishing 
 gradations or undefined limits, at which the obvious mode of 
 action fades away ; to detect the continuing existence of the 
 phenomena we are obliged to have recourse to other than 
 ordinary methods of investigation, and we frequently apply 
 other and different names to the effects so recognised. 
 
 Thus sound is motion ; and although in the earlier periods 
 of philosophy the identity of sound and motion was not traced 
 out, and they were considered distinct affections of matter 
 indeed, at the close of the last century a theory was advanced 
 that sound was transmitted by the vibrations of an ether we 
 now so readily resolve sound into motion, that to those who 
 are familiar with acoustics, the phenomena of sound imme- 
 diately present to the mind the idea of motion, i.e. motion of 
 ordinary matter. 
 
 Again, with regard to light : no doubt now exists that 
 light moves or is accompanied by motion. Here the pheno- 
 mena of motion are not made evident by the ordinary sensuous 
 perception, as, for instance, the motion of a visibly moving 
 projectile would be, but by an inverse deduction from known 
 relations of motion to time and space : as all observation 
 teaches us that bodies in moving from one point in space to 
 another occupy time, we conclude that, wherever a continuing 
 phenomenon is rendered evident in two different points of space 
 at different times, there is motion, though we cannot see the 
 
r6 CORRELATION OF PHYSICAL FORCES. 
 
 progression. A similar deduction convinces us of the motion 
 of electricity. 
 
 As we in common parlance speak of sound moving, although 
 sound is motion, it requires no great stretch of imagination to 
 conceive light and electricity as motions, and not as things 
 of moving. If one end of a long bar metal be struck, a sound 
 is soon perceptible at the other end. This we now know to be 
 a vibration of the bar ; sound is but a word expressive of the 
 mode of motion impressed on the bar ; so one end of a column 
 of air or glass subjected to a luminous impulse gives a per- 
 ceptible effect of light at the other end : this can equally be 
 conceived to be a vibration or transmitted motion of particles 
 in the transparent column : this question will, however, be 
 further discussed hereafter ; for the present we will confine 
 ourselves to motion within the limits to which the term is 
 usually restricted. 
 
 With the perceptible phenomena of motion the mental 
 conception has been invariably associated to which I have 
 before alluded, and to which the term force is given the 
 which conception, when we analyse it, refers us to some antece- 
 dent motion. If we except the production of motion by heat, 
 light, &c., which will be considered in the sequel, when w r e see 
 a body moving we look to motion having been communicated 
 to it by matter which has previously moved. 
 
 Of absolute rest Nature gives us no evidence : all matter, 
 as far as we can ascertain, is ever in movement, not merely in 
 masses, as with the planetary spheres, but also molecularly, 
 or throughout its most intimate structure : thus every altera- 
 tion of temperature produces a molecular change throughout 
 the whole substance heated or cooled ; slow chemical or elec- 
 trical actions, actions of light or invisible radiant forces, are 
 always at play, so that as a fact we cannot predicate of any 
 portion of matter that it is absolutely at rest. Supposing, 
 however, that motion is not an indispensable function of 
 matter, but -that matter can be at rest, matter at rest would 
 never of itself cease to be at rest ; it would not move unless 
 impelled to such motion by some other moving body, or body 
 which has moved. This proposition applies not merely to 
 impulsive motion, as when a ball at rest is struck by a moving 
 
MOTION. ij 
 
 body, or pressed by a spring which has previously been moved, 
 but to motion caused by attractions such as magnetism or 
 gravitation. Suppose a piece of iron at rest in contact with 
 a magnet at rest ; if it be desired to move the iron by the 
 attraction of the magnet, the magnet or the iron must first be 
 moved ; so before a body falls it must first be raised. A 
 body at rest would therefore continue so for ever, and a body 
 once in motion would continue so for ever, in the same direc- 
 tion and with the same velocity, unless impeded by some other 
 body, or affected by some other force than that which origi- 
 nally impelled it. These propositions may seem somewhat 
 arbitrary, and it has been doubted whether they are necessary 
 truths ; they have for a long time been received as axioms, 
 and there can at all events be no harm in accepting them as 
 postulates. It is however very generally believed that if the 
 visible or palpable motion of one body be arrested by its 
 impact on another body, the motion ceases, and the force which 
 produced it is annihilated. 
 
 Now, the view which I venture to submit is, that force 
 cannot be annihilated, but is merely subdivided or altered in 
 direction or character. First, as to direction. Wave your 
 hand : the motion, which has apparently ceased, is taken up 
 by the air, from the air by the walls of the room, &c., and so 
 by direct and reacting waves, continually comminuted, but 
 never destroyed, It is true that, at a certain point, we lose 
 all means of detecting the motion, from its minute subdivision, 
 which defies our most delicate means of appreciation, but we 
 can indefinitely extend our power of detecting it accordingly 
 as we confine its direction, or increase the delicacy of our 
 examination. Thus, if the hand be moved in unconfined air, 
 the motion of the air would not be sensible to a person at a 
 few feet distance ; but if a piston of the same extent of sur- 
 face as the hand be moved with the same rapidity in a tube, 
 the blast of air may be distinctly felt at several yards' distance. 
 There is no greater absolute amount of motion in the air in 
 the second than in the first case, but its direction is restrained, 
 so as to make the means of detection more facile. By carry- 
 ing on this restraint, as in the air-gun, we get a power of 
 detecting the motion, and of moving other bodies at far greater 
 
 C 
 
1 8. CORRELATION OF PHYSICAL FORCES. 
 
 distances. The puff of air which would in the air-gun project 
 a bullet a quarter of a mile, if allowed to escape without its 
 direction being restrained, as by the bursting of a bladder, 
 would not be perceptible at a yard distance, though the same 
 absolute amount of motion be in both the cases impressed on 
 the surrounding air. 
 
 It may, however, be asked, what becomes of force when 
 motion is arrested or impeded by the counter-motion of another 
 body ? This is generally believed to produce rest, or entire 
 destruction of motion, and consequent annihilation of force: 
 so indeed it may, as regards the motion of the masses, but a 
 new force, or new character of force, now ensues, the exponent 
 of which, instead of visible motion, is heat. I venture to 
 regard the heat which results from friction or percussion as a 
 continuation of the force which was previously associated with 
 the moving body, and which, when this impinges on another 
 body, ceasing to exist as gross, palpable motion, continues to 
 exist as heat. 
 
 Thus, let two bodies, A and B, be supposed to move in 
 opposite directions (putting for the moment out of the question 
 all resistance, such as that of the air, &c.), if they pass each 
 other without contact each will move on for ever in its res- 
 pective direction with the original velocity, but if they touch 
 each other the velocity of the movement of each is reduced, 
 and each becomes heated : if this contact be slight, or such as 
 to occasion but a slight diminution of their velocity, as when 
 the surfaces of the bodies are oiled, then the heat is slight ; 
 but if the contact be such as to occasion a great diminution 
 of motion, as in percussion, or as when the surfaces are 
 roughened, then the heat is great, so that in all cases the 
 resulting heat is proportionate to the diminished velocity. 
 Where, instead of resisting and consequently impeding the 
 motion of the body A, the body B gives way, or itself takes 
 up the motion originally communicated to A, then we have 
 less heat in proportion to the motion of the body B, for here 
 the operation of the force continues in the form of palpable 
 motion : thus the heat resulting from friction in the axle of a 
 wheel is lessened by surrounding it by rollers ; these take up 
 the primary motion of the axle, and the less, by this means, 
 
MOTION. I9 
 
 the initial motion is impeded, the less is the resulting heat. 
 Again, if a body move in a fluid, although some heat is pro- 
 duced, the heat is apparently trifling, because the particles of 
 the fluid themselves move, and continue the motion originally 
 communicated to the moving body : for every portion of 
 motion communicated to them this loses an equivalent, and 
 where both lose, then an equivalent of heat results. 
 
 As the converse of this proposition, it should follow that 
 the more rigid the bodies impinging on each other the greater 
 should be the amount of heat developed by friction, and so 
 we find it. Flint, steel, hard stones, glass, and metals are 
 those bodies which give the greatest amount of heat from 
 friction or percussion ; while water, oil, &c. give little or no 
 heat, and from the ready mobility of their particles lessen its 
 development when they are interposed between rigid moving 
 bodies. Thus, if we oil the axles of wheels, we have more rapid 
 motion of the bodies themselves, but less heat ; if we increase 
 the resistance to motion, as by roughening the points of con- 
 tact, so that each particle strikes against and impedes the 
 motion of others, then we have diminished motion, but in- 
 creased heat ; or if the bodies be smooth, but instead of sliding 
 past each other be pressed closely together and then rubbed, 
 we shall in many cases evolve more heat than by the 
 roughened bodies, as we get a greater number of particles in 
 contact and a greater resistance to the initial motion. I cannot 
 present to my mind any case of heat resulting from friction 
 which is not explicable by this view : friction, according to 
 it, is simply impeded motion. The greater the impediment, 
 the more force is required to overcome it, and the greater is 
 the resulting heat ; this resulting heat being a continuation 
 of indestructible force, capable, as we shall presently see, of 
 reproducing palpable motion, or motion of definite masses. 
 
 Whatever be the nature of the bodies, rough or smooth, 
 solid or liquid, provided there be the same initial force, and 
 the whole motion be ultimately arrested, there should be the 
 same amount of heat developed, though where the motion is 
 carried on through a great number of points of matter we do 
 not so sensibly perceive the resulting heat from its greater 
 dissipation. The friction of fluids produces heat, an effect first 
 
 c 2 
 
2d CORRELATION OF PHYSICAL FORCES. 
 
 noticed, I believe, by Mayer. The total heat produced by the 
 friction of fluids should, therefore, it will be said, be equal to 
 that produced by the friction of solids ; for although each 
 particle produces little heat, the motion being readily taken 
 up by the neighbouring particles, yet by the time the whole 
 mass has attained a state of rest there has been the same 
 impeding of the initial motion as by the friction of solids if 
 produced by the same initial force. If the heat be viewed in 
 the aggregate, and allowance be made for the specific thermal 
 capacity of the substances employed, it probably is the same, 
 though apparently less ; the heat in the case of solids being 
 manifested at certain defined points, while in that of fluids it 
 is dissipated, both the time and space during and through 
 which the motion is propagated differ in the two cases, so that 
 the heat in the latter case is more readily carried off by sur- 
 rounding bodies. 
 
 If the body be elastic, and by its reaction the motion im- 
 pressed on it by the initial force be continued, then the heat 
 is proportionately less ; and were a substance perfectly elastic 
 and no resistance opposed to it by the air or other matter, 
 then the movement once impressed would be perpetual, and 
 no heat would result. A ball of caoutchouc bandied about 
 for many minutes between a racket and a wall is not per- 
 ceptibly heated, while a leaden bullet projected by a gun 
 against a wall is rendered so hot as to be intolerable to the 
 touch : in the former case, the motion of the mass is con- 
 tinued by the reaction due to its elasticity ; in the latter the 
 motion of the mass is extinguished and heat ensues. 
 
 A pendulum started in the exhausted receiver of an air- 
 pump continues its oscillation for hours or even days ; the 
 friction at its point of suspension and the resistance of the air 
 is minimised, and the heat is therefore imperceptible, but these 
 trifling resistances in the end arrest the motion of the mass, 
 the one giving it out as heat, the other conveying the force to 
 the receiver, and thence to surrounding bodies. Similar 
 reasoning may be applied to the oscillation of a coiled spring 
 and balance-wheel. 
 
 To wind up a clock a certain amount of force is expended 
 by the muscles ; this force is given back by the descent of 
 
MOTION. 21 
 
 the weight, the wheels move, the pendulum is kept oscillating, 
 heat is generated at each point of friction, and the surrounding 
 air is set in motion, a part of which is made obvious to us by 
 the ticking sound. But it will be said, if instead of allowing 
 the weight to act upon the machinery, the cord by which it 
 is suspended be cut, the weight drops and the force is at an 
 end. By no means, for in this case the house is shaken by 
 the concussion, and thus the force and motion are continued, 
 while in the former case the weight reaches the ground quietly, 
 and no evidence of force or motion is manifested by its im- 
 pact, the whole having been previously dissipated. 
 
 If the initial motion, instead of being arrested by the impact 
 of other bodies, as in friction or percussion, is impeded by 
 confinement or compression, as where the dilatation of a gas 
 is prevented by mechanical means, heat equally results : thus 
 if a piston is used to compress air in a closed vessel, the com- 
 pressed air and, from it, the sides of the vessel, will be heated : 
 the air being unable to take up and carry on the original 
 motion, communicates molecular motion or expansion to all 
 bodies in contact with it ; and, conversely, if we expand air 
 by mechanical motion, as by withdrawing the piston, cold is 
 produced. So when a solid has its particles compressed or 
 brought nearer together, as when a bar of iron is hammered, 
 heat is produced beyond that which is due to percussion 
 alone. In this latter case we cannot very easily effect the 
 converse result, or produce cold by the mechanical dilatation 
 of a solid, though the phenomena of solution, where the par- 
 ticles of a solid are detached from each other, or drawn more 
 widely asunder, give us an approximation to it : in the case 
 of solution cold is produced. 
 
 We are from a very extensive range of observation and 
 experiment entitled to conclude that, with some curious 
 exceptions to be presently noticed, whenever a body is com- 
 pressed or brought into smaller dimensions it is heated, i.e. 
 it expands neighbouring substances. Whenever it is dilated 
 or increased in volume it is cooled, or contracts neighbouring 
 substances. 
 
 Mr. Joule has made several experiments for the pur- 
 pose of ascertaining what quantity of heat is produced 
 
22 CORRELATION OF PHYSICAL FORCES. 
 
 by a given mechanical action. His mode of experimenting 
 is as follows : An apparatus formed of floats or paddles of 
 brass or iron is made to rotate in a bath of water or mercury. 
 The power which gives rise to this rotation is a weight raised 
 like a clock-weight to a certain height ; this by acting during 
 its fall on a spindle and pulley communicates motion to the 
 paddle-wheel, the water or mercury serving as a friction 
 medium and calorimeter ; and the heat is measured by a 
 delicate mercurial thermometer. The results of his experi- 
 ments he considers prove that a fall of 772 Ibs. through a 
 space of one foot is able to raise the temperature of one pound 
 of water through one degree of Fahrenheit's thermometer. 
 Mr. Joule's experiments are of extreme delicacy he tabu- 
 lates to the thousandth part of a degree of Fahrenheit, and a 
 large number of his thermometric data are comprehended 
 within the limits of a single degree. Other experimenters have 
 given very different numerical results, but the general opinion 
 seems to be that the numbers given by Mr. Joule are the 
 nearest approximation to the truth yet obtained. 
 
 Hitherto I have made no distinction as to the physical 
 character of the bodies impinging on each other ; but Nature 
 gives us a remarkable difference in the character or mode of 
 the force eliminated by friction, accordingly as the bodies 
 which impinge are homogeneous or heterogeneous : if the 
 former, heat alone is produced; if the latter, electricity. 
 
 We find, indeed, instances given by authors of electricity 
 resulting from the friction of homogeneous bodies ; but, as I 
 stated in my original Lectures, I have not found such facts 
 confirmed by my own experiments, and this conclusion has 
 been corroborated by some experiments of Professor Erman, 
 communicated to the meeting of the British Association in 
 the year 1845, in which he found that no electricity resulted 
 from the friction of perfectly homogeneous substances ; as, 
 for instance, the ends of a broken bar. Such experiments as 
 these will, indeed, be seldom free from slight electrical cur- 
 rents, on account of the practical difficulty of fulfilling the 
 condition of perfect homogeneity in the substances themselves, 
 their size, their temperature, &c. ; but the effects produced 
 are very trifling and vary in direction, and the resultant effect 
 
MOTION. 23 
 
 is nought. Indeed, it would be difficult to conceive the con- 
 trary. How could we possibly image to the mind or describe 
 the direction of a current from the same body to the same 
 body, or give instructions for a repetition of the experiment ? 
 It would be unintelligible to say that in rubbing to and fro 
 two pieces of bismuth, iron, or glass a current of electricity 
 circulated from bismuth to bismuth, or from iron to iron, or 
 from glass to glass ; for the question immediately occurs 
 from which bismuth to which does it circulate ? And should 
 this question be answered by calling one piece A, and the 
 other B, this would only apply to the particular specimens 
 employed, the distinctive appellation denoting a distinction 
 in fact, as otherwise A could be substituted for B, and the 
 bar to which the positive electricity flowed would in turn be- 
 come the bar to which the negative electricity flowed. We 
 may say that it circulates from rough glass to smooth, from 
 cast iron to wrought, for here there is not homogeneity. It 
 is moreover conceivable, though not experimentally proved, 
 that when the motion is continuous in a definite direction, 
 electricity may result from the friction of homogeneous bodies. 
 If A and B rub against each other, revolving in opposite 
 directions, concentric currents of positive and negative elec- 
 tricity may be conceived circulating within the metals, and 
 be described by reference to the direction of their motion ; 
 this indeed would be a different phenomenon from those we 
 have been considering ; but without some distinction between 
 the two substances in quality or direction, the electrical effects 
 are indescribable, if not inconceivable. 
 
 When, however, homogeneous bodies are fractured or even 
 rubbed together, phenomena are observed to which the term 
 electricity is applied ; a flash or line of light appears at the 
 point of friction, which by some is called electrical, by others 
 phosphorescent. 
 
 I have myself observed a remarkable case of the kind in 
 the caoutchouc fabric now commonly used for waterproof 
 clothing : if two folds of this substance be allowed to cohere 
 so as partly to unite and present a difficulty of separation, 
 then, on stripping the one from the other, or tearing them 
 asunder, a line of light will follow the line of separation. 
 
24 CORRELATION OF PHYSICAL FORCES. 
 
 If this class of phenomena be electrical, it is electricity 
 determined as it is generated ; there is no dual character 
 impressed on the matter acting, the flash is electrical, as a 
 spark from the percussion of flint is electrical, or as the slow 
 combustion of phosphorus, or any other case of the develop- 
 ment of heat and light. It seems to be better to class this 
 phenomenon under the categories of heat and light than 
 under that of electricity, the latter word being retained for 
 those cases where a dual or polar character of force is mani- 
 fested. In experiments which have been made by the friction 
 of similar substances where the one appears positively and 
 the other negatively electrical, there will be found some 
 difference in the mode of rubbing, by which the molecular 
 state of the bodies is in all probability changed, making one 
 a dissimilar substance from the ether ; thus it is said by 
 Bergman n, that when two pieces of glass are rubbed so that 
 all the parts of one pass over one part of the other, the former 
 is positive and the latter negative. It is obvious that in this 
 case the rubbing in one is confined to a line, and that must 
 be more altered in molecular structure at the line of friction 
 than the one where the friction is spread over the whole 
 surface : so if a ribbon be drawn transversely over another 
 ribbon, the substances are not, qua the rubbing action, identi- 
 cal ; so again, in the rupture of crystals, we are dealing with 
 substances having a polar arrangement of particles the sur- 
 faces of the fragments cannot be assumed to be molecularly 
 identical. 
 
 The development of electricity by the common electrical 
 machine arises, as far as I can understand it, from the separa- 
 tion or rupture of contiguity between dissimilar bodies ; a 
 metallic surface, the amalgam of the cushion, is in contact 
 with glass ; these two bodies act upon each other by the force 
 of cohesion ; and when, by an external mechanical force, this is 
 ruptured, as it is at each moment of the motion of the glass 
 plate or cylinder, electricity is developed in each : were they 
 similar bodies, heat only would be developed. 
 
 According to the experiments of Mr. Sullivan electricity 
 may be produced by vibration alone if the substance vibrating 
 be composed either of dissimilar metals, as a wire partly of 
 
MOTION. 25 
 
 iron and partly of brass, caused to emit a musical sound, or of 
 the same metal if its parts be not homogeneous, as a piece of 
 iron one portion of which is hard and crystallised, and the 
 other soft and fibrous ; the current resulting appears to be 
 due to the vibration, and not to heat engendered, as it ceases 
 immediately with the vibration. 
 
 I venture therefore to think that in our present state of 
 knowledge, where the mutually impinging bodies are homo- 
 geneous, heat and not electricity is the result of friction and 
 percussion ; where the bodies impinging are heterogeneous, we 
 may safely state that electricity is always produced by friction 
 or percussion, although heat in a greater or less degree accom- 
 panies it ; but when we come to the question of the ratio in 
 which frictional electricity is produced, as determined by the 
 different characters of the substances employed, we find very 
 complex results. Bodies may differ in so many particulars 
 which influence more or less the development of electricity, 
 such as their chemical constitution, the state of their surfaces, 
 their state of aggregation, their transparency or opacity, their 
 power of conducting electricity, &c., that the normcs of their 
 action are very difficult of attainment. As a general rule, it 
 may be said that the development of electricity is greater 
 when the substances employed are broadly distinct in their 
 physical and chemical qualities, and more particularly in their 
 conducting powers ; but up to the present time the laws 
 governing such development have not been even approximately 
 determined. 
 
 I have said, in reference to the various forces or affections 
 of matter, that either of them may, mediately or immediately, 
 produce the others ; and this is all I can venture to predicate 
 of them in the present state of science. Where at present 
 no immediate relation is established between any of them, 
 electricity generally forms the intervening link. 
 
 Motion, then, will directly produce heat and electricity, and 
 electricity, being produced by it, will produce magnetism a 
 force which is always developed by electrical currents, at right 
 angles to the direction of those currents, as will be subse- 
 quently more fully explained. Light also is readily produced, 
 to all appearance, by motion, directly, when accompanying the 
 
26 CORRELATION OF PHYSICAL FORCES. 
 
 heat of friction, or mediately, by electricity resulting from mo- 
 tion, as in the electrical spark, which has most of the attributes 
 of solar light, differing from it only in those respects in which 
 light differs when emanating from different sources or seen 
 through different media ; for instance, in the position of the 
 fixed lines in the spectrum or in the ratios of the spaces 
 occupied by rays of different refrangibility. In the decom- 
 positions and compositions which the terminal points pro- 
 ceeding from the conductors of an electrical machine develope 
 when immersed in different chemical media, we get the pro- 
 duction of chemical affinity by electricity, of which motion is 
 the initial source. Lastly, motion may be again reproduced 
 by the forces which have emanated from motion ; thus, the 
 divergence of the electrometer, the revolution of the electrical 
 wheel, the deflection of the magnetic needle, are, when 
 resulting from frictional electricity, palpable movements re- 
 produced by the intermediate modes of force, which have 
 themselves been originated by motion. 
 
HEAT. 
 
 IF we now take HEAT as our starting-point, we shall find that 
 the other modes of force may be readily produced by it. To 
 take motion first : this is so generally, I think I may say inva- 
 riably, the immediate effect of heat, that we may almost, if 
 not entirely, resolve heat into motion, and view it as a 
 mechanically repulsive force, a force antagonistic to attraction 
 of cohesion or aggregation, and tending to move the particles 
 of all bodies, or to separate them from each other. 
 
 It may be well here to premise, that in using the terms 
 ' particles ' or ( molecules/ which will be frequently employed in 
 this Essay, I do not use them in the sense of the atomist, or mean 
 to assert that matter consists of indivisible particles or atoms. 
 By many who adopt the atomic doctrine the term ( molecule ' 
 is used to signify a definite congeries of atoms, forming an 
 integral constituent of -matter, somewhat as a brick may be 
 said to be a congeries of sand atoms, but a structural element 
 of a house. The word ' molecule ' will be used by me for the 
 necessary purpose of contradistinguishing the action of the 
 indefinitely minute physical elements of matter from that of 
 masses having a sensible magnitude, much in the same way as 
 the terms ' lines ' or ' points ' are used in an abstract sense ; 
 though there does not exist, in fact, a thing which has length 
 and breadth without thickness, and though a thing without 
 parts or dimensions is nothing. 
 
 If we put aside the sensation which heat produces in our 
 own bodies, and regard heat simply in relation to its effect 
 upon inorganic matter, we find that, with a very few excep- 
 tions, which I shall presently notice, the effects of what is 
 called heat are simply an expansion of the matter acted upon, 
 and that the matter so expanded has the power by its own 
 contraction of communicating expansion to all bodies in conti- 
 
28 CORRELATION OF PHYSICAL FORCES. 
 
 guity with it. Thus, if the body be a solid : for instance, iron ; 
 a liquid, say water ; or a gas, say atmospheric air each of 
 these, when heated, is expanded in every direction : in the two 
 former cases, by increasing the heat to a certain point, we 
 change the physical character of the substance, the solid 
 becomes a liquid, and the liquid becomes a gas ; these, how- 
 ever, are still expansions, particularly the latter, when, at a 
 certain period, the expansion becomes rapidly and indefinitely 
 greater. But what is, in fact, commonly done in order to heat 
 a substance, or to increase the heat of a substance ? It 
 is merely approximated to some other heated, that is, to 
 some other expanded substance, which latter is cooled 
 or contracted as the former expands. Let us now divest 
 the mind of the impression that heat is in itself anything 
 substantive, and suppose that these phenomena are regarded 
 for the first time, and without any preconceived notions on 
 the subject ; let us introduce no hypothesis, but merely 
 express as simply as we can the facts of which we have 
 become cognisant ; to what do they amount ? To this, that 
 matter has pertaining to it a molecular repulsive power, a 
 power of dilatation, which is communicable by continuity or 
 proximity. 
 
 Heat thus viewed, is motion, and this molecular motion we 
 may readily change into the motion of masses, or motion in 
 its most ordinary and palpable form : for example, in the 
 steam-engine, thepiston and all its concomitant masses of matter 
 are moved by the molecular dilatation of the vapour of water. 
 
 To produce continuous motion there must be an alternate 
 action of heat and cold ; a given portion of air, foi instance, 
 heated beyond the temperature of the circumambient air, is 
 expanded. If now it be made to act on a movable piston, it 
 moves this to a point at which the tension or elastic force of 
 the confined air equals that of the surrounding air. If the 
 confined air be kept at this point, the piston would remain 
 stationary ; but if it be cooled, the external air exercising then 
 a greater relative degree of pressure, the piston returns 
 towards its original position ; just as it will be seen, when we 
 come to the magnetic force, that a magnet placed in a parti- 
 cular position produces motion in iron near it ; but to make 
 
HEAT. 29 
 
 this motion continuous, or to obtain an available mechanical 
 power, the magnet must be demagnetised, or else a stable 
 equilibrium is obtained. 
 
 In the case of the piston moved by heated air, the motion 
 of the mass becomes the exponent of the' amount of heat 
 i.e. of the expansion or separation of the molecules ; and we 
 do not, by any of our ordinary methods, test heat in any other 
 way than by its purely dynamical action. The various modi- 
 fications of the thermometer and pyrometer are all measurers 
 of heat by motion ; in these instruments liquid or solid bodies 
 are expanded and elongated, i.e. moved in a definite direction, 
 and, either by their own visible motion, or by the motion of 
 an attached index, communicate to our senses the amount of 
 the force by which they are moved. There are, indeed, some 
 delicate experiments which tend to prove that a repulsive 
 action between separate masses is produced by heat. Fresnel 
 thought he had proved that mobile bodies heated in an 
 exhausted receiver repelled each other to sensible distances ; 
 but his experiments have not been confirmed ; and Baden 
 Powell found that the coloured rings usually called Newton's 
 rings change their breadth and position, when the glasses 
 between which they appear are heated, in a manner which 
 showed that the glasses repelled each other. M. Faye's theory 
 of comets is based on "some such repellent force. There is, 
 however, some difficulty in presenting these phenomena to 
 the mind in the same aspect as the molecular repulsive action 
 of heat. 
 
 The phenomena of what is termed latent heat have been 
 generally considered as strongly in favour of that view which 
 regards heat either as actual matter, or, at all events, as a 
 substantive entity, and not a motion or affection of ordinary 
 matter. 
 
 The hypothesis of latent matter is, I venture with diffidence 
 to think, a dangerous one it is something like the old prin- 
 ciple of Phlogiston : it is not tangible, visible, audible ; it is, in 
 fact, a mere subtle mental conception, and ought, I submit, 
 only to be received on the ground of absolute necessity, the 
 more so as these subtleties are apt to be carried on to other 
 natural phenomena, and so they add to the hypothetical 
 
30 CORRELATION OF PHYSICAL FORCES. 
 
 scaffolding which is seldom requisite, and should be sparingly 
 used, even in the early stages of discovery. As an instance 
 I think a striking one, of the injurious effects of this, I will 
 mention the analogous doctrine of ' invisible light ;' and I do 
 this meaning no disrespect to its distinguished author, any 
 more than, in discussing the doctrine of latent heat, I can be 
 supposed, in the slightest degree, to aim at detracting from 
 the merits of the illustrious investigators of the facts which 
 that doctrine seeks to explain. Is not * invisible light ' a con- 
 tradiction in terms ? Has not light ever been regarded as that 
 agent which affects our visual organs ? Invisible light, then, 
 is darkness, and if it exist, then is darkness light. I know it 
 may be said, that one eye can detect light where another 
 cannot ; that a cat may see where a man cannot ; that an 
 insect may see where a cat cannot ; but then it is not invisible 
 light to those who see it : the light, or rather the object seen 
 by the cat, may be invisible to the man, but it is visible to 
 the cat, and, therefore, cannot abstractedly be said to be 
 invisible. If we go further, and find an agent which affects 
 certain substances similarly to light, but does not, as far as we 
 are aware, affect the visual organs of any animal, then is it 
 not an erroneous nomenclature which calls such an agent 
 light ? There are many cases in which a deviation from the 
 once accepted meaning of words has so gradually entered into 
 common usage as to be unavoidable, but I cannot but think 
 that additions to such cases should as far as possible be 
 avoided, as injurious to that precision of language which is 
 one of the safest guards to knowledge, and from the absence 
 of which physical science has materially suffered. 
 
 Let us now shortly examine the question of latent heat, 
 and see whether the phenomena may not be as well, if not 
 more satisfactorily, explained without the hypothesis of latent 
 matter, an idea presenting many similar difficulties to that of 
 invisible light, though more sanctioned by usage. Latent 
 heat is supposed to be the matter of heat, associated, in a 
 masked or dormant state, with ordinary matter, not capable 
 of being detected by any test so long as the matter with 
 which it is associated remains in the same physical state, but 
 communicated to or absorbed from other bodies, when the 
 
HEAT. 3 r 
 
 matter with which it is associated changes its state. To take 
 a common example : a pound or given weight of water at 
 172, mixed with an equal weight of water at 32, will acquire 
 a mean temperature, or 102 ; while water at 1/2, mixed with 
 an equal weight of ice at 32, will be reduced to 32. By the 
 theory of latent heat this phenomenon is thus explained : In 
 the first case, that of the mixture of water with water, both 
 the bodies being in the same physical state, no latent heat is 
 rendered sensible, or sensible heat latent ; but in the second, 
 the ice changing its condition from the solid to the liquid state, 
 abstracts from the liquid as much heat as it requires to main- 
 tain it in the liquid state, which it renders latent, or retains 
 associated with itself, as long as it remains liquid, but of 
 which heat no evidence can be afforded by any thermoscopic 
 test. 
 
 I believe this and similar phenomena, where heat is con- 
 nected with a change of state, may be explained and distinctly 
 comprehended without recourse to the conception of latent 
 heat, though it requires some effort of the mind to divest 
 itself of this idea, and to view the phenomena simply in their 
 dynamical relations. To assist us in so viewing them, let us 
 first parallel with purely mechanical actions certain simple 
 effects of heat, where change of state (I mean such change as 
 from the solid to the liquid, or liquid to the gaseous state) is 
 not concerned. Thus, place within a receiver a bladder, and 
 heat the air within to a higher temperature than that without 
 it, the bladder expands ; so, force the air mechanically into it 
 by the air-pump, the bladder expands ; cool the air on the 
 outside, or remove its pressure mechanically by an exhausting 
 pump, the bladder also expands ; conversely, increase the 
 external repellent force, either by heat or mechanical pressure, 
 and the bladder contracts. In the mechanical effects, the 
 force which produced the distension is derived from, and at 
 the expense of, the mechanical power employed, as from 
 muscular force, from gravitation, from the reacting elasticity 
 of springs, or any similar force by which the air-pump may be 
 worked. In the heating effects, the force is derived from the 
 chemical action in the lamp or source of heat employed. 
 Let us next consider the experiment so arranged that the 
 
32 CORRELATION OF PHYSICAL FORCES. 
 
 force, which produces expansion in the one case, produces a 
 correlative contraction in the other : thus, if two bladders, 
 with a connecting neck between them, be half-filled with air, 
 as the one is made to contract by pressure the other will 
 dilate, and vice versa ; so a bladder partly filled with cold air, 
 and contained within another filled with hot air, expands, 
 while the space between the bladders contracts, exhibiting a 
 mere transfer of the same amount of repulsive force, the 
 mobility of the particles, or their mutual attraction, being the 
 same in each body ; in other words, the repulsive force acts in 
 the direction of least resistance until equilibrium is produced ; 
 it then becomes a static or balanced, instead of a dynamic or 
 motive force. 
 
 Let us now consider the case where a solid is to be changed 
 to a liquid, or a liquid to a gas ; here a much greater amount 
 of heat or repulsive force is required, on account of the cohe- 
 sion of the particles to be separated. In order to separate 
 the particles of the solid, precisely as much force must be 
 parted with by the warmer liquid body as keeps an equal 
 quantity of it in its liquid state ; it is, indeed, only with a 
 more striking line of demarcation, the case of the hot and cold 
 bladder a part of the repellent power of the hot particles 
 is transferred to the cold particles, and separates them in their 
 turn, but the antagonist force of cohesion or aggregation 
 necessary to be overcome, being in this case much stronger, 
 requires and exhausts an exactly proportionate amount of 
 repellent force mechanically to overcome it ; hence the dif- 
 ferent effect on a body such as the common thermometer, the 
 expanding liquid of which does not undergo a similar change 
 of state. Thus, in the example above given, of the mixture 
 of cold with hot water, the hot and cold water and the mercury 
 of the thermometer being all in a liquid state before, and 
 remaining so after contact, the resulting temperature is an 
 exact mean ; the hot water contracts to a certain extent, the 
 cold water expands to the same extent, and the thermometer 
 either sinks or rises the same number of degrees, accordingly 
 as it had been previously immersed in the cold or in the hot 
 solution, its mercury gaining or losing an equivalent of repellent 
 force. In the second instance, viz. the mixture of ice with hot 
 
HEAT. 33 
 
 water, the substance we use as an indicator, i.e. mercury, does 
 not undergo the same physical change as do those whose 
 relations of volume we are examining. The force viewing 
 heat simply as mechanical force which is employed in loosen- 
 ing or tearing asunder the particles of the solid, ice, is ab- 
 stracted from the liquid, water, and from the liquid mercury 
 of the thermometer, and in proportion as this force meets with 
 a greater resistance in separating the particles of a solid than 
 of a liquid, so the bodies which yield the force suffer propor- 
 tionately a greater contraction. 
 
 If we compare the action of heat on the two substances, 
 water and mercury alone, and throw out of our consideration 
 the ice, we shall be able to apply the same view : thus, if a 
 given source of heat be applied to water containing a mercu- 
 rial thermometer, both the water and mercury gradually 
 expand, but in different degrees ; at a certain point the 
 attractive force of the molecules of the water is so far 
 overcome that the water becomes vapour. At this point, the 
 heat or force, meeting with much less resistance from the 
 attraction of the particles of steam than from those of the 
 mercury, expends itself upon the former ; the mercury does 
 not further expand, or expands in an infinitesimally small 
 degree, and the steam expands greatly. As soon as this 
 arrives at a point where circumambient pressure causes its 
 resistance to further expansion to be equal to the molecular 
 resistance to expansion in the mercury of the thermometer, 
 the latter again rises, and so both go on expanding in an 
 inverse 'ratio to the resisting force. If the circumambient 
 pressure be increased, as by confining the w^ter at the com- 
 mencement of the experiment within a less expansible body 
 than itself, such as a metallic chamber, then the mercury of 
 the thermometer continues to rise ; and if the experiment 
 were continued, the water being confined and not the mercury, 
 until we have arrived at a degree of repulsive force which is 
 able to overcome the cohesive power of the mercury, so that 
 this expands into vapour, then we get the converse effect ; the 
 force expends itself upon the mercury, which expands indefi- 
 nitely, as the water did in the first case, and the water does 
 not expand at all. 
 
 D 
 
34 CORRELATION OF PHYSICAL FORCES. 
 
 Another very usual mode of regarding the subject may 
 embarrass at first sight, but a little consideration will show 
 that it is explicable by the same doctrine. Water which has 
 ice floating in it will give, when measured by the thermometer, 
 the same temperature as the ice ; i.e. both the water and ice 
 contract the mercury of the thermometer to the point conven- 
 tionally marked as 32. It may be said, how is this recon- 
 cilable with the dynamical doctrine, for according to that 
 the solid should take from the mercury of the thermometer 
 more repulsive power than the liquid ; consequently, the ice 
 should contract the mercury more than the water ? 
 
 My answer is, that in the proposition as thus stated, the 
 quantities of the water, ice, and mercury are not taken into 
 consideration, and hence a necessary dynamical element is 
 neglected : if the element of quantity be included, this objec- 
 tion will not apply. Let the thermometer, for instance, con- 
 tain 13^ oz. of mercury, and stand at 100 ; if placed in contact 
 with an unlimited quantity of ice at 32, the mercury will sink 
 to 32. If the same thermometer be immersed in an unlimited 
 quantity of water at 32, the mercury sinks also to 32; not 
 absolutely, perhaps, because, however great the quantity of 
 water or ice, it will be somewhat raised in temperature by the 
 warmer mercury. This elevation of temperature above 32 
 will be smaller in proportion as the quantity of water or ice 
 is larger than the quantity of mercury ; and, as we know of no 
 intermediate state between ice and water, the contact of a 
 thermometer at a temperature above the freezing point with 
 any quantity of ice exactly at the freezing point would, theo- 
 retically speaking, liquefy the whole, provided it had sufficient 
 time ; for as every portion of that ice would in time have its 
 temperature raised by the contact of the warmer body, and as 
 any elevation of temperature above the freezing point liquefies 
 ice, every portion should be liquefied. Practically speaking, 
 however, in both cases, that of the water and of the ice, 
 when the quantity is indefinitely great the thermometer falls 
 to 32. 
 
 Now place the same thermometer at 100 successively in 
 one oz. of water at 32, and in one of ice at 32 ; we shall 
 find in the former case it will be lowered only to 54, .and in 
 
HEAT. 35 
 
 the latter to 32 ; apply to this the doctrine of repulsive force, 
 and we get a satisfactory explanation 
 
 In the first case, the quantity both of ice and water being 
 indefinitely great in respect to the mercury, each reduces it to 
 its own temperature, viz. 32, and the ice cannot reduce the 
 mercury below 32, because the latter would receive back re- 
 pulsive power from the newly-formed water, and this would 
 become ice ; in the second case, where the quantities are 
 limited, the mercury does lose more repulsive power by the 
 ice than by the water, and the observations made in reference 
 to the first illustration apply. 
 
 The above doctrine is beautifully instanced in the experi- 
 ment of Thilorier, by which carbonic acid is solidified. Car- 
 bonic acid gas, retained in a strong vessel under great pressure, 
 is allowed to escape from a small orifice ; the sudden expan- 
 sion requires so great a supply of force, that in furnishing 
 the demands of the expanding gas certain other portions of 
 the gas contract to such an extent as to solidify : thus, we 
 have reciprocal expansion and. contraction going on in one 
 and the same substance, the time being too limited for the 
 whole to assume a uniform temperature, or in other words a 
 uniform extent of expansion. 
 
 It has been observed with reference to heat thus viewed, 
 that it would be as correct to say, that heat is absorbed, or 
 cold produced by motion, as that heat is produced by it. 
 This difficulty ceases when the mind has been accustomed to 
 regard heat and cold as themselves motion, i.e. as correlative 
 expansions and contractions, each being evidenced by relation, 
 and being inconceivable as an abstraction. 
 
 For instance, if the piston of an air-pump be drawn down 
 by a weight, cold is produced in the receiver. It may be 
 here said that. a mechanical force, and the motion consequent 
 upon it, produces cold ; but heat is produced on the opposite 
 side of the piston, if a receiver be adapted so as to retain the 
 compressed air. Assuming them to be equivalent to each 
 other, the force of the falling weight would be expressed by 
 the heat of friction of the piston against its tube, and by the 
 tension or power of reaction of the compressed against the 
 dilated air. If the heat due to compression be made to perform 
 
 D 2 
 
36 CORRELATION OF PHYSICAL FORCES. 
 
 mechanical work, it would pro tanto be consumed, and could 
 not restore the temperature to the dilated air ; but if it per- 
 form no work, no heat is lost. Mr. Joule has experimentally 
 proved this proposition. 
 
 In commencing the subject of heat, I asked my reader to 
 put out of consideration the sensations which heat produces 
 in our own bodies : I did this because these sensations are 
 likely to deceive, and have deceived many as to the nature 
 of heat. These sensations are themselves occasioned by 
 similar expansions to those which we have been considering ; 
 the liquids of the body are expanded, i.e. rendered less viscid 
 by heat, and from their more ready flow we obtain the sen- 
 sation of agreeable warmth. By a greater degree of heat, 
 their expansion becomes too great, giving rise to a sense of 
 pain, and if pushed to extremity, as with the heat which pro- 
 duces a burn, the liquids of the body are dissipated in vapour, 
 and an injury or destruction of the organic structure takes 
 place. A similar though converse effect may be produced by 
 intense cold ; the application of frozen mercury to the animal 
 body produces a burn similar to that produced by great heat, 
 and accompanied with a similar sensation. 
 
 Doubtless other actions than those above-mentioned inter- 
 fere in producing the sensations of heat and cold ; but I 
 think it will be seen that these will not affect the arguments 
 as to the nature of heat. The phenomenal effects will be 
 found unaltered : heat will still be found to be expansion, cold 
 to be contraction ; and the expansion and contraction are, as 
 with the two bladders of air, correlative i.e. we cannot ex- 
 pand one body, A, without contracting some other body, B ; 
 we cannot contract A without expanding B, assuming that we 
 view the bodies with relation to heat alone, and suppose no 
 other force to be manifested. 
 
 I have said that there are a few exceptions as to heat being 
 always manifested by an expansion of matter. One class of 
 these exceptions is only apparent : moist clay, animal or vege- 
 table fibre, and other substances of a mixed nature, which 
 contain matter of diverse character, some of which is more 
 and some less volatile, i.e. expansible, are contracted on the 
 application of heat ; this arises from the more volatile matter 
 
HEAT. 37 
 
 being dissipated in the form of vapour or gas ; and the inter- 
 stices of the less volatile being thus emptied, the latter con- 
 tracts by its own cohesive attraction, giving thus & prima facie 
 appearance of contraction by heat. The pyrometer of Wedg- 
 wood is explicable on this principle. 
 
 The second class of exceptions, though much more limited 
 in extent, is less easily explained. Water, fused bismuth, 
 and probably some other substances (though the fact as to 
 them is not clearly established), expand as they approach 
 very near to the freezing or solidifying point. The most pro- 
 bable explanation of these exceptions is, that at the point of 
 maximum density the molecules of these bodies assume a 
 polar or crystalline condition ; that by the particles being 
 thus arranged in linear directions like chevaux de fn'se, inter- 
 stitial spaces are left, containing matter of less density, so that 
 the specific density of the whole mass is diminished. 
 
 Some experiments of Dr. Tyndall on the physical pro- 
 perties of ice seem to favour this view. When a sunbeam, 
 concentrated by a lens, is allowed to fall on a piece of 
 apparently homogeneous ice, the path of the rays is in- 
 stantly studded with numerous luminous spots like minute 
 air-bubbles, and the planes of freezing are made manifest 
 by these and by small fissures. Stars or flower-like figures 
 of six petals appear parallel to the planes of freezing, and 
 seemingly spreading out from a central bubble. These 
 flowers are formed of water. When the ice is melted in 
 warm water no air is given off from the bubbles, so they 
 seem to be vacuous ; it is, however, possible that extremely 
 minute particles of air sufficient to form foci for the melting 
 points of ice might be dissolved by the water as soon as it 
 surrounds them. Be this as it may, the existence of these 
 points throughout the ice, where it gives way to the heat 
 of the solar beam, if it does not prove actual vacuous or 
 aeriform spaces to exist in ice, proves that it is not homo- 
 geneous, that its structure is probably definitely crystalline, 
 and that the matter composing it is in different degrees of 
 aggregation, so that its mean specific gravity might well be less 
 than that of water. 
 
 We cannot examine piecemeal the ultimate structure of 
 
38 CORRELATION OF PHYSICAL FORCES. 
 
 matter, but in addition to the fact that the bodies which 
 evince this peculiarity are bodies which, when solidified, ex- 
 hibit a very marked crystalline character, there are experi- 
 ments which show that water between the point of maximum 
 density and its point of solidification polarises light circularly; 
 showing, if these experiments be correct, a structural alteration 
 in water, and one analogous to that possessed by certain 
 crystalline solids, and to that possessed by water itself, where 
 it is forcibly made to assume a polarised condition by the 
 influence of magnetism. 
 
 The accuracy of these results has, however, been doubted, 
 and the experiments have not succeeded when repeated by 
 very experienced hands. Whether this be so or not, and 
 whether the above explanation of the exception to the other- 
 wise invariable effect of expansion by heat be or be not 
 regarded as admissible, must be left to the judgment and 
 experience of each individual who thinks upon the subject ; 
 at all events, no theory of heat yet proposed removes the 
 difficulty, and therefore it equally opposes every other view of 
 the phenomena of heat, as it does that which I have here 
 considered, and which regards heat as communicable expan- 
 sive force. 
 
 As certain bodies expand in freezing,, and indeed in some 
 cases before they arrive at the temperature at which they solidify, 
 we get the apparent anomaly that the motion or mechanical 
 force generated by heat or change of temperature is reversed 
 in direction when we arrive at the point of change from the 
 solid to the liquid state. Thus, a piece of ice, at the tem- 
 perature of Zero, Fahrenheit, would expand by heat, and 
 produce a mechanical force by such expansion until it ar- 
 rives at 32 ; but then by an increment of heat it contracts ; 
 and if the first expansion had moved a piston upward s, the 
 subsequent contraction would bring it back to a certain 
 extent, or move it downwards, an apparent negation of the 
 force of heat. 
 
 Again, with water above 39, i.e. above its point of maxi- 
 mum density, a progressive increment of cold or decrement 
 of heat would produce contraction to a certain point, and then 
 expansion or a mechanical force in an opposite direction. 
 
HEAT. 39 
 
 Thus not only heat or the expansive force given to other 
 bodies by a body cooling would be given out by water 
 freezing, but also the force due to the expansion in the body 
 itself, and force would thus seem to be got out of nothing ; 
 but if water in a confined space be gradually cooled, the ex- 
 pansion attendant on its cooling as it approaches the freezing 
 point would occasion pressure amongst its particles, and thence 
 tend to antagonise the force of dilatation produced in them 
 by cooling, or to resist their tendency to freeze ; or in other 
 words the pressure would tend to liquefaction, and conversely 
 to the usual effect of pressure, produce cold instead of heat, 
 and thus neutralise some of the heat yielded by the cooling 
 body. Hence we find that it requires a lower temperature to 
 freeze water under pressure than when exempt from it, or 
 that the freezing point is lowered as the pressure increases for 
 bodies which expand in freezing an effect first predicted by 
 Mr. J. Thomson, and experimentally verified by Sir W. 
 Thomson ; while, as shown by M. Bunsen, the converse effect 
 takes place with bodies which contract in freezing ; here the 
 pressure co-operates with the effects of cold, both tending to 
 approximate the particles, and such substances solidify at a 
 higher temperature in proportion as the pressure is greater ; 
 so that we might expect a body of this class, which under the 
 ordinary pressure of the air is at a temperature just above its 
 freezing point, to solidify by being submitted to pressure 
 alone, the temperature being kept constant. 
 
 A similar class of exception to the general effect of heat 
 in expanding bodies is presented by vulcanised caoutchouc. 
 This has been observed by Mr. Gough, and, indeed, wag 
 pointed out to me many years ago by Mr. Brockedon, to be 
 heated when stretched, and cooled when unstretched. 
 
 Mr. Joule finds that its specific gravity is less when 
 stretched than when unstretched, and that when heated in its 
 stretched state it shortens, presenting in this particular condi- 
 tion a similar series of relations to those which are presented 
 by water near or at its freezing point. 
 
 With the exception of this class of phenomena, which offer 
 difficulties to any theory which has been proposed, the general 
 phenomena of heat may, I believe, be explained upon a purely 
 
40 CORRELATION OF PHYSICAL FORCES. 
 
 dynamical view, and more satisfactorily than by having re- 
 course to the hypothesis of latent matter. Many, however, 
 of the phenomena of heat are involved in much mystery, 
 particularly those connected with specific heat or that relative 
 proportion of heat which equal weights of different bodies 
 require to raise them from a given temperature to another 
 given temperature, which appears to depend in some way 
 hitherto inexplicable upon the molecular constitution of 
 different bodies. 
 
 The view of heat which I have taken, viz. to regard it 
 simply as a communicable molecular repulsive force, is sup- 
 ported by many of the phenomena to which the term specific 
 or relative heat is applied ; for example, bodies as they 
 increase in temperature increase in specific heat. The ratio 
 of this increase in specific heat is greater with solids than with 
 liquids, although the latter are more dilatable ; an effect pro- 
 bably depending upon the commencement of fusion. Again, 
 those metals whose rate of expansion increases most rapidly 
 when they are heated, increase most in specific heat ; and 
 their specific heat is reduced by percussion, which, by approxi- 
 mating their particles, makes them specifically more dense. 
 When, however, we examine substances of very different 
 physical characters, we find that their specific heats have no 
 relation to their density or rate of expansion by heat ; their 
 differences of specific heat must depend upon their intimate 
 molecular constitution in a manner accounted for (as far as I 
 am aware) by no theory of heat hitherto proposed. 
 
 In the greater number, probably in all solids and liquids, 
 the expansion by heat is relatively greater as the temperature 
 is higher ; or, preserving the view of expansion and contrac- 
 tion, if two equal portions of the same substance be juxtaposed 
 at different temperatures, the hotter portion will contract a 
 little more than the colder will expand ; from this fact, viz. 
 that the coefficient of expansion increases in a given body 
 with the temperature, and from other considerations, Dr. Wood 
 has argued, with much apparent reason, that the nearer the 
 particles of bodies are to each other, the less they require to 
 move to produce a given expansion or contraction in those of 
 
HEAT. 41 
 
 another body. His mode of reasoning, if I rightly conceive 
 it, may be concisely put as follows : 
 
 As bodies contract by cold, it is clear that, in a given 
 body, the lower the temperature the nearer are the particles ; 
 and, as the coefficient of expansion increases with the tempe- 
 rature, the lower the temperature of the substance be, the less 
 the particles require to move, or approach to or recede from 
 each other, so as to compensate the correlative recession or 
 approach of the particles in a hotter portion of the same sub- 
 stance, that is, in another portion of the same substance in 
 which the particles are more distant from each other. The 
 amount of approximation or recession of the particles of a 
 body, in other words, its change of bulk by a given change of 
 temperature, being thus in a given substance an index of the 
 relative proximity of its particles, may it not be so of all 
 bodies ? The proposition is very ingeniously argued by Dr. 
 Wood, but the argument is based upon certain hypotheses as 
 to the sizes and distances of atoms, which must be admitted 
 as postulates by those who adopt his conclusions. Dr. Wood 
 seeks by means of this theory to explain the heat produced by 
 chemical combination, and I shall endeavour to give a sketch 
 of his mode of reasoning when I arrive at that part of my 
 subject. 
 
 Although the comparative effects of specific heat may not 
 be satisfactorily explicable by any known theory, the absolute 
 effect of heat upon each separate substance is simply expan- 
 sion, but when bodies differing in their physical characters are 
 used, the rate of expansion varies, if measured by the corre- 
 lative contractions exhibited by the substances producing it. 
 Though I am obliged, in order to be intelligible, to talk of 
 heat as an entity, and of its conduction, radiation, &c., yet 
 these expressions are, in fact, inconsistent with the dynamic 
 theory which regards heat as motion and nothing else ; thus 
 conduction would be simply a progressive dilatation or motion 
 of the particles of the conducting substance, radiation an un- 
 dulation or motion of the particles of the medium through 
 which the heat is said to be transmitted, &c. ; and it is a 
 strong argument in favour of this theory, that for every diver- 
 
42 CORRELATION OF PHYSICAL FORCES. 
 
 sity in the physical character of bodies, and for every change 
 in the structure and arrangement of particles of the same body, 
 a change is apparent in the thermal effects. Thus gold con- 
 ducts heat, or transmits the motion called heat, more readily 
 than copper, copper than iron, iron than lead, lead than 
 porcelain, &c. 
 
 So when the structure of a substance is not homogeneous, 
 we have a change in the conduction of heat by different parts 
 which is dependent upon the structure. This is beautifully 
 shown with bodies whose structure is symmetrically arranged, 
 as in crystals. Senarmont has shown that crystals conduct heat 
 differently in different directions with reference to the axis of 
 symmetry, but definitely in definite directions. His mode of 
 experimenting is as follows : A plate of the crystal is cut in 
 a direction, for one set of experiments parallel, and for another 
 at right angles to the axis ; a tube of platinum is inserted 
 through the centre of the plate, and bent at one extremity, so 
 as to be capable of being heated by a lamp without the heat 
 which radiates from the lamp affecting the crystal ; the surfaces 
 of the plate of crystal are covered with wax. When the 
 platinum is heated, the direction of the heat conducted by the 
 crystal is made known by the melting of the wax, and a curved 
 line is visible at the juncture of the solid and liquid wax. This 
 curve, with homogeneous substances, as glass or zinc, is a circle; 
 it is also a circle on plates of calc spar cut perpendicular to the 
 axis of symmetry ; but on plates cut parallel to the axis of 
 symmetry, and having their plane perpendicular to one of the 
 faces of the primitive rhombohedron,the curves are well-defined 
 ellipses, having their longer axes in the direction of the axis of 
 symmetry, showing that this axis is a direction of greater con- 
 ductibility. From experiments of this character the inference is 
 drawn, that ' in media constituted like crystals of the rhombo- 
 hedral system, the conducting power varies in such a manner, 
 that, supposing a centre of heat to exist within them, and the 
 medium to be indefinitely extended in all directions, the 
 isothermal surfaces are concentric ellipsoids of revolution 
 round the axis of symmetry, or at least surfaces differing but 
 little therefrom.' 
 
 Knoblauch has further shown, that radiant heat is absorbed 
 
HEAT. 43 
 
 in different degrees, according as its direction is parallel or 
 perpendicular to the axis of a crystal. 
 
 If we select a substance of a different but also of a definite 
 structure, such as wood, we find that heat progresses through 
 it with more or less rapidity, according to its direction with 
 reference to the fibre of the wood : thus Decandolle and De 
 la Rive found that the conduction was better in a direction 
 parallel to the fibre than in one transverse to it ; and Dr. 
 Tyndall has added the fact, that the conduction is better in a 
 direction transverse to the fibre and layers of the wood than 
 when transverse to the fibre but parallel to the layers, though 
 in both these directions the conduction is inferior to that 
 following the direction of the fibre. Thus, in the three possible 
 directions in which the structure of wood may be contemplated, 
 we have three different degrees of progression for heat. 
 
 In the above examples we see, as we shall see farther on 
 with reference to all the so-called imponderables, that the 
 phenomena depend upon the molecular structure of the matter 
 affected ; and although these facts are not absolutely incon- 
 sistent with the theory which supposes them to be fluids or 
 entities, it will, I think, be found to be far more consistent 
 with that which views them as motion. Heat, which we are 
 at present considering, cannot be insulated : we cannot remove 
 the heat from a substance and retain it as heat ; we can only 
 transmit it to another substance, either as heat or as some 
 other mode of force. We only know certain changes of 
 matter, for which changes heat is a generic name ; the thing 
 heat is unknown. 
 
 Heat having been shown to be a force capable of producing 
 motion, and motion to be capable of producing the other modes 
 of force, it necessarily follows that heat is capable, mediately, 
 of producing them : I will, therefore, content myself with 
 enquiring how far heat is capable of immediately producing 
 the other modes of force. It will immediately produce 
 electricity, as shown in the beautiful experiments of Seebeck, 
 one of which I have already cited, which experiments proved, 
 that when dissimilar metals are made to touch, or are soldered 
 together, and heated at the point of contact, a current of 
 electricity flows through the metals having a definite direction 
 
44 CORRELATION OF PHYSICAL FORCES. 
 
 according to the metals employed, which current continues as 
 long as an increasing temperature is gradually pervading the 
 metals, ceases when the temperature is stationary, and flows 
 in the contrary direction with the decrement of temperature. 
 
 Another class of phenomena which have been generally 
 attributed to the effects of radiant heat, and to which, from 
 this belief, the term thermography has been applied, may also, 
 in their turn, be made to exhibit electrical effects effects 
 here of Franklinic or static electricity, Seebeck's experiments 
 showing effects of voltaic or dynamic electricity. 
 
 If polished discs of dissimilar metals say, zinc and copper 
 -be brought into close proximity, and kept there for some 
 time, and either of them has irregularities upon its surface, a 
 superficial outline of these irregularities is traceable upon the 
 other disc, and vice versa. Many theories have been framed 
 to account for this phenomenon, but whether it be due or not 
 to thermic radiations, the relative temperature of the discs, 
 their relative capacities and conducting and radiating powers 
 for heat, undoubtedly influence the phenomena. 
 
 Now, if two such discs in close proximity be connected 
 with a delicate electroscope, and then suddenly separated, the 
 electroscope is affected, showing that the reciprocal radiation 
 from surface to surface has produced electrical force. I cite 
 this experiment in treating of heat as an initial force, because 
 at present the probabilities are in favour of thermic radiation 
 producing the phenomenon. The origin of these so-called 
 thermographic effects is, however, a question open to much 
 doubt, and needs much further experiment. When I first 
 published the experiment which showed that the mere 
 approximation of discs of dissimilar metals would give rise to 
 electrical effects, I mentioned that I considered the fact of 
 the superficial change upon the surface of metals in proximity, 
 and, a fortiori, in contact, would explain the development of 
 electricity in Volta's original contact experiment, without 
 having recourse to the contact theory, i.e. a theory which 
 supposes a force to be produced by mere contact of dissimilar 
 metals without any molecular or chemical change. I have 
 seen nothing to alter this view. Mr. Gassiot has repeated 
 and verified my experiment with more delicate apparatus and 
 
HEAT. 45 
 
 under more unexceptionable circumstances ; and without 
 saying that radiant heat is the initial force in this case, we 
 have evidence, by the superficial change which takes place 
 in bodies closely approximated, that some molecular change 
 is taking place, some force is called into action by their 
 proximity, which produces changes in matter as it expends, or 
 rather transmits itself; and, therefore, is not a force without 
 molecular change, as the supposed contact force would be. 
 The force in this, as in all other cases, is not created, but 
 developed by the action of matter on matter, and not annihi- 
 lated, as it is shown by this experiment to be convertible into 
 another mode of force. 
 
 To say that heat will produce light, is to assert a fact 
 apparently familiar to everyone, but there may be some reason 
 to doubt whether the expression to produce light is correct in 
 this particular application ; the relation between heat and light 
 is not analogous to the correlation between these and the other 
 four affections of matter ; heat and light appear to be rather 
 modifications of the same force than distinct forces mutually 
 dependent. The modes of action of radiant heat and of light 
 are so similar, both being subject to the same laws of reflec- 
 tion, refraction, double refraction, and polarisation, that their 
 difference appears to exist more in the manner in which they 
 affect our senses than in their modes of physical action. 
 
 The experiments of Melloni, which have mainly contributed 
 to demonstrate the analogies of heat and light, afford a beau- 
 tiful instance of the assistance which the progress of one 
 branch of physical science renders to that of another. The 
 discoveries of Oersted and Seebeck led to the construction of 
 an instrument for measuring temperature incomparably more 
 delicate than any previously known. To distinguish it from 
 the ordinary thermometer, this instrument is called the tliermo- 
 multiplicr. It consists of a series of small bars of bismuth 
 and antimony, forming one zigzag chain of alternations 
 arranged parallel to each other, and forming together a 
 cylinder or prism; so that the points of junction, which are 
 soldered, shall be all exposed at the bases of the cylinder : the 
 two extremities of this series are united to a galvanometer 
 that is, a coil of wire surrounding a freely-suspended mag- 
 
46 CORRELATION OF PHYSICAL FORCES. 
 
 netic needle, the direction of which is parallel to the convolu- 
 tions of the wire. When radiant heat impinges upon the 
 soldered ends of the multiplier, a thermo-electric current is 
 induced in each pair ; and, as all these currents tend to cir- 
 culate in the same direction, the energy of the whole is 
 increased by the co-operating forces : this current, traversing 
 the helix of the galvanometer, deflects the needle from paral- 
 lelism by virtue of the electro-magnetic tangential force, and 
 the degree of this deflection serves as the index of the 
 temperature. 
 
 Bodies examined by these means show a remarkable dif- 
 ference between their transcalescence or power of transmitting 
 heat, and their transparency : thus, perfectly transparent alum 
 arrests more heat than quartz so dark-coloured as to be 
 opaque ; again alum coupled with green glass Melloni found 
 was capable of transmitting a beam of brilliant light, while, 
 with the most delicate thermoscope, he could detect no indi- 
 cations of transmitted heat : on the other hand, rock salt, the 
 most transcalescent body known, may be covered with soot 
 until perfectly opaque, and yet be found capable of trans- 
 mitting a considerable quantity of heat. Radiant heat, when 
 transmitted through a prism of rock-salt, is found to* be un- 
 equally refracted, as is the case with light ; and the rays of 
 heat thus elongated into what is, for the sake of analogy, 
 called a spectrum, are found to possess similar properties to 
 the primary or coloured rays of light. Thus rock-salt is to 
 heat what colourless glass is to light : it transmits heat of all 
 degrees of refrangibility ; alum is to heat as red glass to light : 
 it transmits the least, and stops the most refrangible rays ; 
 and rock-salt covered with soot represents blue glass, trans- 
 mitting the most, and stopping the least refrangible rays. 
 
 Certain bodies, again, reflect heat of different refrangibi- 
 lity ; thus, paper, snow, and lime, although perfectly white 
 that is, reflecting light of all degrees of refrangibility, reflect 
 respectively heat only of certain degrees ; while metals, which 
 are coloured bodies that is, bodies which reflect light only 
 of certain degrees of refrangibility reflect heat of all degrees. 
 Radiant heat incident upon substances which doubly refract 
 light is doubly refracted ; and the emergent rays are polarised 
 
HEAT. 47 
 
 in planes at right angles to each other, as is the case with 
 light. 
 
 The relation of radiation to absorption also holds good 
 with light as with heat : with the latter it has been long known 
 that the radiating power of different substances is directly 
 proportional to their absorptive and inverse to their reflective 
 power ; or rather, that the sum of the heat radiated and re- 
 flected is a constant quantity. Further, it has been shown 
 by Mr. Balfour Stewart, that the absorption of heat is propor- 
 tional to its radiation, both as to quality and quantity. 
 
 Light presents us with similar relations. Coloured glass, 
 when heated so as to be luminous, emits the same light 
 which at ordinary temperatures it absorbs : thus red glass gives 
 out or radiates a greenish light, and green glass a red tint. 
 
 The flame of substances containing sodium yields a yellow 
 light of such purity that other colours exposed to it appear 
 black a phenomenon shown by the familiar experiment of 
 exposing a picture of bright colours, other than yellow, to the 
 flame of spirits of wine with which common salt is mixed : the 
 picture loses its colours, and appears to be black and white. 
 When the prismatic spectrum of such a flame is examined, it 
 is found to exhibit two bright yellow lines at a certain fixed 
 position. If a source of light be employed which gives no 
 lines in its spectrum, and the light of this incandescent sub- 
 stance be made to pass through the sodium flame, two dark 
 lines will appear in the spectrum precisely coincident in posi- 
 tion with the yellow lines which were given by the sodium 
 flame itself. The same relation of absorption to radiation is 
 therefore shown here : the substance absorbs that light which 
 it yields when it is itself the source of light, The same is 
 true of other substances, the spectra of which exhibit respec- 
 tively lines of peculiar colour and position. Now, the solar 
 prismatic spectrum is traversed by a great number of dark 
 lines ; and Kirchhoff has deduced from considerations such as 
 those which I have shortly stated, that these dark lines in the 
 solar spectrum are due to metals or other substances existing 
 in an atmosphere around the sun which absorb the light from 
 a central incandescent nucleus, each metal absorbing that 
 
48 CORRELATION OF PHYSICAL FORCES. 
 
 light which would appear as a bright line or lines in the spec- 
 trum produced by its own light. 
 
 By comparing the positions of the bright lines in the 
 spectra of various metals with those of the dark lines in the 
 solar spectrum, several of them are found to be in identically 
 the same place : hence it is inferred, and the inference seems 
 reasonable, that the metals which show luminous lines in their 
 spectra, identical in position with dark lines in the solar spec- 
 trum, exist in the sun, and are diffused in a gaseous state in 
 its atmosphere. It does not seem to me necessary to this 
 conclusion to assume that the sun is a solid mass of incandes- 
 cent matter : it may well be that what we term the photo- 
 sphere or luminous envelope of the sun has surrounding it a 
 more diffuse atmosphere containing vaporised metals, and that 
 the mass of the sun itself may be in a different state, and not 
 necessarily in an incandescent temperature ; indeed, the pro- 
 tuberances and red light seen at the period of total eclipses 
 afford some evidence of an atmosphere exterior to the photo- 
 sphere. It would, however, be out of place here to speculate 
 on these subjects : the point which concerns us is the analogy 
 of heat and light, which these discoveries illustrate. Kirch- 
 hoff has carried the analogy farther, by showing that a 
 plate of tourmaline absorbs the polarised ray which when 
 heated it radiates. Thus, the phenomena of light are imitated 
 closely by those of radiant heat ; and the same theory which 
 is considered most plausibly to account for the phenomena of 
 the one, will necessarily be applied to the other agent, and in 
 each case molecular change is accompanied by a change in 
 the phenomenal effects. 
 
 In certain cases heat appears to become partially con- 
 verted into light, by changing the matter affected by heat : 
 thus gas may be heated to a very high point without produc- 
 ing light, or producing it to a very slight degree ; but the 
 introduction of solid matter for instance, the metal platinum 
 into the highly-heated gas instantly exhibits light. Whether 
 the heat is converted into light, or whether it is concentrated 
 and increased in intensity by the solid matter so as to become 
 visible, may be open to some doubt : the fact of solid matter 
 when ignited by the oxyhydrogen jet, decomposing water, 
 
HEAT. 49 
 
 as will be presently explained, would seem to indicate that 
 the heat was rendered more intense by condensation in 
 the solid matter, as water is in this case decomposed by a 
 heated body, which body has itself been heated by the com- 
 bining elements of water. The apparent effect, however, of 
 the introduction of solid incombustible matter into heated gas, 
 is a conversion of heat into light. 
 
 Dr. Tyndall, by passing light from the voltaic arc through 
 solutions of iodine, separates the invisible rays of heat from 
 the luminous rays, and then reproduces light by receiving the 
 former on platinum foil. 
 
 If we concentrate into a focus by a large lens a dim light, 
 we increase the intensity of the light. Now, if a heated body 
 be taken, which, to the unassisted eye, has just ceased to be 
 visible, it seems probable that by collecting and condensing 
 by a lens the different rays which have so ceased to be visible, 
 light would reappear at the focus. The experiment is, for 
 reasons obvious to those acquainted with optics, a difficult one, 
 and to be conclusive, should be made on a large scale, and 
 with a very perfect lens of large diameter and short focus. I 
 have obtained an approximation to the result in the following 
 manner : In a dark room a platinum wire is brought just to 
 the point of visible ignition by a constant voltaic battery ; it 
 is then viewed, at a short distance, through an opera-glass of 
 large aperture applied to one eye, the other being kept open. 
 The wire will be distinctly visible to that eye which regards it 
 through the opera-glass, and at the same time totally invisible 
 to the other and naked eye. It may be said with some jus- 
 tice that such experiments prove little more than the fact 
 already known, viz. that by increasing the intensity of heat, 
 light is produced ; they however exhibit this effect in a 
 more striking form, as bearing on the relations of heat and 
 light. 
 
 With regard to chemical affinity and magnetism, perhaps 
 the only method by which in strictness the force of heat may 
 be said to produce them is through the medium of electricity, 
 the thermo-electrical current, produced, as before described, 
 by heating dissimilar metals, being capable of deflecting the 
 magnet, of magnetising iron, and exhibiting the other mag- 
 
 E 
 
So CORRELATION OF PHYSICAL FORCES. 
 
 netic effects, and also of forming and decomposing chemical 
 compounds, and in some proportion to the progression of heat : 
 this has not, indeed, as yet been proved to bear a measurable 
 quantitative relation to the other forces thus produced by it, 
 because so little of the heat is utilised or converted into electri- 
 city, much being dissipated, without change in the form of heat. 
 
 Heat, however, directly affects and modifies both the mag- 
 net and chemical compounds ; the union of certain chemical 
 substances is induced by heat, as, for instance, the formation 
 of water by the union of oxygen and hydrogen gases : in other 
 cases this union is facilitated by heat, and in many instances, 
 as in ammonia and its salts, it is weakened or antagonised. 
 In many of these cases, however, the force of heat seems more 
 a determining than a producing influence ; yet to be this, it 
 must have an immediate relation with the force whose reaction 
 it determines : thus, although gunpowder, touched with an 
 ignited wire, subsequently carries on its own combustion or 
 chemical combination, independently of the original source of 
 heat, yet the chemical affinities of the first portion touched 
 must be exalted by, and at the cost of, the heat of the wire ; 
 for to disturb even an unstable equilibrium requires a force in 
 direct relation with those which maintain the equilibrium. 
 
 Shortly after the first edition of this essay was published, I 
 communicated to the Royal Society some experiments by 
 which an important exception to the general effect of heat on 
 chemical affinity is removed, and the results of which induce 
 a hope that a generalised relation will ultimately be established 
 between heat, chemical affinity, and physical attraction. I 
 find that if a substance capable of supporting intense heat 
 and incapable of being acted upon by water or either of its 
 elements such a substance, for instance, as platinum, or iri- 
 dium be raised to a high point of ignition and then im- 
 mersed in water, bubbles of permanent gas ascend from it, 
 which on examination are found to consist of mixed oxygen 
 and hydrogen in the proportions in which they form water. 
 The temperature at which this is effected is, according to Dr. 
 Robinson, who has since written a valuable paper on the 
 subject, = 2 3 86. Now, when mixed oxygen and hydrogen 
 are exposed to a temperature of about 800, they combine 
 
HEAT. 51 
 
 and form water; heat therefore appears to act differently upon 
 these elements according to its intensity, in one case producing 
 composition, in the other decomposition. No satisfactory 
 means of reconciling this apparent anomaly have been pointed 
 out : the best approximation to a hypothesis which I can 
 frame is by assuming that the constituent molecules of water 
 are, below a certain temperature, in a state of stable equili- 
 brium ; that the molecules of mixed or oxyhydrogen gas are, 
 above a certain temperature, also in a state of stable equili- 
 brium, but of an opposite character ; while below this latter 
 temperature the molecules of mixed gas are in a state of un- 
 stable equilibrium, somewhat similar to that of the fulminates 
 or similar bodies, in which a slight derangement subverts the 
 nicely-balanced forces. 
 
 If, for instance, we suppose four molecules, A, B, C, D, to 
 be in a balanced state of equilibrium between attracting and 
 repelling forces, the application of a repulsive force between 
 B and C, though it may still farther separate B and C, will 
 approximate B to A and C to D, and may bring them re- 
 spectively within the range of attractive force ; or, supposing 
 the repulsive force to be in the centre of an indefinite sphere 
 of particles, all these, excepting those immediately acted on 
 by the force, will be approximated, and having from attraction 
 assumed a state of stable equilibrium, they will retain this, 
 because the repulsive force divided by the mass is not capable 
 of overcoming it. But if the repulsive force be increased in 
 quantity and of sufficient intensity, then the attractive force 
 of all the molecules may be overcome, and decomposition 
 ensue. Thus, water or steam below a certain temperature, 
 and mixed gas above a certain temperature, may be supposed 
 to be in the state of stable equilibrium, whilst between these 
 limiting temperatures, the equilibrium of oxyhydrogen gas is 
 unstable. 
 
 This, it must be confessed, is but a crude mode of explain- 
 ing the phenomena, and requires the assumption, that chemical 
 attraction or affinity, as it is awkwardly termed, has similar 
 attributes to physical attraction. Chemical affinity holds 
 good between the particles of matter when it is in a gaseous 
 state, and though gases do not give evidence of the physical 
 
 E 2 
 
52 CORRELATION OF PHYSICAL FORCES. 
 
 attraction of cohesion or aggregation, yet both gases and solids 
 expand or contract according to the inverse contraction or 
 expansion of other neighbouring bodies, and so far resemble 
 each other in their relations to heat and cold. The extent to 
 which such expansion or contraction can be carried, seems to 
 be limited only by the correlative state of other bodies ; these, 
 again, by others, and so on, as far as we may judge, throughout 
 the universe. 
 
 Adopting the explanation above given of the decomposition 
 of water by heat, heat would have the same relation to che- 
 mical affinity as it has to physical attraction ; its immediate 
 tendency is antagonistic to both, and it is only by a secondary 
 action that chemical affinity is apparently promoted by heat. 
 This hypothesis would account for heat promoting changes of 
 the equilibrium of chemical affinity among mixed compound 
 substances, by decomposing certain compounds and separating 
 elementary constituents whose affinity is greater, when they 
 are brought within the sphere of attraction for the substance 
 with which they are mixed, than for those with which they 
 were originally chemically united : thus an intense heat being 
 applied to a mixture of chlorine and the vapour of water, 
 occasions the production of muriatic acid, liberating oxygen. 
 
 Carrying out this view, it would appear that a sufficient 
 intensity of heat might yield indefinite powers of decomposi- 
 tion ; and there seems some probability of bodies now supposed 
 to be elementary, being decomposed or resolved into further 
 elements by the application of heat of sufficient intensity ; or, 
 reasoning conversely, it may fairly be anticipated that bodies, 
 which will not enter into combination at a certain temperature, 
 will enter into combination if their temperature be lowered, 
 and that thus new compounds may be formed by a proper 
 disposition of their constituents when exposed to an extremely 
 low temperature, and the more so if compression be also 
 employed. 
 
 The term 'dissociation' has been applied by M. St. Clair 
 Deville to my experiment of the separation of oxygen and 
 hydrogen by heat, and to other cases, in which heat separates 
 the constituents of a substance without any of them combining 
 with another body, thus contradistinguishing these effects from 
 
HEAT. 53 
 
 those of ordinary chemical decomposition where the affinity of 
 other substances is brought into play. 
 
 In considering the effect of heat as a mechanical force, it 
 would be expected, a priori, and independently of any theory 
 of heat which may be adopted, that a given amount of heat 
 acting on a given material must produce a given amount of 
 motive power ; and the next question which occurs to the 
 mind is, whether the same amount of heat would produce the 
 same amount of mechanical power, whatever be the material 
 acted on or affected by the heat. I will endeavour to reason 
 this out on the view of heat which I have advocated. Heat 
 has been considered in this essay as itself motion or mechanical 
 power, and quantity of heat as measured by motion. Thus, if 
 by a given contraction of a body (say hot mercury) air within 
 a cylinder having a movable piston be expanded, the piston 
 moves, and in this case the expansion or motion of the material 
 (say iron) of the cylinder itself and of the air surrounding it is 
 commonly neglected. As the air dilates it becomes colder ; 
 in other words, by undergoing expansion itself it loses its 
 power of making neighbouring bodies expand ; but if the 
 piston be forcibly kept down, the expansive power due to 
 the mercury continues to communicate itself to the iron and to 
 the surrounding air, which become hotter than they would 
 if the piston had given way. 
 
 Now, in the above case, if the air be confined and its 
 volume unchanged, will the expansion of the iron, assuming 
 that it can be utilised, produce an exactly equivalent mecha- 
 nical effect to that which the expansion of the air would pro- 
 duce if the heat be entirely confined to it ? 
 
 Assuming that (with the exception of bodies which expand 
 in freezing, where, through a limited range of temperature, the 
 converse effects obtain) whenever a body is compressed it is 
 heated, i.e. it expands neighbouring substances ; whenever it 
 is dilated or increased in volume it is cooled, i.e. it contracts 
 neighbouring substances the conclusion appears to me in- 
 evitable that the mechanical power produced by heat will be 
 definite, or the same for a given amount and intensity of heat, 
 whatever be the substance acted on. 
 
 Thus, let A be a definite source of heat, say a pound of 
 
54 CORRELATION OF PHYSICAL FORCES. 
 
 mercury at the temperature of 400 ; let B be another equal 
 and similar source of heat : suppose A be employed to raise a 
 piston by dilating the air beneath it, and B to raise another 
 piston by the dilatation of the vapour of water. Imagine the 
 pistons attached to a beam, so that they oppose each other's 
 action, and thus represent a sort of calorific balance. If A 
 being applied to air could conquer B, which is applied to 
 water, it would depress or throw back the piston of the latter, 
 and, by compressing the vapour, occasion an increase of tem- 
 perature ; this, in its turn, would raise the temperature of the 
 source of heat, so that we should have the anomaly that a 
 pound of mercury at 400 could heat another pound of mer- 
 cury at 400 to 401, or to some point higher than its original 
 temperature, and this without any adventitious aid : it will be 
 obvious that this is contradictory to the whole range of our 
 experience. 
 
 The above experiment is ideal, and stated for the object 
 of giving a more precise form to the reasoning ; to bring the 
 idea more prominently into relief, all statements as to quan- 
 tities, specific heats, &c., so as to yield comparative results for 
 given materials, are omitted. The argument may be thus 
 stated in another form, viz. that by no mechanical appliance 
 or difference of material acted on can a given source of heat 
 be made to produce more heat than it originally possessed ; 
 and that, if all be converted into mechanical power, an excess 
 cannot be supposed, for that could be converted into a surplus 
 of heat, and be a creation of force ; and a deficit cannot be 
 supposed, for that would be annihilation offeree. I cannot, 
 however, see how the theoretical conception could be verified 
 by experiment ; the enormous weights and the complex 
 mechanical contrivances requisite to give the measure of power 
 yielded by matter in its less dilatable forms, would be far 
 beyond our present experimental resources. It would also 
 be difficult to prevent the interference of molecular forces, 
 the overcoming of which expends a part of the mechanical 
 power generated, but which could hardly be made to appear 
 in the result. We could not, for instance, practically realise 
 the above conception by the construction of a machine which 
 should act by the expansion and contraction of a bar of 
 
HEAT. 55 
 
 iron, and produce a power equal to that of a steam engine 
 supplied with an equal quantity of heat. 
 
 Carnot, who wrote in 1824 an essay on the motive power 
 of heat, regarded the mechanical power produced by heat as 
 resulting from a transfer of heat from one point to another, 
 without any ultimate loss of heat. Thus, in the action of an 
 ordinary steam-engine, the heat from the furnace having 
 expanded the water of the boiler and raised the piston, a 
 mechanical motion is produced ; but this cannot be continued 
 without the removal of the heat, or the contraction of the 
 expanded water. This is done by the condenser, and the 
 piston descends. But then we have apparently transferred the 
 heat from the furnace to the condenser, and in the transfer 
 effected mechanical motion. 
 
 Should the mechanical motion produced by heat be con- 
 sidered as the effect of a simple transference of heat from 
 one point to another, or as the result of a conversion of heat 
 into the mechanical force of which this motion is the evidence ? 
 This question leads to the following : does the heat which 
 generates the mechanical power return to the thermal machine 
 as heat, or is it conveyed away by the work performed ? 
 
 If a definite quantity of air be heated it is expanded, and 
 by its expansion it cools or loses some of its power of commu- 
 nicating heat to neighbouring bodies. That which we should 
 have called heat if the expansion of the air had been pre- 
 vented, we call mechanical effect, or may regard as converted 
 into mechanical effect ceasing to be heat ; but, throwing out 
 of the question nervous sensation, this expansion or mecha- 
 nical effect is all the evidence we have of heat, for if the 
 air is allowed to expand freely, this expansion becomes the 
 index of the heat ; if the air be confined, the expansion of 
 the matter of the vessel confining it, or of the mercury of a 
 thermometer in contact with it, &c., are the indices of the 
 heat. 
 
 If, again, the air which has been expanded be, by mecha- 
 nical pressure or by other means, restored to its original bulk, 
 it is capable of heating or expanding other substances to a 
 degree to which it would not be equal, if it had remained 
 in its expanded state. To produce continuous motion, or the. 
 
56 CORRELATION OF PHYSICAL FORCES. 
 
 up-and-down stroke of a piston, we must, as I have already 
 stated, heat and cool, just as with a magnetic machine we 
 must magnetise and demagnetise in order to produce a con- 
 tinuous mechanical effect ; and although, from the impos- 
 sibility of insulating heat, some heat is apparently lost in the 
 process, the result may be said to be effected by the transfer 
 of heat from the hot to the cold body, from the furnace to 
 the condenser. But we may equally well say that the heat 
 has been converted into mechanical force, and the mechanical 
 force back into heat ; the effects are always correlative, as 
 are the mechanical effects of an air-pump, with which, as 
 we dilate the air on one side, we condense it on the other ; 
 and as we cannot dilate without the reciprocal condensation, 
 so we cannot heat without the reciprocal cooling, or vice versa. 
 
 Hitherto the resistances of the piston or of any superim- 
 posed weight have been thrown out of consideration, or, what 
 amounts to the same thing, it has been assumed that the 
 weight raised by the piston has descended with it. The heat 
 has not merely been employed in dilating the air or vapour, 
 but in raising the piston with its weight If, as the vapour 
 is cooled, the weight be permitted to descend, its mechanical 
 force restores the heat lost by the dilatation ; but in this case 
 no part of the power has been abstracted so as to be em- 
 ployed for any practical purpose ; this question then follows, 
 what takes place with regard to the initial heat, if, after the 
 ascent of the piston, the weight be removed so as not to help 
 the piston in its descent, but to fall upon a lever or produce 
 some extraneous mechanical effect ? 
 
 To answer this question, let us suppose a weight to rest on 
 a piston which confines air at a definite temperature, say for 
 example 50, in a cylinder, the whole being assumed to be 
 absolutely non-conducting for heat. A part of the heat of 
 this confined air will be due to the pressure, since, as we have 
 seen, compression of an elastic fluid produces heat. 
 
 Suppose, now, the confined air to be heated to 70, the 
 piston with its superincumbent weight will ascend, and the 
 temperature, in consequence of the dilatation of the air, will 
 be somewhat lowered, say to 69 (we will assume, for the sake 
 
. HEAT. 57 
 
 of simplicity, that the heat engendered by the friction of the 
 piston compensates the force lost by friction). 
 
 The piston having reached its maximum of elevation, let 
 a cold body or condenser take away 20 from the temperature 
 of the confined air ; the piston will now descend, and by the 
 compression which the weight on it produces, will restore the 
 i lost by dilatation, and when the piston reaches its original 
 position the temperature of the air will be restored to 50. 
 Suppose this experiment repeated up to the maximum rise of 
 the piston ; but when the piston is at its full elevation, and 
 the cold body applied, let the weight be tilted off, so as to 
 drop upon a wheel, or be used for other mechanical pur- 
 poses. The descending piston will not now reach its original 
 point without more heat being abstracted ; in consequence of 
 the removal of the weight, there will not be the same force to 
 restore the i, and the temperature will be 49, or some 
 fraction short of the original 50. If this were otherwise, then, 
 as the weight in falling may be made to produce heat by fric- 
 tion, we should have more heat than at first, or a creation of 
 heat out of nothing in other words, perpetual motion. 
 
 Let us now assume that this 20 supplied in the first 
 instance was yielded by a body at 90, of such size and mate- 
 rial that its total capacity for heat is equal to that of the mass 
 of confined air : this body would be reduced in temperature 
 to 70, in other words, our furnace would have lost 20 of 
 heat. Let the cold body of the same size and material, used 
 as a condenser, be at 30. In the first experiment, the body 
 at 30 would bring back the piston to its original point ; but 
 in the second experiment, or that where the weight has been 
 removed, the body at 30 would not suffice to restore the 
 piston : to effect this, the cold body or condenser must be at 
 a lower temperature. 
 
 The question in Carnot's theory, which is not experi- 
 mentally resolved, and which presents extreme experimental 
 difficulty, is the following : Granted that a piston with a super- 
 imposed weight be raised by the thermic expansion of confined 
 gas or vapour below it ; if the elastic medium be restored to its 
 original temperature by cooling, the weight -in depressing the 
 
58 CORRELATION OF PHYSICAL FORCES. 
 
 piston will restore that portion of the heat which has been 
 lost by the expansion, and by the mechanical effect consequent 
 thereon ; but if the weight be removed when at its maximum 
 of elevation, and the piston be brought back to its starting 
 point by a necessarily cooler body than could restore it if the 
 weight were not removed, would the return of the piston now 
 yield to the cooling body the heat which had been lost by the 
 dilatation, or, in other words, should we in pulling the piston 
 down by cold reacquire the same quantity of heat as would 
 be restored by pressing it down by mechanical force ? The 
 argument from the impossibility of perpetual motion would 
 say no, for if all the heat were restored, the mechanical effect 
 produced by the fail of the weight, or the heating effect 
 which might be made to result from this mechanical power, 
 would be got from nothing. 
 
 Then follows another question, viz. whether, where an 
 external or derived mechanical effect has been obtained, 
 would the return of the piston, effected without the weight or 
 external force to assist it, but solely by the colder body, give 
 to this latter the same number of thermometric degrees as had 
 been lost by the hot body in the first instance ? Suppose, for 
 instance, the cold body in our experiment to be at 20 instead 
 of 30, would this body gain 20, and then reach the tempera- 
 ture of 40 when the piston is brought back, or would its tem- 
 perature be higher or lower than 40 ? The argument from 
 the impossibility of perpetual motion does not apply here, for 
 it does not necessarily follow that 20, on the thermometric 
 scale from 20 to 40, represents an equal amount of force to 
 20 on the scale from 70 to 90, and therefore it is quite con- 
 ceivable that we may lose 20 from the furnace, and gain 20 
 in the condenser, and yet have obtained a certain amount of 
 derived mechanical power. It will also follow, upon a con- 
 sideration of the above imaginary experiments, that the greater 
 the mechanical power required, the greater should be the dif- 
 ference between the temperature of the furnace and that of 
 the condenser ; but the exact relation in temperature between 
 these for a given mechanical effect, has not, as far as I am 
 aware, been satisfactorily established by experiment, though 
 it has been shown that steam at high pressure produces, com- 
 
HEAT. 59 
 
 paratively, a greater mechanical effect for the same number of 
 degrees than steam at low pressure. 
 
 Carnot, assuming the number of degrees of temperature to 
 be restored, but at a lower point of the thermometric scale, 
 termed this the fall (chute) of caloric. The mechanical effect 
 of heat, on this view, may be likened to that of a series of 
 cascades on water-wheels. The highest cascade turns a wheel, 
 and produces a given mechanical effect ; the water which has 
 produced this cannot again effect it at the same level without 
 being carried back to its original elevation, i.e. without an 
 extra force being employed equivalent to, or rather a fraction 
 more than the force of the descending water ; but though its 
 power is spent with reference to the first wheel, the same 
 water may, by falling over a new precipice upon a second 
 wheel, again reproduce the same mechanical effect (strictly 
 speaking, rather more, for it has approached the centre of 
 gravity), and so on, until no lower fall can be attained. So 
 with heat : it involves no necessity of assuming perpetual 
 motion, to suppose that, after a given mechanical effect, pro- 
 duced by a certain loss of heat, the number of degrees lost 
 from the original temperature may be restored to the con- 
 denser, but at a lower point of the thermometric scale. 
 
 If work has been done, i.e. if force has been parted with, 
 the original temperature itself cannot be restored, but there is 
 no a priori impossibility in the same number of degrees of 
 heat as have been converted into work being conveyed to a 
 condensing body so cold that, when it receives this heat, it 
 will still be below the original temperature to which the work- 
 producing heat was added. 
 
 In the theory of the steam-engine, this subject possesses a 
 great practical interest. Watt supposed that a given weight 
 of water required the same quantity of what is termed total 
 heat (that is, the sensible added to the latent heat) to keep it 
 in the state of vapour, whatever was the pressure to which it 
 was subjected, and, consequently, however its expansive force 
 varied. Clement Desormes was supposed to have experi- 
 mentally verified this law. If this were so, vapour raising a 
 piston with a weight attached would produce mechanical 
 power ; and yet, the same heat existing as at first, there would 
 
60 CORRELATION OF PHYSICAL FORCES. 
 
 be no expenditure of the initial force ; and if we suppose that 
 the heat in the condenser was the real representative of the 
 original heat, we should get perpetual motion. Southern sup- 
 posed that the latent heat was constant, and that the heat of 
 vapour under pressure increased as the sensible heat. M. 
 Dcspretz, in 1832, made some experiments, which led him to 
 the conclusion that the increase was not in the same ratio as 
 the sensible heat, but that yet there was an increase ; a result 
 confirmed and verified with great accuracy by M. Regnault, in 
 some recent and elaborate researches. What seems to have 
 occasioned the error in Watt and Clement Desormes* experi- 
 ments was, the idea involved in the term latent heat ; by 
 which, supposing the phenomenon of the disappearance of 
 sensible heat to be due to the absorption of a material sub- 
 stance, that substance, ' caloric,' was thought to be restored 
 when the vapour was condensed by water, even though the 
 water was not subjected to pressure ; but to estimate the total 
 heat of vapour under pressure the vapour should be condensed 
 while subjected to the same pressure as that under which it is 
 generated, as was done in M. Despretz and M. Regnault's 
 experiments. 
 
 M. Seguin, in 1839, controverted the position that derived 
 power could be got by the mere transfer of heat, and by calcu- 
 lation from certain known data, such as the so-called law of 
 Mariotte, viz. that the elastic force of gases and vapours 
 increased directly with the pressure ; and assuming that for 
 vapour between 100 and 150 centigrade, each degree of 
 elevation of temperature was produced by a thermal unit, he 
 deduced the equivalent of mechanical work capable of being 
 performed by a given decrement of heat ; and thus concluded 
 that, for ordinary pressures, about one gramme of water losing 
 one degree centigrade would produce a force capable of raising 
 a weight of 500 grammes through a space of one metre ; this 
 estimate is a little beyond that given conversely by the experi- 
 ments of Mr. Joule, already stated, in which the heat produced 
 by a given amount of mechanical action is estimated. I am 
 not aware that the amount of mechanical work which is pro- 
 duced by a given quantity of heat has been directly established 
 by experiment, though some approximative results in parti- 
 
HEAT. 61 
 
 cular cases have been given. Theoretically it should be the 
 same that is to say, if a fall of 772 Ibs. through a space of 
 one foot will raise the temperature of I Ib. of water through 
 one degree of Fahrenheit, then the fall in the temperature of 
 I Ib. of water through one degree of Fahrenheit should be 
 able to raise 772 Ibs. through a space of one foot. The calcu- 
 lations of M. Seguin are not far from this, but since the elabo- 
 rate experiments of M. Regnault he has expressed some 
 doubt of the correctness of his former estimate, as by these 
 experiments it appears that, within certain limits, for elevating 
 the temperature of compressed vapour by one degree, no more 
 than about three-tenths of a degree of total heat is required ; 
 consequently, the equivalent multiplied in this ratio would be 
 1,666 grammes instead of 500. Other investigators have 
 given numbers more or less discordant ; so that, without 
 giving any opinion on their different results, this question may 
 be considered at present far from settled. M. Regnault 
 himself does not give the law by which the ratio of heat 
 varies with reference to the pressure, a subject involving 
 questions of which experiments on the mechanical effects 
 of elastic fluids seem to offer the most promising means 
 of solution. In a former edition I inclined to this opinion 
 from the greater visible effects of small changes in tempe- 
 rature on gases than on solids. When we read on a 
 mercurial thermometer to the i,oooth or even looth of a 
 degree, and aim at quantitative results, errors may well 
 arise from the minuteness of the measurement, but with 
 gases minute differences of temperature are cognizable and 
 measurable. Messrs. Joule and Thomson, however, in a 
 paper on the thermal effects of fluids in motion, 1854, thus 
 express themselves on the relation between the heat evolved 
 and the work spent in compressing a gas kept at a constant 
 temperature : ' This relation is not a relation of simple 
 mechanical equivalence, as was supposed by Mayer in his 
 " Bemerkungen ueber die Krafte der Unbeleben Natur," 
 in which he founded on it an attempt to evaluate numerically 
 the mechanical equivalent of the thermal unit. The heat 
 evolved may be less than, equal to, or greater than the 
 equivalent of the work spent, according as the work pro- 
 
62 CORRELATION OF PHYSICAL FORCES. 
 
 duces other effects in the fluid than heat, produces only heat, 
 or is assisted by molecular forces in generating heat, and 
 according to the quantity of heat, greater than, equal to, or 
 less than that held by the fluid in its primitive condition, 
 which it must hold to keep itself at the same temperature 
 when compressed. The a priori assumption of equivalence 
 for the case of air, without some special reason from theory or 
 experiment, is not less unwarrantable than for the case of any 
 fluid whatever subjected to compression. Yet it may be 
 demonstrated that water below its temperature of maximum 
 density (39* I Fahrenheit), instead of evolving any heat at all 
 when compressed actually absorbs heat, and at higher tem- 
 peratures evolves heat in greater or less, but probably always 
 very small, proportion to the equivalent of the work spent ; 
 while air, as will be shown presently, evolves always, at least 
 when kept at any temperature between o and 100 cent, 
 somewhat more heat than the work spent in compressing it 
 could alone create.' 
 
 These remarks show some of the difficulties, among many, 
 which present themselves in the way of ascertaining a me- 
 chanical equivalent of heat ; the most striking is perhaps the 
 expansion of water, and some other substances, at a certain 
 temperature by cold, or, if the expression be preferred, by the 
 absorption of heat by a cold body. 
 
 Thus, assume a cylinder of water at its point of maximum 
 density, 39, which \vater supports a piston with a weight on 
 it. Now let a substance colder than the water, say ice at 25, 
 be brought into contact with the cylinder, the water will 
 expand by the cooling effect of the ice, and produce media 
 nical force, raising the weight. 
 
 An engine the converse of the ordinary steam or calorific 
 engines might thus be constructed, in which cold would be 
 apparently the motive power, and instead of a furnace supply- 
 ing the power, and the steam or air being abstracted by cold 
 water, ice might act as the initiating power, and warm 
 water be used as the abstractor. Even here the effects may 
 be viewed as caused by an abstraction of heat ; but they are 
 not the less the converse of the ordinary thermal effects. 
 Cold may be produced by mechanical force, e.g. the compres- 
 
HEAT. 63 
 
 sion of water between 32 and 39, or by the expansion of 
 gases. It seems difficult to ascertain that in any experiments 
 some cold is not produced by elongation, expansion, or other 
 molecular change, which may neutralise some of the heat 
 produced. To call the work done by ice in cooling water 
 a mechanical effect of heat, the calorific sign must be 
 changed ; and though the phenomena may be entirely con- 
 sistent with the conservation of force, and no force may be 
 created or destroyed, the work done by the water, appa- 
 rently expanded by cold, is in the same direction to that 
 which is effected by heat, when the substance is above or 
 below the given temperature. I have stated in previous 
 editions of this work that ' Although, taking the phenomena 
 as they are known to exist, the mechanical laws may be 
 deduced, yet in any physical conception of the nature of heat 
 expansion by cold has always been a great stumbling-block 
 to me, and, I believe, to many others ;' and, although it may 
 be true, that, assuming a given amount of mechanical force to 
 be all converted into heat, or a given amount of heat into 
 mechanical force, there must be a relation of equivalence, I 
 feel by no means certain that the true equivalent of heat for 
 arrested motion has yet been attained. Mechanical force is 
 now used in the arts for producing cold, and although it 
 doubtless at the same time produces heat, the cold, if it be 
 not equivalent to the heat, has to be deducted or estimated in 
 calculating the result It may turn out that in some cases it 
 is a more correct expression to say that mechanical force pro- 
 duces a disturbance of the equilibrium of temperature than 
 that it produces heat. 
 
 Molecular vibrations also, which cannot be called heat (just 
 as undulations of a certain slowness or rapidity cannot be 
 called sound or light), carry off some of the force employed, 
 and electrical currents or inductive effects may convey away 
 some, so that to say that a given dynamic force has, in any 
 experiments hitherto made, been entirely converted into heat, 
 and heat only, is a proposition of which, to my mind at least, 
 there is not conclusive evidence. 
 
 To make it sure that a given motion of a given body has 
 all been converted into heat, the motion itself should be 
 
64 CORRELATION OF PHYSICAL FORCES. 
 
 entirely arrested ; any residual motion has to be deducted 
 from the producing force. Experiments to verify this are 
 difficult of execution, and even of conception. If a weight, 
 the descent of which produces friction, fall very rapidly with 
 slight resistance, the heat from friction is very slight, and the 
 impact when the weight reaches the ground and force thence 
 dispersed is great. If it fall with extreme slowness, the heat is 
 also experimentally slight, for the cooling effects of surround- 
 ing matter go on nearly pari passu with the heating ; there is, 
 therefore, a practical degree of velocity at which the measured 
 heat is a maximum ; but how can it, in any given experimental 
 investigation, be affirmed with certainty that this point has 
 been attained, and if so, that the heat so measured is the real 
 mechanical equivalent ; motion not being actually arrested, 
 and the dispersion of force, by the residual impact on the 
 weight reaching its destination, being almost incapable of 
 measurement in terms of heat ? 
 
 I have endeavoured to give a proof (by showing the 
 anomaly to which the contrary conclusion would lead) that, 
 whatever amount of mechanical power is produced by one 
 mode of application of heat, the same should, in theory, be 
 equally produced by any other mode. But in practice the 
 difference is immense ; and therefore it becomes a question of 
 great interest practically to ascertain what is the most con- 
 venient medium on which to apply the heat employed, and 
 the best machinery for economising it. One great problem 
 to be solved is the saving of the heat which the steam in 
 ordinary engines, after having done its work, carries into the 
 condenser, or, in the high-pressure engine, into the air. It is 
 argued you have a large amount of fuel consumed to raise 
 water to the boiling-point, at which its efficiency as a motive 
 agent commences. After it has done a small portion of work, 
 and while it still retains a very large portion of the heat 
 originally communicated to it, you reject it, and have to start 
 again with a fresh portion of steam which has similarly 
 exhausted fuel in other words, you throw away all, and 
 more than all the heat which has been employed in raising 
 the water to the boiling-point. Various plans have been 
 devised to remedy this. Using again the warm water of the 
 
HEAT. 65 
 
 condenser to feed the boiler, regains a part, but a very small 
 part, of the heat. Employing the steam first for a high 
 pressure, and then before its rejection or condensation using 
 it for a low pressure cylinder, is a second mode ; a third is to 
 use the steam, after it has done its work on the piston, as a 
 source of heat or second furnace, to boil ether, or some liquid 
 which evaporates at a lower temperature than water. These 
 plans have certain advantages ; but the complexity of ap- 
 paratus, the danger from combustion of ether, and other 
 reasons, have hitherto precluded their general adoption. 
 Under the term * regenerating engine ' various ingenious com- 
 binations have lately been suggested, and some experimental 
 engines tried, with what success it is perhaps too early at 
 present to pronounce an opinion. The fundamental notion 
 on which this class of engine is based is that the vapour or 
 air, when it has performed a certain amount of work, as by 
 raising a piston, should, instead of being condensed or blown 
 off, be retained and again heated to its original high tem- 
 perature, and then used de novo \ or that it should impart its 
 heat to some other substance, and the latter in turn impart it 
 to the fresh vapour about to act. The latter plan has been 
 proposed by Mr. Ericsson : he passes the air which has done 
 its work through layers of wire gauze, which are heated by 
 the rejected air, and through which the next charge of air is 
 made to pass. M. Seguin and Mr. Siemens have constructed 
 machines upon the former principle, which are said to have 
 given good experimental results. There is, however, a theo- 
 retical difficulty in all these, not affecting their capability of 
 acting, but affecting the question of economy, which it does 
 not seem easy to escape from. Whether the heated air or 
 vapour be retained, or whether it yield its heat to a metallic 
 or other substance, this heat must exercise its usual repulsive 
 force, and this must re-act either against the returning piston 
 or against the incoming vapour, and require a greater pressure 
 in that to neutralise it. Vapour raising a piston and pro- 
 ducing mechanical force effects this with decreasing power in 
 proportion as the piston is moved. At a certain point the 
 piston is arrested, or the stroke, as it is termed, is completed, 
 but there is still compressed vapour in the cylinder capable 
 
 F 
 
66 CORRELATION OF PHYSICAL FORCES. 
 
 of doing work, but so little that it is, and must in practice, be 
 neglected ; if this compressed vapour be retained, the piston 
 cannot be depressed without an extra force capable of over- 
 coming the resistance of this, so to speak, semi-compressed 
 vapour, in addition to that which is requisite to produce the 
 normal work of the machine ; and in whatever way the 
 residual force be retained, it must either be antagonised at a 
 loss of power for the initial force, or at most can only yield 
 the more feeble power which it would have originally given if 
 it had been allowed to act for a longer stroke on the piston. 
 It may be that a portion of this residual force may be 
 economised ; indeed, this is done when the boiler is charged 
 with warm water from the condenser, instead of with cold 
 water ; but some, indeed a notable loss, seems inevitable. 
 
 Without farther discussing the various inventions and 
 theories on this subject, which are daily receiving increased 
 development, it may be well to point out how far nature 
 distances art in its present state. According to some careful 
 estimates, the most economical of our furnaces consume from 
 ten to twenty times as much fuel to produce the same 
 quantity of heat as an animal produces ; and Matteucci found 
 that, from a given consumption of zinc in a voltaic battery, 
 a far greater mechanical effect could be produced by making 
 it act on the limbs of a recently-killed frog, notwithstanding 
 the manifold defects of such an arrangement and its inferiority 
 to the action of the living animal, than when the same battery 
 was made to produce mechanical power, by acting on an 
 electro-magnetic or other artificial motor apparatus. The 
 ratio in his experiments was nearly six to one. Thus in all 
 our artificial combinations we can but apply natural forces, 
 and with far inferior mechanism to that which is perceptible 
 in the economy of nature. 
 
 Nature is made better by no mean ; 
 But nature makes that mean ; so o'er that art, 
 Which you say adds to nature, is an art 
 That nature makes. 
 
 The term ' nature ' being, in fact, an abstract expression 
 for the changes and adaptations which have been brought 
 
HEAT. 67 
 
 about in an immeasurably long time by the mutual inter- 
 actions of physical forces, the which, by eliminating impedi- 
 ments, have produced a machinery more perfect than the 
 rapid performances of art. Even with inorganic forces the 
 perfection of natural action and natural selection is wonderful. 
 The following instance may help to explain my meaning : 
 In some of the fissures of the limestone rocks to the west of 
 Oystermouth, near Swansea, may be seen frills formed by 
 bleached oyster shells, which in course of ages have been 
 washed by the tide into them. So perfect is the filling up of 
 these fissures that hardly a crevice is left between the shells, 
 and so close the fitting that great force is required to remove 
 a single shell. A frill of many yards' length is thus formed 
 which it would much puzzle a human artificer to imitate. 
 
 A speculation has been thrown out by Sir W. Thomson, 
 that, as a certain amount of heat results from mechanical 
 action, chemical action, &c., and these other forces resolve 
 themselves into heat, as this heat is radiated into space, there 
 must be a gradual diminution of temperature for the earth, 
 by which expenditure, however slow, being continuous, it 
 would ultimately be cooled to a degree incompatible with the 
 existence of animal and vegetable life in short, that the 
 earth and the planets of our system are parting with more 
 heat than they receive, and are therefore progressively cool- 
 ing. Geological researches seemed at first to support this 
 view, as they showed that the climate of many portions of 
 the terrestrial surface was at remote periods hotter than at 
 the present time: the animals whose fossilised remains are 
 found in ancient strata have their organism adapted to what 
 we should now term a hot climate.* Mr. Balfour Stewart 
 sums up the views of Thomson as follows : ' We are led to 
 look at a beginning in which the particles of matter were in a 
 diffuse chaotic state, but endowed with the power of gravita- 
 tion, and we are led to look to an end in which the whole 
 universe will be one equally heated inert mass, and from 
 which everything like life or motion or beauty will have 
 utterly gone away.' I have some difficulty in seeing how, on 
 
 * ^ee, however, Mr. Croll's views, post. 
 F 2 
 
68 CORRELATION OF PHYSICAL FORCES. 
 
 this supposition, without creation of force, the universe got out 
 of the diffuse chaotic state ; for its tendency must, ex hypothesi, 
 have always been to remain in or return to it ; and if all other 
 forces resolve themselves into heat, and heat is nowhere re- 
 concentrated, but disperses itself through the universe until 
 all acquires an uniform temperature, the theory of the Con- 
 servation of Energy is erroneous, for, as at the ultimate equal 
 distribution all will be immobile and unchangeable, every 
 year, month, hour or minute that state is being gradually 
 approached, and so every minute some amount of active force 
 is being lost ; thus force, or energy (the term preferred) is not 
 ' conserved,' but is in gradual progress of neutralisation, which 
 virtually amounts to annihilation of force. There are so 
 many circumstances of difficulty attending cosmical specula- 
 tions, that but little reliance can be placed upon the most 
 profound. We know not the original source of terrestrial 
 heat, still less that of solar heat ; we know not whether or not 
 systems of planets may be so constituted as to communicate 
 forces, inter se, so that forces which have hitherto escaped 
 detection may be in a continuous or recurring state of inter- 
 change. 
 
 The movements produced by mutual gravitation may be 
 the means of calling into existence molecular forces within 
 the substances of the planets themselves. As neither from 
 observation, nor from deduction, can we fix or conjecture any 
 boundary to the universe of stellar orbs, as each advance in 
 telescopic power gives us a new shell, so to speak, of stars, we 
 may regard our globe, in the limit, as surrounded by a sphere 
 of matter radiating and absorbing heat, light, and possibly 
 other forces. 
 
 Such stellar radiations would not, from the evidence we 
 have at present, appear sufficient to supply the loss of heat 
 by terrestrial radiations ; but it is quite conceivable that the 
 whole solar system may pass through portions of space 
 having different temperatures, as was suggested, I believe, by 
 Poisson ; that as we have a terrestrial summer and winter, so 
 there may be a solar or systematic summer and winter, in 
 which case the heat lost during the latter period might be 
 restored during the former, The amount of the radiations 
 
HEAT. 69 
 
 of the celestial bodies may again, from changes in their 
 positions, vary through epochs which are of enormous dura- 
 tion as regards the existence of the human species ; and 
 while some are giving off heat, others, differently constituted, 
 may be absorbing it. Mr. Matthieu Williams has suggested 
 that the sun, in its motion through space (a motion of which 
 there is now tolerably good evidence), condenses and brings 
 within the range of chemical affinity and combustion the 
 attenuated matter which fills the interplanetary spaces. As 
 to whether this view will turn out to be well-founded or not it 
 would be premature to express an opinion. That the change 
 of position of the sun in space may produce some effect of 
 condensation or friction on the interplanetary medium and on 
 meteoric bodies interspersed through it, seems not impro- 
 bable ; and however visionary conjectures on the illimitable 
 universe may be, the notion that the universe is gradually 
 equalising its heat and tending to a chaos of uniform tem- 
 perature and equilibrium of force, although it may be sup- 
 ported by the phenomena we see immediately around us, 
 seems to me like those views of the instability of the solar 
 system which Laplace negatived, and would require far more 
 cogent proof than any at present given. Though the whole 
 subject will probably never be grasped by human intelligence, 
 enlarged observation may prove that phenomena seeming to 
 tend in one direction will turn out to be recurrent, though never 
 absolutely identical in their recurrence : that there is, through- 
 out the universe, gradual change but no finality. 
 
 The views of Laplace, adopted by many, supposed the 
 planets to have been formed by a gradual condensation of 
 nebulous matter. A modification of this view might, perhaps, 
 be suggested, viz. that worlds or systems, instead of being 
 created as wholes at definite periods, are gradually changing 
 by atmospheric additions or subtractions, or by accretions or 
 diminutions arising from nebulous substance or from meteoric 
 bodies, so that no star or planet could at any time be said 
 to be created or destroyed, or to be in a state of absolute 
 stability, but that some may be increasing, others dwindling 
 away, and so throughout the universe, in the past as in the 
 future. When, however, questions relating to cosmogony, or 
 
yo CORRELATION OF PHYSICAL FORCES. 
 
 to the beginning or end of worlds, are contemplated from a 
 physical point of view, the period of time over which our 
 experience, in its most enlarged sense, extends, is so inde- 
 finitely minute with reference to that which must be 'required 
 for any notable change, even in our own planet, that a variety 
 of theories may be framed equally incapable of proof or of 
 disproof. We have no means of ascertaining whether many 
 changes, which endure in the same direction for a term 
 beyond the range of human experience, are really continuous 
 or only secular variations, which may be compensated for at 
 periods far beyond our ken, so that in such cases the question 
 of comparative stability or change can at best be only 
 answered as to a term which, though enormous with reference 
 to our computations, sinks into nothing with reference to 
 cosmical time, if cosmical time be not eternity. Subjects 
 such as these, though of a kind on which the mind delights 
 to speculate, appear, with reference to any hope of attaining 
 reliable knowledge, far beyond the reach of any present or 
 immediately prospective capacity of man. 
 
ELECTRICITY. 
 
 ELECTRICITY is that affection of matter or mode of force 
 which most distinctly and beautifully brings into relation other 
 modes of force, and exhibits to a great extent, in a quantita- 
 tive form, its own relation with them, and their reciprocal 
 relations with it and with each other. From the manner in 
 which the peculiar force called electricity is seemingly trans- 
 mitted through certain bodies, such as metallic wires, the term 
 current is commonly used to denote its apparent progress. It 
 is very difficult to present to the mind any theory which will 
 give a definite conception of \\smodus agendi : the early theo- 
 ries regard its phenomena as produced either by a single fluid 
 idio-repulsive but attractive of all matter, or else as produced 
 by two fluids, each idio-repulsive but attractive of the other. 
 No substantive theory has been proposed other than these 
 two ; but although this is the case, I think I shall not be un- 
 supported by many who have attentively studied electrical 
 phenomena, in viewing them as resulting, not from the action 
 of a fluid or fluids, but as a molecular polarisation of ordinary 
 matter, or as matter acting by attraction and repulsion in a 
 definite direction. Thus, the transmission of the voltaic cur- 
 rent in liquids is viewed by Grotthus as a series of chemical 
 affinities acting in a definite direction ; for instance, in the 
 electrolysis of water, i.e. its decomposition when placed be- 
 tween the poles or electrodes of a voltaic battery, a molecule 
 of oxygen is supposed to be displaced by the exalted attrac- 
 tion of the neighbouring electrode ; the hydrogen liberated 
 by this displacement unites with the oxygen of the conti- 
 guous molecule of water ; this in turn liberates its hydrogen, 
 and so on ; the current being nothing else than this molecular 
 transmission of chemical affinity. 
 
 There is strong reason for believing that, with some ex- 
 ceptions, such as fused metals, liquids do not conduct elec- 
 tricity without undergoing decomposition ; for even in those 
 
72 CORRELATION OF PHYSICAL FORCES. 
 
 extreme cases where a trifling effect of conduction is appa- 
 rently produced without the usual elimination of substances at 
 the electrodes, the latter, when detached from the circuit, show, 
 by the counter-current which they are capable of producing 
 when immersed in a fresh liquid, that their superficial state has 
 been changed, doubtless by the determination to the surfaces 
 of minute layers of substances having opposite chemical cha- 
 racters. The question whether or not a minute conduction in 
 liquids can take place unaccompanied by chemical action, has 
 however been much agitated, and may be regarded as inter 
 apices of the science. 
 
 Assuming for the moment electrolysis to be the only 
 known electrical phenomenon, electricity would appear to 
 consist in transmitted chemical action. All the evidence we 
 have is, that a certain affection of matter or chemical change 
 takes place at certain distant points of space, the change at 
 one point having a definite relation to the change at the other, 
 and being capable of manifestation at any intermediate points. 
 
 If, now, the electrical effect called induction be examined, 
 the phenomena will be found equally opposed to the theory 
 of a fluid; and consistent with that of molecular polarisation. 
 When an electrified conductor is brought near another which 
 is not electrified, the latter becomes electrified by influence or . 
 induction, as it is termed, the nearest parts of each of these 
 two bodies exhibiting states of electricity of the contrary deno- 
 minations. Until this subject was investigated by Faraday, 
 the intervening non-conducting body or dielectric was sup- 
 posed to have no influence, and the effect was attributed to 
 the repulsion .at a distance of the electrical fluid. Faraday 
 showed that these effects differed greatly according to the 
 dielectric that was interposed. Thus they were more exalted 
 with sulphur than with shellac ; more with shellac than with 
 glass, &c. Matteucci, though differing from Faraday as to 
 the explanation he gave, added some experiments which prove 
 that the intervening dielectric is molecularly polarised. Thus 
 a number of thin plates of mica are superposed like a pack of 
 cards : metallic plates are applied to the outer facings, and 
 one of them electrified, so that the apparatus is charged like 
 a Leyden phial. Upon separating the plates with insulating 
 
ELECTRICITY. 73 
 
 handles, each plate is separately electrified, one side of it being 
 positive and the other negative, showing very neatly and de- 
 cisively a polarisation throughout the intervening substances 
 by the effect of induction. 
 
 Indeed, chemical action or electrolysis may, as I have 
 shown, be transmitted by induction across a dielectric sub- 
 stance, such as glass, but apparently only while the glass is 
 being charged with electricity. A wire passing through and 
 hermetically sealed into a glass tube, a short portion only pro- 
 jecting, is made to dip into water contained in a Florence flask ; 
 the flask is immersed in water to an equal depth with that 
 within it ; the wire and another similar wire dipping into the 
 outer water are made to communicate metallically with the 
 powerful electrical machine known as Rhumkorf's coil ; 
 bubbles of gas instantly ascend from the exposed portions 
 of the wires, but cease after a certain time, and are renewed 
 when, after an interval of separation, the coil is again con- 
 nected with the wires. 
 
 The following interesting experiment by Mr. Karsten 
 affords evidence in corroboration of the molecular changes 
 consequent upon electrisation : A coin is placed on a pack of 
 thin plates of glass, and then electrified. On removing the 
 coin and breathing on the glass plate, an impression of the 
 coin is perceptible ; this shows a certain molecular change 
 on the surface of the glass opposed to the plate, or of the 
 vapours condensed on such surface. This effect might and 
 has been interpreted as arising from a film of greasy deposit 
 supposed to exist on the plate ; the impressions, however, have 
 been proved to penetrate to certain depths below the surface, 
 and not to be removed by polishing. 
 
 The following result, however, goes farther : On sepa- 
 rating carefully the glass plates, images of the coin can be de- 
 veloped on each of the surfaces, showing that the molecular 
 change has been transmitted through the substance of the 
 glass ; and we may thence reasonably suppose that a piece of 
 glass, or other dielectric body, if it could be split up while 
 under the influence of electric induction, would exhibit some 
 molecular change at each side of each lamina, however 
 minute the subdivision. 
 
74 CORRELATION OF PHYSICAL FORCES. 
 
 I have succeeded in farther extending this experiment, and 
 in permanently fixing the images thus produced by electricity. 
 Between two carefully cleaned glass plates is placed a word or 
 device cut out of paper or tinfoil ; sheets of tinfoil a little 
 smaller than the glass plates are placed on the outside of each 
 plate, and these coatings are brought into contact with the 
 terminals -of a Rhumkorf coil. After electrisation for a few 
 seconds, the glasses are separated, and their interior surfaces 
 exposed to the vapour of hydrofluoric acid, which acts che- 
 mically on glass ; the portions of the glass not protected by 
 the paper device are corroded, while those so protected are 
 untouched or less affected by the acid, so that a permanent 
 etching is thus produced, which nothing but disintegration of 
 the glass will efface. 
 
 Some farther experiments of mine on this subject bring 
 out in a still more striking manner these curious molecular 
 changes. One of the plates of glass having been electrified 
 in the manner just mentioned, is coated, on the side impressed 
 with the invisible electrical image, with a film of iodised col- 
 lodion in the manner usually adopted for photographic pur- 
 poses ; it is then in a dark room immersed in a solution 
 of nitrate of silver ; then exposed to diffuse light for a few 
 seconds. On pouring over the collodion the usual solution of 
 pyrogallic acid, the invisible electrical image is brought out 
 as a dark device on a light ground, and can be permanently 
 fixed by hyposulphite of soda. The point worthy of observa- 
 tion in this experiment is, that this permanent image exists 
 in the collodion film, which can be stripped off the glass, dried, 
 and placed on any other surface, so that the molecular change 
 consequent on electrisation has communicated, by contact or 
 close proximity, a change to the film of collodion correspond- 
 ing in form with that on the glass, but undoubtedly of a che- 
 mical nature. Electricity has, moreover, in this experiment 
 so modified the surface of glass, that it can, in its turn, mo- 
 dify the structure of another substance so as to alter the 
 relation of the latter to light. It would require a curious 
 complication of hypothetic fluids to explain this ; but if elec- 
 tricity and light be supposed to be affections of ordinary 
 ponderable matter, the difficulty is only one of detail. 
 
ELECTRICITY. 75 
 
 If, again, we examine the electricity of the atmosphere, 
 when, as is usually the case, it is positive with respect to that 
 of the earth, we find that each successive stratum is positive 
 to those below it and negative to those above it ;. and the 
 converse is the case when the electricity of the atmosphere is 
 negative with respect to that of the earth. 
 
 If other electrical phenomena be selected, other changes in 
 the matter affected will be found to have taken place. The 
 electric spark, the brush, and similar phenomena, the old 
 theories regarded as actual emanations of the matter or 
 fluid, Electricity ; I venture to regard them as produced by an 
 emission of the material itself from whence they issue, and 
 a molecular action of the gas, or intermedium, through or 
 across which they are transmitted. 
 
 The colour of the electric spark, or of the voltaic arc (i.e. 
 the flame which plays between the terminal points of a power- 
 ful voltaic battery), is dependent upon the substance of the 
 metal, subject to certain modifications of the intermedium : 
 thus, the electric spark or arc from zinc is blue ; from silver, 
 green ; from iron, red and scintillating ; precisely the colours 
 afforded by these metals in their ordinary combustion. A 
 portion of the metal is also found to be actually transmitted 
 with every electric or voltaic discharge : in the latter case, 
 indeed, where the quantity of matter acted upon is greater 
 than in the former, the metallic particles emitted by the 
 electrodes or terminals can be readily collected, tested, or even 
 weighed. Moreover, the transverse lines in the spectrum of 
 the electric discharge differ for different metals used as ter- 
 minals. It would thus appear that the electrical discharge 
 arises, at least in part, from an actual repulsion and severance 
 of the electrified matter itself, which flies off at the points of 
 least resistance. 
 
 A careful examination of the phenomena attending the 
 electric spark or the voltaic arc, which latter is the electric 
 disruptive discharge acting on greater portions of matter, 
 tends to modify considerably our previous idea of the nature 
 of the electric force as a producer of ignition and combustion. 
 The voltaic arc is perhaps, strictly speaking, neither ignition 
 nor combustion. It is not simply ignition ; because the 
 
76 CORRELATION OF PHYSICAL FORCES, 
 
 matter of the terminals is not merely brought, to a state of 
 incandescence, but is physically separated and partially trans- 
 ferred from one electrode to another, much of it being dissipated 
 in a vaporous state. It is not combustion ; for the phenomena 
 will take place independently of atmospheric air, oxygen gas, 
 or any of the bodies usually called supporters of combustion, 
 combustion being in fact chemical union attended with heat 
 and light. In the voltaic arc we may have no chemical union ; 
 for if the experiment be performed in an exhausted receiver, 
 or in nitrogen, the substance forming the electrodes is con- 
 densed, and precipitated upon the interior of the vessel in, 
 chemically speaking, an unaltered state. Thus, to take a very 
 striking example : if the voltaic discharge be taken between 
 zinc terminals in an exhausted receiver, a fine black powder 
 of zinc is deposited on the sides of the receiver ; this can be 
 collected, and takes fire readily in the air by being touched 
 with a match, or ignited wire, instantly burning into white 
 oxide of zinc. To an ordinary observer, the zinc would appear 
 to be burned twice first in the receiver, where the pheno- 
 menon presents all the appearance of combustion, and secondly 
 in the real combustion in air. With iron the experiment is 
 equally instructive. Iron is volatilised by the voltaic arc in 
 nitrogen or in an exhausted receiver ; and when a scarcely 
 perceptible film has lined the receiver, this is washed with an 
 acid, which then gives, with ferrocyanide of potassium, the 
 Prussian-blue precipitate. In this case we readily distil iron, 
 a metal by ordinary means fusible only at a very high 
 temperature. 
 
 Another strong evidence that the voltaic discharge consists 
 of the material itself of which the terminals are composed, is 
 the peculiar rotation which is observed in the light when iron 
 is employed, the magnetic character of this metal causing its 
 molecules to rotate by the influence of the voltaic current. 
 
 If we increase the number of reduplications in a voltaic 
 series, we increase the length of the arc, and also increase its 
 intensity or power of overcoming resistance. With a battery 
 consisting of a limited number, say 100 reduplications, the 
 discharge will not pass from one terminal to the other without 
 first bringing them into contact, but if we increase the number 
 
ELECTRICITY. 77 
 
 of cells to 400 or 500, the discharge will pass from one ter- 
 minal to the other before they are brought into contact. 
 The difference between what is called franklinic electricity, 
 or that produced by an ordinary electrical machine, and 
 voltaic electricity, or that produced by the ordinary voltaic 
 battery, is that the former is of much greater intensity than 
 the latter, or has a greater power of overcoming resistance ; 
 but, assuming an equal initial power, it acts upon a much 
 smaller quantity of matter. If, then, a voltaic battery be 
 formed with a view to increase the intensity and lessen the 
 quantity, the character of the electrical phenomena approxi- 
 mates those of the electrical machine. In order to effect this, 
 the sizes of the plates of the battery, and thence the quantity 
 of matter acted on in each cell, must be reduced, but the 
 number of reduplications increased. Thus if in a battery of 
 100 pairs of plates each plate be divided, and the battery be 
 arranged so as to form 200 pairs, each being half the original 
 size, the quantitative effects are diminished, and the effects of 
 intensity increased. By carrying on this sub-division, dimi- 
 nishing the sizes and increasing the number, as is the case in 
 the voltaic piles of Deluc and Zamboni, effects are ultimately 
 produced similar to those of franklinic electricity, and we 
 thus gradually pass from the voltaic arc to the spark or electric 
 discharge. 
 
 This discharge, as I have already stated, has a colour 
 depending in part upon the nature of the terminals employed. 
 If these terminals be highly polished, a spot will be observed, 
 even in the case of a small electric spark, at the points from 
 which the discharge emanates. The matter of the terminals 
 is itself affected ; and a transmission of this matter across the 
 intervening space is detected by the deposition of minute 
 quantities of the metal or substance composing the one upon 
 the other terminal. 
 
 If the gas or elastic medium between the terminals be 
 changed, a change takes place in the length or colour of the 
 discharge, showing an affection of the intervening matter. If 
 the gas be rarefied, the discharge gradually changes with the 
 .degree of rarefaction, from a spark to a luminous glow or 
 diffuse light, differing in colour in different gases, and capable 
 
78 CORRELATION OF PHYSICAL FORCES. 
 
 of extending to a much greater distance than when it takes 
 place in air of the ordinary density. Thus, in highly-attenu- 
 ated air a discharge may be made to pass across six or seven 
 feet of space, while in air of the ordinary density it would not 
 pass across an inch. An observer regarding the beautiful 
 phenomena exhibited by this electric discharge in attenuated 
 gas, which, from some degree of similarity in appearance to 
 the Aurora Borealis, has been called the electric Aurora, 
 would have some difficulty in believing such effects could be 
 due to an action of ordinary matter. The amount of gas 
 present is extremely small ; and the terminals, to a cursory 
 examination, show no change after long experimenting. It is 
 therefore not to be wondered at that the first observers of this 
 and similar phenomena regarded electricity as in itself some- 
 thing as a specific entity or fluid. Even in this extreme 
 case, however, upon a more careful examination we shall find 
 that a change does take place, both as regards the gas and as 
 regards the terminals. Let one of these consist of a highly- 
 polished metal a silver plate is one of the best materials for 
 the purpose and let the discharges in attenuated atmospheric 
 air take place from a point, say a common sewing-needle, 
 to the surface of the polished silver plate ; it will be found 
 that this is gradually changed in appearance opposite the 
 point it is oxidated, and gradually more and more corroded 
 as the discharge is continued. 
 
 If now the gas be changed, and highly-rarefied hydrogen 
 be substituted for the rarefied air, all other things remaining 
 the same, upon passing the discharges as before the oxide 
 will be cleared off the plate, and the polish to a great extent 
 restored not entirely, because the silver has been disinte- 
 grated by the oxidation and the portion which has been 
 affected by the discharge will present a somewhat different 
 appearance from the remainder of the plate. 
 
 A question will probably here occur to the reader : What 
 will be the effect if there be not an oxidating medium pre- 
 sent, and the experiment be first performed in a rarefied gas 
 which possesses no power of chemically acting on the plate ? 
 In this case there will still be a molecular change or disinte- 
 gration of the plate ; the portion of it acted on by the discharge 
 
ELECTRICITY. 79 
 
 will present a different appearance from that which is beyond 
 its reach, and a whitish film, somewhat similar to that seen on 
 the mercurialised portions of a daguerreotype, will gradually 
 appear on the portion of the plate affected by the discharge. 
 If the gas be a compound, as carbonic oxide, or a mixture, 
 as oxygen and hydrogen, and consequently contain elements 
 capable of producing oxidation and reduction, then the effect 
 upon the plate will depend upon whether it be positive or 
 negative ; in the former case it will be oxidated, in the latter 
 the oxide, if existing, will be reduced. This effect will also 
 take place in atmospheric air, if it be highly rarefied, and 
 can hardly be explained otherwise than by a molecular polar- 
 isation of the compound gas, If, again, the metal be reduced 
 to a small point, and be of such material that the gas cannot 
 act chemically upon it, it can yet be shown to be disintegrated 
 by the electric spark. Thus, let a fine platinum wire be her- 
 metically sealed in a glass tube, and the extremity of the tube 
 and the wire ground to a flat surface, so as to expose a section 
 only of the wire ; after taking the discharge from this for some 
 time, it will be found that the platinum wire is worn away, 
 and that its termination is sensibly below the level of the glass. 
 If the discharges from such a platinum wire be taken in gas 
 contained in a narrow tube, a cloud or film consisting of a 
 deposit of platinum will be seen on the part of the tube sur- 
 rounding the point. 
 
 Another curious effect which, in addition to the above, I 
 have detected in the electrical discharge in attenuated media, 
 is that when passing between terminals of a certain form, as 
 from a wire placed at right angles to a polished plate, the dis- 
 charge possesses certain phases or fits of an alternate character, 
 so that, instead of impressing an uniform mark on a polished 
 plate, a series of concentric rings is formed. 
 
 Priestley observed that, after the discharge of a Leyden 
 battery, rings consisting of fused globules of metal were formed 
 on the terminal plates ; in my experiments made in attenu- 
 ated media, alternate rings of oxidation and deoxidation are 
 formed. Thus, if the plate be polished, coloured rings of 
 oxide will alternate with rings of polished or unoxidated sur- 
 face ; and if the plate be previously coated with an uniform 
 
8o CORRELATION OF PHYSICAL FORCES. 
 
 film of oxide, the oxide will be removed in concentric spaces, 
 and increased in the alternate ones, showing a lateral alterna- 
 tion of positive and negative electricity, or electricity of 
 opposite character in the same discharge. 
 
 It would be hasty to assert that in no case can the elec- 
 trical disruptive discharge take place without the terminals 
 being affected. I have, however, seen no instance of such a 
 result where the discharge has been sufficiently prolonged, 
 and the terminals in such a state as could be expected to 
 render manifest slight changes. 
 
 The next question which would occur in following out the 
 enquiry which has been indicated would probably be : What 
 is the action upon the gas itself ? Is this changed in any 
 manner ? 
 
 In answer to this, it must be admitted that, in the present 
 state of experimental knowledge on this subject, certain gases 
 only, appear to leave permanent traces of their having been 
 changed by the discharge, while others, if affected by it 
 which, as will be presently seen, there are reasons to believe 
 they are return to their normal state immediately after the 
 discharge. 
 
 In the former class we may place many compound gases, 
 as ammonia, olefiant gas, protoxide of nitrogen, deutoxide of 
 nitrogen, and others, which are decomposed by the action of 
 the discharge. Mixed gases are also chemically combined by 
 it: for instance, oxygen and hydrogen unite and form water; 
 common air gives nitric acid ; chlorine and aqueous vapour give 
 oxygen, the chlorine uniting with the hydrogen of the water. 
 
 But, farther than this, in the case of certain elementary 
 gases a permanent change is effected by the electrical dis- 
 charge. Thus, oxygen submitted to the discharge is partially 
 changed into the substance called ozone, a substance now 
 considered to be an allotropic condition of oxygen ; and there 
 is reason to believe that, when the change takes place, there 
 is a definite polar condition of the gas, and that definite por- 
 tions of it are affected that in a certain sense one portion 
 of the oxygen bears temporarily to the other the relation 
 which hydrogen ordinarily does to oxygen. 
 
 If the discharge be passed through the vapour of phos- 
 
ELECTRICITY. 81 
 
 phorus in the vacuum of a good air-pump, a deposit of allo- 
 tropic phosphorus soon coats the interior of the receiver, 
 showing an analogous change to that produced in oxygen ; 
 and in this case a series of transverse bands or stratifications 
 appear in the discharge, showing a most striking alteration 
 in its physical character, dependent on the medium across 
 which it is transmitted. These effects were first observed by 
 me in the year 1852. They have since been much examined 
 by Continental philosophers, and much extended by Mr. 
 Gassiot ; but no entirely satisfactory rationale of them has yet 
 been given. 
 
 There are many gases which either do not show any per- 
 manent change, or (which is more probably the case) the 
 changes produced in them by the electrical discharge have 
 not yet been detected. Even with these gases, however, the 
 difference of colour, of length, or of the different position of a 
 certain dark space or spaces which appear in the discharge, 
 shows that the discharge differs for different media. We never 
 find that the discharge has itself added to or subtracted from 
 the total weight of the substances acted on : we find no evi- 
 dence of a fluid but the visible phenomena themselves ; and 
 those we may account for by the change which takes place in 
 the matter affected. 
 
 I have here, as elsewhere, used words of common accepta- 
 tion, such as ' matter affected by the discharge/ &c., though, 
 upon the view I am suggesting, the discharge is itself this 
 affection of matter ; and the writing these passages affords, 
 to me at least, a striking instance of how much ideas are 
 bound up in words, when, to express a view differing from 
 the received one, words involving the received one are neces- 
 sarily used. 
 
 I pass now to the effect of the transmission of electricity 
 by the class of the best conducting bodies, such as the metals 
 and carbon ; here, though we cannot at present give the exact 
 character of the motion impressed upon the particles, there 
 are yet many experiments which show that a change takes 
 place in such substances when they are affected by electricity. 
 
 Let discharges from a Leyden jar or battery be passed 
 through a platinum wire, too thick to be fused by the dis- 
 
 G 
 
82 CORRELATION OF PHYSICAL FORCES. 
 
 charges, and free from constraint, it will be found that the 
 wire is shortened ; it has undergone a molecular change, and 
 apparently been acted on by a force transverse to its length. 
 If the discharges be continued, it gradually gathers up in small 
 irregular bends or convolutions. So with voltaic electricity : 
 place a platinum wire in a trough of porcelain, so that when 
 fused it shall retain its position as a wire, and then ignite it 
 by a voltaic battery. As it reaches the point of fusion it will 
 snap asunder, showing a contraction in length, and conse- 
 quently a distension or increase in its transverse dimensions. 
 Perform the same experiment with a lead wire, which can be 
 more readily kept in a state of fusion, and follow it, as it con- 
 tracts, by the terminal wires of the battery ; it will be seen to 
 gather up in nodules, which press on each other like a string 
 of beads of a soft material which have been longitudinally 
 compressed. 
 
 As we increase the thickness of the wires in these experi- 
 ments with reference to the electrical force employed, we lessen 
 the perceptible effect ; but even in this case we shall be 
 enabled safely to infer that some molecular change accom- 
 panies the transmission of electricity : the wires are heated in 
 a degree decreasing as their thickness increases but by 
 increasing the delicacy of our tests as the heating effects 
 decrease in intensity, we may indefinitely detect the augmenta- 
 tion of temperature accompanying the passage of electricity 
 and wherever there is augmentation of temperature there* 
 must be expansion or change of position of the molecules. 
 
 Again, it has been observed that wires which have for a 
 long time transmitted electricity, such as those which have 
 served as conductors for atmospheric electricity, have their 
 texture changed, and are rendered brittle. In this observa- 
 tion, however, though made by a skilful electrician, M. Peltier, 
 the effects of exposure to the atmosphere, to changes of tem- 
 perature, &c., have not been sufficiently eliminated to/ender 
 it worthy of entire confidence. There are, however, other 
 experiments which show that the elasticity of metals is 
 changed by the passage through them of the electric current. 
 
 Thus, M. Wertheim has, from an elaborate series of 
 experiments, arrived at the conclusion that there is a tern- 
 
ELECTRICITY. 83 
 
 porary diminution in the coefficient of elasticity in wires while 
 they are transmitting the electric current, which is independent 
 of the heating effect of the current. 
 
 M. Dufour has made a considerable number of experiments 
 with the view of ascertaining if any permanent change in 
 metals is effected by electrisation. He arrives at the curious 
 result that in a copper wire through which a feeble voltaic 
 current has passed for several days, a notable diminution in 
 tenacity takes place ; while, in an iron wire, the tenacity is 
 increased ; and that these effects were more perceptible when 
 the wires had been electrised for a long time (nineteen 
 days) than for a short time (four days), The copper wire 
 was, in his experiment, not perfectly pure ; so that the effect 
 or a portion of it, might be due to the state of alloy : in the 
 case of iron, the magnetic character of the metal would pro- 
 bably modify the effects, and might account for the opposite 
 character of the results with these two metals, 
 
 Matteucci has made experiments on the conduction of 
 electricity by bismuth in directions respectively parallel or 
 transverse to the planes of principal cleavage, and he finds 
 that bismuth conducts electricity and heat better in the direc- 
 tion of the cleavage planes than in that transverse to them. 
 
 Many other experiments have been made both on the 
 production of thermo-electric currents by two portions of the 
 same crystalline metal, but with the planes of crystallisation 
 arranged in different directions relatively to each other, and 
 also on the differences in conduction of heat and electricity 
 according to the direction in which they are transmitted with 
 reference to the planes of crystallisation ; the results varying 
 with the arrangements. 
 
 It is found, moreover, that the slightest difference in 
 homogeneity in the same metal enables it when heated to pro- 
 duce a thermo-electric current, and that metals in a state of 
 fusion, in which state they may be presumed to be homoge- 
 neous throughout, give no thermo-electric current : thus, hot in 
 contact with cold mercury has been shown by Matteucci to 
 give no thermo-electric current, and the same is the case with 
 portions of fused bismuth unequally heated. 
 
 The fact that the molecular structure or arrangement of a 
 
 G 2 
 
8 4 CORRELATION OF PHYSICAL FORCES. 
 
 body influences indeed, I may say determines its con- 
 ducting power, is by no means explained by the theory of a 
 fluid ; but if electricity be only a transmission of force or 
 motion, the influence of the molecular state is just what would 
 be expected. Carbon, in a transparent crystalline state, as 
 diamond, is as perfect a non-conductor as we know ; while in 
 an opaque amorphous state, as graphite or charcoal, it is one 
 of the best conductors : thus, in the one state, it transmits 
 light and stops electricity, 'in the other it transmits electricity 
 and stops light. 
 
 It is a circumstance worthy of remark, that the arrange- 
 ment of molecules, which renders a solid body capable of 
 transmitting light, is most unfavourable to its transmission of 
 electricity, transparent solids being very imperfect conductors 
 of electricity ; so gases readily transmit light, but are amongst 
 the worst conductors of electricity, if indeed, properly speak- 
 ing, they can be said to conduct at all. 
 
 The conduction of electricity by different classes of bodies 
 has been generally regarded as a question of degree : thus 
 metals were viewed as perfect conductors, charcoal less so, 
 water and other liquids as imperfect conductors, &c. But, in 
 fact, though between one metal and another the mode of 
 transmission may be the same and the difference one of 
 degree, a different molecular effect obtains, when we contrast 
 
 o * 
 
 metals with electrolytic liquids and these with gases. 
 
 Attenuated gases may be, in one sense, regarded as non- 
 conductors, in another as conductors ; ' thus, if gold-leaves be 
 made to diverge, by electrical repulsion, in air at ordinary 
 pressure, they in a short time collapse ; while in highly-rarefied 
 air, or what is commonly termed a vacuum, they remain 
 divergent for days ; and yet electricity of a certain degree of 
 tension passes readily across attenuated air, and with difficulty 
 across air of ordinary density. 
 
 Again, where the electrical terminals are brought to a 
 state of visible ignition, there are symptoms of the transmission 
 of electricity of low tension across gases ; but no such effects 
 have been detected at lower temperatures. All this presents 
 a strong argument in favour of the transmission of electricity 
 across gases being effected by convection in the disruptive 
 
ELECTRICITY, 85 
 
 discharge, and not by a conduction similar to that which takes 
 place either with metals or with electrolytes. 
 
 The ordinary attractions and repulsions of electrified 
 bodies present no more difficulty when regarded as being pro- 
 duced by a change in the state or relations of the matter 
 affected, than do the attractions of the earth by the sun, or. of 
 a leaden ball by the earth ; the hypothesis of a fluid is not 
 considered necessary for the latter, and need not be so for the 
 former class of phenomena. How the phenomena are produced 
 to which the term attraction is applied is still a mystery. 
 Newton, speaking of it, says : < What I call attraction may be 
 performed by impulse, or by some other means unknown to 
 me. I use that word here to signify only in general any 
 force by which bodies tend towards one another, whatsoever 
 be the cause.' If we suppose a fluid to act in attractions and 
 repulsions, the imponderable fluid must drag or push the 
 matter with it : thus when we feel a stream of air rushing 
 from an electrified metallic point, each molecule of air con- 
 tiguous to the point being repelled, another takes its place, 
 which is in its turn repelled ; how -does a hypothetic fluid 
 assist us here ? If we say the electrical fluid repels itself, or 
 the same electricity repels itself, we must go farther and assert, 
 that it not only repels itself, but either communicates its 
 repulsive force to the particles of the air, or carries with it the 
 particle of air in its passage. Is it not more easy to assume 
 that the particle of air is in such a state that the ordinary 
 forces which keep it in equilibrium are disturbed by the 
 electrical force, or force in a definite direction communicated 
 to it, and that thus each particle jn turn recedes from the 
 point ? As this latter force is increased, not only does the 
 particle of air which was contiguous to the metallic point re- 
 cede, but the cohesion of the extreme particles of metal may 
 be overcome to such an extent that these are detached, and 
 the brush or spark may consist wholly or in part of minute 
 particles of the metal itself thrown off. Of this there is some 
 evidence, though the point can hardly be considered as proved. 
 A similar effect undoubtedly takes place with voltaic elec- 
 tricity, acting upon a terminal immersed in a liquid ; thus if 
 metallic terminals of a powerful voltaic battery be immersed 
 
86 CORRELATION OF PHYSICAL FORCES. 
 
 in water, metal, or the oxide of metal, is forcibly detached, 
 producing great heat at the point of disruption, 
 
 If we apply ourselves to the effect of electricity in the ani- 
 mal economy, we find that the first rationale given of the con- 
 vulsive effect produced by transmission through the living or 
 recently killed animal was, that electricity itself, something sub- 
 stantive, passed rapidly through the body, and gave rise to the 
 contractions ; step by step we are now arriving at the convic- 
 tion that consecutive particles of the nerves and muscles are 
 affected. Thus the contractions which the prepared leg of a 
 frog undergoes at the moment it is submitted to the voltaic 
 current, cease after a time if the current be continued, and are 
 renewed on breaking the circuit, i.e. at the moment when the 
 current ceases to traverse it. The excitability of a nerve, 
 moreover, or its power of producing muscular contraction, is 
 weakened or destroyed by the transmission of electricity in 
 one direction, while the excitability is increased by the trans- 
 mission of electricity in the opposite direction ; showing that 
 the fibre or matter itself of the nerve is changed by electrisa- 
 tion, and changed in a manner bearing a direct relation to the 
 other effects produced by electricity. 
 
 Portions of muscle and of nerve present different electrical 
 states with reference to other portions of the same muscle or 
 nerve ; thus the external part of a muscle bears the same rela- 
 tion to the internal part as platinum does to zinc in the vol- 
 taic battery ; and delicate galvanoscopes will show electrical 
 effects when interposed in a conducting circuit connecting the 
 surface of a nerve with its interior portions. Matteucci has 
 proved that a species of voltaic pile may be formed by a series 
 of slices of muscle, so arranged that the external part of one 
 slice may touch the internal part of the next, and so on. 
 
 Lastly, the magnetic effects produced by electricity also 
 show a change in the molecular state of the magnetic sub- 
 stance affected ; as we shall see when the subject of magnetism 
 is discussed. 
 
 I have taken in succession all the known classes of elec- 
 trical phenomena'; and, as far as I am aware, there is not a 
 single electrical effect, where, if a close investigation be insti- 
 tuted, and the materials chosen in a state for exhibiting 
 
ELECTRICITY. 87 
 
 minute changes, evidence of molecular change will not be 
 detected ; so that, excepting those cases where infinitesimally 
 small quantities of matter are acted on, and our means of de- 
 tection fail, electrical effects are known to us only as changes 
 of ordinary matter. It seems to me as easy to imagine these 
 changes to be effected by a force acting in definite directions, 
 as by a fluid which has no independent or sensible existence, 
 and which, it must be assumed, is associated with, or exerts a 
 force acting upon ordinary matter, or matter of a different 
 order from the supposed fluid. As the idea of the hypothetic 
 fluid is pursued, it gradually vanishes, and resolves itself into 
 the idea of force. The hypothesis of matter without weight 
 presents in itself, as I believe, fatal objections to the theories 
 of electrical fluids, which are entirely removed by viewing 
 electricity as force or motion, and not as matter. 
 
 If it be said that the effects we have been considering may 
 still be produced by a fluid, and that this fluid acts upon ordi- 
 nary matter in certain cases, polarising the matter affected or 
 arranging its particles in a definite direction, whilst in others, 
 by its attractive or repulsive force, it carries with it portions of 
 matter ; then, if the fluid in itself be incapable of recognition 
 by any test, if it be only evidenced by the changes which it 
 operates in ponderable matter, the words fluid and force be- 
 come identical in meaning; we may as well say that the 
 attraction of gravitation or weight is occasioned by a fluid, as 
 that electrical changes are so. 
 
 When, as is constantly done in common parlance, a house 
 is said to be struck, windows broken, metals fused or dissipated 
 by the electrical fluid, are not the expressions used such as, if 
 not sanctioned by habit, would seem absurd ? In all the cases 
 of injury done by lightning there is no fluid perceptible ; the 
 so-called sulphurous odour is either ozone developed by the 
 action of electricity on atmospheric air, or the vapour of some 
 substance dissipated by the discharge ; does it not then seem 
 more consonant with experience to regard these effects as pro- 
 duced by force, as we have analogous effects produced by 
 admitted forces, in cases where no one would invoke the aid 
 of a hypothetic fluid for explanation ? For instance, glasses 
 may be broken by electrical discharges ; so may they by sono- 
 
88 CORRELATION OF PHYSICAL FORCES. 
 
 rous vibrations. Metals electrified or magnetised will emit a 
 sound ; so they will if struck, or if a musical note with which 
 they can vibrate in unison be sounded near to them. 
 
 Even chemical decomposition, in cases of feeble affinity, 
 may be produced by purely mechanical effects. A number of 
 instances of this have been collected by M. Becquerel ; and 
 substances whose constituents are held together by feeble affi- 
 nities such as iodide of nitrogen and similar compounds, are 
 decomposed by the vibration occasioned by sound. 
 
 If, instead of being regarded as a fluid or imponderable 
 matter sui generis t electricity be regarded as the motion of an 
 ether, equal difficulties are encountered. Assuming ether to 
 pervade the pores of all bodies, is the ether a conductor or 
 non-conductor ? If the latter that is, if the ether be inca- 
 pable of transmitting the electrical wave the ethereal hypo- 
 thesis of electricity necessarily fails ; but if the motion of the 
 ether constitute what we call conduction of electricity, then 
 the more porous bodies, or those most permeable by the ether, 
 should be the best conductors. But this is not the case. If, 
 again, the metal and the air surrounding it are both pervaded 
 by ether, why should the electrical wave affect the ether in the 
 metal, and not stir that in the gas ? To support an ethereal 
 hypothesis of electricity, many additional and hardly recon- 
 cilable hypotheses must be imported. 
 
 The fracture and comminution of a non-conducting body, 
 the fusion or dispersion of a metallic wire by the electrical 
 discharge, are effects equally difficult to conceive upon the 
 hypothesis of an ethereal vibration, as upon that of a fluid, but 
 are necessary results of the sudden subversion of molecular 
 polarisation, or of a sudden or irregular vibratory movement of 
 the matter itself. We see similar effects to those of electricity 
 produced by sonorous vibrations, which might be called con- 
 duction and nonconduction of sound. One body transmits 
 sound easily, another stops or deadens it, as it is termed i.e. 
 disperses the vibrations, instead of continuing them in the 
 same direction as the primary impulse ; and solid bodies 
 may, as has been above observed, be shivered by sudden im- 
 pulses of sound in those cases where all the parts of the body 
 cannot uniformly carry on the undulatory motion. 
 
ELECTRICITY. 89 
 
 The progressive stages in the History of Physical Philo- 
 sophy will account in a great measure for the adoption by the 
 early electricians of the theories of fluids. 
 
 The ancients, when they witnessed a natural phenomenon, 
 removed from ordinary analogies, and unexplained by any 
 mechanical action known to them, referred it to a soul, a spi- 
 ritual or preternatural power ; thus amber and the magnet 
 were supposed by Thales to have a soul ; the functions of 
 digestion, assimilation, &c., were supposed by Paracelsus to 
 be effected by a spirit (the Archseus). Air and gases were 
 also at first deemed spiritual, but subsequently they became 
 invested with a more material character ; and the same words, 
 Trvevpa, spirit, &c., were used to signify the soul or a gas ; the 
 very word gas, from geist, a ghost or spirit, affords us an in- 
 stance of the gradual transmission of a spiritual into a physical 
 conception ; the words ' give up the ghost,' ' expire,' had the 
 same confusion of ideas, and the rough conception of death 
 seemed to have been that the last long breath of the dying was 
 the expulsion or separation of the soul from the body ; indeed 
 it is so represented in ancient illuminated missals, where a faint 
 spectre is depicted as issuing from the mouth of the dying 
 man. 
 
 The establishment by Torricelli of the ponderable char- 
 acter of air and gas, showed that substances which had been 
 deemed spiritual and essentially different from ponderable 
 matter were possessed of its attributes. A less superstitious 
 mode of reasoning ensued, and now aeriform fluids were 
 shown to be analogous in many of their actions to liquids or 
 known fluids. A belief in the existence of other fluids, differ- 
 ing from air as this differed from water, grew up, and when a 
 new phenomenon presented itself, recourse was had to a hypo- 
 thetic fluid for explaining the phenomenon and connecting it 
 with others ; the mind once possessed of the idea of a fluid, 
 soon invested it with the necessary powers and properties, 
 and grafted upon it a luxurious vegetation of imaginary 
 offshoots. 
 
 In what I am here throwing out I wish to guard myself 
 from being supposed to state that the course of theory, histo- 
 rically viewed, followed exactly the dates of the discoveries 
 
90 CORRELATION OF PHYSICAL FORCES. 
 
 which were effectual in changing its character; sometimes a 
 discovery precedes, at other times it succeeds a change in the 
 general course of thought ; sometimes, and perhaps most fre- 
 quently, it does both i.e. the discovery is the result of a ten- 
 dency of the age and of the continually improved methods of 
 observation, and when made, it strengthens and extends the 
 views which have led to it. I think the phases of thought 
 which physical philosophers have gone through will be found 
 generally such as I have indicated, and that the gradual accu- 
 mulation of discoveries which has taken place during the more 
 recent periods, by showing what effects can be produced by 
 dynamical causes alone, is rapidly tending to a general dyna- 
 mical theory into which that of the imponderable fluids pro- 
 mises ultimately to merge. 
 
 Commencing with electricity as an initiating force, we get 
 motion directly produced by it in various forms ; for instance, 
 in the attraction and repulsion of bodies, evidenced by mobile 
 electrometers, such as that of Cuthbertson, where large masses 
 are acted on ; the rotation of the fly-wheel, another form of 
 electrical repulsion, is also a mode of palpable, visible motion. 
 
 It would follow, from the reasoning in this essay, that 
 when electricity performs any mechanical work which does not 
 return to the machine, electrical power is lost. It would be 
 unsuitable to the scope of this work to give the mathematical 
 labours of M. Clausius and others here ; but the following 
 experiment, which I devised for making the result evident to 
 an audience at the Royal Institution, will form a useful illus- 
 tration : A Leyden jar, of one square foot coated surface, 
 has its interior connected with a Cuthbertson's electrometer, 
 between which and the outer coating of the jar are a pair of 
 discharging balls fixed at a certain distance (about half an 
 inch apart). Between the Leyden jar and the prime conductor 
 is inserted a small unit jar of nine square inches surface, the 
 knobs of which are o - 2 inch apart. 
 
 The balance of the electrometer is now fixed by a stiff 
 wire inserted between its knobs, and the Leyden jar charged 
 by discharges from the unit jar. After a certain number 
 of these, say twenty, the discharge of the large jar takes 
 place across the half-inch interval. This may be viewed 
 
ELECTRICITY. 91 
 
 as the expression of electrical power received from the unit 
 jar. The experiment is now repeated, the wire between the 
 knobs having been removed, and therefore the ' tip/ or the 
 raising of the weight, is performed by the electrical repulsion 
 and attraction of the two pairs of balls. At twenty discharges 
 of the unit jar the balance is subverted, and one attracting 
 knob drops upon the other ; but no discharge takes- place, 
 showing that some electricity has been lost or converted into 
 the mechanical power which raised the balance. 
 
 By another mode of expression the electricity may be sup- 
 posed to be masked or analogous to latent heat, and it would 
 be restored if the ball were brought back without discharge by 
 extraneous force. If the discharge or other electrical effects 
 were the same in both cases, then, since the raising of the 
 ball or weight is an extra mechanical effort, and since the 
 weight is capable by its fall of producing electricity, heat, 
 or other force, it would seem that force could be got out of 
 nothing, or perpetual motion obtained. 
 
 The above experiment is suggestive of others of a similar 
 character, which may be indefinitely varied. Thus I have 
 found that two balls made to diverge by electricity do not 
 give to an electrometer the same amount of electricity as 
 they do if, whilst similarly electrified, they are kept forcibly 
 together. This experiment is the converse of the former one. 
 There is an advantage in electrical experiments of this class 
 as compared with those on heat, viz. that though there is no 
 perfect insulation for electricity, yet our means of insulating 
 it are immeasurably superior to any attainable for heat. 
 
 Electricity directly produces heat, as shown in the ignited 
 wire, the electric spark, and the voltaic arc ; in the latter, the 
 most intense heat with which we are acquainted so intense, 
 indeed, that it cannot be measured as every sort of matter 
 is dissipated by it. 
 
 In the phenomenon of electrical ignition, as shown by a 
 heated conjunctive wire, the relation of force and resistance, 
 and the correlative character of the two forces, electricity and 
 heat, are strikingly demonstrated. Let a thin wire of platinum 
 join the terminals of a voltaic battery of suitable power, the wire 
 will be ignited, and a certain amount of chemical action will 
 
92 CORRELATION OF PHYSICAL FORCES. 
 
 take place in the cells of the battery a definite quantity of 
 zinc being dissolved and of hydrogen eliminated in a given 
 time. If now the platinum wire be immersed in water, the 
 heat will, from the circulating currents of the liquid, be more 
 rapidly dissipated, resistance in the wire is lessened, and we 
 shall instantly find that the chemical action in the battery will 
 be increased, more zinc will be dissolved, and more hydrogen 
 eliminated for the same time ; the heat being conveyed away 
 by the water, more chemical action is required to generate it, 
 just as more fuel is required for boiling in proportion as evapo- 
 ration is more rapid. 
 
 Reverse the experiment, and instead of placing the wire 
 in water, place it in the flame of a spirit-lamp, so that the 
 force of heat meets with greater resistance to its dissipation. 
 We now find that the chemical action is less than in the first 
 or normal experiment. If the wire be placed in other different 
 gaseous or liquid media, we shall find that the chemical 
 action of the battery will be proportioned to the facility with 
 which the heat is circulated or radiated by these media, and 
 we thus establish an alternating reciprocity of action between 
 these two forces : a similar reciprocity may be established be- 
 tween electricity and motion, magnetism and motion, and so 
 of other forces. If it cannot be realised with all, it is pro- 
 bably because we have not yet eliminated interfering actions. 
 If we carefully think over the matter, we shall, unless I am 
 much mistaken, arrive at the conclusion that it cannot be 
 otherwise, unless it be supposed that a force can arise from 
 nothing can exist without antecedent force. 
 
 In the phenomenon of the voltaic arc, the electric spark, 
 &c., to which I have already adverted, electricity directly pro- 
 duces light of the greatest intensity of any artificially obtained. 
 It directly produces magnetism, as shown by Oersted, who first 
 distinctly proved the connection between electricity and mag- 
 netism. These two forces act upon each other, not in straight 
 lines, as all other known forces do, but in a rectangular direc- 
 tion ; that is, bodies affected by dynamic electricity, or the 
 conduits of an electric current, tend to place magnets at 
 right angles to them ; and, conversely, magnets tend to place 
 bodies conducting electricity at right angles to them. Thus an 
 
ELECTRICITY. 93 
 
 electric current appears to have a magnetic action, in a direc- 
 tion cutting its own at right angles ; or, supposing its section 
 to be a circle, tangential to it ; if, then, we reverse the posi- 
 tion, and make the electric current form a series of tangents 
 to an imaginary cylinder, this cylinder should be a magnet. 
 This is effected in practice by coiling a wire as a helix or spiral, 
 and this, when conducting an electrical current, is to all intents 
 and purposes a magnet. A soft iron core placed within such 
 a helix has the property of concentrating its power, and then 
 we can, by connection or disconnection with the source of 
 electricity, instantly make or unmake a most powerful 
 magnet. 
 
 We may figure to the mind electrified and magnetised 
 matter, as lines of which the extremities repel each other in a 
 definite direction ; thus, if a line A B represent a wire affected 
 by electricity, and superposed on C D, a wire affected by mag- 
 netism, the extreme points A and B will be repelled to the 
 farthest distances from the points C and D, and the line A B 
 be at right angles to the line C D ; and so, if the lines be sub- 
 divided to any extent, each will have two extremities or poles 
 repulsive of those of the other. If the line of matter affected 
 by electricity be a liquid, and consequently have entire mobi- 
 lity of particles, a continuous movement will be produced by 
 magnetisation, each particle successively tending, as it were, 
 to fly off at a tangent from the magnet : thus, place a flat dish 
 containing acidulated water on the poles of a powerful mag- 
 net, immerse the terminals of a voltaic battery in the liquid 
 just above the magnetic poles, so that the lines of electricity 
 and of magnetism coincide ; the water will now assume a 
 movement at right angles to this line, flowing continuously, as 
 if blown by an equatorial wind, which may be made east or 
 west with reference to the magnetic poles by altering the 
 direction of the electrical current : a similar effect may be 
 produced with mercury. These cases afford an additional 
 argument to those previously mentioned, of the particles of 
 matter being affected by the forces of electricity and mag- 
 netism in a way irreconcilable with the fluid or ethereal 
 hypothesis. 
 
 The representation of transverse direction by magnetism 
 
94 CORRELATION OF PHYSICAL FORCES. 
 
 and electricity appears to have led Coleridge to parallel it by 
 the transverse expansion of matter, or length and breadth, 
 though he injured the parallel by adding galvanism as depth ; 
 whether a third force exists which may bear this relation to 
 electricity and magnetism is a question upon which we have 
 no evidence. 
 
 The ratio which the attractive magnetic force produced 
 bears to the electric current producing it has been investigated 
 by many experimentalists and mathematicians. The data are 
 so numerous and so variable, that it is difficult to arrive at 
 definite results. Thus, the relative size of the coil and the 
 iron, the temper or degree of hardness of the latter, its shape, 
 Or the proportion of length to diameter, the number of coils 
 surrounding it, the conducting power of the metal of which 
 the coils are formed, the size of the keeper or iron in which 
 magnetism is induced, the degree of constancy of the battery, 
 &c., complicate the experiments. 
 
 The most trustworthy general relation which has been 
 ascertained is, that the magnetic attraction is as the square of 
 the electric force ; a result due to the researches of Lenz and 
 Jacobi, and also of Sir W. S. Harris. 
 
 Lastly, electricity produces chemical affinity ; and by its 
 agency we are enabled to obtain effects of analysis or syn- 
 thesis with which ordinary chemistry does not furnish us. Of 
 these effects we have examples in the brilliant discoveries, by 
 Davy, of the alkaline metals, and in the peculiar crystalline 
 compounds made known by Crosse and Becquerel ; though 
 the latter are probably not always the definite result of the 
 electric current, but are formed by the chemical changes 
 produced by its bringing the molecules of the substances 
 experimented on within the range of aggregation. 
 
95 
 
 LIGHT. 
 
 IN entering on the subject of LIGHT, it will be well to 
 describe briefly, and in a manner, as far as may be, inde- 
 pendent of theory, the effects to which the term polarisation 
 has been applied. 
 
 When light is reflected from the surface of water, glass, or 
 many other media, it undergoes a change which disables it 
 from being again similarly reflected in a direction at right 
 angles to that at which it has been originally reflected. Light 
 so affected is said to be polarised ; it will always be capable 
 of being reflected in planes parallel to the plane in which it 
 has been first reflected, but incapable of being reflected in 
 planes at right angles to that plane. At planes having a 
 direction intermediate between the original plane of reflection, 
 and a plane at right angles to it, the light will be capable of 
 being partially reflected, and more or less so according as the 
 direction of the second plane of reflection is more or less 
 coincident with the original plane. Light, again, when passed 
 through a crystal of Iceland spar, is what is termed doubly 
 refracted, i.e. split into two divisions or beams, each having 
 half the luminosity of the original incident light ; each of 
 these beams is polarised in planes at right angles to each 
 other ; and if they be intercepted by the mineral tourmaline, 
 one of them is absorbed, so that only one polarised beam 
 emerges. Similar effects may be produced by certain other 
 reflections or refractions. A ray of light once polarised in a 
 certain plane continues so affected throughout its whole sub- 
 sequent course ; and at any indefinite distance from the point 
 where it originally underwent the change, the direction of the 
 plane will be the same, provided the media through which it 
 is transmitted be air, water, or certain other transparent sub- 
 stances which need not be enumerated. If, however, the 
 
96 CORRELATION OF PHYSICAL FORCES. 
 
 polarised ray, instead of passing through water, be made to 
 pass through oil of turpentine, the definite direction in which 
 it is polarised will be found to be changed ; and the change of 
 direction will be greater according to the length of the column, 
 of interposed liquid. Instead of being an uniform plane, it will 
 have a curvilinear direction, similar to that which a strip of 
 card would have if forced along two opposite grooves of a rifle- 
 barrel. This curious effect is produced in different degrees by 
 different media. The direction also varies ; the rotation, as it 
 is termed, of the plane being sometimes to the right hand 
 and sometimes to the left, according to the peculiar molecular 
 character of the medium through which the polarised ray is 
 transmitted. These effects will be presently reverted to. 
 
 Light is, perhaps, that mode of force the reciprocal rela- 
 tions of which with the others have been the least traced out. 
 Until the discoveries of Niepce, Daguerre, and Talbot, very little 
 could be definitely predicated of the action of light in producing 
 other modes of force. Certain chemical compounds, among 
 which stand pre-eminent the salts of silver, have the property 
 of suffering decomposition when exposed to light. If, for 
 instance, recently formed chloride of silver be submitted to 
 luminous rays, a partial decomposition ensues ; the chlorine 
 is separated and expelled by the action of light, and the silver 
 is precipitated. By this decomposition the colour of the sub- 
 stance changes from white to purple. If now, paper be 
 impregnated with chloride of silver, which can be done by a 
 simple chemical process, then partially covered with an 
 opaque substance, a leaf for example, and exposed to a strong 
 light, the chloride will be decomposed in all those parts of the 
 paper where the light is not intercepted, and we shall have, 
 by the action of light, a white image of the leaf on a purple 
 ground. If similar paper be placed in the focus of a lens in a 
 camera-obscura, the objects there depicted will decompose the 
 chloride, just in the proportion in which they are luminous, 
 and thus, as the most luminous parts of the image will most 
 darken the chloride, we shall have a picture of the objects 
 with reversed lights and shadows. The picture thus produced 
 would not be permanent, as subsequent exposure would 
 darken the light portion of the picture : to fix it, the paper 
 
LIGHT. 97 
 
 must be immersed in a solution which has the property of 
 dissolving chloride of silver, but not metallic silver. Iodide 
 of potassium will effect this ; and the paper being washed and 
 dried will then preserve a permanent image of the depicted 
 objects. This was the first and simple process of Mr. Talbot ; 
 but it is defective as to the purposes aimed at, in many points. 
 First, it is not sufficiently sensitive, requiring a strong light 
 and a long time to produce an image ; secondly, the lights 
 and shadows are reversed ; and thirdly, the coarse structure 
 of the finest paper does not admit of the delicate traces of 
 objects being distinctly impressed. These defects have been 
 to a great extent remedied by a process subsequently disco- 
 vered by Mr. Talbot, and which bears his name, and which 
 has led to the collodion process, and others unnecessary to be 
 detailed here. 
 
 The photographs of M. Daguerre, with which all are now 
 familiar, are produced by holding a plate of highly-polished 
 silver over iodine. A thin film of iodide of silver is thus 
 formed on the surface of the metal ; and when these iodized 
 plates are exposed in the camera, a chemical alteration takes 
 place. The portions of the plate on which the light has im- 
 pinged, part with some of the iodine, or are otherwise changed 
 for the theory is somewhat doubtful so as to be capable of 
 ready amalgamation. When, therefore, the plate is placed over 
 the vapour of heated mercury, the mercury attaches itself to 
 the portions affected by light, and gives them a white frosted 
 appearance ; the intermediate tints are less affected, and those 
 parts where no light has fallen, by retaining their original 
 polish, appear dark : the iodide of silver is then washed off by 
 hyposulphite of soda, which has the property of dissolving it, 
 and there remains a picture in which the lights arid shadows 
 are as in nature, and the molecular uniformity of the metallic 
 surface enables the most microscopic details to be depicted 
 with perfect accuracy. By using chloride of iodine, or bro- 
 mide of iodine, instead of iodine, the equilibrium of chemical 
 forces is rendered still more unstable, so that images may be 
 taken in an indefinitely short period a period practically 
 instantaneous. 
 
 It would be foreign to the object of this essay to enter upon 
 
 n 
 
98 CORRELATION OF PHYSICAL FORCES. 
 
 the many beautiful details into which the science of photo- 
 graphy has branched out, and the many valuable discoveries 
 and practical applications to which it has led. The short state- 
 ment I have given above is perhaps superfluous, as, though 
 the effects were new and surprising at the period when these 
 Lectures were delivered, photographic processes have no\v 
 become familiar, not only to the cultivator of science, but to 
 the artist and amateur : the important point for consideration 
 here is that light will chemically or molecularly affect matter. 
 Not only will the particular compounds above selected as in- 
 stances be changed by the action of light ; but a vast number of 
 substances, both elementary and compound, are notably affected 
 by this agent, even those apparently the most unalterable in 
 character, such as metals : so numerous, indeed, are the sub- 
 stances affected, that it has been supposed, not without reason, 
 that matter of every description is altered by exposure to light. 
 The permanent impression stamped on the molecules of 
 matter by light can be made to repeat itself by the same 
 agency, but always with decreasing force. Thus a photograph 
 placed opposite a camera containing a sensitive plate will be re- 
 produced, but if the size of the image be equal to the picture, 
 the second picture will be fainter in delineation than the first, 
 and so on. Thus again, a photograph taken on a dull day can- 
 not, by being placed in bright sunshine, be made to reproduce 
 a second photograph of the same size and more distinctly 
 marked than itself; I at least have never succeeded in such 
 reproduction, and I am not aware that others have : the image 
 loses in intensity as light itself does by each transmission. 
 The surface of the metal or paper may give a brighter image 
 from its being exposed to a more intense light, but the photo- 
 graphic details are limited to the intensity of the first impres- 
 sion, or rather to something short of this. A question of 
 theoretical interest arises from the consideration of these 
 reproduced photographs. We know that the luminosity of 
 the image at the focus of a telescope is limited by the area 
 of the object-glass. The image of any given object cannot 
 be intensified by throwing upon it extraneous light ; it is 
 indeed diminished in intensity, and when for certain purposes 
 astronomers illuminate the fields of their telescopes, they are 
 
LIGHT. 99 
 
 obliged to be contented with a loss of intensity in the telescopic 
 image. 
 
 Now, let us suppose that the minutest details in the 
 image of an object seen in a given telescope, and with a given 
 power, are noted ; that then a photographic plate is placed in 
 the focus of the same telescope so as to obtain a permanent 
 impression of the image which has been viewed by the eye- 
 glass. Could the observer, by throwing a beam of condensed 
 light upon the photograph, enable himself to bring out fresh 
 details ? or, in other words, could he use with advantage a 
 higher power applied to the illuminated photograph than that 
 which he used for direct vision ? 
 
 It is, perhaps, hardly safe to answer a priori this question ; 
 but the experiment of reproducing photographs would seem 
 to show that more detail than that produced by the initial 
 light cannot be obtained, and that we cannot expect to increase 
 telescopic power by photography, though we may render 
 observations more convenient, may by its means fix images 
 seen on rare and favourable occasions, and may preserve per- 
 manent and infallible records of the state of astronomical 
 objects at the periods of observation. 
 
 The effect of light on chemical compounds affords us a 
 striking instance of the extent to which a force, ever active, 
 may be ignored through successive ages of philosophy. If 
 we suppose the walls of a large room covered with photogra- 
 phic apparatus, the small amount of light reflected from the 
 face of a person situated in its centre would simultaneously 
 imprint his portrait; on a multitude of recipient surfaces. Were 
 the cameras absent, but the room coated with photographic 
 paper, a change would equally take place in every portion of 
 it, though not a reproduction of form and figure. As other 
 substances not commonly called photographic are known to 
 be affected by light, the list of which might be indefinitely 
 extended, it becomes a curious object of contemplation to con- 
 sider how far light is daily operating changes in ponderable 
 matter how far a force, for a long time recognised only in its 
 visual effects, may be constantly producing changes in the 
 earth and atmosphere, in addition to the changes it produces 
 in organised structures, which are now beginning to be exten- 
 
 H 2 
 
ioo CORRELATION OF PHYSICAL FORCES. 
 
 sively studied. Every portion of light may be supposed to 
 write its own history by a change more or less permanent in 
 ponderable matter. 
 
 The late Mr. George Stephenson had a favourite idea, which 
 would now be recognised as more philosophical than it was 
 in his day, viz. that the light, which we nightly obtain from 
 coal or other fuel, was a reproduction of that which had at one 
 time been absorbed by vegetable structures from the sun. In 
 one sense we now know this to be true ; the rays of the sun 
 enable the leaves of plants to decompose carbonic acid gas, 
 absorbing and assimilating the carbon, and allowing the 
 oxygen to escape. Mr. Balfour Stewart supposes that the 
 want of photographic power in rays reflected from the leaves 
 of growing plants is due to the energy of the rays having 
 been used up in decomposing the carbonic acid. The carbon 
 thus segregated by the sun's rays is ready to give out heat 
 and light, whenever it may be recombined with oxygen by 
 combustion. The conviction that the transient gleam leaves 
 its permanent impress on the world's history, also leads the 
 mind to ponder over the many possible agencies of which we 
 of the present day may be as ignorant as the ancients were of 
 the chemical action of light. 
 
 I have used the term light, and affected by light, in speak- 
 ing of photographic effects ; but, though the phenomena 
 derived their name from light, it has been doubted by many 
 competent investigators whether the phenomena of photo- 
 graphy are not mainly dependent upon a separate agent 
 accompanying light, rather than upon light itself. It is, 
 indeed, difficult not to believe that a picture, taken in the 
 focus. of a camera-obscura, and which represents to the eye all 
 the gradations of light and shade shown by the original 
 luminous image, is not an effect of light ; certain it is, how- 
 ever, that the different coloured rays exercise different actions 
 upon various chemical compounds, and that the effects on 
 many, perhaps on all of them, are not proportionate in inten- 
 sity to the effects upon the visual organs. Those effects, 
 however, appear to be more of degree than of specific differ- 
 ence ; and, without pronouncing myself positively upon the 
 question, hitherto so little examined, I think we may, at all 
 
LIGHT. ioi 
 
 events in some cases, regard the action on photographic com- 
 pounds as resulting from a function of light. So viewing it, 
 we get light as an initiating force, capable of producing, 
 mediately or immediately, the other modes of force. Thus, 
 it immediately produces chemical action ; and having this, 
 we at once acquire a means of producing the others. At my 
 Lectures in 1843, I showed an experiment by which the pro- 
 duction of all the other modes of force by light is exhibited : 
 I may here shortly describe it. A prepared daguerreotype 
 plate is enclosed in a box filled with water, having a glass 
 front with a shutter over it. Between this glass and the plate 
 is a gridiron of silver wire ; the plate is connected metallically 
 with one extremity of a galvanometer coil, and the gridiron of 
 wire with one extremity of a Breguet's helix an elegant instru- 
 ment formed by a coil of two metals, the unequal expansion of 
 which indicates slight changes in temperature the other 
 extremities of the galvanometer and helix are connected by 
 a wire, and the needles brought to zero. As soon as a beam 
 of either daylight or the oxyhydrogen light is, by raising the 
 shutter, permitted to impinge upon the plate, the needles are 
 deflected. Thus, light being the initiating force, we get chemical 
 action on the plate, electricity circulating through the wires, 
 magnetism in the coil, heat in the helix, and motion in the 
 needles. 
 
 If two plates of platinum be placed in acidulated water, 
 and 'connected with a delicate galvanometer, the needle of this 
 is always deflected, a result due to films of gas or other 
 matter on the surface of the platinum, which no cleaning can 
 remove. If, after the needle has returned to zero, which will 
 not be the case for some hours or even days, one of the plati- 
 num surfaces be exposed to light, a fresh deflection of the 
 needle takes place, due, as far as I have been able to resolve 
 it, to an augmentation of the chemical action which had occa- 
 sioned the original deflection, for the deviation is in the same 
 direction. If, instead of white light, coloured light be per- 
 mitted to impinge on the plate, the deviation is greater with 
 blue than with red or yellow light, showing, in addition to 
 other tests, that the effect is not due to the heat of the sun's 
 rays, as the calorific effects of light are greater with red 
 
102 CORRELATION OF PHYSICAL FORCES. 
 
 than with blue light, while the chemical effects are the 
 inverse. 
 
 There are other apparently more direct agencies of light 
 in producing electricity and magnetism, such as those observed 
 by Morichini and others, as well as its effects upon crystallisa- 
 tion; but these results have hitherto been of so indefinite a 
 character, that they can only be regarded as presenting fields 
 for experiment, and not as proving the relations of light to 
 the other forces. 
 
 Light would seem directly to produce heat in the pheno- 
 mena of what is termed absorption of light : in some of these 
 we find that heat is developed in proportion to the disappear- 
 ance of light. It was thought, and Franklin's experiment of 
 different coloured cloths on snow exposed to the sun seemed 
 to prove it, that heat was produced in a direct ratio to the 
 absorption of light, but Dr. Tyndall has shown that this 
 result does not depend on the colour, but on the chemical 
 and physical character of the substance exposed, and that the 
 heating effect is due mainly to non-luminous radiation. The 
 heating powers of different colours are by no means in exact 
 proportion to the intensity of their light as affecting the visual 
 organs. Thus, red light when produced by refraction from 
 a prism of glass, produces greater heating effect than 
 yellow light in the phenomena of absorption, as has been 
 observed by Sir W. Herschel. The red rays appear to pro- 
 duce a dynamic effect greater than any of the others ; thus 
 they penetrate water to a greater depth than the other 
 colours ; but, according to Dr. Seebeck, we get a further ano- 
 maly, viz. that when light is refracted by a prism of water 
 the yellow rays produce the greater heating effect. The 
 subject, therefore, requires much more experiment before we 
 can ascertain the rationale of the action of the forces of light 
 and heat in this class of phenomena. 
 
 In a former edition of this essay I suggested the following 
 experiment on this subject : Let a beam of light be passed 
 through two plates of tourmaline, or similar substance, and 
 the temperature of the second plate, or that on which the light 
 last impinges, be examined by a delicate thermoscope, first 
 when it is in a position to transmit the polarised beam coming 
 
LIGHT. 103 
 
 from the first plate, and secondly when it has been turned 
 round through an arc of 90, and the polarised beam is 
 absorbed. I expected that, if the experiment were carefully 
 performed, the temperature of the second plate would be more 
 raised in the second case than in the first, and that it might 
 afford interesting results when tried with light of different 
 colours. I met with difficulties in procuring a suitable ap- 
 paratus, and was endeavouring to overcome them, when I 
 found that Knoblauch had, to some extent, realised this result. 
 He finds that, when a solar beam, polarised in a certain 
 plane, is transmitted perpendicularly to the axis of a crystal 
 of brown quartz or tourmaline, the heat is transmitted in a 
 smaller proportion than when the beam passes along the 
 direction of the axis of the crystal. 
 
 It is generally as far as I am aware, universally true 
 that, while light continues as light, even though reflected or 
 transmitted by different media, little or no heat is developed; 
 and, as far as we can judge, it would appear that, if a medium 
 were perfectly transparent, or if a surface perfectly reflected 
 light, not the slightest heating effect would take place ; but, 
 wherever light is absorbed, then heat takes its place, affording 
 us, apparently, an instance of the conversion of light into 
 heat, and of the fact that the force of light is not, in fact, 
 annihilated, but merely changed in character, becoming, in 
 this instance, converted into heat by impinging on solid 
 matter, as in the instance mentioned in treating of heat, this 
 force was shown to be converted into light by impinging on 
 solid matter. But the different effects of different substances 
 in transmitting and reflecting heat and light make this diffi- 
 cult of proof. One experiment, indeed, of Melloni, already 
 mentioned, would seem to show that light may exist in a 
 condition in which it does not produce heat, which our 
 instruments are able to detect ; but some doubt has recently 
 been thrown on the accuracy of this experiment ; probably 
 the substances themselves through which the light is trans- 
 mitted would be found to have been heated. 
 
 The recipient body, or that upon which light impinges, 
 seems to exercise nearly as important an influence on our 
 perceptions of light as the emittent body, or that from which 
 
io4 CORRELATION OF PHYSICAL FORCES. 
 
 the light first proceeds. The recent experiments of Sir John 
 Herschel and Mr. Stokes show that radiant impulses, which, 
 falling on certain bodies, give no effect of light, become 
 luminous when falling on other bodies. 
 
 Thus, let ordinary solar light be refracted by a prism (the 
 best material for which is quartz), and the spectrum received 
 on a sheet of paper, or of white porcelain ; looking on the 
 paper, the eye detects no light beyond the extreme violet 
 rays. If, therefore, an opaque body be interposed so as just 
 to cut off the whole visible spectrum, the paper would be 
 dark or invisible, with the exception of some slight illumina- 
 tion from light reflected from dust in the air and from sur- 
 rounding bodies. Substitute for that portion of the paper 
 which was beyond the visible spectrum a piece of glass 
 tinged by the oxide of uranium, and the glass is perfectly 
 visible ; so with a bottle of sulphate of quinine, or of the 
 juice of horse-chesnuts, or even paper soaked in these latter 
 solutions. Other substances exhibit in different degrees this 
 effect, which is termed fluorescence ; and among the sub- 
 stances which have hitherto been considered perfectly analo- 
 gous as to their appearance when illuminated, notable differ- 
 ences are discovered. Thus it appears that emanations which 
 give no impression of light to the eye, when impinging on 
 certain bodies, become luminous when impinging on others. 
 We might imagine a room so constructed that such emana- 
 tions alone are permitted to enter it, which would be dark or 
 light according to the substance with which the walls were 
 coated, though in full daylight the respective coatings of the 
 walls would appear equally white ; or, without altering the 
 coating of the walls, the room exposed to one class of rays 
 might be rendered dark by windows which would be trans- 
 parent to another class. 
 
 If, instead of solar light, the electrical light be employed 
 for similar experiments, an equally striking effect can actually 
 be produced. A design, drawn on white paper with a solu- 
 tion of sulphate of quinine and tartaric acid, is invisible by 
 ordinary light, but appears with beautiful distinctness when 
 illuminated by the electric light. Thus, in pronouncing upon 
 a luminous effect, regard must be had to the recipient as well 
 
LIGHT. 105 
 
 as to the emittent body. That which is, or becomes, light 
 when it falls upon one body is not light when it falls upon 
 another. Probably the retinae of the eyes of different persons 
 differ to some extent in a similar manner ; and the same 
 substance, illuminated by the same spectrum, may present 
 different appearances to different persons, the spectrum 
 appearing more elongated to the one than to the other, so 
 that what is light to the one is darkness to the other. A 
 dependence on the recipient body may also, to a great extent, 
 be predicated of heat. Let two vessels of water, the contents 
 of the one clear and transparent, of the other tinged by some 
 colouring matter, be suspended in a summer's sun ; in a very 
 short time a notable difference of temperature will be observed, 
 the coloured having become much hotter than the clear liquid. 
 If the first vessel be placed at a considerable distance from 
 the surface of the earth, and the second near the surface, the 
 difference is still more considerable. Carrying on this ex- 
 periment, and suspending the first over the top of a high 
 mountain, and the second in a valley, we may obtain so great 
 a difference of temperature, that animals whose organisation 
 is suited for the one temperature could not live in the other, 
 and yet both are exposed to the same luminous rays at the 
 same time, and substantially at the same distance from the 
 emittent body the substance nearer the sun is, in fact, colder 
 than the more remote. So, with regard to the medium trans- 
 mitting the influence : a greenhouse may have its tempe- 
 rature, or the vegetable growth within it, considerably varied 
 by changing the glass of which its roof is made. 
 
 These effects have an important bearing on certain cos- 
 mical questions which have lately been much discussed, and 
 should induce the greatest caution in forming opinions on 
 such subjects as light and heat on the sun's surface, the tem- 
 perature of the planets, &c. The latter may depend as much 
 upon their physical constitution as upon their distance from 
 the sun. Indeed, the planet Mars gives us a highly probable 
 argument for this ; for, notwithstanding that it is half as far 
 again from -the sun as the earth is, the increase of the white 
 tracts at its poles during its winter, and their diminution 
 during its summer, show that the temperature of the surface 
 
io6 CORRELATION OF PHYSICAL FORCES. 
 
 of this planet oscillates about that of the freezing-point of 
 water, as do the analogous zones of our planet. It is true in 
 this we assume that the substance thus changing its state 
 is water ; but considering the many close analogies of this 
 planet with the earth, and the identity in appearance of these 
 very effects with what takes place on the earth, it seems not 
 an improbable assumption. 
 
 So it by no means necessarily follows, that because Venus 
 is nearer to the sun than the earth, that planet is hotter than 
 our globe. The force emitted by the sun may take a differ- 
 ent character at the surface of each different planet, and 
 require different organisms or senses for its appreciation. 
 Myriads of organised beings may exist imperceptible to our 
 vision, even if we were among them ; and we might be 
 equally imperceptible to them ! 
 
 However vain it may be, in the present state of science, 
 to speculate upon such existences, it is equally vain to assume 
 identity or close approximations to our own forms in those 
 beings which may people other worlds. Reasoning from 
 analogy, or from final causation, if that be admitted, we may 
 feel convinced that the gorgeous globes of the universe are 
 not unpeopled deserts ; but whether the denizens of other 
 worlds are more or less powerful, more or less intelligent, 
 whether they have attributes of a higher or lower class than 
 ourselves, is at present an utterly hopeless guessing. 
 
 Specific gravity and intelligence have no necessary con- 
 nection. On our own planet five senses, and a mean density 
 equal to that of water, are not invariably associated with 
 intellectual or moral greatness, and the many arguments 
 which have been used to prove that suns and planets other 
 than the earth are uninhabited, or not inhabited by intel- 
 lectual beings, might, mutatis mutandis, equally be used by 
 the denizens of a sun or planet to prove that this world was 
 uninhabited. 
 
 Men are too apt, because they are men, because their 
 existence is the one thing of all importance to themselves, to 
 frame schemes of the universe as though it was formed for 
 man alone : painted by an artist of the sun, a man might not 
 
LIGHT. 107 
 
 represent so prominent an object of creation as he does when 
 represented by his own pencil. 
 
 It seems to me no extravagant thought that, if alcohol 
 and hereditary disease do not destroy the human race, a time 
 may come when, from what will be known of the character- 
 istics of some of the planets, a reasonable hypothesis of the 
 characters of their inhabitants may be framed. 
 
 At the beginning of the last century, if it had been stated 
 that a man could, from inspection of a fragment of bone, pro- 
 nounce a well-founded opinion on the form and habits of the 
 animal of which it had been part, such a conjecture would 
 have been deemed ridiculous. 
 
 Could not the doctrine of Cuvier be extended ? To a 
 man who had never seen a fish, it would at first have seemed 
 impossible that an animal could live under water, immersion 
 in which had always, to his observation, proved fatal to ter- 
 restrial creatures ; but a more far-seeing philosopher might 
 argue from analogy that, as each part of the terrestrial sur- 
 face was peopled with different animals varying, according to 
 their organic adaptation, from a centipede to a kangaroo, it 
 might not be improbable that the depths of an ocean might 
 be inhabited by a suitable sentient creature. He might be 
 led, by the observation of analogies, to form some idea of 
 what would be the most suitable organism for respiration and 
 locomotion under water, and so arrive at a not unapt idea of 
 a fish. 
 
 The same mode of reasoning might lead him on thus : 
 ' I see that the greater part of the surface of the globe I live 
 on has organic beings, which, by some process, have not 
 only become adapted to great varieties of climate, but which 
 inhabit earth, air, and water. I see that a certain solution of 
 salt in water which would be fatal to a trout is necessary to a 
 mackerel. I therefore cannot conclude that, because a planet 
 has a more attenuated atmosphere, a greater degree of heat, 
 or a less degree of attraction of gravitation, it is therefore 
 uninhabited.' He would be led to conclude, from all the 
 variety of organic forms which people the present surface, 
 Jakes, rivers, oceans, and atmosphere of this planet, that it is 
 
io8 CORRELATION OF PHYSICAL FORCES. 
 
 more than probable that planets, not differing in their con- 
 dition so much from this as salt water, in which lives a whale, 
 does from the air in which an eagle disports himself, are 
 not barren while this is densely thronged with suitable 
 organisms ; he would think it improbable that, by a singular 
 accident, his lot should have been cast on the only planet 
 which is crowded at the present time with living organisms, 
 and whose history shows that it has been so crowded in the 
 past, that some of its rocks consist of a congeries of organic 
 remains. Would it not appear reasonable to deduce the 
 probability that there are other worlds, not peopled by crea- 
 tures identical with or very similar to those which struggle 
 for each spot of this earth, but as varied from those and 
 from each other as the nature of their surroundings may 
 require ; and thence, finding by more perfect acquaintance 
 the inevitable connection of surrounding conditions with the 
 character of the organisms capable of existing among them, 
 might he not more or less approximately deduce, from know- 
 ledge of the conditions, the character of the organisms ? 
 
 As by the relation of inorganic matter to light, heat, &c. 
 we have now tolerably clear proofs of the substances of which 
 the sun and fixed stars are composed, it seems only a more 
 perfected knowledge that may give the more complex deduc- 
 tions which I venture, at the risk of being thought fanciful, to 
 indicate as possible in the distant future. 
 
 Light was regarded, by what was termed the corpuscular 
 theory, as being in itself a specific matter emanating from 
 luminous bodies, and producing the effects of sensation by 
 impinging on the retina. This theory gave way to the undu- 
 latory one, which is generally adopted in the present day, 
 and which regards light as resulting from the undulation of a 
 specific fluid to which the name of ether has been given, 
 which hypothetic fluid is supposed to pervade the universe, 
 and to permeate the pores of all bodies. 
 
 In a lecture delivered in January 1842, when I first pub- 
 licly advanced the views advocated in this essay, I stated that 
 it appeared to me more consistent with known facts to regard 
 light as resulting from a vibration or motion of the molecules 
 of matter itself, rather than from a specific ether pervading it ; 
 
LIGHT. 109 
 
 just as sound is propagated by the vibrations of wood, or as 
 waves are by water. I am not here speaking of the character 
 of the vibrations of light, sound, or water, which are doubtless 
 very different from each other, but am only comparing them 
 so far as they illustrate the propagation of force by motion in 
 the matter itself. 
 
 I was not aware, at the time that I first adopted the 
 above, view, and brought it forward in my Lectures, that the 
 celebrated Leonard Euler had published a somewhat similar 
 theory ; and, though I suggested it without knowing that it 
 had been previously advanced, I should have hesitated in re- 
 producing it, had I not found that it was sanctioned by 
 so eminent a man as Euler, who can hardly be supposed 
 to have overlooked any irresistible argument against it the 
 more so in a matter so much controverted and discussed as 
 the undulatory theory of light was in his time. 
 
 Although this theory has, been considered defective by a 
 philosopher of high repute, I cannot see the force of the argu- 
 ments by which it has been assailed ; and therefore, for the 
 present, though with diffidence, I adhere to it. The fact 
 itself of the correlation of the different modes of force is to my 
 mind a very cogent argument in favour of their being affec- 
 tions of the same matter ; and though electricity, magnetism, 
 and heat might be viewed as produced by undulations of the 
 same ether as that by means of which light is supposed to be 
 produced, yet this hypothesis offers greater difficulties with 
 regard to the other affections than with regard to light : many 
 of these difficulties I have already alluded to when treating of 
 electricity ; thus, conduction and non-conduction are not ex- 
 plained by it ; the transmission of electricity through long 
 wires in preference to the air which surrounds them, and 
 which must be at least equally pervaded by the ether, is irre- 
 concilable with such an hypothesis. The phenomena exhi- 
 bited by these forces afford, as I think, equally strong evidence 
 with those of light, of ordinary matter acting from particle to 
 particle, and having no action at sensible distances. I have 
 already instanced the experiments of Faraday on electrical 
 induction, showing it to be an action of contiguous particles, 
 which are strongly in favour of this view, and many experi- 
 
no CORRELATION OF PHYSICAL FORCES. 
 
 ments which I have made on the voltaic arc, some of which 
 I have mentioned in this essay, are, to my mind, confirmatory 
 of it. 
 
 If it be admitted that one of the so-called imponderables is 
 a mode of motion, then the fact of its being able to produce 
 the others, and be produced by them, renders it highly diffi- 
 cult to conceive some as molecular motions and others as 
 fluids or undulations of an ether. To the main objection of 
 Dr. Young, that all bodies would have the properties of solar 
 phosphorus if light consisted in the undulations of ordinary 
 matter, it may be answered that so many bodies have this 
 property, and with so great variety in its duration, that non 
 constat all may not have it, though some for a time so short 
 that the eye cannot detect it. M. E. Becquerel has made 
 many experiments which support this view; the fact of the 
 phosphorescence by insolation of a large number of bodies, 
 and the phenomena of fluorescence afford evidence of dense 
 matter being thrown into a state of undulation, or at all events 
 molecularly affected by the impact of light, and is therefore 
 an argument in support of the view which I am advocating. 
 Dr. Young admits that the phenomena of solar phosphorus 
 appear to resemble greatly the sympathetic sounds of musical 
 instruments, which are agitated by other sounds con- 
 veyed to them through the air, and I am not aware that 
 he gives any explanation of these effects on the ethereal 
 hypothesis. 
 
 Some curious experiments of M. Niepce de St. Victor 
 seem also to present an analogy in luminous phenomena to 
 sympathetic sounds. An engraving which has been kept for 
 some days in the dark is half-covered by an opaque screen, 
 and then exposed to the sun ; it is then removed from the 
 light, the screen taken away, and the engraving placed oppo- 
 site, and at a short distance from, photographic paper ; an 
 inverted image of that portion of the engraving which has 
 been exposed to the sun is produced on the photographic 
 paper, while the part which had been covered by the screen is 
 not reproduced. If the engraving, after exposure, is allowed 
 to remain in contact with white paper for some hours, and the 
 white paper is then placed upon photographic paper, a faint 
 
LIGHT. in 
 
 image of the exposed portion of the engraving is reproduced : 
 similar results are produced by mottled marble exposed to the 
 sun ; an invisible tracing on paper by a fluorescent body, sul- 
 phate of quinine, is, after insolation, reproduced on photogra- 
 phic paper, &c. Insolated paper retains the power of 
 producing an impression for a very long period, if it is kept in 
 an opaque tube hermetically closed. 
 
 It is right to observe that these effects are supposed by 
 many to be due to chemical emanations proceeding from 
 the substances exposed to the sun, and which are be- 
 lieved to have undergone some chemical change by this 
 exposure ; this \vould still be a molecular effect, but it is 
 desirable to await further experiment before forming a de- 
 cided opinion. 
 
 The analogies in the progression of sound and light are 
 very numerous : each proceeds in straight lines, until inter- 
 rupted ; each is reflected in the same manner, the angles of 
 incidence and reflexion being equal ; each is alternately nulli- 
 fied and doubled in intensity by interference ; each is capable 
 of refraction when passing between media of different density : 
 this last effect of sound, long ago theoretically determined, 
 has been experimentally proved by Mr. Sondhauss, who con- 
 structed a lens of films of collodion, which, when filled with 
 carbonic acid, enabled him to hear the ticking of a watch 
 placed at one focus of the lens, the ear of the experimenter 
 being at the opposite focus. The ticking was not heard when 
 the watch was moved aside from the focal point, though it 
 remained at an equal distance from the ear. An experiment 
 of M. Dove seems, indeed, to show an effect of polarisation of 
 sound. 
 
 The phenomena presented by heat, viewed according to 
 the dynamic theory, cannot be explained by the motion of an 
 imponderable ether, but involve the molecular actions of ordi- 
 nary ponderable matter. The doctrine of propagation by un- 
 dulations of ordinary matter is very generally admitted by 
 those who support the dynamical theory of heat ; but the 
 analogies of the phenomena presented by heat and light are 
 so close, that I cannot see how a theory applied to the one 
 agent should not be applicable to the other. When heat is 
 
ii2 CORRELATION OF PHYSICAL FORCES. 
 
 transmitted, reflected, refracted, or polarised, can we view 
 that as an affection of ordinary matter, and when the same 
 effects take place with light, view the phenomena as produced 
 by an imponderable ether, and by that alone ? To account 
 for the phenomena of radiant heat the existence of an ether 
 has been considered as necessary as for those of light, but we 
 find that even with radiant heat the medium through which it 
 is transmitted exercises an all-important influence on its ab- 
 sorption and transmission. In the year 1848 I showed that 
 the variation in the cooling effect of different gases in 
 which an ignited platinum wire was immersed was very re- 
 markable, and appeared to depend more on their chemical and 
 molecular constitution than on their physical characteristics, 
 such as density, &c. Dr. Tyndall, in an elaborate series of 
 papers, has shown that there are remarkable differences in the 
 degree of absorption and radiation of heat by gases and 
 vapours. These results are, to my mind, strongly corrobora- 
 tive of the view which dispenses with the hypothetic ether, 
 and regards the imponderables as modes of motion of matter 
 in different states of attenuation or of aggregation. 
 
 An objection that immediately occurs to the mind in re- 
 ference to the ethereal hypothesis of light is, that the most 
 porous bodies are opaque ; cork, charcoal, pumice-stone, dried 
 and moist wood, &c., all very porous and very light, are all 
 opaque. This objection is not so superficial as it might seem 
 at first sight. The theory which assumes that light is an 
 undulation of an ethereal medium pervading gross matter, 
 assumes the distances between the molecules or atoms of mat- 
 ter to be very great. Matter has been likened by Democritus, 
 and by many modern philosophers, to the starry firmament, 
 in which, though the individual monads are at immense dis- 
 tances from each other, yet they have in the aggregate a 
 character of unity, and are firmly held by attraction in their 
 respective positions and at definite distances. Now, if matter 
 be built up of separate molecules, then, as far as our know- 
 ledge extends, the lightest bodies would be those in which the 
 molecules are at the greatest distances, and those in which any 
 undulation of a pervading medium would be the least inter- 
 fered with by the separated particles such bodies should 
 
LIGHT. 113 
 
 consequently be the most transparent. It may be argued 
 that the size and want of continuity of the particles in porous 
 bodies arrests or disperses the undulations, but this argument 
 would apply on the ethereal hypothesis to all bodies. 
 
 If the analogy of the starry firmament held good, in this 
 case an undulation or wave proportioned to the individual 
 monads would be broken up by the number of them, and the 
 very appearance of continuity which results, as in the Milky 
 Way, from each point of vision being occupied by one of the 
 monads, would show that at some portion of its progress the 
 wave is interrupted by one of them, so that the whole may be 
 viewed in some respect as a sheet of ordinary matter interposed 
 in the ethereal expanse. 
 
 Even then, if it be admitted that a highly elastic medium 
 pervades the interspaces, the separate masses as a whole 
 must exercise an important influence on the progress of the 
 wave. 
 
 Sound or vibrations of air meeting with a screen, or, as it 
 were, sponge of diffused particles, would be broken up and 
 dispersed by them ; but if they be sufficiently continuous to 
 take up the vibration and propagate it themselves, the sound 
 continues comparatively unimpaired. 
 
 With regard, however, to liquid and gaseous bodies, there 
 are very great difficulties in viewing them as consisting of 
 separate and distant molecules. If, for instance; we assume 
 with Young that the particles in water are comparatively as 
 distant from each other as 100 men would be if dispersed at 
 equal distances over the surface of England, the distance of 
 these particles, when the water is expanded into steam, would 
 be increased more than forty times, so that the 100 men would 
 be reduced to two, and by further increasing the temperature 
 this distance may be indefinitely increased ; adding to the 
 effects of temperature, rarefaction by the air-pump, we may 
 again increase the distance, so that, if we assume any original 
 distance, we ought, by expansion, to increase it to a point at 
 which the distance between molecule and molecule should 
 become measurable. But no extent of rarefaction, whether 
 by heat or the air-pump, or both, makes the slightest change 
 in the apparent continuity of matter ; and gases, I find, retain 
 
 I 
 
ii4 CORRELATION OF PHYSICAL FORCES. 
 
 their peculiar character, as far as a judgment can be formed 
 from their effect on the electric spark, throughout any extent 
 of rarefaction which can experimentally be applied to them : 
 thus the electric spark in protoxide of nitrogen, however 
 attenuated, presents a crimson tint, that in carbonic oxide a 
 greenish tint. 
 
 Without, however, entering on the metaphysical enquiry as 
 to the constitution of matter (or whether the atomic philo- 
 sophers or the followers of Boscovich are right), a problem 
 which probably human appliances will never solve ; and even 
 admitting that an ethereal medium, not absolutely imponder- 
 able, as asserted by many, but of extreme tenuity, pervades 
 matter, still ordinary or non-ethereal matter itself must exer- 
 cise a most important action upon the transmission of light ; 
 and Dr. Young, who opposed the theory of Euler, that light was 
 transmitted by undulations of gross matter itself, as sound is, 
 was afterwards obliged to call to his assistance the vibrations 
 of the ponderable matter of the refracting media, to explain why 
 rays of all colours were not equally refracted, and other diffi- 
 culties. One of his arguments in support of the existence of 
 a permeating ether was, ' that a medium resembling in many 
 properties that which has been denominated Ether does exist, 
 is undeniably proved by the phenomena of electricity.' This 
 seems to me, if I may venture to say so, of anything proceed- 
 ing from so eminent a man, scarcely logical : it is supporting 
 one hypothesis by another, and considering that to be proved 
 which its most strenuous advocates admit to be surrounded 
 by very many difficulties. 
 
 If it be said that there is not sufficient elasticity in ordinary 
 matter for the transmission of undulations with such velocity 
 as light is known to travel, this may be so if the vibrations be 
 supposed exactly analogous to those of sound ; but that mole- 
 cular motion can travel with equal and even greater velocity 
 than light, is shown by the rapidity with which electricity 
 traverses a metal wire, where each particle of metal is undoubt- 
 edly affected. It has, moreover, been shown by the experi- 
 ments of Mr. Latimer Clarke upon a length of wire of 760 
 miles, that whatever be the intensity of electrical currents, 
 they are propagated with the same velocity, provided the 
 
LIGHT. 115 
 
 effects of lateral induction be the same a striking analogy 
 with one of the effects observed in the propagation of light 
 and sound. The effects observed by MM. Fizeau and Foucault, 
 of the slower progression of light in proportion as the trans- 
 mitting medium is more dense, seem to me in favour of the 
 view here advocated ; as a greater degree of heat would be 
 produced by light in proportion to the density of the medium, 
 force would be thus carried off, and the molecular system 
 disturbed, so that the progress of the motion should be more 
 slow ; but so many considerations enter into this question, and 
 the phenomena are so extremely complex, that it would be 
 rash to hazard any positive opinion. 
 
 Dr. Young ultimately came to the conclusion that it was 
 simplest to consider the ethereal medium, together with the 
 material atoms of the substance, as constituting together a 
 compound medium denser than pure ether, but not more 
 elastic. Ether, or highly attenuated matter, might thus be 
 viewed as performing the functions which oil does with tracing- 
 paper, giving continuity to the particles of gross matter, and 
 in the interplanetary spaces forming itself the medium which 
 transmits the undulations. 
 
 Since the period when Huyghens, Euler, and Young, the 
 fathers of the undulatory theory, applied their great minds to 
 this subject, a mass of experimental data has accumulated, all 
 tending to establish the propositions, that whenever matter 
 transmitting or reflecting light undergoes a structural change, 
 the light itself is affected, and that there is a connection or 
 parallelism between the change in the matter and the change 
 in the affection of light, and conversely that light will modify 
 or change the structure of matter and impress its molecules 
 with new characteristics. 
 
 Transparency, opacity, refraction, reflection, and colour 
 were phenomena known to the ancients, but sufficient attention 
 does not appear to have been paid by them to the molecular 
 states of the bodies producing these effects ; thus the transpa- 
 rency or opacity of a body appears to depend entirely upon 
 its molecular arrangement. If striae occur in a lens or 
 glass through which objects are viewed, the objects are dis- 
 torted : increase the number of these striae, the distortion is 
 
n6 CORRELATION OF PHYSICAL FORCES. 
 
 so increased that the objects become invisible, and the glass 
 ceases to be transparent, though remaining translucent ; but 
 alter completely the molecular structure, as by slow solidifi- 
 cation, and it becomes opaque. Take, again, an example of a 
 liquid and a gas : a solution of soap is transparent, air is 
 transparent, but agitate them together so as to form a froth 
 or lather, and this, though consisting of two transparent bodies, 
 is opaque : the light being dashed about by minute reflections 
 and refractions in different directions. From the same cause 
 the reflection of light from the surface of these bodies, when 
 so intermixed, is strikingly different from its reflection before 
 mixture, in the one case giving to the eye a mere general 
 effect of whiteness, in the other the images of objects in their 
 proper shapes and colours. 
 
 To take a more refined instance : nitrogen is perfectly 
 colourless, oxygen is perfectly colourless, but chemically 
 united in certain proportions they form nitrous acid, a gas 
 which has a deep orange-brown colour. I know not how the 
 colour of this gas, or of such gases as chlorine or vapour of 
 iodine, can be accounted for by the ethereal hypothesis, with- 
 out calling in aid molecular affections of the matter of these 
 gases. 
 
 Colour in many instances depends upon the thickness of 
 the plate or film of transparent matter upon which light is 
 incident ; as in all those cases which are termed the colours 
 of thin plates, of which the soap-bubble affords a beautiful 
 instance. 
 
 When we arrive at the more recent discoveries of double 
 refraction and polarisation, the effects of light are found to 
 trace out as it were the structure of the matter affected, and 
 the crystalline form of a body can be determined by the effects 
 which a minute portion of it exercises on a ray of light. 
 
 Let a piece of good glass be placed in what is called a 
 polariscope (i.e. an instrument in which light that has under- 
 gone polarisation is transmitted through the substance to be 
 examined, and the emergent light is afterwards submitted to 
 another substance capable of polarising light, or, as it is 
 termed, an analyser), no change in effect will be observed. 
 Remove the glass, heat it and suddenly or quickly cool it so 
 
LIGHT. 117 
 
 as to render it unannealed, in which state its molecules are in 
 a state of tension or strain and the glass highly brittle, and 
 on replacing it in the polariscope, a beautiful series of colours 
 is perceptible. Instead of subjecting the glass to heat and 
 sudden cooling, let it be bent or strained by mechanical pres- 
 sure, and the colours will be equally visible, modified accord- 
 ing to the direction of the flexure, and indicating by their 
 course the curves where the molecular state has been changed 
 by pressure. So I have found that if tough glue be elongated 
 and allowed to cool in a stretched state, it doubly refracts 
 light, and the colours are shown as in the instance of glass. 
 
 Submit a series of crystals to the same examination, and 
 different figures will be formed by different crystals, bearing 
 a constant and definite relation to the structure of the par- 
 ticular crystal examined, and to the direction in which, with 
 reference to crystalline form, the ray crosses the crystal. 
 
 In the crystallised salts of paratartaric acid, M. Pasteur 
 noticed two sets of crystals which were hemihedral in opposite 
 directions, i.e. the crystals of one set were to those of the 
 other as to their own image reflected in a mirror ; on making 
 a separate solution of each of these classes of crystals, he found 
 that the solution of the one class rotated the plane of polarisa- 
 tion to the right, while that of the other class rotated it to the 
 left, and that a mixture in proper proportions of the two solu- 
 tions produced no deviation in the plane of polarisation. Yet 
 all these three solutions are what is termed isomeric, that is, 
 have as far as can be discovered the same chemical constitution. 
 
 In the above, and in innumerable other cases, it is seen that 
 an alteration in the structure of a transparent substance alters 
 the character and effects of the transmitted light. The pheno- 
 mena of photography prove that light alters the structure of 
 matter submitted to it ; with regard even to vision itself, the 
 persistence of images on the retina of the eye would seem to 
 show that its structure is changed by the impact of light, the 
 luminous impressions being as it were branded on the retina, 
 and the memory of the vision being the scar of such brand. 
 The science of photography has reference mainly to solid sub- 
 stances, yet there are many instances of liquid and gaseous 
 bodies being changed by the action of light : thus hydrocyanic 
 
n8 CORRELATION OF PHYSICAL FORCES. 
 
 acid, a liquid, undergoes a chemical change and deposits a 
 solid carbonaceous compound by the action of light. Chlorine 
 and hydrogen gases, when mixed and preserved in darkness, 
 do not unite, but when exposed to light rapidly combine, 
 forming hydrochloric acid. 
 
 The above facts and many others might have been ad- 
 duced go far to connect light with motion of ordinary matter, 
 and to show that many of the evidences which our senses 
 receive of the existence of light result from changes in matter 
 itself. When the matter is in the solid state, these changes 
 are more or less permanent ; when in the liquid or gaseous 
 state, they are temporary in the greater number of instances, 
 unless there be some chemical change effected, which is, as it 
 were, seized upon during its occurrence, and a resulting com- 
 pound formed, more stable than the original compound or 
 mixture. 
 
 I might weary my reader with examples, showing that, in 
 every case which we can trace out, the effects of light are 
 changed by any and every change of structure, and that light 
 has a definite connection with the structure of the bodies 
 affected by it. I cannot but think that it is a strong assump- 
 tion to regard ether, a purely hypothetical creation, as changing 
 its elasticity or density for each change of structure, and to 
 regard it as penetrating the pores of bodies of whose porosity 
 we have in many cases no proof; the which pores must, more- 
 over, have a definite and peculiar communication, also assumed 
 for the purpose of the theory. 
 
 Ether is a most convenient medium for hypothesis : thus, 
 if to account for a given phenomenon the hypothesis requires 
 that the ether be more elastic, it is said to be more elastic ; 
 if more dense, it is said to be more dense ; if it be required by 
 hypothesis to be less elastic, it is pronounced to be less elastic ; 
 and so on. The advocates of the ethereal hypothesis certainly 
 have this advantage, that the ether, being hypothetical, can 
 have its characters modified or changed without any possibility 
 of disproof either of its existence or modifications. 
 
 It may be that the refined mathematical labours on light, 
 as on electricity, have given an undue and adventitious 
 strength to the hypotheses on which they are based. M. 
 
LIGHT. 119 
 
 Comte thus expresses himself on this subject: 'Mathema- 
 ticians, too frequently taking the means for the end, have 
 embarrassed natural philosophy with a crowd of analytical 
 labours founded upon hypotheses extremely hazardous, or even 
 upon conceptions purely visionary, and consequently sober- 
 minded people can see in them nothing more than simple 
 mathematical exercises, of which the abstract value is some- 
 times very striking, without their influence in the slightest 
 degree accelerating the natural progress of physics.' 
 
 An objection to which the view I have been advocating is 
 open, and at first sight a formidable one, is the necessity 
 involved in it of an universal plenum ; for if light, heat, elec- 
 tricity, &c., be affections of ordinary matter, then matter must 
 be supposed to be everywhere where these phenomena are 
 apparent, and consequently there can be no vacuum. 
 
 These forces are transmitted through what are called vacua, 
 or through interplanetary spaces, where matter, if it exist, 
 must be in a highly attenuated state. 
 
 It may be safely stated that hitherto all attempts at pro- 
 curing a perfect vacuum have failed. The ordinary air-pump 
 gives us only highly rarefied air ; and, by the principle of 
 construction, even of the best, the operation depends upon the 
 indefinite expansion of the volume of air in the receiver ; even 
 in the vacuum which is formed in this, so great is the tendency 
 of matter to fill up space, that I have observed distilled water 
 contained in a vessel within the exhausted receiver of a good 
 air-pump has a taste of tallow, derived from the grease, or an 
 essential oil contained in it, which is used to form an air-tight 
 junction between the edges of the receiver and the pump-plate. 
 
 The Torricellian vacuum, or that of the ordinary baro- 
 meter, is filled with the vapour of mercury ; but it might be 
 worth the trouble to ascertain what would be the effect of a 
 good Torricellian vacuum, when the mercury in the tube is 
 frozen, which might, without much difficulty, be now effected 
 by the use of solid carbonic acid and ether ; the only pro- 
 bable difficulty would be the different rates of contraction of 
 mercury and glass, at such a degree of cold, and more particu- 
 larly the contraction of mercury at the period of its solidifica- 
 tion. Davy, indeed, endeavoured to form a vacuum, in a 
 
120 CORRELATION OF PHYSICAL FORCES. 
 
 somewhat similar manner, over fused tin, with but partial 
 success ; he also made many other attempts to obtain a perfect 
 vacuum ; his main object being to ascertain what would be 
 the effect of electricity across empty space : he admits that he 
 could not succeed in procuring a vacuum, but found electricity 
 much less readily conducted or transmitted by the best vacuum 
 he could procure than by the ordinary Boylean vacuum. 
 
 Morgan found no conduction by a good Torricellian 
 vacuum ; and, although Davy does not seem to place much 
 reliance on Morgan's experiments, there was one point in which 
 they were less liable to error than those of Davy. Morgan, 
 whose experiments seem to have been carefully conducted, 
 operated with hermetically-sealed glass tubes and by induced 
 electricity, while Davy sealed a platinum wire into the ex- 
 tremity of the tube in which he sought to produce a vacuum. 
 I have found in very numerous experiments which I made to 
 exclude air from water, that platinum wires, most carefully 
 sealed into glass, allow liquids to pass between them and the 
 glass ; and this gives some reason to believe that gases may 
 equally pass through ; indeed, I have observed such effect in 
 the gas battery when it has been in action for a long period. 
 Davy supposed that the particles of bodies may be detached, 
 and so produce electrical effects in a vacuum ; and such effects 
 would more readily take place in his experiments, where a 
 wire projected into the exhausted space, than in Morgan's, 
 where the induced electricity was diffused over the surface of 
 the glass. 
 
 M. Masson found that the barometric vacuum does not 
 conduct a current of electricity, or even a discharge, unless 
 the tension is considerable and sufficient to detach particles 
 from the electrodes ; and by adopting a plan of Dr. Andrews, 
 viz. absorbing carbonic acid by potash, Mr. Gassiot has suc- 
 ceeded in forming vacua across which the powerful discharge 
 from the Rhumkorf coil will not pass. 
 
 The limited but somewhat varying height at which the 
 Aurora Borealis appears (according to such parallactic mea- 
 surements as can be made), is not improbably due to the trans- 
 mission of electricity, between the polar and equatorial 
 regions of the atmosphere, being only effected at a certain 
 
LIGHT. 121 
 
 degree of rarefaction, the atmosphere below the observed 
 height being too dense and that above too rare to allow the 
 electricity to pass. 
 
 The odour which many metals, such as iron, tin, and zinc, 
 emit, and the so-called thermographic radiations, can hardly 
 be explained upon any other theory than the evaporation of 
 an infinitesimally small portion of the metal itself. 
 
 So universal is the tendency of matter to diffuse itself into 
 space, that it gave rise to the old saying that nature abhors a 
 vacuum ; an aphorism which, though cavilled at and ridiculed 
 by the self-sufficiency of some modern philosophers, contains 
 in a terse though somewhat metaphorical form of expression, 
 a comprehensive truth, and evinces a large extent of obser- 
 vation in those who, with few of the advantages which we 
 possess, first generalised by this sentence the facts of which 
 they had become cognisant. 
 
 It has been argued that if matter were capable of infinite 
 divisibility, the earth's atmosphere would have no limit, and 
 that consequently portions of it would exist at points of space 
 where the attraction of the sun and planets would be greater 
 than that of the earth, and whence it would fly off to those 
 bodies and form atmospheres around them. This was sup- 
 posed to be negatived by the argument of the well-known 
 paper of Dr. Wollaston ; in which, from the absence of notable 
 refraction near the margin of the sun and of the planet 
 Jupiter, he considered himself entitled to conclude that the 
 expansion of the earth's atmosphere had a definite limit, and 
 was balanced at a certain point by gravitation : this deduction 
 has been shown to be inconclusive by Dr. Whewell, and has 
 also been impugned upon other grounds by Dr. Wilson. There 
 is a point not adverted to in these papers, and which Wol- 
 laston does not seem to have considered, viz. that there is no 
 evidence that the apparent discs of the sun and of Jupiter 
 show us their real discs or bodies. Sir W. Herschel regards 
 the margin of the visible discs as that of clouds or a peculiar 
 state of atmosphere, and the rapidly changing character of the 
 apparent surfaces renders some such conclusion necessary. If 
 this be so, refraction of an occulted star could not be detected 
 at all event c . ?V the denser portion of the atmosphere. . 
 
122 CORRELATION OF PHYSICAL FORCES. 
 
 Sir W. Herschel's observations go to prove that the sun 
 and Jupiter have dense atmospheres, while Wollaston's were 
 believed to prove that they have no appreciable atmospheres. 
 The results of recent observations with the spectroscope are in 
 favour of the former conclusion ; the chromosphere being 
 found to extend far beyond the defined disc of the sun. 
 
 If it be admitted, or considered proved, that the sun and 
 planets have atmospheres and little doubt now exists on this 
 point then the grounds upon which Wollaston founded his 
 arguments are untenable ; and there appears no reason why 
 the atmosphere of the different planets should not be, with 
 reference to each other, in a state of equilibrium. Ether, 
 which term we may apply to the highly-attenuated matter 
 existing in the interplanetary spaces, being an expansion of 
 some or all of these atmospheres, or of the more volatile por- 
 tions of them, would thus furnish matter for the transmission 
 of the modes of motion which we call light, heat, &c. ; and 
 possibly minute portions of these atmospheres may, by gradual 
 accretions and subtractions, pass from planet to planet, form- 
 ing a link of material communication between the distant 
 monads of the universe. 
 
 The assumption of the universal presence of matter is 
 common to the theory of the transmission of light by the 
 undulations of ordinary matter and to the other two theories, 
 which equally presuppose the non-existence of a vacuum ; for, 
 according to the emissive or corpuscular theory, the vacuum 
 is filled by the matter itself, of light, heat, &c. ; according to 
 the ethereal it is filled by the all-penetrating ether. Of the 
 existence of matter in the interplanetary spaces we have some 
 evidence in the diminishing periods of comets ; and where, 
 from its highly attenuated state, the character of the medium 
 by which the forces are conveyed cannot be tested, the term 
 ether is a very admissible generic name for such medium. 
 
 Newton has some curious passages on the subject-matter 
 of light. In the ' Queries to the Optics ' he says : 
 
 ' Are not gross bodies and light convertible into one 
 another, and may not bodies receive much of their activity 
 from the particles of light which enter their composition ? 
 . . . The changing of bodies into light and light into 
 
LTGHT. 123 
 
 bodies is very conformable to the course of Nature, which 
 seems delighted with transmutations. Water, which is a very 
 fluid, tasteless salt, she changes by heat into vapour, which is 
 a sort of air, and by cold into ice, which is a hard, pellucid, 
 brittle, fusible stone, and this stone returns into water by heat, 
 and vapour returns into water by cold. . . . And, among 
 such various and strange transmutations, why may not Nature 
 change bodies into light, and light into bodies ? ' 
 
 Newton has here seemingly in his mind the emissive theory 
 of light ; but the passages might be applied to either theory ; 
 the analogy he saw in the change of state of matter, as in ice, 
 water, and vapour, with the hypothetic change into light, is 
 very striking, and would seem to show that he regarded the 
 change or transmutation of which he speaks as one analogous to 
 the known changes of state, or consistence, in ordinary matter. 
 
 The difference between the view which I am advocating 
 and that of the ethereal theory as generally enunciated is, 
 that the matter which in the interplanetary spaces serves as 
 the means of transmitting by its undulations light and heat, I 
 should regard as possessing the properties of ordinary, or as it 
 has sometimes been called gross matter, and particularly 
 weight; though, from its extreme rarefaction, it would mani- 
 fest these properties in an indefinitely small degree ; whilst, 
 near to or on the surface of the earth, that matter attains a 
 density cognisable by our means of experiment, and the dense 
 matter is itself, in great part, if not entirely, the conveyer of 
 the undulations in which these agents consist. Doubtless, in 
 very many of the forms which matter assumes it is porous, , 
 and pervaded by more volatile essences, which may differ as 
 much in kind as matter does. In these cases a composite 
 medium, such as that indicated by Dr. Young, would result ; 
 but even on such a supposition, the denser matter would pro- 
 bably exercise the more important influence on the undula- 
 tions. Returning to the somewhat strained hypothesis, that 
 the particles of dense matter in a so-called solid are as distant 
 as the stars in heaven, still a certain depth or thickness of 
 such solid would present at every point- of space a particle or 
 rock in the successive progress of a wave, which particles, to 
 carry on the movement, must vibrate in unison with it. 
 
1 24 CORRELA TION OF PHYSICAL FOR CES. 
 
 At the utmost, our assumption, on the one hand, is, 
 that wherever light, heat, &c., exist, ordinary matter exists, 
 though it may be so attenuated that we cannot recognise it 
 by the test of gravitation ; and that to the expansibility of 
 matter no limit can be assigned. On the other hand, a 
 specific matter without weight must be assumed, of the exist- 
 ence of which there is no evidence but in the phenomena 
 for the explanation of which its existence is supposed. To 
 account for the phenomena the ether is assumed, and to 
 prove the existence of the ether the phenomena are cited. 
 For these reasons, and others above given, I think that the 
 assumption of the universality of ordinary matter is the least 
 gratuitous. 
 
 Qvcev TL TOV Trai'TOQ Kerov ireXei ove 
 
 A question has often occurred to me and possibly to others : 
 Is the continuance of a luminous impulse in the interplanetary 
 spaces perpetual, or does it after a certain distance dissipate 
 itself and become lost as light I do not mean by mere diver- 
 gence directly as the squares of the distances it travels, but 
 does the physical impulse itself lose force as it proceeds ? 
 Upon the view I have advocated, and indeed upon any undu- 
 latory hypothesis, there must be some resistance to its pro- 
 gress ; and unless the matter or ether in the interplanetary 
 spaces be infinitely elastic, and there be no lateral action of a 
 ray of light, there must be some loss. That it is exceedingly 
 minute is proved by the distance light travels. Stars whose 
 parallax is ascertained are at such a distance from the earth 
 that their light, travelling at the rate of 192,500 miles in a 
 second, takes more than ten years to reach the earth ; so that 
 we see them as they existed ten years ago. The distance of 
 most visible stars is probably far greater than this, and yet 
 their brilliance is great, and increases when their rays are 
 collected by the telescope in proportion ceteris paribus to the 
 area of the object-glass or speculum. There is, however, an 
 argument of a somewhat speculative character, by which light 
 would seem to be lost or transformed, into some other force 
 in the interplanetary spaces. 
 
 Every increase of space-penetrating power in the telescope 
 
LIGHT. 125 
 
 gives us a new field of visible stars. If this expansion of the 
 stellar universe go on indefinitely and no light be lost, then, 
 assuming the fixed stars to be of an average equal brightness 
 with our sun, and to fill up every point of space, and that no 
 light be lost other than that by divergence, the night ought to 
 be equally luminous with the day ; for though the light from 
 each point diminishes in intensity as the square of the dis- 
 tance, the number of luminous points would increase as the 
 square of the distance, and thus occupy the whole visible 
 space around us ; and if every point of space is occupied by 
 an equally brilliant point of light, the distance of the points 
 from us becomes immaterial. The loss of light intercepted 
 by stellar bodies would make no difference in the total quan- 
 tity of light reaching us, for each of these would yield from its 
 own self-luminosity at least as much light as it intercepted. 
 Light may, however, be intercepted by non-luminous bodies, 
 such as planets revolving round the self-luminous stars ; but, 
 making every allowance for these, it is difficult to understand 
 why we get so little light at night from the stellar universe, 
 without assuming that some light is lost in its progress 
 through space not lost absolutely, for that would be an 
 annihilation of force but converted into some other mode of 
 motion. 
 
 It may be objected that this hypothesis assumes the stellar 
 universe to be illimitable : if pushed to its extreme so as to 
 make the light of night equal that of day, provided no stellar 
 light be lost, it does make this assumption ; but even this seems 
 a far more rational assumption to make than that the stellar 
 universe is limited. Our experience gives no indication of a 
 limit ; each improvement in telescopic power gives us new 
 realms of stars or of nebulse, which, if not stellar clusters, are 
 at all events self-luminous matter ; and if we assume a limit, 
 what is it ? We cannot conceive a physical boundary, for 
 then immediately comes the question, what bounds the 
 boundary ? and to suppose the stellar universe to be bounded 
 by infinite space or by infinite chaos, that is to say, to suppose 
 a spot for it would then become so of matter in definite 
 forms, with definite forces, and probably teeming with definite 
 organic beings, plunged in a universe of nothing, is, to my 
 
126 CORRELATION OF PHYSICAL FORCES. 
 
 mind at least, far more unphilosophical than to suppose a 
 boundless universe of matter existing in forms and actions more 
 or less analogous to those which, as far as our examination 
 goes, pervades space. But without speculating on topics in 
 which the mind loses itself, it may not unreasonably be 
 expected that a greater amount of light would reach us from 
 the surrounding self-luminous spheres, were not some portion 
 lost as light by its action on the medium which conveys the 
 impulses. What force this becomes, or what it effects, it 
 would be vain to speculate upon. 
 
 After advancing the above conjecture, I found that Struve 
 had been led, from astronomical observations, to the conclu- 
 sion that some light is lost in the interplanetary spaces ; he 
 gives as an approximation one per cent, as lost by the passage 
 of light from a star of the first magnitude, assuming a mean 
 or average distance. 
 
127 
 
 MAGNETISM. 
 
 MAGNETISM, as was proved by the important discovery of 
 Faraday, will produce electricity, but with this condition, 
 that being in itself static, to produce a dynamic force, motion 
 must be superadded to it : without this it is, in fact, direc- 
 tive, not motive, altering the direction of other forces, but 
 not, in strictness, initiating them. It is difficult to convey a 
 definite notion of the force of magnetism, and of the mode in 
 which it affects other forces. The following illustration 
 may give a rude idea of magnetic polarity. Suppose a 
 number of wind-vanes, say of the shape of arrows, with the 
 spindles on which they revolve arranged in a row, but the 
 vanes pointing in various directions : a wind blowing from the 
 same point with an uniform velocity will instantly arrange 
 these vanes in a definite direction, the arrow-heads or narrow 
 parts pointing one way, the swallow-tails or broad parts 
 another. If they be delicately suspended on their spindles, a 
 very gentle breeze will so arrange them, and a very gentle 
 breeze will again deflect them ; or, if the wind cease, and they 
 have been originally subject to other forces, such as gravity 
 from unequal balancing, they will return to irregular posi- 
 tions, themselves creating a slight breeze by their return. 
 Such a state of things will represent the state of the molecules 
 of soft iron ; electricity acting on them not indeed in straight 
 lines, but in a definite direction produces a polar arrange- 
 ment, which they will lose as soon as the dynamic inducing 
 force is removed. 
 
 Let us now suppose the vanes, instead of turning easily, to 
 be more stiffly fixed to the axes, so as to be turned with diffi- 
 culty : it will require a stronger wind to move them and 
 arrange them definitely ; but when so arranged, they will 
 retain their position ; and should a gentle breeze spring up in 
 
128 CORRELATION OF PHYSICAL FORCES. 
 
 another direction, it will not alter their position, but will itself 
 be definitely deflected. Should the conditions of force and 
 stability be intermediate, both the breeze and the vanes will be 
 slightly deflected ; or, if there be no breeze, and the spindles 
 be all moved in any direction, preserving their parallel rela- 
 tion, they will themselves create a breeze. Thus it is with 
 the molecules of hard iron or steel in permanent magnets : 
 they are polarised with greater difficulty than those of soft 
 iron, but, when so polarised, they cannot be affected by a 
 feeble current of electricity. Again, if the magnets be moved, 
 they themselves originate a current of electricity ; and, lastly, 
 the magnetic polarity and the electric current may be both 
 mutually affected, if the degrees of mobility and stability be 
 intermediate 
 
 The above instance will, of course, be taken only as an 
 approximation, and not as binding me to any closer analogy 
 than is generally expected of a mechanical illustration. It is 
 difficult to convey by words a definite idea of the dual or 
 antithetic character of force involved in the term polarity. 
 The illustration I have employed may, I hope, somewhat aid 
 in elucidating the manner in which magnetism acts on the 
 other dynamic forces ; i.e., definitely directing them, but not 
 initiating them, except while in motion. 
 
 Magnets being moved in the direction of lines, joining 
 their poles, produce electrical currents in such neighbouring 
 bodies as are conductors of electricity, in directions transverse 
 to the line of motion ; and if the direction of motion or the 
 position of the magnetic poles be reversed, the current of 
 electricity flows in a reverse direction. So if the magnet be 
 stationary, conducting bodies moved across any of the lines of 
 magnetic force (i.e. lines in the direction of which the mutual 
 action of the poles of the magnet would place minute portions 
 of iron) have currents of electricity developed in them, the 
 direction of which is dependent upon that of the motion of 
 the substance with reference to the magnetic poles. Thus, as 
 bodies affected by an electrical current are definitely moved 
 by a magnet in proximity to them, so conversely bodies 
 moved near a magnet have an electrical current developed in 
 them. Magnetism can, then, through the medium of electri- 
 
MAGNETISM. 129 
 
 city, produce heat, light, and chemical affinity. Motion it can 
 directly produce under the above conditions ; i.e. a magnet 
 being itself moved will move other ferreous bodies ; these will 
 acquire a static condition of equilibrium, and be again moved 
 when the magnet is also moved. By motion or arrested mo- 
 tion only, could the phenomena of magnetism ever have become 
 known to us. A magnet, however powerful, might rest for 
 ever unnoticed and unknown, unless it were moved near to 
 iron, or iron moved near to it, so as to come within the sphere 
 of its attraction. 
 
 But even with other than either magnetic or electrified 
 substances, all bodies will be moved when placed near the 
 poles of very powerful magnets some taking a position axi- 
 ally, or in the line from pole to pole of the magnet ; others 
 equatorially, or in a direction transverse to that line the 
 former being attracted, the latter apparently repelled, by the 
 poles of the magnet. These effects, according to the views 
 of Faraday, show a generic difference between the two classes 
 of bodies, magnetics and diamagnetics ; according to others, 
 a difference of degree or a resultant of magnetic action ; the 
 less magnetic substance being forced into a transverse position 
 by the magnetisation of the more magnetic medium which 
 surrounds it. 
 
 According to the view given above, magnetism may be 
 produced by the other forces, just as the vanes in the instance 
 given are definitely deflected, but cannot produce them ex- 
 cept when in motion : motion, therefore, is to be regarded in 
 this case as the initiative force. Magnetism will, however, 
 directly affect the other forces light, heat, and chemical 
 affinity, and change their direction or mode of action, or, at 
 all events, will so affect matter subjected to these forces, that 
 their direction is changed. Since these lectures were deli- 
 vered, Faraday has discovered a remarkable effect of the 
 magnetic force in occasioning the deflection of a ray of 
 polarised light. 
 
 If a ray of polarised light pass through water, or through 
 any transparent liquid or solid which does not alter or turn 
 aside the plane of polarisation, and the column, say of water, 
 
 K 
 
130 CORRELATION OF PHYSICAL FORCES. 
 
 through which it passes be subjected to the action of a 
 powerful magnet, the line of magnetic force, or that which 
 would unite the poles of the magnet, being in the same 
 direction as the ray of polarised light, the water acquires, with 
 reference to the light, similar, though not quite identical, pro- 
 perties to oil of turpentine the plane of polarisation is ro- 
 tated, and the direction of this rotation is changed by changing 
 the direction of the magnetic force : thus, if \ve suppose a 
 polarised ray to pass first in its course the north pole of the 
 magnet, then between that and the south pole, it will be de- 
 flected, or curved to the right ; while if it meet the south pole 
 first in its course, it will, in its journey between that and the 
 north pole, be turned to the left. If the substance through 
 which the ray is transmitted be of itself capable of deflecting 
 the plane of polarisation, as, for instance, oil of turpentine, then 
 the magnetic influence will increase or diminish this rota- 
 tion, according to its direction. A similar effect to this is 
 observed with polarised heat when the medium through which 
 it is transmitted is subjected to magnetic influence. 
 
 Whether this effect of magnetism is rightly termed an 
 effect upon light and heat, or is a molecular change of the 
 matter transmitting the light and heat, is a question the reso- 
 lution of which must be left to the future ; at present, the 
 answer to it would depend upon the theory we adopt. If the 
 view of light and heat which I have stated be adopted, then 
 we may fairly say that magnetism, in these experiments, 
 directly affects the other forces ; for light and heat being, 
 according to that view, motions of ordinary matter, magnetism, 
 in affecting these movements, affects the forces which occa- 
 sion them. If, however, the other theories be adhered to, it 
 would be more consistent with the facts to view these results 
 as exhibiting an action upon the matter itself, and the heat 
 and light as secondarily affected. 
 
 When substances are undergoing chemical changes, and a 
 magnet is brought near them, the. direction or lines of action 
 of the chemical force will be changed. There are many old 
 experiments which probably depended on this effect, but 
 which were erroneously considered to prove that permanent 
 magnetism could produce or increase chemical action : these 
 
MAGNETISM. 131 
 
 have been extended and explained by Mr. Hunt and Mr. 
 Wartmann, and are now better understood. 
 
 The above cases are applicable to the subject of the pre- 
 sent Essay, inasmuch as they show a relation to exist between 
 magnetic and the other forces, which relation is, in all proba- 
 bility, reciprocal ; but in these cases there is not a production 
 of light, heat, or chemical affinity, by magnetism alone, but a 
 change in their direction or mode of action. 
 
 There is, however, that which may be viewed as a dynamic 
 condition of magnetism ; i.e. its condition at the commence- 
 ment and the termination, or during the increment or decre- 
 ment of its development. While iron or steel is being 
 rendered magnetic, and as it progresses from its non-magnetic 
 to its maximum magnetic state, or recedes from its maximum 
 to zero, it exhibits a dynamic force ; the molecules are, it may 
 be inferred, in motion. Similar effects can then be produced 
 to those which are produced by a magnet whilst in motion. 
 
 An experiment which I published in 1845 tends, I think, 
 to illustrate this, and in some degree to show the character of 
 the motion impressed upon the molecules of a magnetic 
 metal at the period of magnetisation. A tube filled with 
 the liquid in which magnetic oxide of iron had been prepared, 
 and terminated at each end by plates of glass, is surrounded 
 by a coil of coated wire. To a spectator looking through this 
 tube a flash of light is perceptible whenever the coil is elec- 
 trised, and less light is transmitted when the electrical current 
 ceases, showing a symmetrical arrangement of the minute par- 
 ticles of magnetic oxide while under the magnetic influence. 
 In this experiment it should be borne in mind that the par- 
 ticles of oxide of iron are not shaped by the hand of man, as 
 would be the case with iron filings, or similar minute portions 
 of magnetic matter, but being chemically precipitated, are of 
 the form given to them by nature. 
 
 While magnetism is in the state of change above described 
 it will produce the other forces ; but it may be said, while 
 magnetism is thus progressive, some other force is acting on 
 it, and therefore it does not initiate : this is true, but the same 
 may be said of all the other forces ; they have no commence- 
 ment that we can trace. We must ever refer them back to 
 
 K 2 
 
i3? CORRELATION OF PHYSICAL FORCES. 
 
 some antecedent force equal in amount to that produced, and 
 therefore the word initiation cannot in strictness apply, but 
 must only be taken as signifying the force selected as the 
 first : this is another reason why the idea of abstract causa- 
 tion is inapplicable to physical production. To this point I 
 shall again advert. 
 
 Electricity may thus be produced directly by magnetism, 
 either when the magnet as a mass is in motion, or when its 
 magnetism is commencing, increasing, decreasing, or ceasing ; 
 and heat may seemingly be directly produced by magnetism. 
 Mr. Joule and Mr. Van Breda showed that iron was heated 
 when magnetised or demagnetised, and I, not knowing of their 
 experiments, took some pains to prove this, to eliminate 
 courses of error and to show that the heating effect took place 
 with other magnetic metals than iron. This was done first 
 by subjecting a bar of iron, nickel, or cobalt to the influence 
 of a powerful electro-magnet, which was rapidly magnetised 
 and demagnetised in reverse directions, the electro-magnet 
 itself being kept cool by cisterns of water, so ^that the mag- 
 netic metal subjected to the influence of magnetism was raised 
 to a higher temperature than the electro-magnet itself, and 
 could not, therefore, have acquired its increased temperature 
 by conduction or radiation of heat from the electro-magnet ; 
 and secondly, by rotating a permanent steel magnet with 
 its poles opposite to a bar of iron, a thermo-electric pile being 
 placed opposite the latter. 
 
 It may, however, be fairly contended that in all these 
 experiments the heating effect was the result of electrical 
 currents developed by the magnet, and not directly by the 
 magnetism. 
 
 Dr. Maggi covered a plate of homogeneous soft iron with 
 a thin coating of wax mixed with oil ; a tube traversed the 
 centre, through which the vapour of boiling water was passed. 
 The plate was made to rest on the poles of an electro-magnet, 
 with card interposed. When the iron is not magnetised, the 
 melted wax assumes a circular form, the tube occupying the 
 centre, but when the electro-magnet is put in action, the curve 
 marking the boundary of the melted substance changes its 
 form and becomes elongated in a direction transverse to the 
 
MAGNETISM. 133 
 
 line joining the poles, showing apparently that the conducting 
 power of the iron for heat is changed by magnetisation. 
 
 Thus we get heat produced by magnetism and the conduc- 
 tion of heat altered by it in a direction having a definite 
 relation to the direction of the magnetism. Is it necessary to 
 call in aid ether or the substance ' caloric ' to explain these 
 results ? Is it not more rational to regard the calorific effects 
 as changes in the molecular arrangements of the matter sub- 
 jected to magnetism ? 
 
 There is no obvious reason why magnetism, in the dynamic 
 state, i.e. either when the magnet is in motion, or when the 
 magnetic intensity is varying, may not also directly produce 
 chemical affinity and light, though up to the present time such 
 has not been proved to be the case ; the reciprocal effect, also, 
 of the direct production of magnetism by light or heat has 
 not yet been experimentally established. 
 
 I have used, in contradistinction, the terms dynamic and 
 static to represent the different states of magnetism. The 
 applications I have made of these terms may be open to some 
 exception, but I know of no other words which will so nearly 
 express my meaning. 
 
 The static condition of magnetism resembles the static 
 condition of other forces : such as the state of tension existing 
 in the beam and cord of a balance, or in a charged Leyden 
 phial. One of the old definitions of force was, that which 
 caused change in motion ; and yet even this definition presents 
 a difficulty : in a case of static equilibrium, such, for instance, 
 as that which obtains in the two arms of a balance, we get the 
 idea of force without any palpable apparent motion : whether 
 there be really an absence of motion may be a doubtful 
 question, as such absence would involve in this case perfect 
 elasticity, and, in all other cases, a stability which, in a long 
 course of time, nature generally negatives, showing, as I 
 believe, an inseparable connection of motion with matter, and 
 an impossibility of a perfectly immobile or durable state. So 
 with magnetism ; I believe no magnet can exist in an abso- 
 lutely stable state, though the duration of its stability will be 
 proportionate to its original resistance to assuming a polarised 
 condition. This, however, must be taken merely as a matter 
 
i 3 4 CORRELATION OF PHYSICAL FORCES. 
 
 of opinion : we have, in support of it, the general fact that 
 magnets do deteriorate in the course of years ; and we have 
 the further general fact of the instability, or fluxional state, 
 of all nature, when we have an opportunity of fairly investi- 
 gating it at different and remote periods : in many cases, how- 
 ever, the action is so slow that the changes escape human 
 observation, and, until this can be brought to bear over a pro- 
 portionate period of time, the proposition cannot be said to 
 be experimentally or inductively proved, but must be left to 
 the mental conviction of those who examine it by the light of 
 already acknowledged facts. 
 
 All cases of static force present the same difficulty : thus, 
 two springs pressing against each other would be said to be 
 exercising force ; and yet there is no resulting action, no heat, 
 no light, &c. 
 
 So if gas be compressed by a piston, at the time of com- 
 pression heat is given off; but when this is abstracted, 
 although the pressure continues, no further heat is eliminated. 
 Thus, by an equilibrium produced by opposing forces, motion 
 is locked up, or in abeyance, as it were, and may be again de- 
 veloped when the forces are relieved from the tension. But 
 in the first instance, in producing the state of tension, force has 
 to be employed ; and as we have said in treating of mecha- 
 nical force, so with the other forces the original change which 
 disturbs equilibrium produces other changes which go on 
 without end. Thus, by the act of charging a Leyden phial, 
 the cylinder, the rubber, and the adjoining portions of the 
 electrical machine have each and all their states changed, and 
 thence produce changes in surrounding bodies ad infinitum ; 
 when the jar is discharged, converse changes are again produced. 
 
 The term potential energy has of late been much used to 
 signify locked up force, and though I am far from denying its 
 occasional utility, I am a little fearful of its extension ; like 
 other words, it has only a relative application ; a heated globe 
 has a potential energy capable of becoming active when 
 brought within colder surroundings, but none if all around it 
 be of the same temperature as itself. A piece of ice may be 
 said to have a potential energy when brought into warmer 
 surroundings. Oxygen has a potential energy when brought 
 
MAGNETISM. 135 
 
 into relation with hydrogen. Hydrogen has it when brought 
 into relation with oxygen. Mixed oxygen and hydrogen have 
 an enormous power, if ignited when the surrounding media 
 are at a certain temperature, but if these are above the tem- 
 perature at which decomposition of water by heat takes place, 
 the potential energy of the gases in relation to each other is 
 nil] they cannot combine by the application of heat, and they 
 must be chilled to evolve active force. 
 
 Dr. Odling has shown that these and other substances on 
 entering into combination necessarily evolve heat, and it seems 
 probable that, if the heat surrounding them be greater than 
 that evolved by their combination, the heat they would evolve 
 cannot be conveyed away, and consequently that they would 
 not combine. 
 
 A stone on a hill has a potential energy with reference to 
 the valley beneath, and when it has reached that it has a 
 potential energy with reference to the bottom of a mine. 
 
 Every particle of matter has in it a locked up or potential 
 energy when it is in equilibrium, but active when the equili- 
 brium is disturbed. It is capable as the case may be of heat- 
 ing, of cooling, of oxidating, of deoxidating, of evaporating, 
 of condensing, &c., according to the state of surrounding bodies. 
 
 A weight raised from the earth not only has the potential 
 energy of its tendency to fall, but, as I have before observed, 
 changes the centre of gravity of the earth (which may be said 
 to have thus acquired new potential energy), and alters its 
 relation to the solar and other systems. The whole universe 
 has thus acquired new potential energy when a stone has 
 been lifted by a crane and placed on a wall. Could we not 
 hope to arrive at more accurate scientific ideas if we expressed 
 ourselves in terms of actual motion and discarded latent or 
 non-acting force ? Thus, if by raising a stone the centre of 
 gravity of the earth be changed, the sun and planets, &c. are 
 affected ; so that when we produce what is called a state of 
 potential energy, we are actually eliminating dynamic force 
 from other bodies. 
 
 Were the state of things contemplated by Sir W. Thomson 
 (p. 67) to be the ultimate result of all other energies being 
 resolved into uniform diffused heat, the total energy in the 
 
136 CORRELATION OF PHYSICAL FORCES. 
 
 universe would be the same as at present, but it would really 
 be the same result as though there were no energy at all, as 
 none could act, all would be in a state of unchanging stable 
 equilibrium ' and everything be nought.' 
 
 As with heat, light, and electricity, daily accumulating 
 observations tend to show that each change in the phenomena 
 to which these names are given is accompanied by a change 
 either temporary or permanent in the matter affected by them ; 
 so many experiments on magnetism have connected magnetic 
 phenomena with a molecular change in the subject-matter. 
 In addition to the cases previously given the following may be 
 mentioned out of many. M. Wertheim has shown that the 
 elasticity of iron and steel is altered by magnetisation ; the 
 coefficient elasticity of in iron being temporarily, in steel per- 
 manently diminished. 
 
 He has also examined the effects of torsion upon mag- 
 netised iron, and concludes, from his experiments, that in a 
 bar of iron arrived at a state of magnetic equilibrium, tem- 
 porary torsion diminishes the magnetism, and that the 
 untwisting or return to its primitive state restores the original 
 degree of magnetisation. 
 
 M. Guillemin observed that a bar slightly curved by its 
 own weight is straightened by being magnetised. Mr. Page 
 and Mr. Marrion discovered that a sound is emitted when iron 
 or steel is rapidly magnetised or demagnetised ; and Mr. 
 Joule found that a bar of iron is slightly elongated by mag- 
 netisation. 
 
 Again, with regard to diamagnetic bodies. M. Matteucci 
 found that the mechanical compression of glass altered the 
 rotatory power of magnetism upon a ray of polarised light 
 which the glass transmitted. He further considered that a 
 change took place in the temper of portions of glass which he 
 submitted to the influence of powerful magnets. 
 
 The same arguments which have been submitted to the 
 reader as to the other affections of matter being modes of 
 molecular motion, are therefore equally applicable to mag- 
 netism. 
 
137 
 
 v - 
 
 Library. 
 
 CHEMICAL AFFINITY. 
 
 CHEMICAL AFFINITY, or the force by which dissimilar bodies 
 tend to unite and form compounds differing generally in 
 character from their constituents, is that mode of force of which 
 the human mind has hitherto formed the least definite idea. 
 The word itself affinity is ill- chosen, its meaning, in this 
 instance, bearing no analogy to its ordinary sense ; and the 
 mode of its action is conveyed by certain conventional expres- 
 sions, no dynamic theory of it worthy of attention having been 
 adopted. Its action so modifies and alters the character of 
 matter, that the changes it induces have acquired, not perhaps 
 very logically, a generic contradistinction from other material 
 changes, and we thus use, as contradistinguished, the terms 
 physical and chemical. 
 
 The main distinction between chemical affinity and physi- 
 cal attraction or aggregation, is the difference of character of 
 the chemical compound from its components. This is, how- 
 ever, but a vague line of demarcation ; in many cases, which 
 would be classed by all as chemical actions, the change of 
 character is but slight ; in others, as in the effects of neutrali- 
 sation, the difference of character would be a result which would 
 be expected to follow from physical attraction of dissimilar 
 substances, the previous characters of the constituents depend- 
 ing upon this very attraction or affinity : thus an acid corrodes 
 because it tends to unite with another body ; when united, its 
 corrosive power, i.e. its tendency to unite, being satiated, it 
 cannot, so to speak, be further attracted, and it necessarily 
 loses its corrosive power. But there are other cases where no 
 such result could a priori be anticipated, as where the attrac- 
 tion or combining tendency of the compound is higher than 
 that of any of its constituents ; thus, who could, by physical 
 
138 CORRELATION OF PHYSICAL FORCES. 
 
 reasoning, anticipate a substance like nitric acid from the com- 
 bination of nitrogen and oxygen ? 
 
 The nearest approach, perhaps, that we can form to a 
 comprehension of chemical action, is by regarding it (vaguely, 
 perhaps) as a molecular attraction or motion. It will directly 
 produce motion of definite masses, by the resultant of the 
 molecular changes it induces : thus, the projectile effects of 
 gunpowder may be cited as familiar instances of motion pro- 
 duced by chemical action. It may be a question whether, in 
 this case, the force which occasions the motion of the mass is 
 a conversion of the force of chemical affinity, or whether it is 
 not, rather, a liberation of other forces existing in a state of 
 static equilibrium, and having been brought into such state 
 by previous chemical actions ; but, at all events, through the 
 medium of electricity chemical affinity may be directly and 
 quantitatively converted into the other modes of force. By 
 chemical affinity, then, we can directly produce electricity ; 
 this latter force was, indeed, said by Davy to be chemical 
 affinity acting on masses : it appears rather to be chemical 
 affinity acting in a definite direction through a chain of par- 
 ticles ; but by no definition can the exact relation of chemical 
 affinity and electricity be expressed ; for the latter, however 
 closely related to the former, yet exists where the former 
 does not, as in a metallic wire, which, when electrified, or 
 conducting electricity, is, nevertheless, not chemically altered, 
 or, at least, not known to be chemically altered. 
 
 Volta, the antitype of Prometheus, first enabled us defi- 
 nitely to relate the forces of chemistry and electricity. When 
 two dissimilar metals in contact are immersed in a liquid 
 belonging to a certain class, and capable of acting chemically 
 on one of them, what is termed a voltaic circuit is formed, 
 and, by the chemical action, that peculiar mode of force 
 called an electric current is generated, which circulates from 
 metal to metal, across -the liquid, and through the points of 
 contact. 
 
 Let us take, as an instance of the conversion of chemical 
 force into electrical, the following, which I made known some 
 years ago. If gold be immersed in hydrochloric acid, no 
 chemical action takes place. If gold be immersed in nitric 
 
CHEMICAL AFFINITY. 139 
 
 acid, no chemical action takes place ; but mix the two acids, 
 and the immersed gold is chemically attacked and dissolved : 
 this is an ordinary chemical action, explained as the result of 
 a double chemical affinity. In hydrochloric acid, which is 
 composed of chlorine and hydrogen, the affinity of chlorine 
 for gold being less than its affinity for hydrogen, no change 
 takes place ; but when the nitric acid is added, this latter 
 containing a great quantity of oxygen in a state of feeble 
 combination, the affinity of oxygen for hydrogen opposes 
 that of hydrogen for chlorine, and then the affinity of the 
 latter for gold is enabled to act, the gold combines with the 
 chlorine, and chloride of gold remains in solution in the 
 liquid. Now, in order to exhibit this chemical force in the 
 form of electrical force, instead of mixing the liquids, place 
 them in separate vessels or compartments, but so that they 
 may be in contact, which may be effected by having a porous 
 material, such as unglazed porcelain, amianthus, &c., between 
 them. Immerse in each of these liquids a strip or wire of 
 gold : as long as the pieces of gold remain separated, no 
 chemical or electrical effect takes place ; but the instant they 
 are brought into metallic contact, either immediately or by 
 connecting each with the same metallic wire, chemical action 
 takes place- the gold in the hydrochloric acid is dissolved, 
 electrical action also takes place, the nitric acid is deoxidised 
 by the transferred hydrogen, and a current of electricity may 
 be detected in the metals, or connecting metal, by the appli- 
 cation of a galvanometer or any instrument appropriate for 
 detecting such effect. 
 
 There are few, if any, chemical actions which cannot be 
 experimentally made to produce electricity : the oxidation of 
 metals, the burning of combustibles, the combination of oxy- 
 gen and hydrogen, &c., may all be made sources of electricity. 
 The common mode in which the electricity of the voltaic 
 battery is generated is by the chemical action of the oxygen of 
 water upon zinc ; this action is increased by adding certain 
 acids to the water, which enable it to act more powerfully upon 
 the zinc, or in some cases act themselves upon it ; and one 
 of the most powerful chemical actions known that of nitric 
 acid upon oxidable metals is that which produces the most 
 
MO CORRELATION OF PHYSICAL FORCES. 
 
 powerful voltaic battery, a combination which I made known 
 in the year 1839 ; indeed, we may safely say, that when the 
 chemical force is utilised, or not wasted, but all converted 
 into electrical force, the more powerful the chemical action, 
 the more powerful is the electrical action which results. 
 
 If, instead of employing manufactured products or educts, 
 such as zinc and acids, we could realise as electricity the 
 whole of the chemical force which is active in the combustion 
 of cheap and abundant raw materials, such as coal, wood, fat, 
 &c., with air or water, we should obtain one of the greatest 
 practical desiderata, and have at our command a mechanical 
 power in every respect superior in its applicability to the 
 steam-engine. 
 
 I have shown that the flame of the common blowpipe 
 gives rise to a very marked electrical current, capable not 
 only of affecting the galvanometer, but of producing chemical 
 decomposition : two plates or coils of platinum are placed, 
 the one in the portion of the flame near the orifice of the jet, 
 or at the points where combustion commences, the other in 
 the full yellow flame where combustion is at its maximum ; 
 this latter should be kept cool, to enable a thermo-electric 
 current, which is produced by the different temperature of the 
 platinum plates, to co-operate with the flame current ; wires 
 attached to the plates of platinum form the terminals or 
 poles. By a row of jets a flame battery may be formed, 
 yielding increased effects ; but in these experiments, though 
 theoretically interesting, so small a fraction of the power, 
 actually at work in the combustion, has been thrown into an 
 electrical form, that there is no immediate promise of a 
 practical result. 
 
 The quantity of the electrical current, as measured by the 
 quantity of matter it acts upon in its different phenomenal 
 effects, is proportionate to the quantity of chemical action 
 which generated it ; and its intensity, or power of overcoming 
 resistance, is also proportionate to the intensity of chemical 
 affinity when a single voltaic pair is employed, or to the 
 number of reduplications when the well-known instrument 
 called the voltaic battery is used. 
 
 The mode in which the voltaic current is increased in 
 
CHEMICAL AFFINITY. 141 
 
 intensity by these reduplications, is in itself a striking instance 
 of the mutual relations and dynamic analogies of different 
 forces. Let a plate of zinc or other metal possessing a strong 
 affinity for oxygen, and another of platinum or other metal 
 possessing little or no affinity for oxygen, be partially im- 
 mersed in a vessel, A, containing dilute nitric acid, but not in 
 contact with each other ; let platinum wires touching each 
 of these plates have their extremities immersed in another 
 vessel, B, containing also dilute nitric acid : as the acid in 
 vessel A is decomposed, by the chemical affinity of the zinc 
 for the oxygen of the acid, the acid in vessel B is also 
 decomposed, oxygen appearing at the extremity of the wire 
 which is connected with the platinum : the chemical power is 
 conveyed or transferred through the wires, and, abstracting 
 certain local effects, for every unit of oxygen which combines 
 with the zinc in the one vessel, a unit of oxygen is evolved 
 from the platinum wire in the other. The platinum wire is 
 thus thrown into a condition analogous to zinc, or has a 
 power given to it of determining the oxygen of the liquid to 
 its surface, though it cannot, as is the case with zinc, combine 
 with it under similar circumstances. If we now substitute for 
 the platinum wire, which was connected with the platinum 
 plate, a zinc wire, we have, in addition to the determining 
 tendency by which the platinum was affected, the chemical 
 affinity of the oxygen in vessel B for the zinc wire : thus we 
 have, added to the force which was originally produced by 
 the zinc of the combination in vessel A, a second force pro- 
 duced by the zinc in vessel B, co-operating with the first ; 
 two pairs of zinc and platinum thus connected produce, there- 
 fore, a more intense effect than one pair ; and if we go on 
 adding to these alternations of zinc, platinum, and liquid, we 
 obtain an indefinite exaltation of chemical power, just as in 
 mechanics we obtain accelerated motion by adding fresh 
 impulses to motion already generated. 
 
 The same rule of proportion which holds good in chemical 
 combinations also obtains in electrical effects, when these are 
 produced by chemical actions. Dalton and others proved 
 that the constituents of a vast number of compound sub- 
 stances always bore a definite quantitative relation to each 
 
142 CORRELATION OF PHYSICAL FORCES. 
 
 other : thus, water, which consists of one part by weight of 
 hydrogen united to eight parts of oxygen, cannot be formed 
 by the same elements in any other than these proportions ; 
 you can neither add to nor subtract from the normal ratio of 
 the elements, without entirely altering the nature of the com- 
 pound. Further, if any element be selected as unity, the 
 combining ratios of other elements will bear an invariable 
 quantitative relation to that and to each other: thus, if 
 hydrogen be chosen as I, oxygen will be 8, chlorine will 
 be 36 ; that is, oxygen will unite with hydrogen in the pro- 
 portion of 8 parts by weight to I, while chlorine will unite 
 with hydrogen in the proportion of 36 to I, or with oxygen 
 in the proportion of 36 to 8. Numbers expressing their com- 
 bining weights, which are thus relative, not absolute, may, by 
 a conventional assent as to the point of unity, be fixed for all 
 chemical reagents ; and, when so fixed, it will be found that 
 bodies, at least in inorganic compounds, generally unite in 
 those proportions, or in simple multiples of them : these 
 proportions are termed Equivalents. 
 
 Now, a voltaic battery, which consists usually of alterna- 
 tions of two metals, and a liquid capable of acting chemically 
 upon one of them, has, as we have seen, the power of pro- 
 ducing chemical action in a liquid connected with it by 
 metals upon which this liquid is incapable of acting : in such 
 case the constituents of the liquid will be eliminated at the 
 surfaces of the immersed metals, and at a distance one from 
 the other. For example, if the two platinum terminals of a 
 voltaic battery be immersed in water, oxygen will be evolved 
 at one and hydrogen at the other terminal, exactly in the 
 proportions in which they form water ; while, to the most 
 minute examination, no action is perceptible in the inter- 
 vening stratum of liquid. It was known before Faraday's 
 time that, while this chemical action was going on in the 
 subjected liquid, a chemical action was going on in the cells 
 of the voltaic battery ; but it was scarcely if at all known 
 that the amount of chemical action in the one bore a constant 
 relation to the amount of action in the other. Faraday proved 
 that it bore a direct equivalent relation : that is, supposing the 
 battery to be formed of zinc, platinum, and water, the amount 
 
CHEMICAL AFFINITY. 143 
 
 of oxygen which united with the zinc in each cell of the 
 battery was exactly equal to the amount evolved at the one 
 platinum terminal, while the hydrogen evolved from each 
 platinum plate of the battery was equal to the hydrogen 
 evolved from the other platinum terminal. 
 
 Supposing the battery to be charged with hydrochloric 
 acid, instead of water, while the terminals are separated by 
 water, then for every 36 parts by weight of chlorine which 
 united with each plate of zinc, eight parts of oxygen would be 
 evolved from one of the platinum terminals : that is, the 
 weights would be precisely in the same relation which Dalton 
 proved to exist in their chemical combining weights. This 
 relation applies to all liquids capable of being decomposed by 
 the voltaic electrical force, thence called Electrolytes ; and as 
 no voltaic effect is produced by liquids incapable of being 
 thus decomposed, it follows that voltaic action is chemi- 
 cal action taking place at a distance, or transferred through 
 a chain of media, and that the chemical equivalent numbers 
 are the exponents of the amount of voltaic action for corre- 
 sponding chemical substances. 
 
 As heat, light, magnetism, or motion, can be produced by 
 the requisite application of the electric current, and as this is 
 definitely produced by chemical action, we get these forces 
 very definitely, though not immediately, produced by chemi- 
 cal action. Let us, however, here enquire, as we have already 
 done with respect to the other forces, how far other forces 
 may directly emanate from chemical affinity. 
 
 Heat is an immediate product of chemical affinity. I 
 know of no exception to the general proposition that all bodies 
 in chemically combining produce heat ; i.e. if solution be not 
 considered as chemical action, and even in that case, when 
 cold results, it is from a change of consistence, as from the 
 solid to the liquid state, and not from chemical action. 
 
 We shall find that the same view of the expenditure of 
 force which we have considered in treating of latent heat 
 holds good as to the expenditure of chemical force when re- 
 garded with reference to the amount of heat or repulsive force 
 which it engenders, the chemical force being here exhausted 
 by mechanical expansion that is, by heat. Thus, in the 
 
144 CORRELATION OF PHYSICAL FORCES. 
 
 chemical action of the .ordinary combustion of coal and oxy- 
 gen, the expenditure of fuel will be in proportion to the ex- 
 pansibility of the substances heated ; water passing freely 
 into the state of steam will consume more fuel than if it 
 be confined and kept at a temperature above its boiling- 
 point. 
 
 Why chemical action produces heat, or what is the action 
 of the molecules of matter when chemically uniting, is a ques- 
 tion upon which many theories have been proposed, and which 
 may possibly be never more than approximately resolved. 
 
 Some authors explain it by the condensation which takes 
 place ; but this will not account for the many instances 
 where, from the liberation of gases, a great increase of volume 
 ensues upon chemical combustion, as in the familiar instance 
 of the explosion of gunpowder ; others explain it as resulting 
 from the union of atmospheres of positive and negative elec- 
 tricity, which are assumed to surround the atoms of bodies ; 
 but this involves hypothesis upon hypothesis. Dr. Wood 
 has lately thrown out a view of the heat of chemical action 
 which is more in accordance with a dynamic theory of heat, 
 and as such demands some notice. Starting with his propo- 
 sition, which I have previously mentioned, ' that the nearer 
 the particles of bodies are to each other the less they require 
 to move to produce a given motion in the particles of 
 another body,' his argument assumes something of this 
 form. 
 
 In the mechanical approximation of the particles of a 
 homogeneous body heat results ; the particles a a of the body 
 A would, by their approximation, produce expansion in the 
 neighbouring body B, the more so in proportion as they 
 themselves were previously nearer to each other. In chemi- 
 cally combining, a a the particles of A are brought into very 
 close proximity with b b the particles of B ; heat should there- 
 fore result, and the greater because the proximity may fairly 
 be assumed to be greater in the case of chemical combination 
 than in that of mechanical compression. In cases, then, where 
 there is no absolute diminution of bulk ensuing on chemical 
 combination, if the greater proximity of the combining par- 
 ticles be such that the correlative expansion ought to be greater 
 
CHEMICAL AFFINITY. 145 
 
 (if there were no chemical combination) than that occupied by 
 the total volume of the new compound, an extra expanding 
 power is evolved, and heat or expansion ought to be produced 
 in surrounding bodies. In other words, if a a could be brought 
 by physical attraction as near each other as they are by 
 chemical attraction brought near to b b, they would, from their 
 increased proximity, produce an expansive power ultra the 
 volume occupied by the actual chemical compound A and B. 
 The question, however, immediately occurs, why should the 
 volume of the compound be limited and not occupy the full 
 space equivalent to the expanding power induced by the con- 
 traction or approximation of the particles ? As the distance 
 of the particles is the resultant of the contending contracting 
 and expanding powers, this result ought to express itself in 
 terms of the actual volume produced by the combination, 
 which it certainly does not. 
 
 Though I see some difficulties in Dr. Wood's theory, and 
 perhaps have not rightly conceived it, his views have to my 
 mind great interest, his mode of regarding natural phenomena 
 being analogous to that which I have in this Essay, and for 
 many years, advocated, viz. to divest physical science as much 
 as possible of hypothetic fluids, ethers, latent entities, occult 
 qualities, &c. 
 
 Many compounds evolve heat by decomposition alone, 
 e.g. chloride and iodide of nitrogen, euchlorine, &c. ; this 
 cannot well be from a falling together of the atoms. My 
 own notion of the heat produced by chemical combination 
 though I scarcely dare venture an opinion upon a sub- 
 ject so controverted is, that it is analogous to the heat of 
 friction, that the particles of matter in close approximation 
 and rapid motion inter se evolve heat as a continuation of the 
 motion interrupted by the friction or intestinal motion of the 
 particles : heat would thus be produced, whether the result- 
 ing compound were of greater or less bulk than the sum of 
 the components, though of course when the compound is of 
 greater bulk less heat would be apparent in neighbouring 
 bodies, the expansion taking place in one of the substances 
 themselves I say in one of them, for it is stated in books of 
 authority that there is no instance of two or more solids or 
 
 L 
 
146 CORRELATION OF PHYSICAL FORCES. 
 
 liquids, or a solid and a liquid, combining and producing a 
 compound which is entirely gaseous at ordinary temperatures 
 and pressures. The substance gun-cotton, however, disco- 
 vered by Schoenbein, very nearly realises this proposition. 
 
 Dr. Andrews has arrived at the conclusion, after careful 
 experiment, that in chemical combinations where acids and 
 alkalies or analogous substances are employed, the amount 
 of heat produced is determined by the basic ingredient, and 
 his experiments have received general assent ; although it 
 should be stated that M. Hess arrived at contrary results, the 
 acid constituent according to his experiments furnishing the 
 measure of the heat developed. 
 
 Light is directly produced by chemical action, as in the 
 flash of gunpowder, the burning of phosphorus in oxygen 
 gas, and all rapid combustions : indeed, wherever intense heat 
 is developed, light accompanies it. In many cases of slow 
 combustion, such as the phenomena of phosphorescence, the 
 light is apparently much more intense than the heat, the 
 former being obvious, the latter so difficult of detection that 
 for a long time it was a question whether any heat was 
 eliminated ; and I am not aware that, at the present day, any 
 thermic effects from certain modes of phosphorescence, such 
 as those of phosphorescent wood, putrescent fish, &c., have 
 been detected. 
 
 Chemical action produces magnetism whenever it is thrown 
 into a definite direction, as in the phenomenon of electrolysis. 
 I may adduce the gas voltaic battery, as presenting a simple 
 instance of the direct production of magnetism by chemical 
 synthesis. Oxygen and hydrogen in that combination chemi- 
 cally unite ; but instead of combining by intimate molecular 
 admixture, as in the ordinary cases, they act upon water, i.e. 
 combined oxygen and hydrogen, placed between them so as 
 to produce a line of chemical action ; and a magnet adja- 
 cent to this line of action is deflected, and places itself at 
 right angles to it. What a chain of molecules does here, there 
 can be no doubt, all the molecules entering into combina- 
 tion would produce in ordinary chemical actions ; but in such 
 cases, the direction of the lines of combination being irregular 
 and confused, there is no general resultant by which the 
 magnet can be affected. 
 
CHEMICAL AFFINITY. 147 
 
 What the exact nature of the transferrence of chemical 
 power across an electrolyte is, we at present know not, nor 
 can we form any more definite idea of it than that given by 
 the theory of Grotthus. We have no knowledge as to the exact 
 nature of any mode of chemical action, and, for the present, 
 must leave it as an obscure action of force, of which future 
 researches may simplify our apprehensions. 
 
 We have seen that an equivalent or proportionate elec- 
 trical effect is produced by a given amount of chemical action ; 
 if we, in turn, produce heat, magnetism, and motion by the 
 electricity resulting from chemical action, we shall be able to 
 measure these forces far more accurately than when they are 
 directly produced, and thus to deduce their equivalent relation 
 to the initial chemical action. Thus, M. Favre, after ascertain- 
 ing the quantity of heat produced by the oxidation of a given 
 quantity of zinc, and finding, as have others, that the heat 
 so produced is the same in amount as when evolved from a 
 voltaic battery by the same consumption of zinc forming its 
 positive element, makes the following experiment. 
 
 A voltaic battery and electro-magnet are immersed in ca- 
 lorimeters, and the heat produced when the connection with 
 the magnet is effected is noted. 
 
 The electro-magnet is then made to raise a weight, and 
 thus perform mechanical work ; and the heat produced is 
 again noted. It is found in the latter case that less heat is 
 evolved than in the former ; a certain quantity of heat has 
 therefore been replaced by the mechanical work ; and by 
 estimating the amount of heat subtracted, and the amount of 
 work produced, he deduces the relative equivalent of work to 
 heat. These experiments give a production of mechanical 
 work by chemical action, not, it is true, a direct production, 
 but, as the heat and work are in inverse ratios, and each has 
 its source in chemical action, they prove that they are definite 
 for a definite amount of chemical action ; and as each is pro- 
 duced respectively by electricity and magnetism, these forces 
 must also bear a definite relation to the initial chemical force. 
 
 The doctrine of definite combining proportions, which so 
 beautifully serves to relate chemistry to voltaic electricity, led 
 to the atomic theory, which, though adopted in its universality 
 
 L 2 
 
148 CORRELATION OF PHYSICAL FORCES. 
 
 by a large majority of chemists, presents great difficulties 
 when extended to all chemical combinations. 
 
 The equivalent ratios in which a great number of substances 
 chemically combine, hold good in so many instances, that the 
 atomic doctrine is believed by many to be universally appli- 
 cable, and called a law ; and yet, when followed in the com- 
 binations of substances whose mutual chemical attractions are 
 very feeble, the relation fades away, and is sought to be 
 recovered by applying a separate and arbitrary multiplier to 
 the different constituents. 
 
 Thus, when it was found that a vast number of substances 
 combined in definite volumes and weights, and in definite 
 volumes and weights only, it was argued that their ultimate 
 molecules or atoms had a definite size and weight, as otherwise 
 there was no apparent reason why this equivalent ratio should 
 hold good ? why, for instance, water should only be formed of 
 two volumes or one unit by weight of hydrogen, and of one 
 volume or eight units by weight of oxygen ? why, unless there 
 were some ultimate limits to the divisibility of its molecules, 
 should not water, or a fluid substance approximating to water 
 in character, be formed by a half, a third, or a tenth part of 
 hydrogen, with eight parts of oxygen ? 
 
 It was perfectly consistent with the atomic view that a 
 substance might be formed with one part combined with eight 
 parts, or with sixteen, or with twenty-four, for in such a sub- 
 stance there would be no subdivision of the (supposed indi- 
 visible) molecule ; and this held good with many compounds : 
 thus, fourteen parts by weight, say grains of nitrogen, will 
 combine respectively with eight, sixteen, twenty-four, thirty- 
 two, and forty parts by weight, or grains, of oxygen. 
 
 So, again, twenty-seven grains of iron will combine with 
 eight grains of oxygen or with twenty-four grains, i.e. three 
 proportionals of oxygen. No compound is known in which 
 twenty-seven grains of iron will combine with two propor- 
 tionals or sixteen grains of oxygen ; but this does not much 
 affect the theory, as such a compound may be yet discovered, 
 or there may be reasons at present unknown why it cannot be 
 formed. 
 
 But now comes a difficulty : twenty-seven parts by weight 
 
CHEMICAL AFFINITY. 149 
 
 of iron will combine with twelve parts by weight of oxygen, 
 and twenty-seven parts of iron will also combine with ten and 
 two-third parts of oxygen. Thus, if we retain the unit of iron 
 we must subdivide the unit of oxygen, or if we retain the unit 
 of oxygen we must subdivide the unit of iron, or we must 
 subdivide both by a different divisor. What then becomes of 
 the notion of an atom or molecule physically indivisible ? 
 
 If iron were the only substance to which this difficulty 
 applied, it might be viewed as an unexplained exception, or 
 as a mixture of two oxides ; or recourse might be had to a 
 more minute subdivision to form the units or equivalents of 
 other substances ; but numerous other substances fall under a 
 similar category ; and in organic combinations, to preserve the 
 atomic nomenclature, we must apply a separate multiplier 
 or divisor to far the greater number of the elementary consti- 
 tuents, i.e. we must divide that which is, exhypothesi, indivisible, 
 
 Thus, to take a more complex substance than any formed 
 by the combination of iron and oxygen, let us select the sub- 
 stance albumen, composed of carbon, hydrogen, nitrogen, 
 oxygen, phosphorus, and sulphur. In this case we must either 
 divide the atoms of phosphorus and sulphur so as to reduce 
 them to small fractions, or multiply the atoms of the other 
 substances by extravagant numbers ; thus, to preserve the 
 unit of one of the constituents of this substance, chemists say 
 it is composed of 400 atoms of carbon, 310 of hydrogen, 120 
 of oxygen, 50 of nitrogen, 2 of sulphur, and I of phosphorus. 
 This is a somewhat extreme case, but similar difficulties will 
 be found in different degrees to prevail among organic com- 
 pounds ; in very many no constituent can be taken as a unit 
 to which simple multiples of any of the others will give their 
 relative proportions. By the mode of notation adopted, if any 
 conceivable substance be selected, it could, whatever be the 
 proportions of its constituents, be formulated as atomic. A 
 solution of an ounce of sugar in a pound of water, in a pound 
 and a half, in a pound and a quarter, in a pound and a tenth, 
 might be expressed in an atomic form, if we select arbitrarily 
 a multiplier or divisor. 
 
 In the case of solution, different proportions can be 
 united up to the point of saturation without any difference 
 
i5o CORRELATION OF PHYSICAL FORCES. 
 
 in the character of the compound, and the same may be 
 predicated to some extent of an acid and an alkali ; but even 
 where the steps are sudden, and compounds only exist with 
 definite proportions, they cannot, in a multitude of cases, be 
 reconciled with the true idea of an atomic combination, i.e. 
 one to one, one to two, one to three, &c. 
 
 Although, therefore, there are facts which show that there 
 is some restrictive law of combination which in numerous cases 
 limits the ratios in which substances will combine, nay, further, 
 which show many instances of a proportion between the com- 
 bining weights of one compound and those of another; although 
 there is also a remarkable simplicity in the combining volumes 
 of numerous gases, there remain very many cases to which 
 the doctrine of atomic combinations cannot fairly be applied. 
 
 That there must be something in the constitution of 
 matter, or in the forces which act on it, to account for the per 
 saltum manner in which chemical combinations take place, is 
 inevitable ; but the idea of atoms does not seem satisfactorily 
 to account for it. 
 
 By selecting a separate multiplier or divisor, chemists may 
 denote every combination in terms derived from the atomic 
 theory ; but they have passed from the original law, which 
 contemplated only definite multiples, and the very hypothetic 
 expressions of atoms, which the apparently simple relations of 
 combining weights first led them to adopt, they are obliged to 
 vary and to contradict in terms, by dividing that which their 
 hypothesis and the expression of it assumed to be indivisible. 
 
 While, therefore, I fully recognise a great natural truth in 
 the definite ratios presented by a vast number of chemical 
 combinations, and in the per saltum steps in which nearly all 
 take place, I cannot join in evading the argument against the 
 atomic theory presented by those combinations which are 
 made to support it only by the application of an arbitrary 
 notation. 
 
 A similar straining of theory seems gradually obtaining 
 in regard to the doctrine of compound radicals. The dis- 
 covery of cyanogen by Gay-Lussac was probably the first 
 inducement to the doctrine of compound radicals : a doctrine 
 which is now generally, perhaps too generally, received in 
 
CHEMICAL AFFINITY. 151 
 
 organic chemistry. As, in the case of cyanogen, a body 
 obviously compound discharged in almost all its reactions the 
 functions of an element, so in many other cases it was found 
 that compound bodies, in which a number of elements existed, 
 might be regarded as binary combinations, by considering 
 certain groups of these elements as a compound radical ; that 
 is, as a simple body when treated of in relation to the other 
 complex substances of which it forms part, and only as non- 
 elementary when referred to its own internal constitution. 
 
 Undoubtedly, by approximating in theory the reactions of 
 inorganic and of organic chemistry, by keeping the mind 
 within the limits of a beaten path, instead of allowing it to 
 wander through a maze of isolated facts, the doctrine of com- 
 pound radicals has been of service ; but, on the other hand, 
 the indefinite variety of changes which may be rung upon the 
 composition of an organic substance, by different associations 
 of its primary elements, makes the binary constituents vary 
 as the "minds of the authors who treat of them, and makes 
 their grouping depend entirely upon the strength of the ana- 
 logies presented to each individual mind. From this cause, 
 and from the extreme license which has been taken in 
 theoretic groupings deduced from this doctrine, a serious 
 question arises whether it may not ultimately, unless carefully 
 restricted, produce confusion rather than simplicity, and be to 
 the student an embarrassment rather than an assistance. 
 
 The term 'law' has not unfrequently been applied to 
 groupings of phenomena such as those to which I have been re- 
 ferring ; it seems to me that a popular misconception as to the 
 meaning of the term prevails. Putting aside what are called 
 necessary truths, such as those proved by mathematics, and 
 the vexed metaphysical question as to whether these are or 
 are not independent of experience, we can, I think, have little 
 doubt that a vast number of those formulae to which the 
 term ' laws ' are commonly applied are merely categories into 
 which our own experience has thrown phenomenal instances. 
 The so-called law of chemical equivalents, the laws of refrac- 
 tion and polarisation of light, of the reciprocal action and re- 
 action of electricity and magnetism, nay, the very grouping 
 of phenomena under the terms electricity, magnetism, &c., 
 
152 CORRELATION OF PHYSICAL FORCES. 
 
 are mere results of collected and collated observations, and 
 all our generalising philosophy is based on an unconscious 
 belief in continuity. In such cases, though the word ' law ' is 
 used, all that can be predicated is that, as far as experience 
 goes, certain results follow certain arrangements of matter. 
 Many of these supposed laws have been found to be erroneous 
 or to be no laws at all, e.g. Marriotte's law as to the 
 expansion of gases, Gay-Lussac's as to the chemical relations 
 of the volumes of gases, &c. ; the word * law/ therefore, except- 
 ing the term be applied to mathematical truths, is merely a 
 generalised expression of certain observed results which as far 
 as our experience goes are invariable ; and when we personify 
 the term ' law/ as by talking of effects taking place in obedience 
 to a law, we use an inaccurate expression, only justifiable as 
 metaphor ; but this use of the term has so warped men's minds 
 that the mass of even educated people believe in law as some- 
 thing separate from and governing the phenomena, the latter 
 being supposed to be subject to the law, whereas this is only 
 an abstract definition of relations derived from the observa- 
 tion of the phenomena themselves. The word ' nature ' is 
 still more personified ; instead of being used to denote, what 
 alone it can denote- namely, things as we see, hear or feel, 
 them, and their relations ascertained by comparison and 
 abstraction nature is treated as a sort of superintending 
 angel who enjoins this, permits that, and forbids the other. 
 
 It may be that natural phenomena, as we come to know 
 them more thoroughly, may resolve themselves into forms 
 which may be mathematically expressed, and which may have 
 the certainty of mathematical truths, as the laws of Kepler and 
 Newton's law of gravitation now appear to have; but we are far 
 from that at present so far as regards the greater number of 
 phenomena, particularly those exhibited by molecular and 
 physiological effects. To bring all within the domain of what 
 are termed necessary truths would require omniscience. 
 
153 
 
 OTHER MODES OF FORCE. 
 
 CATALYSIS, or the chemical action induced by the mere pre- 
 sence of a foreign body, embraces a class of facts which must 
 considerably modify many of our notions of chemical action : 
 thus, oxygen and hydrogen, when mixed in a gaseous state, 
 will remain unaltered for an indefinite period ; but the intro- 
 duction to them of a slip of clean platinum will cause more or 
 less rapid combination, without being in itself in any respect 
 altered. On the other hand, oxygenated water, which is a 
 compound of one equivalent of hydrogen plus two of oxygen, 
 will, when under a certain temperature, remain perfectly 
 stable ; but touch it with platinum in a state of minute divi- 
 sion, and it is instantly decomposed, one equivalent of oxygen 
 being set free. Here, again, the platinum is unaltered, and thus 
 we have both synthesis and analysis effected apparently by 
 the mere contact of a foreign body. It is not improbable that 
 the increased electrolytic power of water by the addition of 
 some acids, such as the sulphuric and phosphoric, where the 
 acids themselves are not decomposed, depends upon a catalytic 
 effect of these acids ; but we know too little of the nature and 
 rationale of catalysis to express any confident opinion on its 
 modes of action, and possibly we may comprehend very dif- 
 ferent molecular actions under one and the same name. In no 
 case does catalysis yield us new power or force : it only deter- 
 mines or facilitates the action of chemical force, and, therefore, 
 is no creation of force by contact. 
 
 The force so developed by catalysis may be converted into 
 a voltaic form thus : in a single pair of the gas battery pre- 
 viously alluded to, one portion of a strip of platinum is im- 
 mersed in a tube of oxygen, the other in one of hydrogen, 
 both the gases and the extremities of the platinum being 
 connected by water or other electrolyte ; a voltaic combina- 
 
154 CORRELATION OF PHYSICAL FORCES. 
 
 tion is thus formed, and electricity, heat, light, magnetism, and 
 motion produced at the will of the experimenter. 
 
 In this combination we have a striking instance of cor- 
 relative expansions and contractions, analogous, though in a 
 much more refined form, to the expansions and contractions 
 by heat and cold detailed in the early part of this Essay, and 
 illustrated by 'the alternate actions of two bladders partially 
 filled with air : thus, as by the effect of chemical combination 
 in each pair of tubes of the gas battery the gases oxygen and 
 hydrogen lose their gaseous character and shrink into water, so 
 at the platinum terminals of the battery, when immersed in 
 water, water is decomposed, and expands into oxygen and 
 hydrogen gases. The correlate of the force which changes 
 gas into liquid at one point of space changes liquid into gas 
 at another, and the exact volume which disappears in the one 
 place reappears in the other ; so that it would appear to an 
 inexperienced eye as though the gases passed through solid 
 wires. 
 
 Gravitation, inertia, and aggregation were but cursorily 
 alluded to in my original Lectures ; their relation to the other 
 modes of force seemed to be less definitely traceable ; but the 
 phenomenal effects of gravitation and inertia, being motion 
 and resistance to motion, in considering motion I have in some 
 degree included their relations to the other forces. 
 
 To my mind gravitation would only produce other force 
 when the motion caused by it is diminished or ceases. Thus, 
 if we suppose a meteor to be a mass rotating in an orbit round 
 the earth, and with no resisting medium, then, as long as that 
 rotation continues, the motion of the meteoric mass itself 
 would be the exponent of the force impelling it ; if there be a 
 resisting medium, part of this motion would be arrested and 
 taken up by the medium, either as motion, heat, electricity, 
 or some other mode of force ; if the meteor approach the earth 
 sufficiently to fall upon it, the perceptible motion of the 
 meteor is stopped, but is taken up by the earth which vibrates 
 through its mass ; part also reappears as heat in both earth 
 and meteor, and part in the change in the earth's position 
 consequent on its increase of gravity, and so on. Gravitation 
 is but the subjective idea, and its relation to other modes of 
 
OTHER MODES OF FORCE. 155 
 
 force seems to me to be identical with that of pressure or 
 motion. Thus, when arrested motion produces heat, it matters 
 not whether the motion has been produced by a falling body, 
 i.e. by gravitation, or a body projected by an explosive com- 
 pound, &c. ; the heat will be the same, provided the mass and 
 velocity at the time of arrest be the same. In no other sense 
 can I conceive a relation between gravitation and the other 
 forces, and, with all diffidence, I cannot agree with those who 
 consider there is a different sort of link. 
 
 Mosottihas mathematically treated of the identity of gravi- 
 tation with cohesive attraction, and Pliicker has succeeded in 
 showing that crystalline bodies are definitely affected by 
 magnetism, and take a position in relation to the lines of 
 magnetic force dependent upon their optical axis or axis of 
 symmetry. 
 
 What is termed the optic axis is a fixed direction through 
 crystals in which they do not doubly refract light, and which 
 direction, in those crystals which have one axis of figure, or a 
 line around which the figure is symmetrical, is parallel to the 
 axis of symmetry. When submitted to magnetic influence 
 such crystals take up a position, so that their optic axis 
 points diamagnetically or transversely to the lines of magnetic 
 force ; and when, as is the case in some crystals, there is more 
 than one optic axis, the resultant of these axes points dia- 
 magnetically. The mineral cyanite is influenced by mag- 
 netism in so marked a manner that when suspended it will 
 arrange itself definitely with reference to the direction of 
 terrestrial magnetism, and may, according to Pliicker, be used 
 as a compass-needle. 
 
 There is scarcely any doubt that the force which is con- 
 cerned in aggregation is the same which gives to matter its 
 crystalline form ; indeed, a vast number of inorganic bodies, 
 if not all, which appear amorphous, are, when closely examined, 
 found to be crystalline in their structure : we thus get a re- 
 ciprocity of action between the force which unites the mole- 
 cules of matter and the magnetic force, and through the 
 medium of the latter the correlation of the attraction of 
 aggregation with the other modes of force may be established. 
 
 I believe that the same principles and mode of reasoning 
 
156 CORRELATION OF PHYSICAL FORCES. 
 
 as have been adopted in this Essay might be applied to the 
 organic as well as the inorganic world ; and that muscular 
 force, animal and vegetable heat, &c. might, and at some time 
 will, be shown to have similar definite correlations ; but I have 
 purposely avoided this subject, as pertaining to a department 
 of science to which I have not devoted my attention. I 
 ought, however, while alluding to this subject, shortly to 
 mention some experiments of Professor Matteucci, communi- 
 cated to the Royal Society in the year 1850, by which it 
 appears that whatever mode of force it be which is propagated 
 along the nervous filaments, this mode of force is definitely 
 affected by currents of electricity. His experiments show 
 that when a current of positive electricity traverses a portion 
 of the muscle of a living animal in the same direction as that 
 in which the nerves ramify i.e. a direction from the brain to 
 the extremities a muscular contraction is produced in the 
 limb experimented on, showing that the nerve of motion is 
 affected ; while, if the current, as it is termed, be made to 
 traverse the muscle in the reverse direction, or towards the 
 nervous centres, the animal utters cries, and exhibits all the 
 indications of suffering pain, scarcely any muscular move- 
 ment being produced, showing that in this case the nerves 
 of sensation are affected by the electric current ; some defi- 
 nite polar condition therefore exists, or is induced in the 
 nerves, to which electricity is co-related, and probably this 
 polar condition constitutes or conveys nervous agency. There 
 are other analogies given in the papers of M. Matteucci, and 
 derived from the action of the electrical organs of fishes, 
 which tend to corroborate and develop the same view. 
 
 By an application of the doctrine of the Correlation of 
 Forces, Dr. Carpenter has shown how a difficulty arising 
 from the ordinary notions of the development of an organised 
 being from its germ-cell may be lessened. It has been thought 
 by many physiologists that the nisus formativus, or organising 
 force of an animal or vegetable structure, lies dormant in the 
 primordial germ-cell. ' So that the organising force required 
 to build up an oak or a palm, an elephant or a whale, is con- 
 centrated in a minute particle only discernible by microscopic 
 aid' 
 
OTHER MODES OF FORCE. 157 
 
 Certain other views of nearly equal difficulty have been 
 propounded. Dr. Carpenter suggests the probability of ex- 
 traneous forces, as heat, light, and chemical affinity, con- 
 tinuously operating upon the material germ ; so that all that 
 is required in this is a structure capable of receiving, directing, 
 and converting these forces into those which tend to the 
 assimilation of extraneous matter and the definite develop- 
 ment of the particular structure. In proof of this position he 
 shows how dependent the process of germ development is 
 upon the presence and agency of external forces, particularly 
 heat and light, and how it is regulated by the measure of 
 these forces supplied to it. 
 
 It certainly is far less difficult so to conceive the supply of 
 force yielded to organised beings in their gradual process of 
 growth, than to suppose a store of dormant or latent force 
 pent up in a microscopic monad. 
 
 As by the artificial structure of a voltaic battery, chemical 
 actions may be made to co-operate in a definite direction, so, 
 by the organism of a vegetable or animal, the mode of motion 
 which constitutes heat, light, &c. may, without extravagance, 
 be conceived to be appropriated and changed into the forces 
 which induce the absorption and assimilation of nutriment, 
 and into nervous agency and muscular power. Indications 
 of similar thoughts may be detected in the writings of 
 Liebig. 
 
 Some difficulty in studying the correlations of vital with 
 inorganic physical forces arises from the effects of sensation 
 and consciousness, presenting a similar confusion to that 
 alluded to, when, in treating of heat, I ventured to suggest 
 that observers are too apt to confound the sensations with the 
 phenomena. Let us apply some of the considerations on 
 force, given in the introductory portion of this Essay, to cases 
 where vitality or consciousness intervenes. When a weight is 
 raised by the hand, there should, according to the doctrine of 
 non-creation of force, have been somewhere an expenditure 
 equivalent to the amount of gravitation overcome in raising 
 the weight. That there is expenditure we can prove, though 
 in the present state of science we cannot measure it. Thus, 
 prolong the effort, raise weights for an hour or two, the vital 
 
158 CORRELATION OF PHYSICAL FORCES. 
 
 powers sink, food, i.e. fresh chemical force, is required to supply 
 the exhaustion. If this supply is withheld and the exertion 
 is continued, we see the consumption of force in the super- 
 vening weakness and emaciation of the body. 
 
 The consciousness of effort, which has formed a topic of 
 argument by some writers when treating of force, and is by 
 them believed to be that which has originated the idea of 
 force, may by the physical student be regarded as feeling is 
 in the phenomena of heat and cold, viz. a sensation of the 
 struggle of opposing molecular motions in overcoming the 
 resistance of the masses to be moved. When we say we feel 
 hot, we feel cold, we feel that we are exerting ourselves, our 
 expressions are intelligible to beings who are capable of expe- 
 riencing similar sensations ; but the physical changes accom- 
 panying these sensations are not thereby explained. Without 
 pretending to know what probably we shall never fully know, 
 the actual modus agendi of the brain, nerves, muscles, &c., we 
 may study vital as we do inorganic phenomena, both by ob- 
 servation and experiment. Thus Sir Benjamin Brodie has 
 examined the effect of respiration on animal heat by inducing 
 artificial respiration after the spinal cord has been severed ; in 
 which case he finds the animal heat declines, notwithstanding 
 the continuance of the chemical action of respiration, carbonic 
 acid being formed as usual ; but he also finds that under such 
 circumstances the struggles or muscular actions of the animal 
 are very great, and sufficient probably to account for the force 
 eliminated by the chemical action in digestion and respiration ; 
 and Liebig, by measuring the amount of chemical action in 
 digestion and respiration, and comparing it with the labour 
 performed, has to some extent established their equivalent 
 relations. 
 
 Mr. Helmholtz has found that the chemical changes which 
 take place in muscles are greater when these are made to 
 undergo contractions than when they are in repose ; and that, 
 as would be expected, the consumption of the matter of the 
 muscle, or, in other terms, the waste of excrementitious 
 matter thrown off, is greater in the former than in the latter 
 case. 
 
 M. Matteucci has ascertained that the muscles of recently 
 
OTHER MODES OF FORCE. 159 
 
 killed frogs absorb oxygen and exhale carbonic acid, and that 
 when they are thrown into a state of contraction, and still 
 more when they perform mechanical work, the absorption is 
 increased ; and he even calculates the equivalents of work so 
 performed. 
 
 M. Beclard finds that the quantity of heat produced by 
 voluntary muscular contraction in man is greater when that con- 
 traction is what he terms static that is, when it produces no 
 external work, but is effort alone than when that effort and 
 contraction are employed dynamically, so as to raise a weight 
 or produce mechanical work. Every year is now adding evi- 
 dence that the forces brought into play by organised beings are 
 traceable to the continuous and mainly to the chemical effects 
 of food, air, light, &c. The assimilation of carbon by plants, 
 that of carbon and nitrogen by animals, together with the 
 slow combustion by inhalation of oxygen ; and the derivation 
 of the forces which animate and keep alive organisms, from 
 the heat and light radiated by the sun, are now becoming 
 accepted beliefs. 
 
 Thus, though we may see no present promise of being able 
 to resolve sensations into their ultimate elements, or to trace, 
 physically, the link which unites volition with exertion or 
 effort, we may hope to approximate the solution of these 
 deeply interesting questions. 
 
 In the same individual the chemical and physical state of 
 the secretions in the warm may be compared with those in 
 the cold parts of the body. The changes in digestion and 
 respiration, when the body is in a state of rest, may be com- 
 pared with those which obtain when it is in a state of activity. 
 The relations with external matter, maintaining, by the con- 
 stant play of natural forces, the vital nucleus, or the organisa- 
 tion by means of which matter and force receive, for a definite 
 period, a definite incorporation and direction, may be ascer- 
 tained, while the more minute structural changes are revealed 
 to us by the ever-improving powers of the microscope ; and 
 thus step by step we may learn that which it is given to us to 
 learn, boundless in its range and infinite in its progress, and 
 therefore never giving a response to the ultimate Why ? 
 
 As the first glimpse of a new star is caught by the eye of 
 
160 CORRELATION OF PHYSICAL FORCES. 
 
 the astronomer while directing his vision to a different point 
 of space, and disappears when steadfastly gazed at, only to 
 have its position and figure ultimately ascertained by the 
 employment of more penetrative powers, so. the first scintilla- 
 tions of new natural phenomena frequently present themselves 
 to the eye of the observer, dimly seen when viewed askance, 
 and disappearing if directly looked for. When new powers 
 of thought and experiment have developed and corrected the 
 first notions, and given a character to the new image probably 
 very different from the first impression, fresh objects are 
 again glanced at in the margin of the new field of vision, 
 which in their turn have to be verified, and again lead to new 
 extensions ; thus, the effort to establish one observation leads 
 to the imperfect perception of new and wider fields of re- 
 search ; and, instead of approaching finality, the more we 
 discover the more infinite appears the range of the undis- 
 covered ! 
 
CONCLUDING REMARKS. 
 
 I HAVE now gone through the affections of matter to which 
 distinct names have been given in our received nomenclature : 
 that other forces may be detected, differing as much from 
 them as they differ from each other, is highly probable, and 
 that when discovered, and their modes of action fully traced 
 out, they will be found to be related inter se, and to these 
 forces as these are to each other, I believe to be as far certain 
 as certainty can be predicted of any future event. 
 
 It may in many cases be a difficult question to determine 
 what constitutes a distinct affection of matter or mode of 
 force. It is highly probable that different lines of demarca- 
 tion would have been drawn between the forces already known, 
 had they been discovered in a different manner, or first 
 observed at different points of the chain which connects them. 
 Thus, radiant heat and light are mainly distinguished by the 
 manner in which they affect our senses : were they viewed 
 according to the way in which they affect inorganic matter, 
 very different notions would possibly be entertained of their 
 character and relation. Electricity, again, was named from 
 the substance in which, and magnetism from the district 
 where, they first happened to be observed, and a chain of inter- 
 mediate phenomena has so connected electricity with gal- 
 vanism that they are now regarded as the same force, differing 
 only in the degree of its intensity and quantity, though for a 
 long time they were regarded as distinct. 
 
 The phenomenon of attraction and repulsion by amber, . 
 which originated the term electricity, is as unlike that of the 
 decomposition of water by the voltaic pile, as any two natural 
 phenomena can well be. It is only because the historical 
 sequence of scientific discoveries has associated them by a 
 number of intermediate links, that they are classed within the 
 
 M 
 
162 CORRELATION OF PHYSICAL FORCES, 
 
 same category. What is called voltaic electricity might equally, 
 perhaps more appropriately, be called voltaic chemistry. I 
 mention these facts to show that the distinction in the name 
 may frequently be much greater than the distinction of the 
 subject which it represents, and vice versa, not as at all object- 
 ing to the received nomenclature on these points ; nor do I 
 say it would be advisable to depart from it : were we to do 
 so, inevitable confusion would result, and objections equally 
 forcible might be found to apply to our new terminology. 
 
 Words, when established to a certain point, become a part 
 of the social mind ; its powers and very existence depend 
 upon the adoption of conventional symbols ; and were these 
 suddenly departed from, or varied, according to individual 
 apprehensions, the acquisition and transmission of knowledge 
 would cease. Undoubtedly, neology is more permissible in 
 physical science than in any other branch of knowledge, 
 because it is more progressive ; new facts or new relations 
 require new names, but even here it should be used with great 
 caution. 
 
 Si forte necesse est 
 
 Indiciis monstrare recentibus abdita rerum, 
 Fingere cinctutis non exaudita Cethegis 
 Continget ; dabiturque licentia, sumpta pudenter. 
 
 Even should the mind ever be led to dismiss the idea of 
 various forces, and regard them as the exertion of one force, 
 or resolve them definitely into motion, still we could never 
 avoid the use of different conventional terms for the different 
 modes of action of this one pervading force. 
 
 Reviewing the series of relations between the various forces 
 which we have been considering, it would appear that in many 
 cases where one of these is excited or exists, all the others are 
 also set in action : thus, when a substance, such as sulphuret 
 of antimony, is electrified, at the instant of electrisation it 
 becomes magnetic in directions at right angles to the lines of 
 electric force ; at the same time it becomes heated to an extent 
 greater or less according to the intensity of the electric force. 
 If this intensity be exalted to a certain point the sulphuret 
 becomes luminous, or light is produced : it expands, conse- 
 quently motion is produced ; and it is decomposed, therefore 
 
CONCLUDING REMARKS. 163 
 
 chemical action is produced. If we take another substance, 
 say a metal, all these forces except the last are developed ; 
 and although we can scarcely apply the term chemical action 
 to a substance hitherto undecomposed, and which, under the 
 circumstances we are considering, enters into no new combi- 
 nation, yet a metal undergoes that species of polarisation 
 which, as far as we can judge, is the first step towards chemical 
 action, and which, if the substance were decomposable, would 
 resolve it into its elements. Perhaps, indeed, some hitherto 
 undiscovered chemical action is produced in substances which 
 we regard as undecomposable : there are experiments, to 
 some of which I have alluded, which tend to show that metals 
 which have been electrised are permanently changed in their 
 molecular constitution. Oxygen, we have seen, is changed by 
 the electric spark into ozone, and phosphorus into allotropic 
 phosphorus, both which changes were for a long time unknown 
 to those familiar with electrical science. 
 
 Thus, with some substances, when one mode of force is 
 produced all the others are simultaneously developed. With 
 other substances, probably with all matter, some of the other 
 forces are developed, whenever one is excited, and all may be 
 so were the matter in a suitable condition for their develop- 
 ment, or our means of detecting them sufficiently delicate. 
 
 This simultaneous production of several different forces 
 seems at first sight to be irreconcilable with their mutual 
 and necessary dependence, and it certainly presents a formid- 
 able experimental difficulty in the way of establishing their 
 equivalent relations ; but when examined closely, it is not in 
 fact inconsistent with the views we have been considering, but 
 is indeed a strong argument in favour of the theory which 
 regards them as modes of motion. 
 
 Let us select one or two cases in which this form of objec- 
 tion may be prominently put forward. A voltaic battery 
 decomposing water in a voltameter, while the same current is 
 employed at the same time to make an electro-magnet, gives 
 nevertheless in the voltameter an equivalent of gas, or de- 
 composes an equivalent of an electrolyte for each equivalent 
 of chemical decomposition in the battery cells, and will give 
 the same ratios if the electro-magnet be removed. Here, at 
 
 M 2 
 
1 64 CORRELATION OF PHYSICAL FORCES. 
 
 first sight, it would appear that the magnetism was an extra 
 force produced, and that thus more than the equivalent power 
 was obtained from the battery. In answer to this objection 
 it may be said, that in the circumstances under which this 
 experiment is ordinarily performed, several cells of the battery 
 are used, and so there is a far greater amount of heat, &c., 
 generated in the cells than is indicated by the effect in the 
 voltameter. If, moreover, the magnet be not interposed, still 
 the magnetic force is equally existent throughout the whole 
 current; for instance, the wires joining the plates will attract 
 iron filings, deflect magnetic needles, &c v and produce 
 diamagnetic effects on surrounding matter. By the iron core 
 a small portion of the force is, indeed, absorbed while it is 
 being made a magnet, and probably less heat is developed in 
 the circuit, but this ceases to be absorbed when the magnet 
 is made; this has been proved by the observation of Mr. 
 Latimer Clarke, who has found that along the wires of the 
 electric telegraph the magnetic needles placed at different 
 stations remained fixed after the connection with the battery 
 was made, and while the electric current acted by induction 
 on surrounding conducting matter, separated from the wires 
 by their gutta-percha coating, so that a sort of Leyden phial 
 was formed ; but as soon as this induction had produced its 
 effect between each station, or, so to speak, the phial was 
 charged, the needles successively were deflected : it is like the 
 case of a pulley and weight, which latter exhausts force while 
 it is being raised ; but when raised, the force is free, and may 
 be used for other purposes. 
 
 If a battery of one cell, just capable of decomposing 
 water and no more, be employed, this will cease to decompose 
 while making a magnet. There must, in every case, be pre- 
 ponderating chemical affinity in the battery cells, either by 
 the nature of its elements or by the reduplication in series, to 
 effect decomposition in the voltameter; and if the point is 
 just reached at which this is effected, and the power is then 
 reduced by any resistance, decomposition ceases : were it 
 otherwise, were the decomposition in the voltameter the ex- 
 ponent of the entire force of the generating cells, and these 
 could independently produce magnetic force, this latter force 
 would be got from nothing, and perpetual motion be obtained. 
 
CONCLUDING REMARKS. 165 
 
 To take another and different example : A piece of zinc 
 dissolved in dilute sulphuric acid gives somewhat less heat 
 than when the zinc has a wire of platinum attached to it, and 
 is dissolved by the same quantity of acid. The argument is 
 deducible that, as there is more electricity in the second than 
 in the first case, there should be less heat ; but as, according 
 to our received theories, the heat is a product of the electric 
 current, and in consequence of the impurity of zinc electricity 
 is generated in the first case molecularly, by what is called 
 local action, though not thrown into a general direction, there 
 should be more of both heat and electricity in the second 
 than in the first case, as the heat and electricity due to the 
 voltaic combination of zinc and platinum are added to that 
 excited on the surface of the zinc, and the zinc should be, as 
 in fact it is, more rapidly dissolved ; so that the extra heat 
 and electricity are produced by extra chemical force. Many 
 additional cases of a similar description might be suggested. 
 But although it is difficult, and perhaps impossible, to restrict 
 the action of any one force to the production of one other 
 force, and of one only yet if the whole of one force, say 
 chemical action, be supposed to be employed in producing its 
 full equivalent of another force, say heat, then as this heat is 
 capable in its turn of reproducing chemical action, and in the 
 limit, a quantity equal or practically all but equal to the 
 initial force : if this could at the same time produce inde- 
 pendently another force, say magnetism, we could, by adding 
 the magnetism to the total heat, get more than the original 
 chemical action, and thus create force or obtain perpetual 
 motion. 
 
 The term Correlation, which I selected as the title of my 
 Lectures in 1843, strictly interpreted, means a necessary 
 mutual or reciprocal dependence of two ideas, inseparable 
 even in mental conception : thus, the idea of height cannot 
 exist without involving the idea of its correlate, depth ; the 
 idea of parent cannot exist without involving the idea of off- 
 spring. The word itself had not been previously used ; and 
 although, as I have said, I object to the introduction of new 
 terms without strong reason, there are a vast variety of 
 physical relations which cannot certainly be so well expressed 
 by any other term. The extent to which it has been since 
 
166 CORRELATION OF PHYSICAL FORCES. 
 
 used has, I think, justified me. Its use has, in my judgment, 
 been carried too far in applying it to subjects quite beyond 
 its fair meaning. There are many facts, one of which cannot 
 take place without involving the other ; one arm of a lever 
 cannot be depressed without the other being elevated the 
 finger cannot press the table without the table pressing the 
 finger. A body cannot be heated without another being 
 cooled, or some other force being exhausted in an equivalent 
 ratio to the production of heat ; a body cannot be positively 
 electrified without some other body being negatively electrified, 
 &c. To such cases the term correlation may be usefully 
 applied, but hardly to adaptations of structure, &c. 
 
 The probability is, that, if not all, the greater number of 
 physical phenomena are, in one sense, correlative, and that, 
 without a duality of conception, the mind cannot form an 
 idea of them : thus, motion cannot be perceived or probably 
 imagined without parallax or relative change of position. 
 The world was believed fixed, until, by comparison with the 
 celestial bodies, it was found to change its place with regard 
 to them : had there been no perceptible matter external to 
 the world, we should never have discovered its motion. In 
 sailing along a river, the stationary vessels and objects on the 
 banks seem to move past the observer : if at last he arrives 
 at the conviction that he is moving, and not these objects, it 
 is by correcting his senses by reflection derived from a more 
 extensive previous use of them : even then he can only form 
 a notion of the motion of the vessel he is in, by its change of 
 position with regard to the objects it passes that is, provided 
 his body partakes of the motion of the vessel, which it only 
 does when its course is perfectly smooth, otherwise the relative 
 change of position of the different parts of the body and the 
 vessel inform him of its alternating, though not fully of its 
 progressive movement. So in all physical phenomena, the 
 effects produced by motion are all in proportion to the relative 
 motion : thus, whether the rubber of an electrical machine be 
 stationary, and the cylinder mobile, or the rubber mobile and 
 the cylinder stationary, or both mobile in different directions, 
 or in the same direction with different degrees of velocity, the 
 electrical effects are, cczteris paribiis, precisely the same, pro- 
 
CONCLUDING REMARKS. 167 
 
 vided the relative motion is the same, and so, without excep- 
 tion, of all other phenomena. The question of whether there 
 can be absolute motion, or, indeed, any absolute isolated 
 force, is purely the metaphysical question of idealism or 
 realism a question for our purpose of little import ; sufficient 
 for the purely physical enquirer, the maxim, ' Denon apparenti- 
 bits et non existentibus eadem est ratio' 
 
 . The sense I have attached to the word Correlation, in 
 treating of physical phenomena, will, I think, be evident, from 
 the previous parts of this Essay, to be that of a necessary 
 reciprocal production ; in other words, that any force capable of 
 producing another may, in its turn, be produced by it nay, 
 more, can be itself resisted by the force it produces, in pro- 
 portion to the energy of such production, as action is ever 
 accompanied and resisted by reaction : thus, the action of an 
 electro-magnetic machine is reacted upon by the magneto- 
 electricity developed by its action. 
 
 To many, however, of the cases we have been considering, 
 the term correlation may be applied in a more strict accord- 
 ance with its original sense : thus, with regard to the forces of 
 electricity and magnetism in a dynamic state, we cannot 
 electrise a substance without magnetising it we cannot mag- 
 netise it without electrising it each molecule, the instant it is 
 affected by one of these forces, is affected by the other ; but, 
 in transverse directions, the forces are inseparable and 
 mutually dependent correlative, but not identical. 
 
 The evolution of one force or mode of force into another 
 has induced many to regard all the different natural agencies 
 as reducible to unity, and as resulting from one force which is 
 the efficient cause of all the others : thus, one author writes to 
 prove that electricity is the cause of every change in matter ; 
 another, that chemical action is the cause of everything ; 
 another, that heat is the universal cause, and so on. If, as I 
 have stated it, the true expression of the fact is, that each 
 mode of force is capable of producing the others, and that 
 none of them can be produced but by some other as an 
 anterior force, then any view which regards either of them as 
 abstractedly the efficient cause of all the rest, is erroneous : 
 the view has, I believe, arisen from a confusion between the 
 
168 CORRELATION OF PHYSICAL FORCES. 
 
 abstract or generalised meaning of the term cause, and its 
 concrete or special sense; the word itself being indiscriminately 
 used in both these senses. 
 
 Another confusion of terms has arisen, and has, indeed, 
 much embarrassed me in enunciating the propositions put 
 forth in these pages, on account of the imperfection of scien- 
 tific language ; an imperfection in great measure unavoid- 
 able, it is true, but not the less embarrassing. Thus, the 
 words, light, heat, electricity, and magnetism, are constantly 
 used in two senses viz. that of the force producing, or the 
 subjective idea of force or power, and of the effect produced, 
 or the objective phenomenon. The word motion, indeed, is 
 only applied to the effect, and not to the force, and the term 
 chemical affinity is generally applied to the force, and not to 
 the effect ; but the other four terms are, for want of a dis- 
 tinct terminology, applied indiscriminately to both. 
 
 I may have occasionally used the same word at one 
 time in a subjective, at another in an objective sense ; all I 
 can say is, that this cannot be avoided without a neology, 
 which I have not the presumption to introduce, or the autho- 
 rity to enforce. Again, the use of the term forces in the 
 plural might be objected to by those who do not attach to 
 the term force the notion of a specific agency, but of one 
 universal power associated with matter, of which its various 
 phenomena are but diversely modified effects. 
 
 Whether the imponderable agents, viewed as force, and 
 not as matter, ought to be regarded as distinct forces or as 
 distinct modes of force, is probably not very material, for, as 
 far as I am aware, the same result would follow either view ; 
 I have therefore used the terms indiscriminately, as either 
 happened to be the more expressive for the occasion. 
 
 Throughout this Essay I have placed motion in the same 
 category as the other affections of matter. The course of 
 reasoning adopted in it, however, appears to me to lead inevi- 
 tably to the conclusion that these affections of matter are 
 themselves modes of motion ; that, as in the case of friction, 
 the gross or palpable motion, which is arrested by the contact 
 of another body, is subdivided into molecular motions or 
 vibrations, which vibrations are heat or electricity as the case 
 
CONCLUDING REMARKS. 169 
 
 may be ; so the other affections are only matter moved or 
 molecularly agitated in certain definite directions. I have 
 already considered the hypothesis that the passage of electri- 
 city and magnetism causes vibrations in an ether permeating 
 the bodies through which the current is transmitted, i.e. the 
 application of the same ethereal hypothesis to these impon- 
 derables which had previously been applied to light ; many, 
 in speaking of some of their effects, admit that electricity and 
 magnetism cause or produce by their passage vibrations in the 
 particles of matter, but regard the vibrations produced as an 
 occasional, though not always a necessary, effect of the pas- 
 sage of electricity, or of the increment or decrement of mag- 
 netism. The view which I have taken is, that such vibrations, 
 molecular polarisations, or motions of some sort from particle 
 to particle, are themselves electricity or magnetism ; or, to 
 express it in the converse, that dynamic electricity and mag- 
 netism are themselves motion, and that permanent magnetism, 
 and static electricity, are conditions of force bearing a similar 
 relation to motion which tension or gravitation do. The so- 
 called imponderables affect matter in all its states. Gases 
 which have transmitted light are altered, e.g. chlorine is ren- 
 dered capable of combining with hydrogen, liquids are altered, 
 peroxalate of iron is chemically changed and gives off carbonic 
 acid, and the light which has produced these effects is less able 
 to produce them a second time. Solids are altered, as shown 
 in the extensive range of photographic effects. So with elec- 
 tricity, compound gases are changed chemically, e.g. ammonia 
 or carbonic acid ; elementary gases are changed allotropically, 
 e.g. phosphorus vapour or oxygen ; liquids are changed, as in 
 the electrolysis of water, &c.; and solids are changed, as in the 
 projection of particles of the terminals, and molecular impres- 
 sions on polished surfaces. Chemical affinity is only known 
 to us by its affecting matter in the three states, and magnetism 
 is proved to affect liquids and solids, though as yet gases are 
 not shown to be altered by it. 
 
 This theory might well be discussed in greater detail than 
 has been used in this work ; but to do this and to anticipate 
 objections would lead into specialties foreign to my present 
 object, in the course of this Essay my principal aim having 
 
170 CORRELATION OF PHYSICAL FORCES. 
 
 been rather to show the relation of forces as evinced by 
 acknowledged facts, than to enter upon any detailed expla- 
 nation of their specific modes of action. 
 
 Probably man will never know the ultimate structure of 
 matter or the minutiae of molecular actions ; indeed, it is 
 scarcely conceivable that the mind can ever attain to this 
 knowledge ; the monad irresolvable by a given microscope 
 may be resolved by an increase in power. Much harm has 
 already been done by attempting hypothetically to dissect 
 matter and to discuss the shapes, sizes, and numbers of 
 atoms, and their atmospheres of heat, ether, or electricity. 
 
 Whether the regarding electricity, light, magnetism, &c., 
 as simply motions of ordinary matter be or be not admissible, 
 certain it is, that all past theories have resolved, and all exist- 
 ing theories do resolve, the actions of these forces into motion. 
 Whether it be that, on account of our familiarity with mo- 
 tion, we refer other affections to it, as to a language which is 
 most easily construed and most capable of explaining them, 
 or whether it be that it is in reality the only mode in which 
 our minds, as contradistinguished from our senses, are able to 
 conceive material agencies, certain it is that since the period 
 at which the mystic notions of spiritual or preternatural 
 powers were applied to account for physical phenomena, all 
 hypotheses framed to explain them have resolved them into 
 motion. Take, for example, the theories of light, to which I 
 have before alluded : one of these supposes light to be a pecu- 
 liar rare matter, emitted from i.e. put in motion by luminous 
 bodies; a second supposes that the specific matter is not 
 emitted from luminous bodies, but is put into a state of vibration 
 or undulation, i.e. motion by them ; and thirdly, light may be 
 regarded as an undulation or motion of ordinary matter, and 
 propagated by undulations of air, glass, &c., as I have before 
 stated. In all these hypotheses, matter and motion are the 
 only conceptions. Nor, even if we accept terms derived from 
 our own sensations, the which sensations themselves may be 
 but modes of motion in the nervous filaments, can we find 
 words to describe phenomena other than those expressive of 
 matter and motion. We in vain struggle to escape from these 
 
CONCLUDING REMARKS. 171 
 
 ideas ; if we ever do so, our mental powers must undergo a 
 change of which at present we see no prospect. 
 
 If we apply to any other force the mode of reasoning 
 which in the course of this Essay has been applied to heat, we 
 shall arrive at the same conclusion, and see that a given source 
 of power can, supposing it to be fully utilised in each case, 
 yield no more by employing it as an exciter of one force than 
 of another. Let us take electricity as an example. Suppose 
 a pound of mercury at 400 be employed to produce a thermo- 
 electric current, and the latter be in its turn employed to pro- 
 duce mechanical force ; if this latter force be greater than that 
 which the direct effect of heat would produce, then it could 
 by compression or friction raise the temperature of the mer- 
 cury itself, or of a similar quantity equally heated, to a higher 
 point than its original temperature, the 400 to 401, for ex- 
 ample, which is obviously impossible; nor, if we admit force 
 to be indestructible, can it produce less than 400, except by 
 some portion of it being converted into another form or mode 
 of force. 
 
 But as the mechanical effect here is produced through the 
 medium of electricity, and the mechanical effect is definite, so 
 the quantity of electricity producing it must be definite also, 
 for unequal quantities of electricity could only produce an 
 equal mechanical effect by a loss or gain of their own force 
 into or out of nothing. The same reasoning will apply to the 
 other forces, and will lead, it appears to me, necessarily and 
 inevitably to the conclusion, that each force is definitely and 
 equivalently. convertible into any other, and that where expe- 
 riment does not give the full equivalent, it is because the initial 
 force has been dissipated, not lost, by conversion into other 
 unrecognised forces. The equivalent is the limit never prac- 
 tically reached. 
 
 The great problem which remains to be solved, in regard 
 to the correlation of physical forces, is this establishment of 
 their equivalents of power, or their measurable relation to a 
 given standard. The progress made in some of the branches 
 of this enquiry has been already noticed. Viewed in their 
 static relations, or in the conditions requisite for producing 
 
172 CORRELATION OF PHYSICAL FORCES. 
 
 equilibrium or quantitative equality of force, a remarkable 
 relation between chemical affinity and heat is that discovered 
 in many simple bodies by Dulong and Petit, and extended to 
 compounds by Neumann and Avogadro. Their researches 
 have shown that the specific heats of certain substances, when 
 multiplied by their chemical equivalents, give a constant quan- 
 tity as product or, in other words, that the combining weights 
 of such substances are those weights which require equal ac- 
 cessions or abstractions of heat, equally to raise or lower their 
 temperature. To put the proposition more in accordance with 
 the view we have taken of the nature of heat : each body has 
 a power of communicating or receiving molecular repulsive 
 power, exactly equal, weight for weight, to its chemical or 
 combining power. For instance, the equivalent of lead is 104, 
 of zinc 33, or in round numbers, as 3 to i: these numbers are 
 therefore inversely the exponents of their chemical power, 
 three times as much lead as zinc being required to saturate 
 the same quantity of an acid or substance combining with it ; 
 but their power of communicating or abstracting heat or re- 
 pulsive power is precisely the same, for three times as much 
 lead as zinc is required to produce the same amount of expan- 
 sion or contraction in a given quantity of a third substance, 
 such as water. 
 
 Again, a great number of bodies chemically combine in 
 equal volumes, i.e. in the ratios of their specific gravities ; but 
 the specific gravities represent the attractive powers of the 
 substances, or are the numerical exponents of the forces 
 tending to produce motion in masses of matter of equal 
 volume towards each other ; while the chemical equivalents 
 are the exponents of the affinities or tendencies of the mole- 
 cules of dissimilar substances to combine and saturate each 
 other ; consequently, here we have in certain cases an equi- 
 valent relation between these two modes of force gravitation 
 and chemical attraction. 
 
 Were the above relations extended into an universal law, 
 we should have the same numerical expression for the three 
 forces of heat, gravity, and affinity ; and as electricity and 
 magnetism are quantitatively related to them, we should 
 have a similar expression for these forces ; but at present the 
 
CONCLUDING REMARKS. 173 
 
 bodies in which this parity of force has been discovered, 
 though in themselves numerous, are small compared with the 
 exceptions, and, therefore, this point can only be indicated as 
 promising a generalisation, should subsequent researches alter 
 our knowledge as to the elements and combining equivalents 
 of matter. 
 
 With regard to what may be called dynamic equivalents, 
 i.e. the definite relation of the motive action of these varied 
 forces upon equivalents of matter, the difficulty of establish- 
 ing them is still greater. If the proposition which I stated at 
 the commencement of this paper be correct, that motion may 
 be subdivided or changed in character, so as to become heat, 
 electricity, &c., it ought to follow that when we collect the 
 dissipated and changed forces, and re-convert them, the initial 
 motion, minus an infinitesimal quantity, affecting the same 
 amount of matter with the same velocity, should be repro- 
 duced, and so of the changes in matter produced by the other 
 forces ; but the difficulties of proving the truth of this by 
 experiment will, in many cases, be all but insuperable ; we 
 cannot imprison motion as we can matter, though we may 
 to some extent restrain its direction. 
 
 The term perpetual motion, which I have not unfrequently 
 employed in these pages, is itself equivocal. If the doctrines 
 here advanced be well founded, all motion is, in one sense, per- 
 petual. In masses whose motion is stopped by mutual con- 
 cussion, heat or motion of the particles is generated ; and thus 
 the motion continues, so that if we could venture to extend 
 such thoughts to the universe, we should assume the same 
 amount of motion affecting the same amount of matter for 
 ever. Where force is made to oppose force, and produce 
 static equilibrium, the balance of pre-existing equilibrium is 
 affected, and fresh motion is started equivalent to that which 
 is withdrawn into a state of abeyance. 
 
 But the term perpetual motion is applied, in ordinary 
 parlance (and in such sense I have used it), to a perpetual 
 recurrent motion, e.g., a weight which by its fall would turn a 
 wheel, which wheel would, in its turn, raise the initial weight, 
 and so on for ever, or until the material of which the machine 
 is made be worn out. It is strange that to common appre- 
 
1 74 CORRELA TION OF PHYSICAL FOR CES. 
 
 hension the impossibility of this is not self-evident : if the 
 initial weight is to be raised by the force it has itself gene- 
 rated, it must necessarily generate a force greater than that 
 of its own weight or centripetal attraction ; in other words, it 
 must be capable of raising a weight heavier than itself ; so 
 that, setting aside the resistance of friction, &c., a weight, to 
 produce perpetual recurrent motion, must be heavier than an 
 equal weight of matter, in short, heavier than itself. 
 
 Suppose two equal weights at each end of an equi-armed 
 lever, there is no motion ; cut off a fraction of one of them, 
 and it rises while the other falls. How, now, is the lesser 
 weight to bring back the greater without any extraneous 
 application of force ? If, as is obvious, it cannot do so in this 
 simple form of experiment, it is a fortiori more impossible if 
 machinery be added, for increased resistances have then to 
 be overcome. Can we again mend this by employing any 
 other force ? Suppose we employ electricity, the initial 
 weight in descending turns a cylinder against a cushion, and 
 so generates electricity ; to make this force recurrent, the 
 electricity so generated must, in its turn, raise the initial 
 weight, or one heavier than it, i.e. the initial weight must, 
 through the medium of the electricity it generates, raise a 
 weight heavier than itself. The same problem, applied to any 
 other forces, will involve the same absurdity ; and yet, simple 
 as the matter seems, the world is hardly yet disabused of an 
 idea little removed from superstition. 
 
 But the importance of the deductions to be derived from 
 the negation of perpetual motion seems scarcely to have 
 impressed philosophers, and we only find here and there a 
 scattered hint of the consequences necessarily resulting from 
 that which to the thinking mind is a conviction. Some of 
 these I have ventured to put forward in the present Essay, 
 but many remain, and will crowd upon the mind of those who 
 pursue the subject. Does not, for instance, the impossibility 
 of perpetual motion, when thought out, involve the de- 
 monstration of the impossibility, to which I have previously 
 alluded, of any event identically recurring ? 
 
 The pendulum in vacua, at each beat, leaves a portion of 
 the force which started it, in the form of heat at its point of 
 
CONCLUDING REMARKS. 175 
 
 suspension ; this force, though ever existent, can never be 
 restored in its integrity to the ball of the pendulum, for in 
 the process of restoration it must affect other matter, and 
 alter the condition of the universe. To restore the initial 
 force in its integrity, everything as it existed at the moment 
 of the first beat of the pendulum must be restored in its 
 integrity; but how can this be? for while the force was 
 escaping from the pendulum by radiating heat from the point 
 of suspension, surrounding matter has not stood still ; the 
 very attraction which caused the beat of the pendulum has 
 changed in degree, for the pendulum is nearer to or farther 
 from the sun, or from some planet or fixed star. 
 
 It might be an interesting and not profitless speculation 
 to follow out as far as may be these and other consequences : 
 it would, I believe, lead us to the conviction that the universe 
 is ever changing, and that, notwithstanding secular recurrences 
 which would, primd facie, seem to replace matter in its original 
 position, nothing, in fact, ever returns or can return to a state 
 of existence identical with a previous state. But the field of 
 enquiry is illimitable. 
 
 The inevitable dissipation or throwing off a portion of the 
 initial force presents a great experimental difficulty in the 
 way of establishing the equivalents of the various natural 
 forces. In the steam-engine, for instance, the heat of the 
 furnace not only expands the water and thereby produces 
 the motion of the piston, but it also expands the iron of the 
 boiler of the cylinder and all surrounding bodies. The force 
 expended in expanding this iron to a very small extent is 
 equal to that which expands the vapour to a very large 
 extent : this expansion of the iron is capable, in its turn, of 
 producing a great mechanical force, which is practically lost. 
 Could all the force be applied to the vapour, an enormous 
 addition of power would be gained for the same expenditure ; 
 and perhaps, even with our present means, more might be 
 done in utilising the expansion of the iron. 
 
 Another great difficulty in experimentally ascertaining the 
 dynamic equivalents of different forces arises from the effects 
 of disruption, elongation, or the overcoming an existing force. 
 Thus, when a part of the initial force employed is engaged in 
 
176 CORRELATION OF PHYSICAL FORCES. 
 
 twisting or tearing asunder matter previously held together by 
 cohesive attraction, or in overcoming gravitation or inertia, 
 the same amount of heat or electricity would not apparently 
 be evolved as if such obstacle were non-existent, and the 
 initial force were wholly employed in producing, not in 
 opposing. There is great difficulty in devising experiments 
 in which some portion of the force is not so employed. 
 
 The initial force, however, that has been employed for such 
 disruption is not lost, as at the moment of disruption the 
 bodies producing it fly off, and carry with them their force. 
 Thus, let two weights be attached to a cord placed across a 
 bar.; when their force is sufficient to break the cord or the 
 bar, the weights fall and strike the earth, making it vibrate, 
 and so conveying away or continuing the force which the 
 bar or cord had been acting on. If, instead of breaking a 
 cord, the weights be employed to bend a bar, their gravitating 
 force, instead of making the earth vibrate, produces heat in 
 the bar, and so with whatever other force may be employed 
 to produce effects of disruption, torsion, &c., so that, though 
 difficult in practice, the numerical problem of the equivalent 
 of the force is not theoretically irresolvable. 
 
 The voltaic battery affords us the best means of ascertaining 
 the dynamic equivalents of different forces, and it is probable 
 that by its aid the best theoretical and practical results will 
 be ultimately attained. 
 
 In investigating the relation of the different forces, I have 
 in turn taken each one as the initial force or starting-point, 
 and endeavoured to show how the force thus arbitrarily 
 selected could mediately or immediately produce and be 
 merged into the others ; but it will be obvious to those who 
 have attentively considered the subject, and brought their 
 minds into a general accordance with the views I have sub- 
 mitted to them, that no force can, strictly speaking, be initial, 
 as there must be some anterior force which produced it : we 
 cannot create force or motion any more than we can create 
 matter. 
 
 Thus, to take an example previously noticed, and trace it 
 backwards : the spark of light is produced by electricity, 
 electricity by motion, and motion is produced by something 
 
CONCLUDING REMARKS. 177 
 
 else, say a steam-engine that is, by heat. This heat is pro- 
 duced by chemical affinity, i.e. the affinity of the carbon of the 
 coal for the oxygen of the air : this carbon and this oxygen 
 have been previously eliminated by the heat of the sun, or by 
 other actions difficult to trace, but of the pre-existence of 
 which we cannot doubt, and in which actions we should find 
 the conjoint and alternating effects of heat, light, chemical 
 affinity, &c. Thus, tracing any force backwards to its ante- 
 cedents, we are merged in an infinity of changing forms of 
 force ; at some point we lose it, not because it has been in 
 fact created at any definite point, but because it resolves 
 itself into so many contributing forces, that the evidence of 
 it is lost to our senses or powers of detection ; just as, in 
 following it forward into the effect it produces, it becomes, 
 as I have before stated, so subdivided and dissipated as to be 
 equally lost to our means of detection. 
 
 Can we, indeed, suggest a proposition, definitely conceivable 
 by the mind, of force without antecedent force ? I cannot, 
 without calling for the interposition of creative power, any 
 more than I can conceive the sudden appearance of a mass of 
 matter come from nowhere, and formed from nothing. The 
 impossibility, humanly speaking, of creating or annihilating 
 matter, has long been admitted, though, perhaps, its distinct 
 reception in philosophy may be set down to the overthrow of 
 the doctrine of Phlogiston, and the reformation of chemistry 
 at the time of Lavoisier. The reasons for the admission of a 
 similar doctrine as to force appear to be equally strong. With 
 regard to matter, there are many cases in which we never 
 practically prove its cessation of existence, yet we do not the 
 less believe in it : who, for instance, can trace, so as to re- 
 weigh, the particles of iron worn off the tire of a carriage 
 wheel ? who can re-combine the particles of wax dissipated 
 and chemically changed in the burning of a candle? By 
 placing matter undergoing physical or chemical changes under 
 special limiting circumstances, we may, indeed, acquire evi- 
 dence of its continued existence, weight for weight and so 
 we may in some instances of force, as in definite electrolysis ; 
 indeed, the evidence we acquire of the continued existence of 
 matter is by the continued exertion of the force it exercises, 
 
 N 
 
178 CORRELATION OF PHYSICAL FORCES. 
 
 as, when we weigh it, our evidence is the force of attraction ; 
 so, again, our evidence of force is the matter it acts upon. 
 Thus, matter and force are correlates, in the strictest sense of 
 the word ; the conception of the existence of the one involves 
 the conception of the existence of the other : the quantity of 
 matter again, and the degree of force, involve conceptions of 
 space and time. But to follow out these abstract relations 
 would lead me too far into the alluring paths of metaphysical 
 speculation. 
 
 That the theoretical portions of this Essay are open to 
 objection I am fully conscious. I cannot, however, but think 
 that the fair way to test a theory is to compare it with other 
 theories, and to see whether, upon the whole, the balance of 
 probability is in its favour. Were a theory open to no objec- 
 tion it would cease to be a theory, and would be called a law ; 
 and were we not to theorise, or to take generalised views of 
 natural phenomena until those generalisations were sure and 
 unobjectionable, science would be lost in a complex mass of 
 unconnected observations, which would probably never disen- 
 tangle themselves. Excess on either side is to be avoided ; 
 although we may often err on the side of hasty generalisation, 
 we may equally err on the side of mere elaborate collection 
 of observations, which, though sometimes leading to a valu- 
 able result, yet, when cumulated without a connecting link, 
 frequently occasion a costly waste of time, and leave the 
 subject to which they refer in greater obscurity than that in 
 which it was involved at their commencement. 
 
 Collections of facts differ in importance, as do theories : 
 the former, in many instances, derive their value from their 
 capability of generalisation ; while, conversely, theories are 
 valuable as methods of co-ordinating given series of facts, and 
 more valuable in proportion as they require fewer exceptions 
 and fewer postulates. Facts may sometimes be as well ex- 
 plained by one view as by another, but without a theory they 
 are unintelligible and incommunicable. Let us use our utmost 
 effort to communicate a fact without using the language of 
 theory, and we fail ; theory is involved in all our expressions ; 
 the knowledge of bygone times is imported into succeeding 
 times by terms involving theoretic conceptions. As the know- 
 
CONCLUDING REMARKS. 179 
 
 ledge of any particular science developes itself, our views of 
 it become more simple ; hypotheses, or the introduction of 
 supposititious views, are more and more dispensed with ; 
 words become applicable more directly to the phenomena, 
 and, losing the hypothetic meaning which they necessarily 
 possessed at their reception, acquire a secondary sense, which 
 brings more immediately to our minds the facts of which they 
 are indices. The scaffolding has served its purpose. The 
 hypothesis fades away, and a theory, or generalised view of 
 phenomena, more independent of supposition, but still full of 
 gaps and difficulties, takes its place. This in its turn, should 
 the science continue to progress, either gives place to a more 
 simple and wider generalisation, or becomes, by the removal 
 of objections, established as what is termed a law. Even in 
 this more advanced stage words importing theory must be 
 used, but phenomena are now intelligible and connected, 
 though expressed by varied forms of speech. 
 
 To think on nature is to theorise ; and difficult it is not to 
 be led on by the continuities of natural phenomena to theories 
 which appear forced and unintelligible to those who have not 
 pursued the same path of thought ; which, moreover, if allowed 
 to gain an undue influence, seduce us from that truth which 
 is the sole object of our pursuit. 
 
 Where to draw the line where to say thus far we may 
 go, and no farther, in any particular class of analogies or rela- 
 tions which Nature presents to us ; how far to follow the 
 progressive indications of thought, and where to resist its 
 allurements, is a question of degree which must depend upon 
 the judgment of each individual or of each class of thinkers ; 
 yet it is consolatory that thought is seldom expended in vain. 
 
 I have throughout endeavoured to discard the hypotheses 
 of subtle or occult entities ; if in this endeavour some of my 
 views have been adopted upon insufficient data, I still hope 
 that this Essay will not prove valueless. 
 
 The conviction that the so-called imponderables are modes 
 of motion, will, at all events, lead the observer of natural 
 phenomena to look for changes in these affections, wherever 
 the intimate structure of matter is changed ; and, conversely, 
 to seek for changes in matter, either temporary or permanent, 
 
 N 2 
 
i8o CORRELATION OF PHYSICAL FORCES. 
 
 whenever it is affected by these forces. I believe he will 
 seldom do this in vain. It was not until I had long reflected 
 on the subject that I ventured to publish my views ; their 
 publication may induce others to think on their subject- 
 matter. They are not put forward with the same objects, nor 
 do they aim at the same elaboration of detail, as memoirs on 
 newly-discovered physical facts : they purport to be a method 
 of mentally regarding known facts, some few of which I have 
 myself made known on other occasions, but the great mass of 
 which have been accumulated by the labours of others, and 
 are admitted as established truths. Everyone has a right to 
 view these facts through any medium he thinks fit to employ, 
 but some theory must exist in the minds of those who reflect 
 upon the many new phenomena which have recently, and more 
 particularly during the present century, been discovered. It 
 is by a generalised or connected view of past acquisitions in 
 natural knowledge that deductions can best be drawn as to 
 the probable character of the results to be anticipated. It is 
 a great assistance in such investigations to be intimately con- 
 vinced that no physical phenomenon can stand alone : each is 
 inevitably connected with anterior changes, and as inevitably 
 productive of consequential changes, each with the other, and 
 all with time and space ; and, either in tracing back these 
 antecedents or following up their consequents, many new 
 phenomena will be discovered, and many existing phenomena 
 hitherto believed distinct will be connected and explained : 
 explanation is, indeed, only relation to something more 
 familiar, not more known i.e. known as to causative or crea- 
 tive agencies. In all phenomena the more closely they are 
 investigated the more are we convinced that, humanly speak- 
 ing, neither matter nor force can be created or annihilated, and 
 that an essential cause is unattainable. 
 
CONTINUITY. 
 
 ADDRESS AS PRESIDENT OF THE BRITISH ASSOCIA- 
 TION FOR THE ADVANCEMENT OF SCIENCE. 
 
 Nottingham, 1866. Third Edition. 
 
 IF our rude predecessors, who at one time inhabited the 
 caverns which surround this town, could rise from their graves 
 and see it in its present state, it may be doubtful whether 
 they would have sufficient knowledge to be surprised. 
 
 The machinery, almost resembling organic beings in deli- 
 cacy of structure, by which are fabricated products of world- 
 wide reputation, the powers of matter applied to give motion 
 to that machinery, are so far removed from what must have 
 been the conceptions of the semi-barbarians to whom I have 
 alluded, that they could not look on them with intelligent 
 wonder. 
 
 Yet this immense progress has all been effected step by 
 step, now and then a little more rapidly than at other times ; 
 but, viewing the whole course of improvement, it has been 
 gradual, though moving in an accelerated ratio. But it is not 
 merely in those branches of natural knowledge which tend 
 to improvements in economical arts and manufactures that 
 science has made great progress. In the study of our own 
 planet and the organic beings with which it is crowded, and 
 in so much of the universe as vision, aided by the telescope, 
 has brought within the area of observation, the present cen- 
 tury has surpassed any antecedent period of equal duration. 
 
 It would be difficult to trace out all the causes which have 
 led to the increase of observational and experimental know- 
 ledge. 
 
 Among the more thinking portion of mankind the gratifi- 
 
1 82 CONTINUITY. 
 
 cation felt by the discovery of new truths, the expansion of 
 faculties, and extension of the boundaries of knowledge, have 
 been doubtless a sufficient inducement to the study of nature ; 
 while, on more practical minds, the reality, the certainty, the 
 progressive character of the acquisitions of natural science, 
 and the enormously increased means which its applications 
 give, have impressed its importance as a minister to daily 
 wants and a contributor to ever-increasing material comforts, 
 luxury, and power. 
 
 Though by no means the only one, yet an important cause 
 of the rapid advance of science is the growth of associations 
 for promoting the progress either of physical knowledge 
 generally, or of special branches of it. Since the foundation 
 of the Royal Society, now more than two centuries ago, a 
 vast number of kindred societies have sprung up in this 
 country and in Europe. The advantages conferred by these 
 societies are manifold : they enable those who are devoted to 
 scientific research to combine, compare, and check their obser- 
 vations ; to assist, by the thoughts of several minds, the pro- 
 motion of the enquiry undertaken ; they contribute from a 
 joint purse to such efforts as their members deem most worthy ; 
 they afford a means of submitting to a competent tribunal 
 notices and memoirs, and of obtaining for their authors and 
 others, by means of the discussions which ensue, information 
 given by those best informed on the particular subject ; they 
 enable the author to judge whether it is worth his while to 
 pursue the subjects he has brought forward ; and they defray 
 the expense of printing and publishing such researches as are 
 thought meritorious. 
 
 These advantages, and others might be named, pertain to 
 the Association the thirty-sixth meeting of which we are this 
 evening assembled to inaugurate; but it has, from its inter- 
 mittent and peripatetic character, advantages which belong 
 to none of the societies which are fixed as to their locality. 
 
 Among these are the novelty and freshness of an annual 
 meeting, which, while it brings together old members of the 
 Association, many of whom only meet on this occasion, 
 always adds a quota of new members, infusing fresh blood, 
 and varying the social character of our meetings. 
 
CONTINUITY. 183 
 
 The visits of distinguished foreigners, whom we have pre- 
 viously known by reputation, is one of the most delightful 
 and improving of the results. The wide field of enquiry, and 
 the character of communications made to the Association, 
 including all branches of natural knowledge, and varying 
 from simple notices of an interesting observation or experi- 
 ment, to the most intricate and refined expositions of scientific 
 research, is another valuable characteristic. 
 
 Lastly, perhaps the greatest advantage resulting from the 
 annual visits of this great parliament to new localities is, that 
 w r hile it imparts fresh local and general knowledge to the 
 visitors, it leaves behind stimulating memories, which rouse 
 into permanent activity dormant or timid minds an effect 
 which, so far from ceasing with the visit of the Association, 
 frequently begins when that visit terminates. 
 
 Every votary of physical science must be anxious to see 
 it recognised by those institutions of his country which can to 
 the greatest degree promote its cultivation and reap from it 
 the greatest benefit. You will probably agree with me that 
 the principal educational establishments on the one hand, and 
 on the other the Government, in many of its departments, are 
 the institutions which may best fulfil these conditions. The 
 more early the mind is trained to a pursuit of any kind, the 
 deeper and more permanent are the impressions received, and 
 the more service can be rendered by the students. 
 
 . ' Quo semel est imbuta recens, servabit odorem 
 Testa diu.' 
 
 Little can be achieved in scientific research without an 
 acquaintance with it in youth ; you will rarely find an instance 
 of a man who has attained any eminence in science who has 
 not commenced its study at a very early period of life. 
 Nothing, again, can tend more to the promotion of science 
 than the exertions of those who have early acquired the rjdos 
 resulting from a scientific education. I desire to make no 
 complaint of the tardiness with which science has been 
 received at our public schools, and, with some exceptions, at 
 our universities. These great establishments have their roots 
 in historical periods, and long time and patient endeavour are 
 
1 84 CONTINUITY. 
 
 requisite before a new branch of thought can be grafted with 
 success on a stem to which it is exotic. Nor should I ever 
 wish to see the study of languages, of history, of all those 
 refined associations which the past has transmitted to us, 
 neglected ; but there is room for both. It is sad to see the 
 number of so-called educated men who, travelling by railway, 
 voyaging by steamboat, consulting the almanac for the time 
 of sunrise or full-moon, have not the most elementary know- 
 ledge of a steam-engine, a barometer, or a quadrant ; and 
 who will listen with a half-confessed faith to the most idle pre- 
 dictions as to weather or cometic influences, while they are in 
 a state of crass ignorance as to the cause of the trade-winds, 
 or the form of a comet's path. May we hope that the slight 
 infiltration of scientific studies, now happily commenced, will 
 extend till it occupies its fair space in the education of the 
 young, and that those who may be able learnedly to discourse 
 on the Eolic digamma will not be ashamed of knowing the 
 principles on which the action of an air-pump, an electrical 
 machine, or a telescope depend, and will not, as Bacon com- 
 plained of his contemporaries, despise such knowledge as 
 something mean and mechanical ? 
 
 To assert that the great departments of Government should 
 encourage physical science may appear a truism, and yet it is 
 but of late that it has been seriously done. Now the habit of 
 consulting men of science on important questions of national 
 interest is becoming a recognised practice ; and in a time, which 
 may seem long to individuals, but is short in the history of a 
 nation, a more definite sphere of usefulness for national pur- 
 poses will, I have no doubt, be provided for those duly quali- 
 fied men who may be content to give up the more tempting 
 study of abstract science for that of its practical applications. 
 In this respect the report of the Kew Committee for this year 
 affords a subject of congratulation to those whom I have the 
 honour to address. The Kew Observatory, the petted child of 
 the British Association, may possibly become an important 
 national establishment ; and if so, while it will not, I trust, 
 lose its character of a home for untrammelled physical 
 research, it will have superadded some of the functions of the 
 
CONTINUITY. 185 
 
 Meteorological Department of the Board of Trade, with a 
 staff of skilful and experienced observers. 
 
 This is one of the results which the general growth of 
 science, and the labours of this Association in particular, have 
 produced ; but I do not propose on this occasion to recapitulate 
 the special objects attained by the Association ; this has been 
 amply done by several of my predecessors ; nor shall I con- 
 fine my address to the progress made in physical science since 
 the time when my most able and esteemed friend and prede- 
 cessor addressed you at Birmingham. In the various reports 
 and communications which will be read at your sections, details 
 of every step which has been made in science since our last 
 meeting will be brought to your notice, and I have no doubt 
 fully and freely discussed. 
 
 I purpose, with your kind permission, to submit to you 
 certain views of what has within a comparatively recent period 
 been accomplished by science, what have been the steps 
 leading to the attained results, and what, as far as we may 
 fairly form an opinion, is the general character pervading 
 modern discovery. 
 
 It seems to me that the object we have in view would be 
 more nearly approached, by each President, chosen as they 
 are in succession as representing different branches of science, 
 giving on these occasions either an account of the progress of 
 the particular branch of science he has cultivated, when that 
 is not of a very limited and special character, or enouncing 
 his own view of the general progress of science ; and though 
 this will necessarily involve much that belongs to recent years, 
 the confining a President to a mere resume of what has taken 
 place since our last meeting would, I venture with diffidence 
 to think, limit his means of usefulness, and render his dis- 
 course rather an annual register than an instructive essay. 
 
 I need not dwell on the commonplace but yet important 
 topics of the material advantages resulting from the applica- 
 tion of science ; I will address myself to what, in my humble 
 judgment, are the lessons we have learned, and the probable 
 prospects of improved natural knowledge. 
 
 One word will give you the key to what I am about to 
 
1 86 CONTINUITY. 
 
 discourse on ; that word is continuity no new word, and used 
 in no new sense, but perhaps applied more generally than it 
 has hitherto been. We shall see, unless I am much mistaken, 
 that the development of observational, experimental, and even 
 deductive knowledge is either attained by steps extremely 
 small and forming really a continuous march ; or, when 
 distinct results apparently separate from any co-ordinate phe- 
 nomena have been attained, that then, by the subsequent 
 progress of science, intermediate links have been discovered 
 uniting the apparently segregated instances with other more 
 familiar phenomena. We shall see that the more we inves- 
 tigate, the more we find that in existing phenomena graduation 
 from the like to the seemingly unlike prevails, and in the 
 changes which take place in time gradual progress is, and 
 apparently must be, the course of nature. 
 
 Let me now endeavour to apply this view to the recent 
 progress of some of the more prominent branches of science. 
 
 In Astronomy, from the time when the earth was consi- 
 dered a flat plain bounded by a flat ocean when the sun, 
 moon, and stars were regarded as lamps to illuminate this 
 plain each successive discovery has brought with it similitudes 
 and analogies between this earth and many of the objects of 
 the universe, with which our senses, aided by instruments, 
 have made us acquainted. I pass, of course, over those dis- 
 coveries which have established the Copernican system as 
 applied to our sun, its attendant planets, and their satellites. 
 The proofs, however, that gravitation is not confined to our 
 solar system, but pervades the universe, have received many 
 confirmations by the labours of members of this Association. 
 I may name those who have held the office of President, Lord 
 Rosse, Lord Wrottesley, and Sir J. Herschel, the two latter 
 having devoted special attention to the orbits of double stars, 
 the former to those probably more recent systems called 
 nebulae. Double stars seem to be orbs analogous to our own 
 sun, and revolving round their common centre of gravity in a 
 conic-section curve, as do the planets with which we are more 
 intimately acquainted ; but the nebulae present more difficulty, 
 and some doubt has been expressed whether gravitation, as 
 we understand it, acts with those bodies (at least those exhi- 
 
CONTINUITY. 187 
 
 biting a spiral form) as it does with us ; possibly some other 
 modifying influence may exist, our present ignorance of which 
 gives rise to the apparent difficulty. There is, however, 
 another class of observations quite recent in its importance, and 
 which has formed a special subject of contribution to the 
 reports and transactions of this Association ; I allude to those 
 on Meteorites, at which our lamented member, and to many of 
 us our valued friend, Prof. Baden Powell, assiduously laboured, 
 for investigations into which a Committee of this Association is 
 formed ; and a series of star-charts for enabling observers of 
 shooting-stars to record their observations was laid before the 
 last meeting of the Association by Mr. Glaisher. 
 
 It would occupy too much of your time to detail the efforts 
 of Bessel, Schwinke, the late Sir J. Lubbock, and others, as 
 applied to the formation of star-charts for aiding the observa- 
 tion of meteorities which Mr. Alexander Herschel, Mr. 
 Brayley, Mr. Sorby, and others are now studying. 
 
 Dr. Olmsted explained the appearance of a point from 
 which the lines of flight of meteors seem to radiate, as being the 
 perspective vanishing-point of their parallel or nearly parallel 
 courses, appearing divergent to an observer on the earth as 
 they approach it. The uniformity of position of these radiant 
 points, the many corroborative observations on the direction, 
 the distances, and the velocities of these .bodies, the circum- 
 stance that their paths intersect the earth's orbit at certain 
 definite periods, and the total failure of all other theories 
 which have been advanced, while there is no substantial objec- 
 tion to this, afford evidence almost amounting to proof that 
 these are cosmical bodies moving in the interplanetary space by 
 gravitation around the sun, and some perhaps around planets. 
 This view gives us a new element of continuity. The universe 
 would thus appear not to have the extent of empty space for- 
 merly attributed to it, but to be studded between the larger 
 and more visible masses with smaller planets, if the term be 
 permitted to be applied to meteorites. 
 
 Observations are now made at the periods at which me- 
 teors appear in greatest numbers at Greenwich, by Mr. 
 Glaisher ; at Cambridge, by Prof. Adams ; and at Hawkhurst, 
 by Mr. Alexander Herschel ; and every preparation is made 
 
1 88 CONTINUITY. 
 
 to secure as much accuracy as can, in the present state of 
 knowledge, be attained by such observations. 
 
 The number of known asteroids, or bodies of a smaller 
 size than what are termed the ancient planets, has been so 
 increased by numerous discoveries, that instead of seven we 
 now count eighty-eight as the number of recognised planets, 
 a field of discovery with which the name of Hind will be 
 ever associated. 
 
 The smallest of these is only twenty or thirty miles in 
 diameter, indeed cannot be accurately measured ; and if we 
 were to apply the same scrutiny to other parts of the hea- 
 vens as. has been applied to the zone between Mars and 
 Jupiter, it is no far-fetched speculation to suppose that, in 
 addition to asteroids and meteorites, many other bodies 
 exist, so that the space occupied by our solar system may be 
 dotted up with planetary bodies varying in size from that of 
 Jupiter (1,240 times larger in volume than the earth) to that 
 of a cannon-ball or even a pistol-bullet. 
 
 The researches of Leverrier on the intra-Mercurial planets 
 come in aid of these views ; and another half-century may, 
 and not improbably will, enable us to ascertain that the now 
 seemingly vacant interplanetary spaces are occupied by smaller 
 bodies, which have hitherto escaped observation, just as the 
 asteroids had until the time of Olbers and Piazzi. But the 
 evidence of continuity as pervading the universe does not stop 
 at the results of telescopic observation ; chemistry and physical 
 optics bring us new proofs. Those meteoric bodies which 
 have from time to time come so far within reach of the earth's 
 attraction as to fall upon its surface, give on analysis metals 
 and oxides similar to those which belong to the structure 
 of the earth they come as travellers bringing specimens of 
 minerals from extra-terrestrial regions. 
 
 In a series of papers recently communicated to the French 
 Academy, M. Daubree has discussed the chemical and mine- 
 ralogical character of meteorites as compared with the rocks 
 of the earth. He finds that the similarity of terrestrial rocks 
 to meteorites increases as we penetrate deeper into the earth's 
 crust, and that some of the deep-seated minerals have a com- 
 position and characteristics almost identical with meteorites 
 
CONTINUITY. 189 
 
 [olivine, herzolite, and serpentine, for instance, closely resemble 
 them] ; that as we approach the surface rocks having similar 
 components with meteorites are found, but in a state of oxida- 
 tion, which necessarily much modifies their mineral character, 
 and which, by involving secondary oxygenised compounds, 
 must also change their chemical constitution. By experiments 
 he has succeeded in forming from terrestrial rocks substances 
 very much resembling meteorites. Thus close relationship, 
 though by no means identity of composition, is established 
 between this earth and those wanderers from remote regions, 
 some evidence, though at present incomplete, of a common 
 origin. 
 
 Surprise has often been expressed that, while the mean 
 specific gravity of this globe is from five to six times that of 
 water, the mean specific gravity of its crust is barely half as 
 great. It has long seemed to me that there is no ground for 
 wonder here. The exterior of our planet is to a considerable 
 depth oxidated ; the interior is in all probability free from 
 oxygen, and whatever bodies exist there are in a reduced or 
 deoxidated state ; if so, the specific gravity of most of them must 
 necessarily be higher than that of their oxides or chlorides, &c. ; 
 we find, moreover, that many of the deep-seated minerals have 
 a higher specific gravity than the average of those on the sur- 
 face ; olivine, for instance, has a specific gravity of 3*3. There 
 is therefore no a priori improbability that the mean specific 
 gravity of the earth should notably exceed that of its surface ; 
 and if we go farther, and suppose the interior of the earth to be 
 formed of the same ingredients as the exterior, minus oxygen, 
 chlorine, bromine, &c., a specific gravity of 5 to 6 would not 
 be an unlikely one. Many of the elementary bodies entering 
 largely into the formation of the earth's crust are as light or 
 lighter than water for instance, potassium, sodium, &c. ; 
 others, such as sulphur, silicon, aluminium, have from two to 
 three times its specific gravity ; others, again, as iron, copper, 
 zinc, tin, seven to nine times ; while others, lead, gold, plati- 
 num, &c., are much more dense ; but, speaking generally, the 
 more dense are the least numerous. There seems no impro- 
 bability in a mixture of such substances producing a mean 
 specific gravity of from 5 to 6, although it by no means 
 
190 CONTINUITY. 
 
 follows, indeed the probability is rather the other way, that the 
 proportions of the substances in the interior of the earth are 
 the same as on the exterior. It might be worth the labour to 
 ascertain the mean specific gravity of all the known minerals 
 on the earth's surface, averaging them on the ratios in which, 
 as far as our knowledge goes, they quantitatively exist, and 
 assuming them to exist without the oxygen, chlorine, &c., 
 with which they are, with some rare exceptions, invariably 
 combined on the surface of the earth : great assistance to the 
 knowledge of the probable constitution of the earth might be 
 derived from such an investigation. 
 
 While chemistry, analytic and synthetic, thus aids us in 
 ascertaining the relationship of our planet to meteorites, its 
 relation in composition to other planets, to the sun, and to 
 more distant suns and systems, is aided by another science, 
 viz. optics. 
 
 That light passing from one transparent medium to another 
 should carry with it evidence of the source from which it 
 emanates, would, until lately, have seemed an extravagant 
 supposition ; but probably (could we read it) everything con- 
 tains in itself a large portion of its own history. 
 
 I need not detail to you the discoveries of KirchhofT, Bun- 
 sen, Miller, Huggins, and others ; they have been dilated on 
 by my predecessor. Assuming that spectrum analysis is a 
 reliable indication of the presence of given substances by the 
 position of transverse bright lines exhibited when they are 
 burnt, and of transverse dark lines when light is transmitted 
 through their vapours, though Pliicker has shown that with 
 some substances these lines vary with temperature, the point 
 of importance in the view I am presenting to you is, that 
 while what may be called comparatively neighbouring cosmi- 
 cal bodies exhibit lines identical with many of those shown by 
 the components of this planet, as we proceed to the more dis- 
 tant appearances of the nebulae we get but one or two of such 
 lines, and we get one or two new lines not yet identified with 
 any known to be produced by substances on this globe. 
 
 Within the last year Mr. Huggins has added to his former 
 researches observations on the spectrum of a comet (comet I 
 of 1866), the nucleus of which shows but one bright line, while 
 
CONTINUITY. 191 
 
 the spectrum formed by the light of the coma is continuous, 
 seeming to show that the nucleus is gaseous, while the coma 
 would consist of matter in a state of minute division shining 
 by reflected light : whether this be solid, liquid, or gaseous is 
 doubtful, but the author thinks it is in a condition analogous 
 to that of fog or cloud. The position in the spectrum of the 
 bright line furnished by the nucleus is the same as that of 
 nitrogen, which line is also shown in some of the nebulae. 
 
 But the most remarkable achievement by spectrum ana- 
 lysis is the record of observations on a temporary star which 
 has shone forth this year in the constellation of the Northern 
 Crown, about a degree S.E. of the star e. When it was first 
 seen, May 12, it was nearly equal in brilliancy to a star of the 
 second magnitude ; when observed by Mr. Huggins and Dr. 
 Miller, May 16, it was reduced to the third or fourth magni- 
 tude. Examined by these observers with the spectroscope, it 
 gave a spectrum which they state was unlike that of any 
 celestial body they had examined. 
 
 The light was compound, and had emanated from two dif- 
 ferent sources. One spectrum was analogous to that of the 
 sun, viz. formed by the light of an incandescent solid or liquid 
 photosphere which had suffered absorption by the vapours of 
 an envelope cooler than itself. The second spectrum consisted 
 of a few bright lines, which indicated that the light by which 
 it was formed was emitted by matter in a state of luminous 
 gas. The observers consider that, from the position of two of 
 the bright lines, the gas must probably be hydrogen, and from 
 their brilliancy compared with the light of the photosphere 
 the gas must have been at a very high temperature. They 
 imagine the phenomena to result from the burning of hy- 
 drogen with some other element, and that from the resulting 
 temperature the photosphere is heated to incandescence. 
 
 There is strong reason to believe that this star is one pre- 
 viously seen by Argelander and Sir J. Herschel, and that it is 
 a variable star of long or irregular period ; it is also notable 
 that some of its spectrum lines correspond with those of several 
 variable stars. The time of its appearance was too short for 
 any attempt to ascertain its parallax ; it would have been im- 
 portant if it could even have been established that it is not a 
 
iQ2 CONTINUITY. 
 
 near neighbour, as the magnitude of such a phenomenon must 
 be inverse to its distance. I forbear to add any speculations 
 as to the cause of this most singular phenomenon. However 
 imperfect the knowledge given us by these observations, it is 
 a great triumph to have caught this fleeting object, and ob- 
 tained permanent records for the use of future observers. 
 
 It would seem as if the phenomenon of gradual change 
 obtained towards the remotest objects with which we are at 
 present acquainted, and that the farther we penetrate into space 
 the more unlike to those we are acquainted with become the 
 objects of our examination sun, planets, meteorites, earth, 
 similarly though not identically constituted, stars differing from 
 each other and from our system, and nebulae more remote in 
 space and differing more in their characters and constitution. 
 
 While we can thus to some extent investigate the physical 
 constitution of the most remote visible substances, may we not 
 hope that some farther insight as to the constitution of the 
 nearest, viz. our own satellite, may be given us by this class 
 of researches ? The question whether the moon possesses any 
 atmosphere may still be regarded as unsolved. If there be any, 
 it must be exceedingly small in quantity and highly attenu- 
 ated. Calculations, made from occultations of stars, on the 
 apparent differences of the semi-diameter of the bright and 
 dark moon, give an amount of difference which might indicate 
 a minute atmosphere, but which Mr. Airy attributes to 
 irradiation. 
 
 Supposing the moon to be constituted of similar materials 
 to the earth, it must be, to say the least, doubtful whether 
 there is oxygen enough to oxidate the metals of which she is 
 composed ; and if not, the surface which we see must be me- 
 tallic, or nearly so. The appearance of her craters is not 
 unlike that seen on the surface of some metals, such as bis- 
 muth, or, according to Professor Phillips, silver, when cooling 
 from fusion, and just previous to solidifying ; and it might be 
 a fair subject of enquiry whether, if there be any coating of 
 oxide on the surface, it may not be so thin as not to disguise 
 the form of the congealed metallic masses, as they may have 
 set in cooling from igneous fusion. M. Chacornac's recent 
 observations lead him to suppose that many of the lunar 
 
CONTINUITY. 193 
 
 craters were the result of a single explosion, which raised the 
 surface as a bubble and deposited its debris around the orifice 
 of eruption. 
 
 The eruptions on the surface of the moon clearly did not 
 take place at one period only, for at many parts of the disk cra- 
 ters may be seen encroaching on and disfiguring more ancient 
 ones, sometimes to the extent of three or four successive 
 displacements. Two important questions might, it seems to 
 me, be solved by an attentive examination of such portions 
 of the moon. By observing carefully with the most powerful 
 telescopes the character of the ridges thus successively formed, 
 the successive states of the lunar surface at different epochs 
 might be elucidated ; and, secondly, as on the earth we should 
 look for actual volcanic action at those points where recent 
 eruptions have taken place, so on the moon the more recently 
 active points being ascertained by the successive displace- 
 ment of anterior formations, it is these points which should 
 be examined for existing disruptive disturbances. Metius 
 and Fabricius might be cited as points of this character, 
 having been found by M. Chacornac to present successive 
 displacements, and to be perforated by numerous channels or 
 cavities. M. Chacornac considers that the seas, as they are 
 called, or smoother portions of the lunar surface, have at some 
 time made inroads on anteriorly formed craters ; if so, a large 
 portion of the surface of the moon must have been in a fused, 
 liquid, semi-liquid, or alluvial state long after the solidifying of 
 other portions of it. It would be difficult to suppose that 
 this state was one of igneous fusion, for this could hardly 
 exist over a large part of the surface without melting up the 
 remaining parts ; on the other hand, the total absence of any 
 signs of water, and of any atmosphere, or, if any there be, it is 
 most attenuated, would make it equally difficult to account 
 for a large diluvial formation. 
 
 Some substances, like mercury on this earth, might have 
 remained liquid after others had solidified by the cooling of 
 the planet ; but the problem is one which needs more examina- 
 tion and study before any positive opinion can be pronounced. 
 
 I cannot pass from the subject of lunar physics without 
 recording the obligation we are under to our late President 
 
 O 
 
194 CONTINUITY. 
 
 for his most valuable observations and for his exertions in 
 organising a band of observers devoted to the examination of 
 our nearest celestial neighbour ; and to Mr. Nasmyth and 
 Mr. De la Rue for their important graphical and photogra- 
 phical contributions to this subject. The granular character 
 of the sun's surface observed by Mr. Nasmyth in 1860 is also 
 a discovery which ought not to be passed over in silence. 
 
 Before quitting the subject of Astronomy I cannot avoid 
 expressing a feeling of disappointment that the achromatic 
 telescope, which has rendered such notable service to this 
 science, still retains in practice the great defect which was 
 known a century ago, at the time of Hall and Dollond, namely, 
 the inaccuracy of definition arising from what was termed the 
 irrationality of the spectrum, or the incommensurate divisions 
 of the spectra formed by flint and crown glass. 
 
 The beautiful results obtained by Blair have remained 
 inoperative from the circumstance that evaporable liquids 
 being employed between the lenses, a want of permanent 
 uniformity in the instrument was experienced ; and notwith- 
 standing the high degree of perfection to which the grinding 
 and polishing object-glasses has been brought by Clarke, 
 Cooke, and Mertz, notwithstanding the greatly improved 
 instrumental manufacture, the defect to which I have adverted 
 remains unremedied and an eyesore to the observer with the 
 refracting telescope. 
 
 We have now a large variety of different kinds of glass 
 formed from different metallic oxides. A list of many such 
 was given by M. Jacquelain a few years back. The last speci- 
 men which I have seen is a heavy high refracting glass formed 
 from the metal thallium by M. Lamy. Among all these, 
 could not two or three be selected which, having appropriate 
 refracting and dispersing powers, would have the coloured 
 spaces of their respective spectra, if not absolutely in the same 
 proportions, at all events much more nearly so than those of 
 flint and crown glass ? Could not, again, oily or resinous 
 substances having much action on the more refrangible rays 
 of the spectrum, such as castor-oil, Canada-balsam, &c., be 
 made use of in combination with glass lenses to reduce if not 
 annihilate this signal defect ? I have succeeded, to some 
 
CONTINUITY. 195 
 
 extent, in experiments with this class of substances. It is not 
 a problem to the solution of which there seems any insuper- 
 able difficulty ; the reason why it has not been solved is, I 
 incline to think, that the great practical opticians have no 
 time at their disposal to devote to long tentative experiments 
 and calculations; and on the other hand the theoretic opticians 
 have not the machinery and the skill in manipulation requisite 
 to give the appropriate degree of excellence to the materials 
 with which they experiment ; yet the result is worth labouring 
 for, as, could the defect be remedied, the refracting telescope 
 would make nearly as great an advance upon its present state 
 as the achromatic did on the single lens refractor. 
 
 While gravitation, physical constitution, and chemical 
 analysis by the spectrum show us that matter has similar 
 characteristics in other worlds than our own, when we pass to 
 the consideration of those other attributes of matter which 
 were at one time supposed to be peculiar kinds of matter 
 itself, or, as they were called, imponderables, but which are 
 now generally, if not universally, recognised as forces or modes 
 of motion, we find the evidence of continuity still stronger. 
 
 When all that was known of magnetism was that a piece 
 of steel rubbed against a particular mineral had the power of 
 attracting iron, and, if freely suspended, of arranging itself 
 nearly in a line with the earth's meridian, it seemed an ex- 
 ceptional phenomenon. When it was observed that amber, if 
 rubbed, had the temporary power of attracting light bodies, 
 this also seemed something peculiar and anomalous. What 
 are now magnetism and electricity as known to us ? Forces so 
 universal, so apparently connected with matter, as to become 
 two of its invariable attributes, and that to speak of matter 
 not being capable of being affected by these forces would 
 seem almost as extravagant as to speak of matter not being 
 affected by gravitation. 
 
 So with light, heat, and chemical affiinity, not merely is 
 every form of matter with which we are acquainted capable 
 of manifesting all these modes of force, but so-called matter 
 supposed incapable of such manifestations would to most 
 minds cease to be matter. 
 
 Further than this it seems to me (though, as I have taken 
 
 o 2 
 
196 CONTINUITY. 
 
 an active part for many years, dating from a quarter of a 
 century, in promoting this view, I may not be considered an 
 impartial judge) that it is now proved that all these forces are 
 so invariably connected inter se and with motion as to be 
 regarded as modifications of each other, and as resolving 
 themselves objectively into motion, and subjectively into that 
 something which is supposed to produce or resist motion, and 
 which we call force. 
 
 I may perhaps be permitted to recall a forgotten experi- 
 ment, which nearly a quarter of a century ago I showed at 
 the London Institution, an experiment simple enough in itself, 
 but which then seemed to me important from the consequences 
 to be deduced from it, and the importance of which will be 
 much better appreciated now than then. 
 
 A train of multiplying wheels ended with a small metallic 
 wheel, which, when the train was put in motion, revolved with 
 extreme rapidity against the periphery of the next wheel, a 
 wooden one. In the metallic wheel was placed a small piece of 
 phosphorus, and as long as the wheels revolved the phosphorus 
 remained unchanged ; but the moment the last wheel was 
 stopped, by moving a small lever attached to it, the phosphorus 
 burst into flame. My object was to show that while motion 
 of the mass continued heat was not generated, but that when 
 this was arrested, the force continuing to operate, the motion 
 of the mass became heat in the particles. The experiment 
 differed from that of Rumford's cannon-boring and Davy's 
 friction of ice in showing that there was no heat while the 
 motion was unresisted, but that the heat was dependent on 
 the motion being impeded or arrested. We have now become 
 so accustomed to this view, that whenever we find motion 
 resisted we look to heat, electricity, or some other force as the 
 necessary and inevitable result. 
 
 It would be out of place here, and treating of matters too 
 familiar to the bulk of my audience, to trace how, by the 
 labours of Oersted, Seebeck, Faraday, Talbot, Daguerre, and 
 others, materials have been provided for the generalisation 
 now known as the correlation of forces or conservation of 
 energy ; while Davy, Rumford, Seguin, Mayer, Joule, Helm- 
 holtz, Thomson, and others (among whom I would not name 
 
CONTINUITY. 197 
 
 myself, were it not that I may be misunderstood and supposed 
 to have abandoned all claim to a share in the initiation of 
 this, as I believe, important generalisation) have carried on 
 the work ; and how, sometimes by independent and, as is 
 commonly the case, nearly simultaneous deductions, some- 
 times by progressive and accumulated discoveries, the doctrine 
 of the reciprocal interaction, of the quantitative relation, and 
 of the necessary dependence of all the forces, has, I think I may 
 venture to say, been established. 
 
 If magnetism be, as it is proved to be, connected with the 
 other forces or affections of matter, if electrical currents 
 always produce, as they are proved to do, lines of magnetic 
 force at right angles to their lines of action, magnetism must 
 be cosmical, for where there is heat and light there is elec- 
 tricity, and consequently magnetism. Magnetism, then, must 
 be cosmical and not merely terrestrial. Could we trace 
 magnetism in other planets and suns as a force manifested in 
 axial or meridional lines, i.e. in lines cutting at right angles 
 the curves formed by their rotation round an axis, it would 
 be a great step ; but it is one hitherto unaccomplished. The 
 apparent coincidences between the maxima and minima of 
 solar spots, and the decennial or undecennial periods of ter- 
 restrial magnetic intensity, though only empirical at present, 
 might tend to lead us to a knowledge of the connection 
 we are seeking ; and Sir Edward Sabine considers that an 
 additional epoch of coincidence has arrived, making the 
 fourth decennial period ; but some doubt is thrown upon 
 these coincidences by the magnetic observations made at 
 Greenwich Observatory. In a paper published in the 'Trans- 
 actions of the Royal Society,' 1863, the Astronomer-Royal 
 says, speaking of results extending over seventeen years, 
 there is no appearance of decennial cycle in the recurrence of 
 great magnetic disturbances ; and Mr. Glaisher last year, in the 
 physical section of this Association, stated that after persever- 
 ing examination he had been unable to trace any connection 
 between the magnetism of the earth and the spots on the sun. 
 
 Mr. Airy, however, in a more recent paper, suggests that 
 currents of magnetic force having reference to the solar hour 
 are detected, and seem to produce vortices or circular disturb- 
 
198 CONTINUITY. 
 
 ances, and he invites further co-operative observation on the 
 subject, one of the highest interest, but at present remaining 
 in great obscurity. 
 
 One of the most startling suggestions as to the consequence 
 resulting from the dynamical theory of heat is that made by 
 Mayer, that by the loss of vis viva occasioned by friction of the 
 tidal waves, as well as by their forming, as it were, a drag upon 
 the earth's rotatory movement, the velocity of the earth's rota- 
 tion must be gradually diminishing, and that thus, unless some 
 undiscovered compensatory action exist, this rotation must 
 ultimately cease and changes hardly calculable take place in 
 the solar system. 
 
 M. Delaunay considers part of the acceleration of the 
 moon's mean motion, which is not at present accounted for by 
 planetary disturbances, to be due to the gradual retardation of 
 the earth's rotation ; to which view, after an elaborate inves- 
 tigation, the Astronomer-Royal has given his assent. 
 
 Another most interesting speculation of Mayer is that with 
 which you are familiar, viz. that the heat of the sun is occasioned 
 by friction or percussion of meteorites falling upon it : there are 
 some difficulties, not perhaps insuperable, in this theory. Sup- 
 posing such cosmical bodies to exist in sufficient numbers, they 
 would, as they revolve round the sun, fall into it, not as an 
 aerolite falls upon the earth, directly by an intersection of orbits, 
 but by the gradual reduction in size of the orbits, occasioned 
 by a resisting medium ; some portion of motive force would 
 be lost, and heat generated in space by friction against such 
 medium. When these bodies arrive at the sun they would, 
 assuming them, like the planets, to have revolved in the same 
 direction, all impinge in a definite direction, and we might ex- 
 pect to see some symptoms of their action in the sun's photo- 
 sphere ; but though this is in a constant state of motion, and 
 the direction of these movements has been carefully investi- 
 gated by Mr. Carrington and others, no such general direction 
 is detected ; and M. Faye, who some time ago wrote a paper 
 pointing out many objections to the theory of solar heat being 
 produced by the^ fall of meteoric bodies into the sun, has 
 recently investigated the proper motions of sun-spots, and 
 believes he has removed certain apparent anomalies and re- 
 
CONTINUITY. 199 
 
 duced their motions to a certain regularity in the motion of 
 the photosphere, attributable to some general action arising 
 from the internal mass of the sun. 
 
 It might be expected that comets, bodies so light and so 
 easily deflected from their course, would show some symptoms 
 of being acted on by gravitation, were such a number of 
 bodies to exist in or near their paths as are pre-supposed in 
 this theory of solar heat. The sun may, however, meet sup- 
 plies of meteoric bodies and condense gaseous matter as it 
 travels in space (as the sun is now fairly proved to do), and so 
 heat may be produced. 
 
 Assuming the undulatory theory of light to be true, and 
 that the motion which constitutes light is transmitted across 
 the interplanetary spaces by a highly elastic ether, then, unless 
 this motion is confined to one direction, unless there be no 
 interference, unless there be no viscosity, as it is now termed, 
 in the medium, and, consequently, no friction, light must lose 
 something in its progress from distant luminous bodies, that is 
 to say, must lose something as light ; for, as all reflecting 
 minds are now convinced that force cannot be annihilated, the 
 force is not lost, but its mode of action is changed. If light, 
 then, is lost as light (and the observations of Struve seem to 
 show this to be so, that in fact a star may be so far distant 
 that it can never be seen, in consequence of its luminous un- 
 dulations becoming extinct), what becomes of the transmitted 
 force lost as light, but existing in some other form ? So with 
 heat : our sun, our earth, and planets are constantly radiating 
 heat into space, so in all probability are the other suns, the 
 stars, and their attendant planets. What becomes of the heat 
 thus radiated into space ? If the universe have no limit and 
 it is difficult to conceive one heat and light should be every- 
 where uniform ; and night would be as light and as warm as 
 day. What becomes of the enormous force thus apparently 
 non-recurrent in the same form ? Does it return as palpable 
 motion ? Does it move or contribute to move suns and planets ? 
 Can it be conceived as a force similar to that which Newton 
 speculated on as universally repulsive and capable of being 
 substituted for universal attraction ? We are in no position 
 at present to answer such questions as these ; but I know of 
 
200 CONTINUITY. 
 
 no problem in celestial dynamics more deeply interesting than 
 this, and we may be no farther removed from its solution than 
 the predecessors of Newton were from the simple dynamical 
 relation of matter to matter which his potent intellect de- 
 tected and demonstrated. 
 
 Passing from extra-terrestrial theories to the narrower field 
 of molecular physics, we find the doctrine of correlation of 
 forces steadily making its way. In the Bakerian Lecture for 
 1863 Mr. Sorby shows, not perhaps a direct correlation of 
 mechanical and chemical forces, but that when, either by 
 solution or by chemical action, a change in volume of the 
 resulting substance as compared with that of its separate 
 constituents is effected, the action of pressure retards or pro- 
 motes the change, according as the substance formed would 
 occupy a larger or smaller space than that occupied by its 
 separate constituents ; the application of these experiments 
 to geological enquiries as to subterranean changes which 
 may have taken place under great pressure is obvious, and 
 we may expect to form compounds under artificial compres- 
 sion which cannot be found under normal pressure. 
 
 In a practical point of view the power of converting one 
 mode of force into another is of the highest importance ; and 
 with reference to a subject which at present, somewhat pre- 
 maturely, perhaps, occupies men's minds, viz. the prospective 
 exhaustion of our coal-fields, there is every encouragement 
 to be derived from the knowledge that we can at will produce 
 heat by the expenditure of other forces ; but, more than 
 that, we may probably be enabled to absorb or store up, as 
 it were, diffused energy for instance, Berthelot has found 
 that the potential energy, as it is termed, of formate of potash 
 is much greater than that of its proximate constituents, caus- 
 tic potash and carbonic oxide. This change may take place 
 spontaneously and at ordinary temperatures, and by such 
 change carbonic oxide becomes, so to speak, re-invested with 
 the amount of potential energy which its carbon possessed 
 before uniting with oxygen, or, in other words, the carbonic 
 oxide is raised as a force-possessor to the place of carbon by 
 the direct absorption or conversion of heat from surrounding 
 matter. 
 
CONTINUITY. 201 
 
 This may bear on certain recent speculations on the dissi- 
 pation of force by heat radiation. 
 
 Here also we have as to force-absorption, an analogous 
 result to that of the formation of coal from carbonic acid and 
 water ; and though this is a mere illustration, and cannot be 
 expected to become economical on a large scale, still it and 
 similar examples may calm apprehension as to future means 
 of supplying heat, should our present fuel become exhausted. 
 As the sun's force, spent in times long past, is now returned to 
 us from the coal which was formed by its light and heat, so 
 the sun's rays, which are daily wasted, as far as we are 
 concerned, on the sandy deserts of Africa, may hereafter by 
 chemical or mechanical means, be made to light and warm 
 the habitations of the denizens of colder regions. The tidal 
 wave is, again, a large reservoir of force hitherto almost 
 unused. . 
 
 The valuable researches of Professor Tyndall on radiant 
 heat afford many instances of the power of localising, if 
 the term be permitted, heat which would otherwise be 
 dissipated. 
 
 The discoveries of Graham, by which atmospheric air, 
 drawn through films of caoutchouc, leaves behind half its 
 nitrogen, or, in other words, becomes richer by half in oxygen, 
 and hence has a much increased potential energy, not only 
 show a most remarkable instance of physical molecular action 
 merging into chemical, but afford us indications of means of 
 storing up force, much of the force used in working the aspi- 
 rator being capable at any period, however remote, of being 
 evolved by burning the oxygen with a combustible. 
 
 What changes may take place in our modes of applying 
 force before the coal-fields are exhausted it is impossible to 
 predict. Even guesses at the probable period of their exhaus- 
 tion are uncertain. There is a tendency to substitute for 
 smelting in metallurgic processes, liquid chemical action, which 
 of course has the effect of saving fuel ; and the waste of fuel 
 in ordinary operations is enormous, and can be much econo- 
 mised by already known processes. It is true that we are, at 
 present, far from seeing a practical mode of replacing that 
 granary of force, the coal-fields ; but we may with confidence 
 
202 CONTINUITY. 
 
 rely on invention being in this case, as in others, born of 
 necessity, when the necessity arises. 
 
 I will not further pursue this subject. At a time when 
 science and civilisation cannot prevent large tracts of country 
 being irrigated by human blood in order to gratify the ambition 
 of a few restless men, it seems an over-refined sensibility to 
 occupy ourselves with providing means for our descendants in 
 the tenth generation to warm their dwellings or propel their 
 locomotives. 
 
 Two very remarkable applications of the convertibility of 
 force have been recently attained by the experiments of Mr. 
 Wilde and Mr. Holz ; the former finds that, by conveying 
 electricity from the coils of a magneto-electric machine to an 
 electro-magnet, a considerable increase of electrical power 
 may be attained, and by applying this as a magneto-electric 
 machine to a second, and this in turn to a third electro-mag- 
 netic apparatus, the force is largely augmented. Of course, to 
 produce this increase, more mechanical force must be used at 
 each step to work the magneto-electric machines ; but provided 
 this be supplied, there hardly seems a limit to the extent to 
 which mechanical may be converted into electrical force. 
 
 Mr. Holz has contrived a Franklinic electrical machine, in 
 which a similar principle is manifested. A varnished glass 
 plate is made to revolve in close proximity to another plate 
 having two or more pieces of card attached, which are elec- 
 trified by a bit of rubbed glass or ebonite ; the moment this is 
 effected, a resistance is felt by the operator who turns the 
 handle of the machine, and the slight temporary electrisation 
 of the card converts into a continuous flood of intense elec- 
 tricity the force supplied by the arm of the operator. 
 
 These results offer great promise of extended application ; 
 they show that, by a mere formal disposition of matter, one 
 force may be converted into another, and that not to the 
 limited extent hitherto attained, but to an extent co-ordinate, 
 or nearly so, with the increased initial force, so that, by a mere 
 change in the arrangement of apparatus, a means of absorbing 
 and again eliminating in a new form a given force may be 
 obtained to an indefinite extent. As we may, in a not very 
 distant future, need, for the daily uses of mankind, heat, light, 
 
CONTINUITY. 203 
 
 and mechanical forge, and find our present resources exhausted, 
 the more we can invent new modes of conversion of forces, the 
 more prospect we have of practically supplying such want. It 
 is but a month from this time that the greatest triumph of 
 force-conversion has been attained. The chemical action 
 generated by a little salt-water on a few pieces of zinc will 
 now enable us to converse with inhabitants of the opposite 
 hemisphere of this planet, and 
 
 ' Put a girdle round about the earth in forty minutes.' 
 The Atlantic Telegraph is an accomplished fact. 
 
 In Physiology very considerable strides are being made by 
 studying the relation of organised bodies to external forces ; 
 and this branch of enquiry has been promoted by the labours 
 of Carpenter, Bence Jones, Playfair, E. Smith, Frankland, and 
 others. Vegetables acted on by light and heat decompose 
 water, ammonia, and carbonic acid, and transform them into, 
 among other substances, oxalate of lime, lactic acid, starch, 
 sugar, stearine, urea, and ultimately albumen ; while the living 
 animal reverses the process, as does vegetable decay, and pro- 
 duces from albumen, urea, stearine, .sugar, starch, lactic acid, 
 oxalate of lime, and ultimately ammonia, water, and carbonic 
 acid. 
 
 As moreover, heat and light are absorbed or converted in 
 forming the synthetic processes going on in the vegetable, so 
 conversely heat and sometimes light is given off by the living 
 animal ; but it must not be forgotten that the line of demar- 
 cation between a vegetable and an animal is difficult to draw, 
 that there are no single attributes which are peculiar to either, 
 and that it is only by a number of characteristics that either 
 can be defined. 
 
 The series of processes above given may be simulated by 
 the chemist in his laboratory ; and the amount of labour 
 which a man has undergone in the course of twenty- four hours 
 may be approximately arrived at by an examination of the 
 chemical changes which have taken place in his body ; changed 
 forms in matter indicating the anterior exercise of dynamical 
 force. That muscular action is produced or supported by 
 
204 CONTIINUTY. 
 
 chemical change would probably now be a generally accepted 
 doctrine ; but while many have thought that muscular power 
 is derived from the oxidation of albuminous or nitrogenised 
 substances, several recent researches seem to show that the 
 latter is rather an accompaniment than a cause of the former, 
 and that it is by the oxidation of carbon and hydrogen com- 
 pounds that muscular force is supplied. Traube has been pro- 
 minent in advancing this view, and experiments detailed in a 
 paper published this year by two Swiss professors, Drs. Fick 
 and Wislicenus, which were made by and upon themselves in 
 an ascent of the Faulhorn, have gone far to confirm it. Having 
 fed themselves, before and during the ascent, upon starch, fat, 
 and sugar, avoiding all nitrogenised compounds, they found 
 that the consumption of such food was amply sufficient to 
 supply the force necessary for their expedition, and that they 
 felt no exhaustion. By appropriate chemical examination 
 they ascertained that there was no notable increase in the 
 oxidation of the nitrogenised constituents of the body. After 
 calculating the mechanical equivalents of the combustion 
 effected, they state, as their first conclusion, that ' the burning 
 of protein substances cannot be the only source of muscular 
 power, for we have here two cases in which men performed 
 more measurable work than the equivalent of the amount of 
 heat which, taken at a most absurdly high figure, could be cal- 
 culated to result from the burning of the albumen.' 
 
 They further go on to state that, so far from the oxidation 
 of albuminous substances being the only source of muscular 
 power, * the substances by the burning of which force is gene- 
 rated in the muscles, are not the albuminous constituents of 
 those tissues, but non-nitrogenous substances, either fats or 
 hydrates of carbon,' and that the burning of albumen is 
 not in any way concerned in the production of muscular 
 power. 
 
 We must not confuse the question of the food which forms 
 and repairs muscle and gives permanent capability of muscular 
 force with that which supplies the requisites for temporary 
 activity ; no doubt the carnivora are the most powerfully con- 
 stituted animals, but the chamois, gazelle, &c., have great tem- 
 porary capacity for muscular exertion, though their food is 
 
CONTINUITY. 205 
 
 vegetable; for concentrated and sustained energy, however, 
 they do not equal the carnivora ; and with the domestic gra- 
 minivora we certainly find that they are capable of performing 
 more continuous work when supplied with those vegetables 
 which contain the greatest quantity of nitrogen. 
 
 These and many similar classes of research show that in 
 chemical enquiries, as in other branches of science, we are gra- 
 dually relieving ourselves of hypothetical existences, which 
 certainly had the advantage that they might be varied to suit 
 the requirements of the theorist. 
 
 Phlogiston, as Lavoisier said with a sneer, was sometimes 
 heavy, sometimes light; sometimes fire in a free state, some- 
 times combined; sometimes passing through glass vessels, 
 sometimes retained by them ; which by its protean changes 
 explained causticity and non-causticity, transparency and 
 opacity, colours and their absence. As phlogiston and similar 
 creations of the mind have passed away, so with hypothetic 
 fluids, imponderable matters, specific ethers, and other inven- 
 tions of entities made to vary according to the requirements 
 of the theorist, I believe the day is approaching when these 
 will be dispensed with, and when the two fundamental con- 
 ceptions of matter and motion will be found sufficient to 
 explain physical phenomena. 
 
 The facts made known to us by geological enquiries, while 
 on the one hand they afford striking evidence of continuity, 
 on the other, by the breaks in the record, may be used as 
 arguments against it. The great question once was, whether 
 these chasms represent sudden changes in the formation of 
 the earth's crust, or whether they arise from dislocations 
 occasioned since the original deposition of strata or from 
 gradual shifting of the areas of submergence. Few geologists 
 of the present day would, I imagine, not adopt the latter 
 alternatives. Then comes a second question, whether, when 
 the geological formation is of a continuous character, the 
 different characters of the fossils represent absolutely per- 
 manent varieties, or may be explained by gradual modifying 
 changes. 
 
 Professor Ansted, summing up the evidence on this head 
 as applied to one division of stratified rocks, writes as follows : 
 
206 CONTINUITY. 
 
 ( Paleontologists have endeavoured to separate the Lias 
 into a number of subdivisions, by the Ammonites, groups of 
 species of those shells being characteristic of different zones. 
 The evidence on this point rests on the assumption of specific 
 differences being indicated by permanent modifications of the 
 structure of the shell. But it is quite possible that these may 
 mean nothing more than would be due to some change in the 
 conditions of existence. Except between the Marlstone and 
 the Upper Lias there is really no palaeontological break, in 
 the proper sense of the words; alterations of form and size 
 consequent on the occurrence of circumstances more or less 
 favourable, migration of species, and other well-known causes, 
 sufficiently account for many of those modifications of the 
 form of the shell that have been taken as specific marks. This 
 view is strengthened by the fact that other shells and other 
 organisms generally show no proof of a break of any import- 
 ance except at the point already alluded to.' 
 
 But, irrespectively of another deficiency in the geological 
 record, which will be noticed presently, the physical breaks in 
 the stratification make it next to impossible to fairly trace the 
 order of succession of organisms by the evidence afforded by 
 their fossil remains. Thus there are nine great breaks in the 
 Palaeozoic series, four in the Secondary, and one in the 
 Tertiary, besides those between Palaeozoic and Secondary and 
 Secondary and Tertiary respectively. Thus, in England there 
 are sixteen important breaks in the succession of .strata, to- 
 gether with a number of less important interruptions. But 
 although these breaks exist, we find pervading the works of 
 many geologists a belief, resulting from the evidence presented 
 to their minds, sometimes avowed, sometimes unconsciously 
 implied, that the succession of species bears some definite 
 relation to the succession of strata. Thus, Professor Ramsay 
 says, that ' in cases of superposition of fossiliferous strata, in 
 proportion as the species are more or less continuous, that is 
 to say, as the break in the succession of life is partial or com- 
 plete, so was the time that elapsed between the close of the 
 lower and the commencement of the upper strata a shorter 
 or a longer interval. The break in life may be indicated not 
 only by a difference in species, but yet more importantly by 
 
CONTINUITY. 207 
 
 the absence of older and appearance of newer allied or unallied 
 genera.' 
 
 Indications of the connection between cosmical studies 
 and geological researches are dawning on us : there is, for 
 instance, some reason to believe that we can trace many 
 geological phenomena to our varying path of rotation round 
 the sun ; thus, more than thirty years ago Sir J. Herschel 
 proposed an explanation of the changes of climate on the 
 earth's surface as evidenced by geological phenomena, founded 
 on the changes of eccentricity in the earth's orbit. 
 
 He said he had entered on the subject ' impressed with 
 the magnificence of that view of geological revolutions which 
 regards them rather as regular and necessary effects of great 
 and general causes, than as resulting from a series of convul- 
 sions and catastrophes regulated by no laws and reducible to 
 no fixed principles.' 
 
 As the mean distance of the earth from the sun is nearly 
 invariable, it would seem at first sight that the mean annual 
 supply of light and heat received by the earth would also be 
 invariable; but according to his calculations it is inversely 
 proportional to the minor axis of the orbit : this would give 
 less heat when the eccentricity of the earth's orbit is approach- 
 ing towards or at its minimum. Mr. Croll has recently shown 
 reason to believe that the climate, at all events in the circum- 
 polar and temperate zones of the earth, would depend, in 
 some degree, on whether the winter of a given region occurred 
 when the earth at its period of greatest eccentricity was in 
 aphelion or perihelion if the former, the annual average of 
 temperature would be lower, if the latter, it would be higher 
 than when the eccentricity of the earth's orbit were less, or 
 approached more nearly to a circle. He calculates the dif- 
 ference in the amount of heat at the period of maximum 
 eccentricity of the earth's orbit to be at nineteen to twenty-six, 
 according as the winter would take place when the earth was 
 in aphelion or in perihelion. His reason may be briefly stated 
 thus: assuming the mean annual heat to be the same, what- 
 ever the eccentricity of orbit, yet if the extremes of heat and 
 cold in summer and winter be greater, a colder climate will 
 prevail, for there will be more snow and ice accumulated in 
 
208 CONTINUITY. 
 
 the cold winter than the hot summer can melt, a result aided 
 by the shelter from the sun's rays produced by the vapour 
 suspended in consequence of aqueous evaporation ; hence we 
 should get glacial periods, when the orbit of the earth is at 
 its greatest eccentricity, at those parts of the earth's surface 
 where it is winter when the earth is in aphelion; carboni- 
 ferous or hot periods where it is winter in perihelion; and 
 normal or temperate periods when the eccentricity of orbit is 
 at a minimum; all these would gradually slide into each 
 other, and would produce at long distant periods alternations 
 of cold and heat, several of which we actually observe in 
 geological records. 
 
 If this theory be borne out, we should approximate to a 
 test of the time which has elapsed between different geological 
 epochs. Mr. Croll's computation of this would make it cer- 
 tainly not less than 100,000 years since the last glacial epoch, 
 a time not very long in geological chronology probably it is 
 much more. 
 
 When we compare with the old theories of the earth, by 
 which the apparent changes on its surface were accounted for 
 by convulsions and cataclysms, the modern view inaugurated 
 by Lyell, your former President, and now, if not wholly, at 
 all events to a great extent adopted, it seems strange that the 
 referring past changes to similar causes to those which are 
 now in operation should have remained uninvestigated until 
 the present century; but with this, as with other branches 
 of knowledge, the most simple is frequently the latest view 
 which occurs to the mind. It is much more easy to invent 
 a Deus ex machina than to trace out the influence of slow 
 continuous change; the love of the marvellous is so much 
 more attractive than the patient investigation of truth, that 
 we find it to have prevailed almost universally in the early 
 stages of science. 
 
 In astronomy we had crystal spheres, cycles,. and epicycles ; 
 in chemistry the philosopher's stone, the elixir vitse, the archaeus 
 or stomach demon, and phlogiston ; in electricity the notion 
 that amber possessed a soul, and that a mysterious fluid could 
 knock down a steeple. In geology a deluge or a volcano was 
 supplied. In palaeontology a new race was created whenever 
 
CONTINUITY. 209 
 
 theory required it : how such new races began, the theorist 
 did not stop to enquire. 
 
 A curious speculator might say to a palaeontologist of even 
 recent date, in the words of Lucretius, 
 
 ' Nam neque de ccelo cecidisse animalia possunt 
 Nee terrestria de salsis exisse lacunis. 
 
 E nihilo si crescere possent, 
 (Turn) fierent juvenes subito ex infantibus parvis, 
 E terraque exorta repente arbusta salirent ; 
 Quorum nil fieri manifestum est, omnia quando 
 Paulatim crescunt, ut par est, semine certo, 
 Crescentesque genus servant ' 
 
 which may be thus freely paraphrased : ' You have abandoned 
 the belief in one primaeval creation at one point of time, you 
 cannot assert that an elephant existed when the first saurians 
 roamed over earth and water. Without, then, in any way limit- 
 ing Almighty power, if an elephant were created without pro- 
 genitors, the first elephant must, in some way or other, have 
 physically arrived on this earth. Whence did he come ? did he 
 fall from the sky (i.e. from interplanetary space) ? did he rise 
 moulded out of a mass of amorphous earth or rock ? did he ap- 
 pear out of the cleft of a tree ? If he had no antecedent pro- 
 genitors, some such beginning must be assigned to him.' I know 
 of no scientific writer who has, since the discoveries of geology 
 have become familiar, ventured to present in intelligible terms 
 any definite notion of how such an event could have occurred : 
 those who do not adopt some view of continuity are content 
 to say God willed it ; but would it not be more reverent and 
 more philosophical to enquire by observation and experiment, 
 and to reason from induction and analogy, as to the proba- 
 bilities of such frequent miraculous interventions ? 
 
 I know I am touching on delicate ground, and that a long 
 time may elapse before that calm enquiry after truth which it 
 is the object of associations like this to promote can be fully 
 attained ; but I trust that the members of this body are suffi- 
 ciently free from prejudice, whatever their opinions may be, to 
 admit an enquiry into the general question whether what we 
 
 P 
 
210 CONTINUITY. 
 
 term species are and have been rigidly limited, and have at 
 numerous periods been created complete and unchangeable, 
 or whether, in some mode or other, they have not gradually 
 and indefinitely varied, and whether the changes due to the 
 influence of surrounding circumstances, to efforts to accom- 
 modate themselves to surrounding changes, to what is called 
 natural selection, or to the necessity of yielding to superior 
 force in the struggle for existence, as maintained by our 
 illustrious countryman Darwin, and other causes, have not so 
 modified organisms as to enable them to exist under changed 
 conditions. I am not going to put forward any theory of my 
 own, I am not going to argue in support of any special 
 theory, but having endeavoured to show how, as science ad- 
 vances, the continuity of natural phenomena becomes more 
 apparent, it would be cowardice not to present some of the 
 main arguments for and against continuity as applied to the 
 history of organic beings. 
 
 As we detect no such phenomenon as the creation or 
 spontaneous generation of vegetables and animals which are 
 large enough for the eye to see without instrumental assist- 
 ance, as we have long ceased to expect to find a Plesiosaurus 
 spontaneously generated in our fish-pond, or a Pterodactyle in 
 our pheasant-cover, the field of this class of research has be- 
 come identified with the field of the microscope, and at each 
 new phase the investigation has passed from a larger to a 
 smaller class of organisms. The question whether among the 
 smallest and apparently the most elementary forms of organic 
 life the phenomenon of spontaneous generation obtains, has 
 recently formed the subject of careful experiment and ani- 
 mated discussion in France. If it could be found that organ- 
 isms of a complex character were generated without progenitors 
 out of amorphous matter, it might reasonably be argued that 
 a similar mode of creation might obtain in regard to larger 
 organisms. Although we see no such phenomenon as the for- 
 mation of an animal such as an elephant, or a tree such as an 
 oak, excepting from a parent which resembles it, yet if the 
 microscope revealed to us organisms, smaller but equally 
 complex, so formed without having been reproduced, it would 
 render it not improbable that such might have been the case 
 
CONTINUITY. 2ii 
 
 with larger organic beings. The controversy between M. Pas- 
 teur and M. Pouchet has led to a very close investigation of 
 this subject, and the general opinion is, that when such; pre- 
 cautions are taken as exclude from the substance submitted to 
 experiment all possibility of germs from the atmosphere being 
 introduced, as by passing the air which is to support the life 
 of the animalculse through tubes heated to redness and other 
 precautions, no formation of organisms takes place. Some 
 experiments of Mr. Child's, communicated to the Royal 
 Society, again throw doubt on the negative results obtained 
 by M. Pasteur ; so that the question may be not finally de- 
 termined, but the balance of experiment and opinion is 
 against spontaneous generation. 
 
 One argument presented by M. Pasteur is well worthy of 
 remark, viz. that in proportion as our means of scrutiny 
 become more searching, heterogeny, or the development 
 of organisms without generation from parents of similar 
 organism, has been gradually driven from higher to lower 
 forms of life, so that if some apparent exceptions still exist 
 they are of the lowest and simplest forms, and these ex- 
 ceptions may probably be removed, as M. Pasteur considers 
 he has removed them, by a more searching investigation. 
 
 If it be otherwise, if heterogeny obtains at all, few will 
 not now admit that at present the result of the most careful 
 experiments shows it to be confined to the more simple 
 organic structures, and that all the progressive and more 
 highly developed forms are, as far as the most enlarged 
 experience shows, generated by reproduction. 
 
 [It has seemed to some an inconsistency that an advocate 
 of continuity or gradual hereditary change in organisms 
 should not be in favour of heterogeny, since, if highly com- 
 plex organisms be formed by gradual evolution from more 
 simple structures, we should ultimately reach a point at which 
 organised forms should be developed from inorganic matter. 
 
 This may be so, could we go far enough back in the his- 
 tory of genesis ; but what I look on with some degree of scep- 
 ticism is the development of complex though minute organ- 
 isms which appear, so to speak, full-blown at their inception, 
 and perfectly capable of reproduction, though not themselves 
 
 p 2 
 
212 CONTINUITY. 
 
 reproductions. That there was a time, and possibly may be 
 now, when inorganic matter, by infinitesimally slow changes, 
 became or becomes organised, I am far from denying. All I 
 hesitate to accept is heterogeny of self-moving reproductive 
 organisms, which seem less complex than larger vitalised 
 beings only in all probability because our microscopes are 
 not powerful enough to detect their structures.] 
 
 The great difficulty which is met with at the threshold of 
 enquiry into the origin of species is the definition of species ; 
 in fact, species can hardly be defined without begging the 
 question in dispute. 
 
 Thus, if species be said to be a perseverance of type inca- 
 pable of blending itself with other types, or, which comes 
 nearly to the same thing, incapable of producing by union 
 with other types offspring of an intermediate character 
 which can again reproduce, we arrive at this result, that when- 
 ever the advocate of continuity shows a blending of what had 
 been hitherto deemed separate species, the answer is, they 
 were considered separate species by mistake ; they do not now 
 come under the definition of species, because they interbreed. 
 
 The line of demarcation is thus ex hypothesi removed a 
 step farther, so that unless the advocate of continuity can, on 
 his side, prove the whole question in dispute, by showing that 
 all can directly or by intermediate varieties reproduce, he is 
 defeated by the definition itself of species. 
 
 On the other hand, if this, or something in fact amounting 
 to it, be not the definition of species if it be admitted that 
 distinct species can, under certain favourable conditions, pro- 
 duce intermediate offspring capable of reproduction, then 
 continuity in some mode or other is admitted. 
 
 The question, then, takes this form : Are there species or 
 are there not ? Is the word to be used as signifying a real, 
 natural distinction, or as a mere convenient designation ap- 
 plied to subdivisions, having a permanence which will pro- 
 bably outlive man's discussions on the subject, but not an 
 absolute fixity ? The same question, in a wider sense, and 
 taking into consideration a much longer time, would be appli- 
 cable to genera and families. 
 
 Actual experiment has done little to elucidate the ques- 
 
CONTINUITY. ,213 
 
 tion, nor, unless we can suppose the experiments continued 
 through countless generations, is it likely to contribute much 
 to its solution. We must therefore have recourse to the 
 enlarged experience or induction from the facts of geology, 
 palaeontology, and physiology, aided by analogy from the 
 modes of action which nature evidences in other departments. 
 
 The doctrine of gradual succession is hardly yet formula- 
 rised ; and though there are some high authorities for certain 
 modifications of such view, the preponderance of authority 
 would necessarily be on the other side. Geology and palae- 
 ontology are recent sciences, and we cannot tell what the 
 earlier authors would have thought or written had the more 
 recently discovered facts been presented to their view. Au- 
 thority, therefore, does not much help us on this question. 
 
 Geological discoveries seemed, in the early period of the 
 science, to show complete extinction of certain species and the 
 appearance of new ones, great gaps existing between the cha- 
 racteristics of the extinct and the new species. As science 
 advanced, these were more or less filled up, and the difficulty 
 in the first instance of admitting unlimited modification of 
 species would seem to have arisen from the comparison of the 
 extreme ends of the scale where the intermediate links or 
 some of them were wanting. 
 
 To suppose a Zoophyte the progenitor of a Mammal,.or to 
 suppose at some particular period of time a highly developed 
 animal to have come out of nothing, or suddenly grown out of 
 inorganic matter, would appear at first sight equally extrava- 
 gant hypotheses. As an effort of Almighty creative power, 
 neither of these alternatives presents more difficulty than the 
 other ; but as we have no means of ascertaining how creative 
 power worked, but by an examination and study of the works 
 themselves, we are not likely to get either view proved to 
 ocular demonstration. A single phase in the progress of 
 natural transmutation would probably require a term far 
 transcending all that embraced by historical records ; and, on 
 the other hand, it might be said, sudden creations, though 
 taking place frequently, if viewed with reference to the immen- 
 sity of time involved in geological periods, may be so rare 
 with reference to our experience, and so difficult of clear 
 
214 CONTINUITY. 
 
 authentication, that the non-observation of such instances 
 cannot be regarded as absolute disproof of their possible 
 occurrence. 
 
 The more the gaps between species are filled up by the 
 discovery of intermediate varieties the stronger becomes the 
 argument for transmutation and the weaker that for succes- 
 sive creations, because the former view then becomes more 
 and more consistent with experience, the latter more discordant 
 from it. As undoubted cases of variation, more or less per- 
 manent from given characteristics, are produced by the effects 
 of climate, food, domestication, &c., the more species are 
 increased by intercalation, the more the distinctions slide 
 down towards those which are within the limits of such ob- 
 served deviations ; while, on the other hand, to suppose the 
 more and more frequent recurrence of fresh creations out of 
 amorphous matter is a multiplication of miracles or special 
 interventions not in accordance with what we see of the uni- 
 form and gradual progress of nature, either in the organic or 
 inorganic world. If we were entitled to conclude that the 
 progress of discovery would continue in the same course, and 
 that species would become indefinitely multiplied, the distinc- 
 tions would become infinitely minute, and all lines of demar- 
 cation would cease, the polygon would become a circle, the 
 succession of points a line. Certain it is that the more we 
 observe the more we increase the subdivision of species, and 
 consequently the number of the supposed creations ; so that 
 new creations become innumerable ; and yet of these we have 
 no one well-authenticated instance, and in no other observed 
 operation of nature have we seen this want of continuity, these 
 frequent per sallum deviations from uniformity, each of which 
 is a miracle. 
 
 The difficulty of producing intermediate offspring from 
 what are termed distinct species and the infecundity in many 
 instances of hybrids are used as strong arguments against 
 continuity of succession ; on the other hand, it may be said 
 long-continued variation through countless generations has 
 given rise to such differences of physical character that repro- 
 duction is difficult in some cases, and. in others impossible. 
 
 Suppose, for instance, M to represent a parent race whose 
 
CONTINUITY. 215 
 
 offspring by successive changes through eons of time have 
 divaricated, and running opposite ways which we may denote 
 by the letters of the alphabet, have produced on the one hand a 
 species A, and on the other a species z : the changes here have 
 been so great that we should never expect directly to reproduce 
 an intermediate between A and z. A and B on the one hand, 
 and Y and z on the other, might reproduce ; but to regain the 
 original type M, we must not only retrocede through all the 
 intermediates, but must have similar circumstances recalled in 
 an inverse order at each phase of retrogression, conditions 
 which it is obviously impossible to fulfil. But though among 
 the higher forms of organic structure we cannot retrace the 
 effects of time and reproduce intermediate types, yet among 
 some of the lower forms we find it difficult to assign any line 
 of specific demarcation ; thus, as a result of the very elaborate 
 and careful investigations of Dr. Carpenter on Foraminifera, 
 he states : ' It has been shown that a very wide range of varia- 
 tion exists among Orbitolites, not merely as regards external 
 form, but also as to plan of development ; and not merely as 
 to the shape and aspect of the entire organism, but also with 
 respect to the size and configuration of its component parts. 
 It would have been easy, by selecting only the most divergent 
 types from amongst the whole series of specimens which I 
 have examined, to prefer an apparently substantial claim on 
 behalf of these to be accounted as so many distinct species. 
 But after having classified the specimens which could be 
 arranged around these types, a large proportion would yet 
 have remained, either presenting characters intermediate 
 between those of two or more of them, or actually combining 
 those characters in different parts of their fabric' ; thus show- 
 ing that no lines of demarcation can be drawn across any part 
 of the series that shall definitely separate it into any number 
 of groups, each characterised by features entirely peculiar to 
 itself.' 
 
 At the conclusion of his enquiry he states 
 
 I. The range of variation is so great among Foraminifera 
 
 as to include not merely the differential characters which sys- 
 
 tematists proceeding upon the ordinary methods have accounted 
 
 specific, but also those upon which the greater part of the 
 
2i6 CONTINUITY. 
 
 genera of this group have been founded, and even in some 
 instances those of its orders. 
 
 II. The ordinary notion of species as assemblages of indi- 
 viduals marked out from each other by definite characters that 
 have been genetically transmitted from original proto-types 
 similarly distinguished, is quite inapplicable to this group ; 
 since even if the limits of such assemblages were extended so 
 as to include what elsewhere would be accounted genera, they 
 would still be found so intimately connected by gradational 
 links that definite lines could not be drawn between them. 
 
 III. The only natural classification of the vast aggregate 
 of diversified forms which this group contains will be one 
 which ranges them according to their direction and degree of 
 divergence from a small number of principal family types ; 
 and any subordinate grouping of genera and species which 
 may be adopted for the convenience of description and nomen- 
 clature must be regarded merely as assemblages of forms 
 characterised by the nature and degree of the modifications 
 of the original type, which they may have respectively 
 acquired in the course of genetic descent from a common 
 ancestry. 
 
 IV. Even in regard to these family types it may fairly be 
 questioned whether analogical evidence does not rather favour 
 the idea of their derivation from a common original than that 
 of their primitive distinctness. 
 
 Mr. H. Bates, when investigating ' The Lepidoptera of the 
 Amazon Valley/ may almost be said to have witnessed the 
 origin of some species of butterflies, so- close have been his 
 observations on the habits of these animals that have led to 
 their variation and segregation, so closely do the results follow 
 his observations, and so great is the difficulty of otherwise 
 accounting for any of the observed facts. 
 
 In the numerous localities of the Amazon region certain 
 gregarious species of butterfly (Heliconided) swarm in incre- 
 dible numbers, almost outnumbering all the other butterflies 
 in the neighbourhood ; the species in the different localities 
 being different, though often to be distinguished by a very 
 slight shade. 
 
CONTINUITY. 217 
 
 In these swarms are to be found, in small numbers, other 
 species of butterflies belonging to as many as ten different 
 genera, and even some moths; and these intruders, though 
 they structurally differ in toto from the swarms they mingle 
 with, and from one another, mimic the Heliconidea so closely 
 in colours, habits, mode of flight, &c., that it is almost im- 
 possible to distinguish the intruders from those they mingle 
 with. The obvious benefit of this mimicry is safety, the 
 intruders hence escaping detection by predatory animals. 
 
 Mr. Bates has extended his observations to the habits of 
 life, food, variations, and geographical range of the species 
 concerned in these mimetic phenomena, and finds in every 
 case corroborative evidence of every variety and species being 
 derivative, the species being modified from place to place to 
 suit the peculiar form of Heliconidea stationed there. 
 
 Mr. Wallace has done similar service to the derivative 
 theory by his observations and writings on the butterflies and 
 birds of the Malay Archipelago, adducing instances of 
 mimetic resemblances strictly analogous to the above ; and 
 adding in further illustration a beautiful series of instances 
 where the form of the wing of the same butterfly is so modified 
 in various islets as to produce changes in their mode of flight 
 that tend to the conservation of the variety by aiding its 
 escape when chased by birds or predacious insects. 
 
 He has also adduced a multitude of examples of geogra- 
 phical and representative races, species, and varieties, forming 
 so graduated a series as to render it obvious that they have 
 had a common origin. 
 
 The effect of food in the formation and segregation of 
 races and of certain groups of insects has been admirably 
 demonstrated by Mr. B. D. Walsh, of North America. 
 
 Mr. McDonnell has been led to the discovery of a new 
 organ in electric fishes from the application of the theory 'of 
 descent, and Dr. Fritz Miiller has published numerous obser- 
 vations showing that organs of very different structure may, 
 through the operation of natural selection, acquire very similar 
 and even identical functions. Sir John Lubbock's diving 
 hymenopterous insect affords a remarkable illustration of 
 
2i8 CONTINUITY. 
 
 analogous phenomena ; it dives by the aid of its wings, and 
 is the only insect of the vast order it belongs to that is at all 
 aquatic. 
 
 The discovery of the Eozoon is of the highest importance 
 in reference to the derivative hypothesis, occurring as it does 
 in strata that were formed at a period inconceivably antece- 
 dent to the presupposed introduction of life upon the globe, 
 and displacing the argument derived from the supposition 
 that at the dawn of life a multitude of beings of high .organi- 
 sation were simultaneously developed (in the Silurian and 
 Cambrian strata). 
 
 Professor A. De Candolle, one of the most distinguished 
 Continental botanists, has, to some extent, abandoned the 
 tenets held in his ' Geographic Botanique,' and favours the 
 derivative hypothesis in his paper on the variation of oaks ; 
 following up a paper by Dr. Hooker, on the oaks of Palestine, 
 showing that some sixteen of them are derivative, he avows 
 his belief that two-thirds of the 300 species of this genus, 
 which he himself describes, are provisional only. 
 
 Dr. Hooker, who had only partially accepted the derivative 
 hypothesis propounded before the publication of ' The Origin 
 of Species through Natural Selection/ at the same time de- 
 clining the doctrine of special creation, has since then cordially 
 adopted the former, and illustrated its principles by applying 
 them to the solution of various botanical questions : first, in 
 reference to the flora of Australia, the anomalies of which he 
 appears to explain satisfactorily by the application of these 
 principles ; and, latterly, in reference to the Arctic flora. 
 
 In the case of the Arctic flora, he believes that originally 
 Scandinavian types were spread over the high northern lati- 
 tudes ; that these were driven southwards during the glacial 
 period, when many of them changed their forms in the struggle 
 that ensued with the displaced temperate plants ; that on the 
 returning warmth, the Scandinavian plants, whether changed 
 or not, were driven again northwards and up to the mountains 
 of the temperate latitudes, followed in both cases by series of 
 pre-existing plants of the temperate Alps. The result is the 
 present mixed Arctic flora, consisting of a basis of more or 
 less changed and unchanged Scandinavian plants, associated 
 
CONTINUITY. 219 
 
 in each longitude with representatives of the mountain flora 
 of the more temperate regions to the south of them. 
 
 The publication of a previously totally unknown flora, that 
 of the Alps of tropical Africa, by Dr. Hooker, has afforded a 
 multitude of facts that have been applied in confirmation of 
 the derivative hypothesis. This flora is found to have relation- 
 ships with those of temperate Europe and North Africa, of 
 the Cape of Good Hope, and of the mountains of tropical 
 Madagascar and Abyssinia, such as can be accounted for on 
 no other hypothesis but that there has been ancient climatal 
 connection and some coincident or subsequent slight changes 
 of specific character. 
 
 The doctrine of Cuvier, every day more and more borne 
 out by observation, that each organ bears a definite relation 
 to the whole of the individual, seems to support the view of 
 indefinite variation. If an animal seeks its food or safety 
 by climbing trees, its claws will become more prehensile, the 
 muscles which act upon those claws must become more de- 
 veloped, the body will become agile by the very exercise which 
 is necessary to it, and each portion of the frame will mould 
 itself to the wants of the animal by the effect on it of the 
 habits of the animal. 
 
 Another series of facts which present an argument in 
 favour of gradual succession, are the phases of resemblance 
 to inferior orders which the embryo passes through in its de- 
 velopment, and the relations shown in what is termed the 
 metamorphosis of plants ; facts difficult to account for on the 
 theory of frequent separate creations, but almost inevitable on 
 that of gradual succession. So also the existence of rudimen- 
 tary and effete organs, which must either be referred to a 
 lusus natures or to some mode of continuous succession. 
 
 The doctrine of typical nuclei seems only a mode of 
 evading the difficulty ; experience does not give us the types 
 of theory ; and, after all, what are these types ? It must be 
 admitted there are none such in reality ; how are we led to the 
 theory of them ? simply by a process of abstraction from 
 classified existences. Having grouped from natural similitudes 
 certain forms into a class, we select attributes common to each 
 member of the class, and call the assemblage of such attri- 
 
220 CONTINUITY. 
 
 butes a type of the class. This process gives us an abstract 
 idea, and we then transfer this idea to the Creator, and make 
 him start with that which our own imperfect generalisation 
 has derived. It seems to me that the doctrine of types is, in 
 fact, a concession to the theory of continuity or indefinite 
 variability ; for the admission that large groups have common 
 characters shows, necessarily, a blending of forms within the 
 scope of the group, which supports the view of each member 
 being derived from some other member of it : if this be so can 
 it be fairly asserted that even the assigned limits of such 
 groups have a definite line of demarcation ? 
 
 [The constant use of different terms has frequently led to 
 a settled conviction that the things signified are as broadly 
 distinct as the terms used. Take, for instance, the terms 
 ' instinct ' and ' reason ; ' for centuries these have been re- 
 garded as distinct mental attributes, and it is only in recent 
 years that the merging of the one into the other has been 
 discussed even as a possibility. 
 
 Is what is called instinct other than a process of reasoning 
 carried on in a limited sphere, and very perfect within that 
 sphere, because from long habit and hereditary transmission 
 it has become stereotyped in the race ? 
 
 The bee has learned gradually by using mere mechanical 
 means to save space by forcing the cylindrical cells of its honey- 
 comb against each other, until by pressing out the intervening 
 matter they become hexagons, and not by a mathematical in- 
 stinct, as it used to be called. By similar transmitted habits he 
 knows the honey-bearing flowers by their colour or their scent. 
 The mind of the higher animals the dog, for instance is more 
 diffuse, but attains less perfection in any limited sphere ; his 
 wants and habits being more various, his brain has more 
 varied power ; but when he buries a bone or points at a par- 
 tridge I see no reason for supposing that he feels compelled by 
 some power extraneous to himself to do these acts, but pro- 
 bably feels as much acting from the impulse of his own mind 
 as an Esquimaux making a 'cache' or lying in wait for a seal. 
 Thoughts which may seem intuitive in the individual are pro- 
 bably the result of experience accumulated and transmitted 
 in the race, and gradually modifying the structure of the brain 
 
CONTINUITY. 221 
 
 in the course of transmission. This has long been my opinion, 
 and, as I understand, it is the view advocated by Mr. Herbert 
 Spencer in different parts of his valuable works.] 
 
 The condition of the earth's surface, or at least of large 
 portions of it, has for long periods remained substantially the 
 same; this would involve a greater degree of fixity in the 
 organisms which have existed during such periods of little 
 change than in those which have come into being during 
 periods of more rapid transition ; for, though rejecting catas- 
 trophes as the modus agendi of nature, I am far from saying 
 that the march of physical changes has been always perfectly 
 uniform. 
 
 There have been, doubtless, what may be termed secular 
 seasons, and there have been local changes of varying degrees 
 of extent and permanence ; from such causes the characteristics 
 of organised beings would be more concentrated in certain di- 
 rections than in others, the fixity of character being in the ratio 
 of the fixity of condition. This would throw natural forms into 
 certain groups which would be more prominent than others, 
 like the colours of the rainbow, which present certain pre- 
 dominant tints, though they merge into each other by 
 insensible gradations. 
 
 While the evidence seems daily becoming stronger in 
 favour of a derivative hypothesis as applied to the succession 
 of organic beings, we are far removed from anything like a 
 sufficient number of facts to show that, at all events within 
 the existing geological periods capable of being investigated, 
 there has been always a progression from a simpler or more 
 embryonic to a more complex type. 
 
 Professor Huxley, though inclined to the derivative hypo- 
 thesis, shows, in the concluding portion of his address to the 
 Geological Society, 1862, a great number of cases in which, 
 though there is abundant evidence of variation, there is none 
 of progression. There are, however, several groups of Verte- 
 brata in which the endoskeleton of the older presents a less 
 ossified condition than that of the younger genera. He cites 
 the Devonian Ganoids, the Mesozoic Lepidosteidse, the 
 Palaeozoic Sharks, and the more ancient Crocodilia and 
 Lacertilia, and particularly the Pycnodonts and Labyrintho- 
 
222 CONTINUITY. 
 
 donts, as instances of this when compared with their more 
 recent representatives. 
 
 The records of life on the globe may have been destroyed 
 by the fusion of the rocks, which would otherwise have pre- 
 served them, or by crystallisation after hydrothermal action. 
 The earlier forms may have existed at a period when this 
 planet was in course of formation, or being segregated or 
 detached from other worlds or systems. We have hardly evi- 
 dence enough to speculate on the subject, but by time and 
 patience we may acquire it. 
 
 Were all the forms which have existed embalmed in rock 
 the question would be solved ; but what a small proportion 
 of extinct forms is so preserved, and must be, if we consider 
 the circumstances necessary to fossilise organic remains ! On 
 the dry land, unwashed by rivers and seas, when an animal 
 or plant dies, it undergoes chemical decomposition, which 
 changes its form ; it is consumed by insects, its skeleton is 
 oxidised and crumbles into dust. Of the myriads of animals 
 and vegetables which annually perish we find hardly an 
 instance of a relic so preserved as to be likely to become a 
 permanent fossil. So again in the deeper parts of the ocean, 
 or of the larger lakes, the few fish there are perish and their 
 remains sink to the bottom, and are there frequently consumed 
 by other marine or lacustrine organisms or are chemically 
 decomposed. As a general rule, it is only when the remains 
 are silted up by marine, fiuviatile or lacustrine sediments that 
 the remains are preserved. Geology, therefore, might be ex- 
 pected to keep for us mainly those organic remains which 
 inhabited deltas or the margins of seas, lakes, or rivers ; here 
 and there an exception may occur, but the mass of preserved 
 relics would be those of creatures so situated ; and so we find 
 it, the bulk of fossil remains consists of fish and amphibia ; 
 shell-fish form the major part of the geological museum, 
 limestone and chalk rocks frequently consisting of little else 
 than a congeries of fossil shells ; plants of reed or rush-like 
 character, fish which are capable of inhabiting shallow waters, 
 and saurian animals, form another large portion of geological 
 remains. 
 
 Compare the shell-fish and amphibia of existing organisms 
 
CONTINUITY. 223 
 
 with the other forms, and what a small proportion they 
 supply ; compare the shell-fish and amphibia of Palaeontology 
 with the other forms, and what an overwhelming majority 
 they yield ! 
 
 There is nothing, as Professor Huxley has remarked, like 
 an extinct order of birds or mammals, only a few isolated 
 instances. It may be said the ancient world possessed a 
 larger proportion of fish and amphibia, and was more suited 
 to their existence. I see no reason for believing this, at least 
 to anything like the extent contended for ; the fauna and 
 flora now in course of being preserved for future ages would 
 give the same idea to our successors. 
 
 Crowded as Europe is with cattle, birds, insects, &c., 
 how few are geologically preserved ! while the muddy or 
 sandy margins of the ocean, the estuaries, and deltas are 
 yearly accumulating numerous Crustacea and mollusca, with 
 some fishes and reptiles, for the study of future palaeonto- 
 logists. 
 
 If this position be right, then, notwithstanding the immense 
 number of preserved fossils, there must have lived an im- 
 measurably larger number of organic beings which are not pre- 
 served, so that the chance of filling up the missing links, except 
 in occasional instances, is very slight. Yet, where circumstances 
 have remained suitable for their preservation, many closely 
 connected species are preserved in other words, while the 
 intermediate types in certain cases are lost, in others they 
 exist. The opponents of continuity lay all stress on the lost 
 and none on the existing links. 
 
 But there is another difficulty in the way of tracing a 
 given organism to its parent forms, which, from our con- 
 ventional mode of deducing genealogies, is never looked upon 
 in its proper light. 
 
 Where are we to look for the remote ancestor of a given 
 form ? Each of us, supposing none of our progenitors to 
 have intermarried with relatives, would have had at or about 
 the period of the Norman Conquest upwards of a hundred 
 million direct ancestors of that generation, and if we add the 
 intermediate ancestors, double that number. As each in- 
 dividual has a male and female parent, we have only to 
 
224 CONTINUITY. 
 
 multiply by two for each thirty years, the average duration of 
 a generation, and it will give the above result. 
 
 Let anyone assume that one of his ancestors at the time 
 of the Norman Conquest was a Moor, another a Celt, and a 
 third a Laplander, and that the remains of these three were 
 preserved, while those of all the others were lost, he would 
 never recognise either of them as his ancestor ; he would only 
 have the one-hundred millionth of the blood of each of them, 
 and, as far as they were concerned, there would be no per- 
 ceptible sign of identity of race. 
 
 But the problem is more complex than that which I have 
 stated. At the time of the Conquest there were hardly a 
 hundred million people in Europe : it follows that a great 
 number of the ancestors of the propositus must have inter- 
 married with relations, and then the pedigree, going back to 
 the time of the Conquest, instead of being represented by 
 diverging lines, would form a network so tangled that no 
 skill could unravel it. The law of probabilities would indicate 
 that any two people in the same country, taken at hazard, 
 would not have many generations to go back before they 
 would find a common ancestor, who, probably, could they 
 have seen him or her in the life, had no traceable resemblance 
 to either of them. Thus, two animals of a very different 
 form, and of what would be termed very different species, 
 might have a common geological ancestor, and yet the skill 
 of no comparative anatomist could trace the descent. 
 
 From the long-continued conventional habit of tracing 
 pedigrees through the male ancestor, we forget, in talking of 
 progenitors, that each individual has a mother as well as a 
 father, and there is no reason to suppose that he has in him 
 less of the blood of the one than of the other. 
 
 The recent discoveries in palaeontology show us that Man 
 existed on this planet at an epoch far anterior to that com- 
 monly assigned to him. The instruments connected with 
 human remains, and indisputably the work of human hands, 
 show that to these remote periods the term civilisation could 
 hardly be applied chipped flints of the rudest construction, 
 probably, in the earlier cases, fabricated by holding an amor- 
 phous flint in the hand and chipping off portions of it by 
 
CONTINUITY. 225 
 
 striking it against a larger stone or rock ; then, as time sug- 
 gested improvements, it would be more carefully shaped, and 
 another stone used as a tool ; then (at what interval we can 
 hardly guess) it would be ground, then roughly polished, and 
 so on subsequently bronze weapons, and nearly the last 
 before we come to historical periods, iron. Such an appa- 
 rently simple invention as a wheel must, in all probability, 
 have been far subsequent to the rude hunting tools or 
 weapons of war to which I have alluded. 
 
 A little step-by-step reasoning will convince the unpre- 
 judiced that what we call civilisation must have been a 
 gradual process ; can it be supposed that the inhabitants of 
 Central America or of Egypt suddenly and what is called 
 instinctively built their cities, carved and ornamented their 
 monuments ? If not, if they must have learned to construct 
 such erections, did it not take time to acquire such learning, 
 to invent tools as occasion required, contrivances to raise 
 weights, rules or laws by which men acted in concert to effect 
 the design ? Did not all this require time ? and if, as the 
 evidence of historical times shows, invention marches with a 
 geometrical progression, then, viewing its progress inversely, 
 how slow must have been the earlier steps ! If even now 
 habit, and prejudice resulting therefrom, vested interests, &c., 
 retard for some time the general application of a new inven- 
 tion, or the adoption of a new social change, what must have 
 been the degree of retardation among the comparatively un- 
 educated beings which then existed ? 
 
 I have of course been able to indicate only a few of the 
 broad arguments on this most interesting subject ; for de- 
 tailed data on which my reasoning is founded, the works of 
 Lamarck, Darwin, Hooker, Huxley, Carpenter, Lyell, and 
 others must be examined. If I lean to the view that the 
 successive changes in organic beings do not take place by 
 sudden leaps, it is, I believe, from no want of an impartial 
 feeling ; but if the facts are stronger in favour of one theory 
 than another, it would be an affectation of impartiality to 
 make the balance appear equipoised. 
 
 The prejudices of education and associations with the past 
 are against this, as against all new views ; and while on the 
 
 Q 
 
226 CONTINUITY. 
 
 one hand a theory is not to be accepted because it is new and 
 primd facie plausible, still to this assembly I need not say 
 that its running counter to existing opinions is not necessarily 
 a reason for its rejection ; the onus probandi should rest on 
 those who advance a new view, but the degree of proof must 
 differ with the nature of the subject. The fair question is, 
 Does the newly-proposed view remove more difficulties, re- 
 quire fewer assumptions, and present more consistency with 
 observed facts than that which it seeks to supersede ? If so, 
 the philosopher will adopt it, and the world will follow the 
 philosopher after many days. 
 
 It must be borne in mind that, even if we are satisfied 
 from a persevering and impartial enquiry that organic forms 
 have varied indefinitely in time, the causa causans of these 
 changes is not explained by our researches ; if it be admitted 
 that we find no evidence of amorphous matter suddenly 
 changed into complex structure, still why matter should be 
 endowed with the plasticity by which it slowly acquires 
 modified structure is unexplained. If we assume that con- 
 tinuous effort, natural selection, or the struggle for existence, 
 coupled with the tendency of like to reproduce like, gives 
 rise to various organic changes, still our researches are at 
 present uninstructive as to why like should produce like, why 
 acquired characteristics in the parent should be reproduced in 
 the offspring. Reproduction itself is still an enigma, and this 
 great question may involve deeper thoughts than it would be 
 suitable to enter upon now. 
 
 Perhaps primd facie the most striking argument in favour 
 of continuity which could be presented to a doubting mind 
 would be the difficulty it would feel in representing to itself 
 any per saltum act of nature. Who would not be astonished 
 at beholding an oak tree spring up in a day, and not from 
 seed or shoot ? We are forced by experience, though often 
 unconsciously, to believe in continuity as to all effects now 
 taking place ; if any one of them be anomalous we endeavour, 
 by tracing its history and concomitant circumstances, to find 
 its cause, i.e. to relate it to antecedent phenomena ; are we 
 then to reject similar enquiries as to the past ? Is it laudable 
 to seek an explanation of present changes by observation, 
 
CONTINUITY. 227 
 
 experiment, and analogy, and yet reprehensible to apply the 
 same mode of investigation to the past history of the earth 
 and of the organic remains embalmed in it ? 
 
 If we disbelieve in sudden creations of matter or force, in 
 the sudden formations of complex organisms now, if we now 
 assign to the heat of the sun an action enabling vegetables to 
 live by assimilating gases, carbon, and amorphous earths into 
 growing structures, why should such effects not have taken 
 place in earlier periods of the world's history, when the sun 
 shone as now, and when the same materials existed for his 
 rays to fall upon ? 
 
 If we are satisfied that continuity is a law of nature, the 
 true expression of the action of Almighty Power, then, though 
 we may humbly confess our inability to explain why matter 
 is impressed with this tendency to gradual structural forma- 
 tion, we should cease to look for special interventions of 
 creative power in changes which are difficult to understand, 
 because, being removed from us in time, their concomitants 
 are lost ; we should endeavour from the relics to evoke their 
 history, and when we find a gap not try to bridge it over with 
 a miracle. 
 
 If it be true that continuity pervades all physical pheno- 
 mena, the doctrine applied by Cuvier to the relations of the 
 different parts of an animal to each other might be capable 
 of great extension. All the phenomena of inorganic and 
 organised matter might be expected to be so inter-related 
 that the study of an isolated phenomenon would lead to a 
 knowledge of numerous other phenomena with which it is 
 connected. As the antiquary deduces from a monolith the 
 tools, the arts, the habits, and epoch of those by whom it is 
 wrought, so the student of science may deduce from a spark of 
 electricity or ray of light the source whence it is generated ; and 
 by similar processes of reasoning other phenomena hitherto 
 unknown may be deduced from their probable relation with 
 the known. But, as with heat, light, magnetism, and electri- 
 city, though we may study the phenomena to which these 
 names have been given, and their mutual relations, we know 
 nothing of their ultimate cause ; so, whether we adopt the view 
 of natural selection, of effort, of plasticity, &c., we know not 
 
 Q 2 
 
228 CONTINUITY. 
 
 why organisms should have this nisus formativus, or why the 
 acquired habit or exceptional quality of the individual should 
 reappear in the offspring. 
 
 Philosophy ought to have no likes or dislikes truth is her 
 only aim ; but if a glow of admiration be permitted to a . 
 physical enquirer, to my mind a far more exquisite sense of 
 the beautiful is conveyed by the orderly development, by the 
 necessary inter-relation and inter-action of each element of 
 the cosmos, and by the conviction that a bullet falling to the 
 ground changes the dynamical conditions of the universe, 
 than can be conveyed by mysteries, by convulsions, or by 
 cataclysms. 
 
 The sense of understanding is to the educated more grati- 
 fying than the love of the marvellous, though the latter need 
 never be wanting to the nature-seeker. 
 
 But the doctrine of continuity is not solely applicable to 
 physical enquiries. 
 
 The same modes of thought which lead us to see con- 
 tinuity in the field of the microscope as in the universe, in 
 infinity downwards as in infinity upwards, will lead us to see 
 it in the history of our own race ; the revolutionary ideas of 
 the so-called natural rights of man, and a priori reasoning 
 from what are termed first principles, are far more unsound 
 and give us far less ground for improvement of the race than 
 the study of the gradual progressive changes arising from 
 changed circumstances, changed wants, changed habits. Our 
 language, our social institutions, our laws, the constitution of 
 which we are proud, are the growth of time, the product of 
 slow adaptations, resulting from continuous struggles. Happily 
 in this country practical experience has taught us to improve 
 rather than to remodel ; we follow the law of nature and avoid 
 cataclysms. 
 
 [My own impression is that the philosophy of the future, 
 not merely as applied to physical forces and the science of 
 organisms, but to the history of the human race, its habits, 
 laws, languages, and possibly thoughts themselves, will be 
 mainly based on the doctrine of continuity, and that instead 
 of enquiries as to why a thing is in the sense of ascertaining 
 its ultimate causation, the research will be into the question 
 
CONTINUITY. 229 
 
 how did it become what it is ? by what steps of change, by 
 what mode of force did the substance, the phenomenon, the 
 organism, the habit or the event arise ?] 
 
 The superiority of Man over other animals inhabiting this 
 planet, of civilised over savage man, and of the more civilised 
 over the less civilised, is proportioned to the extent which his 
 thought can grasp of the past and of the future. His memory 
 reaches farther back, his capability of prediction reaches 
 farther forward in proportion as his knowledge increases. He 
 has not only personal memory which brings to his mind at 
 will the events of his individual life he has history, the 
 memory of the race; he has geology, the history of this 
 planet ; he has astronomy, the geology of other worlds. 
 Whence does the conviction to which I have alluded, that each 
 material form bears in itself the records of its past history, 
 arise ? Is it not from the belief in continuity ? Does not the 
 worn hollow in the rock record the action of the tide, its 
 stratified layers the slow deposition by which it was formed, 
 the organic remains imbedded in it, the beings living at the 
 times these layers were deposited, so that from a fragment of 
 stone we can get the history of a period myriads of years ago ? 
 From a fragment of bronze we may get the history of our 
 race at a period antecedent to tradition. As science advances 
 our power of reading such history improves and is extended. 
 Saturn's ring may help us to a knowledge of how our solar 
 system developed itself, for it as surely contains that history 
 as the rock contains the record of its own formation. 
 
 By this patient investigation how much have we already 
 learned, which the most civilised of ancient human races 
 ignored ! While in ethics, in politics, in poetry, in sculpture, 
 in painting, we have scarcely, if at all, advanced beyond the 
 highest intellects of ancient Greece or Italy, how great are the 
 steps we have made in physical science and its applications ! 
 
 But how much more may we not expect to know ! 
 
 We, this evening assembled, Ephemera as we are, have 
 learned by transmitted labour, to weigh, as in a balance, other 
 worlds larger and heavier than our own, to know the length 
 of their days and years, to measure their enormous distance 
 from us and from each other, to detect and accurately ascertain 
 
230 CONTINUITY. 
 
 the influence they have on the movements of our world and 
 on each other, and to discover the substances of which they 
 are composed ; may we not fairly hope that similar methods 
 of research to those which have taught us so much may give 
 our race farther information, until problems relating not only 
 to remote worlds, but possibly to organic and sentient beings 
 which may inhabit them problems which it might now seem 
 wildly visionary to enunciate may be solved by progressive 
 improvements in the modes of applying observation and 
 experiment, induction and deduction ? 
 
231 
 
 ON A NEW VOLTAIC COMBINATION. 
 
 Phil. Mag., 1839.* 
 
 MR. PoRRETT was, I believe, the first who employed a blad- 
 der to separate the liquids in the operating cell of the voltaic 
 pile. M. Becquerel, by introducing this into the exciting 
 cells, has shown us how to render constant the primitive in- 
 tensity of the battery by preventing cross precipitation ; Mr. 
 Daniell has remedied some practical defects in M. Becquerel's 
 arrangement, and his form of battery is undoubtedly the best 
 of any that have been hitherto proposed. 
 
 In a letter published in the ' Philosophical Magazine ' for 
 February I endeavoured to show, that in addition to the im- 
 mense benefit derived from constancy of action, which was the 
 object aime<4 at by these gentlemen, the combination of four 
 elements was capable of producing a much more powerful 
 development of electricity than that of three, as by this means 
 we have nearly the sum of chemical affinities instead of their 
 difference ; I also there suggested that if the principles I had 
 laid down were true, there was every probability of superior 
 combinations being discovered. I have lately been fortunate 
 enough to hit upon a combination which I have no hesitation 
 in pronouncing much more powerful than any previously 
 known. The experiments which led to it are curious, and 
 possess an interest of their own, as they prove a well-known 
 chemical phenomenon to depend upon electricity, and thus 
 tighten the link which binds these two sciences. The effect to 
 which I allude is the dissolution of gold in nitro-muriatic acid ; 
 this metal, as is well known, not being attacked by either of 
 the acids singly. The following experiments leave, I think, 
 no doubt as to the rationale of this phenomenon : 
 
 * From the Comptes Rendus, Academic des Sciences, Paris, 
 
232 EXPERIMENTAL INVESTIGATIONS. 
 
 1. Into the bottom of a wineglass I cemented the bowl of 
 a tobacco-pipe ; into this was poured pure nitric acid, while 
 muriatic acid was poured into the wine glass to the same 
 level ; in this latter acid two strips of gold-leaf were allowed 
 to remain for an hour, at the end of which time they remained 
 as bright as when first immersed. A gold wire was now made 
 to touch the nitric acid and the extremity of one of the strips 
 of gold-leaf ; this was instantly dissolved, while the other 
 strip remained intact. 
 
 2. The experiment was inverted, but offered some diffi- 
 culty, as the gold would not remain an equally long time in the 
 nitric acid, from the effect of the nitrous gas ; enough, how- 
 ever, was ascertained to prove that to the gold in this acid 
 contact made little or no difference ; while the gold in the 
 muriatic was always dissolved. 
 
 3. A platina arc was used for connection instead of gold ; 
 the effect was the same. 
 
 4. The outside of the pipe was coated with gold-leaf, 
 leaving scarcely any part exposed ; a strip was placed in the 
 muriatic acid as before, and when contact was made with the 
 nitric acid this strip was destroyed, while the coating of gold 
 directly across the line of junction was unhurt. 
 
 5. The nitric acid was stained with a little tournesol ; when 
 contact was made, I could not see that the muriatic acid 
 acquired any of the colour. 
 
 6. Nitrate of copper was used instead of nitric acid ; the 
 effect was the same, but took place more slowly, and I could 
 detect no precipitation on the negative metal. 
 
 7. I now made gold-leaf in muriatic acid the electrodes 
 of a single pair of voltaic metals ; the acid was decomposed 
 and the positive electrode was dissolved. 
 
 From all this I think we may pronounce the action to be 
 as follows : as soon as the electric current is established, both 
 the acids are decomposed, the Tiydrogen of the muriatic unites 
 with the oxygen of the nitric, and the chlorine attacks the gold. 
 
 In all these cases the currents were examined with a gal- 
 vanometer, and in all the gold which was dissolved repre- 
 sented the zinc of an ordinary voltaic combination. The 
 greatest deflection was obtained with platina, gold, and two 
 
NITRIC ACID BATTERY. 233 
 
 acids. It now occurred to me that as gold, platina, and two 
 acids gave so powerful an electric current, a fortiori, the same 
 arrangement, with the substitution of zinc for gold, must form 
 a combination more energetic than any yet known. I delayed 
 not to submit this to experiment, and was gratified with the 
 most complete success. A single pair, composed of a strip of 
 amalgamated zinc an inch long and a quarter of an inch 
 wide, a cylinder of platina three-quarters of an inch high, with 
 a tobacco-pipe bowl and an egg-cup, readily decomposed 
 water acidulated with sulphuric acid. In this battery the 
 action is constant, and there is no precipitation on either 
 metal. It offers the great advantage of being able to utilise 
 the action of concentrated nitric acid. I tried the same 
 arrangement, substituting for the muriatic acid caustic potass, 
 which was suggested to me by a well-known experiment of 
 M. Becquerel : the action was equally powerful ; and I should 
 prefer this arrangement, as there is no necessity for amalga- 
 mating the zinc, but for a fatal objection the nitrate of potass, 
 crystallising in the pores of the earthenware, splits it to 
 pieces ; except, therefore, a new description of diaphragm be 
 discovered, which will bear the action of powerful acids, this 
 combination must be abandoned. 
 
 I diluted the muriatic acid with twice its volume of water, 
 and the effect was not perceptibly inferior. I then tried sul- 
 phuric with four or five times its volume of water ; the intensity 
 was a little diminished, but so little that I should prefer this 
 combination to any other, as cheaper, exercising less local 
 action on the zinc, and by no possibility endangering the pla- 
 tina. The nitric acid may be the common acid of commerce, 
 but must be concentrated. If the hydrogen, instead of being 
 absorbed by the oxygen of the nitric acid, is evolved on 
 the surface of the platina, the energy of action is lowered and 
 is no longer constant. 
 
 Great advantage will be found in employing a cell divided 
 by a porous diaphragm for a decomposing cell ; thus, if oxygen 
 gas be wanted, the positive electrode should be put into dilute 
 sulphuric acid and the negative into concentrated nitric. If 
 chlorine be wanted, the positive into muriatic, the negative 
 into nitric ; if hydrogen, both into muriatic ; the positive one 
 
234 EXPERIMENTAL INVESTIGATIONS. 
 
 being of amalgamated zinc, &c. &c. By this means, and 
 with a small battery of the description I am about to indi- 
 cate, a traveller may carry in his pocket an electro-chemical 
 laboratory. 
 
 I have constructed a small battery, of a circular shape, 
 consisting of seven liqueur-glasses and seven pipe-bowls ; the 
 diameter is four inches, the height one inch and a quarter ; this 
 pocket battery gives about one cubic inch of mixed gases in 
 two minutes. The form of this combination is in effect that of 
 Mr. Daniell ; the connexions, however, never need adjustment 
 but when the worn-out zinc is renewed ; and in a battery 
 which M. Becquerel and myself are about to construct I hope 
 to remedy another defect, viz. the necessity of pouring solu- 
 tions separately into each cell, which is troublesome and inju- 
 rious, from the inequality of strength which results. I have 
 entirely remedied this in a copper and zinc constant battery ; 
 but though the process be simple, it would require more 
 words to describe than a matter of such minor importance is 
 worth. 
 
 ON VOLTAIC REACTION, OR THE PHENOMENA 
 USUALLY CALLED POLARISATION. 
 
 Philosophical Mag.) 1840. Report of Royal Institution Lecture. 
 
 MR. GROVE detailed the first experiments of Volta, Erman, 
 Ritter, and Davy, the more recent ones of De la Rive, the 
 explanation of these by Becquerel, and the confirmation of 
 this latter philosopher's opinion by the experiments of Dr. 
 Schcenbein, Mr. Matteucci, and Mr. Grove himself; all which, 
 as well as the experiments of Mr. Grove on the inactivity of 
 amalgamated zinc, which he proved to be due to the same 
 order of causes, have been already given in full in various 
 numbers of the ' Philosophical Magazine.' All the effects 
 which have generally been included under the term polarisa- 
 tion were proved by Mr. Grove to be traceable to one prin- 
 ciple, viz. the electrolytic transfer of elements having for 
 
VOLTAIC REACTION. 235 
 
 each other a chemical affinity, and the reaction caused by 
 this affinity when the decomposing and transferring power, i.e. 
 the initial voltaic current, is arrested. What we are most 
 anxious to call the attention of our readers to are the effects 
 exhibited by Mr. Grove at the conclusion of his lecture. Two 
 batteries, little differing in construction from that described 
 by him in the ' Lond. and Edinb. Phil. Mag./ were charged 
 some time previously to the lecture, and up to the period of its 
 conclusion remained in perfect inactivity until the circuit was 
 completed. One of these was arranged as a series of five plates, 
 and contained altogether about four square feet of platina foil ; 
 with this the mixed gases were liberated from water at the 
 rate of one hundred and ten cubic inches per minute. A sheet 
 of platinum, one inch wide by twelve long, was heated in the 
 open air through its whole extent, and the usual class of 
 effects produced in corresponding proportion. With the other 
 arrangement, consisting of fifty plates of two inches by four, 
 arranged in single series, a voluminous flame of one inch and 
 a quarter long was exhibited by charcoal points, which showed 
 beautifully the magnetic properties of the voltaic arc, as Dr. 
 Faraday held a piece of iron near it, being attracted and 
 repelled by different portions of the iron : 'bars of different 
 metals were instantly run into globules and dissipated in 
 oxide. It should be borne in mind that all these effects were 
 produced by a battery which did not cover a space of sixteen 
 inches square, and was only four inches highland which had 
 been charged for some hours. 
 
 Mr. Grove adverted to the letter of Prof. Jacobi to Dr. 
 Faraday published in ' Lond. and Edinb. Phil. Mag./ vol. xv. 
 p. 1 6 1, and stated that Mr. Pattison, who navigated the Neva 
 with Prof. Jacobi in October last, had observed that the bat- 
 teries employed were on Mr. Grove's construction, which the 
 Professor without hesitation admitted. 
 
236 EXPERIMENTAL INVESTIGATIONS. 
 
 ON THE INACTION OF AMALGAMATED ZINC IN 
 ACIDULATED WATER.* 
 
 Phil. Mag., August 1839. 
 
 IT is a well-known fact that the common zinc of commerce, 
 when immersed in water acidulated by sulphuric, phosphoric, 
 or muriatic acid, is rapidly dissolved, evolving torrents of 
 hydrogen gas, while zinc with the surface amalgamated 
 remains inactive under similar circumstances, unless touched 
 by another metal placed in the same solution, in which case 
 hydrogen, amounting to the equivalent of the oxygen which 
 unites with the zinc, is evolved from the surface of the 
 associated metal, and the zinc is tranquilly dissolved. 
 
 M. De la Rive observed that pure zinc is much less vigor- 
 ously attacked by diluted acid than commerical zinc, and has 
 thence, after a careful experimental investigation, concluded 
 that the evolution of gas by common zinc arises from the 
 circumstance of its adulteration by other metals ; thus an 
 infinity of minute voltaic circles is established, the particles 
 of zinc being oxidated, while those of the more negative 
 metals evolve hydrogen ('Bib. Univ./ 1830). This explana- 
 tion does not apply to the inactivity of amalgamated zinc, 
 for, as M. Becquerel asks (' Traite,' vol. v. p. 8), ' Why does 
 not mercury, which by its contact with zinc and acidulated 
 water must also form a voltaic combination, produce a similar 
 effect ? ' 
 
 An accidental circumstance led me to some experiments 
 which I think give a satisfactory answer to this question. The 
 circumstance to which I allude, and which has in all proba- 
 bility been observed by many others, was this : in decom- 
 posing by a voltaic battery water acidulated with sulphuric 
 acid, there happened to be a few globules of mercury at the 
 bottom of the operating cell or glass containing the electrodes 
 of platinum. I remarked that whenever the negative electrode 
 touched the mercury it became amalgamated ; at first I attri- 
 
 * Read before the Royal Academy of Sciences of Paris, on June 24, 1839. 
 
INACTION OF AMALGAMATED ZINC. 237 
 
 buted this effect to the reduction of a film of oxide of mercury 
 by the nascent hydrogen, but on touching the negative elec- 
 trode thus amalgamated with the positive one, the latter also 
 became frequently amalgamated. After several experiments 
 I found that mercury which had acted in acidulated water as 
 negative electrode of a voltaic battery possessed the property 
 of amalgamating platinum and iron, and that strips of plati- 
 num, iron, and even steel, which had served as negative elec- 
 trodes, were instantly and perfectly amalgamated by immersion 
 in pure mercury. 
 
 Having cleansed from acid particles several portions of 
 mercury which had been used as negative electrodes, I found 
 that they invariably gave an alkaline reaction, and it now 
 became evident that the increased power of amalgamation 
 proceeded from the mercury being alloyed with an alkaline 
 metal. Remembering the highly electro-positive state of 
 mercury which contains the slightest traces of an alkaline 
 metal, a property first noticed by Sir Humphry Davy, it oc- 
 curred to me that the inaction of amalgamated zinc was the 
 effect of polarisation,* but of one which differed from ordinary 
 cases of polarisation, in that the cations of the electrolyte 
 instead of being . precipitated on the surface of the negative 
 metal, combine with it and render it so completely positive 
 that the current is nullified, and not merely reduced in inten- 
 sity as in other cases. To verify this idea I made the following 
 experiments : 
 
 1. I amalgamated half the surface of a strip of copper, and 
 immersed it and a strip of zinc in water, acidulated with 
 sulphuric or phosphoric acid ; on making the plates touch 
 there was a rapid evolution of gas from the unamalgamated 
 part of the copper, while only a few detached bubbles appeared 
 on the amalgamated portion. 
 
 2. I placed a large globule of mercury (about half an 
 ounce) in the bottom of a glass of acidulated water, and by 
 means of a copper wire, the whole surface of which was 
 amalgamated, made it communicate with one extremity of a 
 
 * I know of no other word to express the effect here alluded to ; the word is 
 used in this sense by most French writers, but, from its numerous applications, is 
 sadly inaccurate. 
 
238 EXPERIMENTAL INVESTIGATIONS. 
 
 galvanometer, while a strip of amalgamated zinc immersed 
 in the same liquid communicated with the other extremity ; 
 at the instant of communication an energetic current was in- 
 dicated, which however immediately diminished in intensity, 
 and at the end of a few minutes the needle returned to zero ; 
 scarcely any gas was evolved, and of the few bubbles which 
 appeared as much could be detected on the surface of the 
 zinc as of the mercury. 
 
 3. With the same arrangement I substituted for the mer- 
 cury a strip of platinum well amalgamated. In this case, as 
 before, after a few minutes the current became null, or so 
 feeble as to require a delicate instrument to indicate its exist- 
 ence ; and if, after the cessation of the current, the zinc was 
 changed for unamalgamated platinum, this latter evolved tor- 
 rents of hydrogen, and the needle indicated a violent current 
 in a contrary direction. 
 
 4. With things arranged as in experiment 2, I employed 
 a solution of sulphate of copper as an electrolyte instead of 
 acidulated water ; an energetic and constant current was pro- 
 duced, the mercury became amalgamated to saturation with 
 reduced copper, and the precipitation of copper upon this 
 amalgam continued as long as crystals of the sulphate were 
 added to the solution. 
 
 By these experiments it appears that mercury, which in its 
 normal state is well known to be inefficient as the positive 
 metal of a voltaic combination, is in many cases equally in- 
 efficient as a negative metal, from its faculty of combining with 
 the cations of the electrolytes, which, by rendering it equally 
 positive with the metal with which it is voltaically associated, 
 cause the opposed forces to neutralise each other. But if, as in 
 experiment 4, the cation of the electrolyte is not of a highly 
 electro-positive character, the zinc (or other associated metal) 
 retains its superior oxidability and the voltaic current is not 
 arrested. The application of these experiments to the pheno- 
 mena presented by amalgamated zinc is evident ; all the hete- 
 rogeneous metals with which the zinc may be adulterated, and 
 which form minute negative elements, being amalgamated, 
 become by polarisation equally positive with the particles of 
 zinc, and consequently without the presence of another metal 
 
INACTION OF AMALGAMATED ZINC. 239 
 
 to complete the circuit, all action is arrested, as in the case of 
 pure zinc. The fact of amalgamated zinc being positive with 
 respect to common zinc, of its precipitating copper from its 
 solutions, and other anomalies, are also explained by these 
 experiments. 
 
 As in a common voltaic combination of zinc and mercury 
 this effect is complicated by the variety of cations which are 
 transferred to the negative metal, for instance, hydrogen, zinc, 
 and alkaline matters,* I was anxious to discover whether 
 hydrogen alone could, by its combination in small quantities 
 with mercury, give it the same electro-positive qualities. Sir 
 Humphry Davy, aided by a costly apparatus, scarcely sucr- 
 ceeded in purifying water of alkaline matters, but the affinity 
 of mercury for the alkaline metals gave me some hopes of 
 attaining this object with less expensive means ; 'to this end I 
 submitted to electrolysation for five days, in a vessel of bees- 
 wax, distilled water acidulated with pure sulphuric acid ; the 
 negative electrode, of amalgamated copper, terminating in a 
 mass of mercury. At the end of this period I removed the 
 mercury, substituting a fresh portion of the same metal which 
 was perfectly pure, and renewed the electrolysation for two 
 hours ; I then quickly enclosed this last mercury in a tube 
 with the purified water ; it always evolved a small quantity of 
 hydrogen, but I could not determine with certainty that its 
 volume bore any given proportion to that of the mercury 
 employed. Although this last portion of mercury when 
 carefully cleaned and tested with reddened litmus-paper gave 
 no alkaline reaction, yet its existence might be suspected as 
 derived from the wax ; and as metallic vessels were obviously 
 objectionable, I sought other means of determining the part 
 played by the hydrogen in this combination. I repeated ex- 
 periment 2, keeping heated, below the boiling point, the 
 vessel containing the zinc and mercury ; my galvanometer 
 gave a tolerably constant deflection of sixty degrees, and the 
 mercury evolved much more hydrogen than when the appa- 
 ratus was cold. Again, I kept for some time a strip of \vell- 
 
 * The current is not so completely null when dilute muriatic acid is the elec- 
 trolyte, as it is with sulphuric or phosphoric acid ; perhaps the sulphur or phos- 
 phorus contributes to the effect ; the difference is, however, but trifling. 
 
240 EXPERIMENTAL INVESTIGATIONS. 
 
 cleaned platinum in hydrogen gas, and then immersed it in 
 mercury ; when either platinum or mercury was moist I 
 perceived a tendency to amalgamation, but none when they 
 were perfectly dry. As I hope to renew this examination 
 with a more perfect apparatus, I will not detail any more of 
 these experiments, but merely state my impression to be that 
 mercury under the influence of a voltaic current is capable of 
 absorbing a very small quantity of hydrogen, which it gives 
 up as soon as the communication is interrupted. 
 
 The probability of a temporary combination of hydrogen 
 with mercury throws some light upon the movements of mer- 
 cury submitted, under an electrolyte, to a voltaic current : the 
 hydruretted particles of mercury are repelled until out of the 
 immediate influence of the current, where they yield their 
 hydrogen, and so on of the rest. In electrolysation with a 
 mass of mercury as negative electrode, the hydrogen is all 
 evolved at the parts most distant from the positive electrode. 
 
 In order to see whether this property of complete polari- 
 sation was proper to mercury, or common to all metals in a 
 state of fusion, I caused two currents, one proceeding from a 
 voltaic pair consisting of zinc and a fused globule of Darcet's 
 alloy, the other from zinc and mercury kept at the same tem- 
 perature, to pass in contrary directions through the wire of a 
 galvanometer ; the current proceeding from the first pair was 
 much more energetic than that from the second, and kept the 
 needle constantly at 85 degrees. I could not repeat the ex- 
 periment with other metals, from the impossibility of keeping 
 them fused without volatilising the electrolyte. As far as 
 this case goes, it would seem that metals possess different 
 polarising capacities. I have before remarked a difference in 
 the proportionate diminution of the current by polarisation 
 with solid metals, and think the subject merits an experimental 
 examination. 
 
241 
 
 ON A VOLTAIC PROCESS FOR ETCHING 
 DAGUERREOTYPE PLATES. 
 
 From the Proceedings of the London Electrical Society, Part II. ; having been 
 read before the Society on August 17, 1841. 
 
 DR. BERRES, of Vienna, was the first, I believe, who pub- 
 lished a process for etching Daguerreotypes : his method was 
 to cover the plates with a solution of gum-arabic, and then to 
 immerse them in nitric acid of a certain strength. I have not 
 seen any plates thus prepared, but the few experiments which 
 I have made with nitric acid have given me a burred and im- 
 perfect outline ; and I have experienced extreme difficulty of 
 manipulation from the circumstance of the acid never attack- 
 ing the plate uniformly and simultaneously. My object, how- 
 ever, in this communication, is not to find fault with a process 
 which I have never perhaps fairly tried or seen tried by 
 experienced hands, and the inventor of which deserves the 
 gratitude of all interested in physical science ; but to make 
 public another, which possesses the advantage of extreme 
 simplicity, which anyone, however unskilled in chemical 
 manipulation, may practise with success, and which produces 
 a perfect etching of the original image ; so much so, that a 
 plate thus etched can scarcely be distinguished from an actual 
 Daguerreotype, preserving all the microscopic delicacy of the 
 finest parts of the impression. 
 
 One sentence will, convey the secret of this process ; it 
 is to make the Daguerreotype the anode* of a voltaic com- 
 bination, in a solution which will not of itself attack either 
 silver or mercury, but of which, when electrolysed, the anion 
 will attack these metals unequally. This idea occurred to me 
 soon after the publication of Daguerre's process ; but, being 
 then in the country, and unable to procure any plates, \ 
 
 * Strictly speaking this is a misapplication of Faraday's term : he applied jt 
 to the surface of the electrolyte ; as, however, all Continental and many English 
 writers (among whom I may name Whewell) have applied it to the positive electrode, 
 and as an expression is most needed for that, I have not hesitated so to apply it. 
 
 R 
 
242 EXPERIMENTAL INVESTIGATIONS. 
 
 allowed the matter to sleep ; and other occupations prevented 
 for some time any recurrence to it. Recently having heard 
 much conversation as to the practicability or impracticability 
 of Daguerrotype engraving, I became anxious to try a few 
 experiments in pursuance of my original notion ; and for this 
 purpose applied in several quarters for Daguerreotypes ; but, 
 thanks to the exclusiveness of M. Daguerre's patent, I found 
 that to procure a sufficient number of plates for any reason- 
 able chance of success was quite out of the question. 
 
 On mentioning the subject to Mr. Gassiot, he, with his 
 usual energy and liberality, offered to procure me a sufficiency 
 of Daguerreotypes ; and it is owing to his zealous and valuable 
 co-operation that I have been able to get such definite results 
 as appear worth publication. 
 
 Five points naturally present themselves to the considera- 
 tion of the experimenter on this subject : first, the quantity 
 of the voltaic current ; secondly, its intensity ; thirdly, the 
 distance between the anode and cathode ; fourthly, the time 
 during which the process should be continued ; and fifthly, 
 the solution to be employed. 
 
 i. With regard to the first element, or quantity, many 
 previous experiments had convinced me that, to give the 
 maximum and most uniform quantitative* action of any vol- 
 taic combination, the electrodes should be of the same size as 
 the generating plates ; in other words, that the sectional area 
 of the electrolyte should be the same throughout the whole 
 voltaic circuit. It seems strange that this point should have 
 been so generally overlooked as it has been : an electrician 
 would never form a battery, one pair of plates of which were 
 smaller than the rest ; and yet the electrodes, which, offering 
 of themselves a resistance to the current from the inoxida- 
 bility of the anode, are, a fortiori, a restriction when of small 
 size, have generally been formed indefinitely smaller than the 
 generating plates ; I therefore, without further experiment, 
 applied this principle to the process about to be detailed. 
 
 * I say quantitative action ; for, where great intensity is required, as in decom- 
 posing alkalis, &c., it may be advisable to narrow the electrodes, so as to present 
 a smaller surface for the reaction of the liberated elements. 
 
ETCHING DAGUERREOTYPES. 243 
 
 2. The intensity of the voltaic current. Here it appeared 
 to me that, as in the electrotype, where the visible action is 
 at the cathode, a certain degree of intensity throws down 
 metal as a crystal, an increased intensity as a metallic plate, 
 and a further intensity as a pulverulent mass ; that degree of 
 intensity which would show on the negative deposit the finest 
 impressions from the cathode, would also produce on the 
 anode the most delicate excavations, and consequently, an 
 intensity which would just fall short of the point of evolving 
 oxygen from the plate to be etched, would be the most likely 
 to succeed ; this point was not, however, adopted without care- 
 ful experiment, the more so, as in one instance Mr. Gassiot 
 succeeded in procuring a very fair etching with a series of 
 ten pairs of the nitric acid battery ; however, the results of 
 repeated experiments, in which the intensity has been varied 
 from a series of sixteen pairs to one of the nitric acid battery, 
 were strongly in favour of the above idea, and consequently 
 went to prove that one pair gives the most efficient degree of 
 intensity for the purpose required. 
 
 3. The distance between the plates. As it was proved 
 by De la Rive that in an electrolytic solution, when the 
 electrodes are at a distance, the action extends a little beyond 
 the parallel lines which would join the bounds of the elec- 
 trodes, and thus, that the current, as it were, diverges and 
 converges, it appeared advisable to approximate the electrodes 
 as nearly as possible, so as to produce uniformity of action 
 over the whole plate. Provided a solution be used which 
 does not evolve gas at the cathode, I am inclined to think 
 that the plates may be with advantage indefinitely approxi- 
 mated ; but as this was not the case with the solution I 
 selected for the greater number of experiments, 0*2 of an 
 inch was fixed on as the distance, in order that the gas 
 evolved from the cathode should not adhere to the anode, 
 and thus interfere with the action. 
 
 4. Time of continuing the operation. This was a matter 
 only to be decided by experiment, and must vary for the vol- 
 taic combination arid solution employed. With a single pair 
 of the nitric acid battery, from twenty-five to thirty seconds 
 was, after a great number of experiments, fixed on as the 
 
 R 2 
 
244 EXPERIMENTAL INVESTIGATIONS. 
 
 proper time ; and as the plate may at any period be removed 
 from the solution and examined, the first experiment should 
 never exceed twenty-five seconds, when, if not complete, the 
 plate may be again subjected to electrolysis. 
 
 5. The solution to be employed. Here a vast field was 
 open, and still is open to future experimentalists. Admitting 
 the usual explanation of the Daguerreotype, which supposes 
 the light parts to be mercury, and the dark silver, the object 
 was to procure a solution which would attack one of these, 
 and leave the other untouched. If one could be found to 
 attack the silver and not the mercury, so much the better ; as 
 this would give a positive engraving, or one with the lights 
 and shadows, as in nature ; while the converse would give a 
 negative one. Unfortunately, silver and mercury are nearly 
 allied in their electrical relations. I made several experiments 
 with pure silver and mercury, used as the anode of a voltaic 
 combination ; but found that any solution which would act 
 on one acted also on the other. All, then, that could be ex- 
 pected was a difference of action. With the Daguerreotype 
 plates I have used the following : 
 
 Dilute sulphuric acid, dilute hydrochloric acid, solution of 
 sulphate of copper, of potash, and of acetate of lead. The 
 object of using acetate of lead was the following : With this 
 solution, peroxide of lead is precipitated upon the anode ; 
 and, this substance being insoluble in nitric acid, it was hoped 
 that the pure silver parts of the plate, being more closely in- 
 vested with a stratum of peroxide than the mercurialised por- 
 tions, these latter would, when immersed in this menstruum, 
 be attacked, and thus furnish a negative etching. I was also 
 not altogether without hopes of some curious effects, from the 
 colour of the thin films thus thrown down ; here, however, I 
 was disappointed : the colours succeeded each other much as 
 in the steel plate used for the metallochrome, but with in- 
 ferior lustre. On immersion in nitric acid of different degrees 
 of dilution the plates were unequally attacked, and the etching 
 burred and imperfect. Of the other solutions, hydrochloric 
 acid was, after many experiments, fixed on as decidedly the 
 best ; indeed, this I expected, from the strong affinity of 
 chlorine for silver. 
 
ETCHING DAGUERREOTYPES. 245 
 
 I will now describe the manipulation which has been em- 
 ployed by Mr. Gassiot and myself, in the laboratory of the 
 London Institution, with very uniform success. A wooden 
 frame is prepared, having two grooves at o - 2 of an inch 
 distance, into which can be slid the plate to be etched, and a 
 plate of platinum of the same size. To ensure a ready and 
 equable evolution of hydrogen, this latter is platinised after 
 Mr. Smee's method ; for, if the hydrogen adhere to any part 
 of the cathode, the opposite portions of the anode are pro- 
 portionably less acted on. The back and edges of the Da- 
 guerreotype are varnished with a solution of shell -lac, which 
 is scraped off one edge to admit of metallic connexion being 
 established. The wooden frame, with its two plates, is now 
 fitted into a vessel of glass or porcelain, filled with a solution 
 of two measures hydrochloric acid, and one distilled water 
 (sp. gr. ri) ; and two stout platinum wires, proceeding from a 
 single pair of the nitric acid battery, are made to touch the 
 edges of the plates, while the assistant counts the time ; this, 
 as before stated, should not exceed thirty seconds. When the 
 plate is removed from the acid it should be well rinsed with 
 distilled water ; and will now (if the metal be homogeneous) 
 present a beautiful sienna-coloured drawing of the original 
 design, produced by a film of the oxychloride formed ; it is 
 then placed in an open dish containing a very weak solution of 
 ammonia, and the surface gently rubbed with very soft cotton, 
 until all the deposit is dissolved ; as soon as this is effected 
 it should be instantly removed, plunged into distilled water, 
 and carefully dried. The process is now complete, and a per- 
 fect etching of the original design will be observed; this, when 
 printed from, gives a positive picture, or one which has its 
 lights and shadows as in nature ; and which is, in this respect, 
 more correct than the original Daguerreotype, as the sides are 
 not inverted. Printing can therefore be directly read ; and in 
 portraits thus taken the right and left sides of the face are in 
 their proper position. There is, however, ex necessitate this 
 difficulty, with respect to prints from Daguerreotypes if 
 the plates be etched to a depth sufficient to produce a very 
 distinct impression, some of the finer lines of the original 
 must inevitably run into each other, and thus the chief beauty 
 
246 EXPERIMENTAL INVESTIGATIONS. 
 
 of these exquisite images is destroyed. If, on the other hand, 
 the process be only continued long enough to leave an exact 
 etching of the original design, which can be done to the mi- 
 nutest perfection, the very cleaning of the plate by the printer 
 destroys its beauty ; and, the molecules of the printing-ink 
 being larger than the depth of the etchings, an imperfect im- 
 pression is produced. For this reason it appeared to me that 
 at present the most important part of this process is the 
 means it offers of multiplying indefinitely Daguerreotypes 
 by means of the electrotype. An ordinary Daguerreotype, 
 it is known, will, when electrotyped, leave a faint impression ; 
 but in so doing it is entirely destroyed, and this impression 
 cannot be perpetuated ; but one thus etched at the voltaic 
 anode will admit of any number of copies being taken from 
 it. To give an idea of the perfect accuracy of these, I may 
 mention, that in one I have taken, on which is a signboard 
 measuring on the electrotype plate 0*1 by 0*06 of an inch, 
 five lines of inscription can, with the microscope, be distinctly 
 read. The great advantages of the voltaic over the chemical 
 process of etching appear to me to be the following : 
 
 1. By the former, an indefinite variety of menstrua may 
 be used ; thus, solutions of acids, alkalies, salts, more espe- 
 cially the haloid class, sulphurets, cyanurets, in fact any ele- 
 ment which may be evolved by electrolysis, may be made to 
 act upon the plate. 
 
 2. The action is generalised, and local voltaic currents are 
 avoided. 
 
 3. The time of operation can be accurately determined, 
 and any required depth of etching produced. 
 
 4. The process can be stopped at any period, and again 
 renewed if desirable. 
 
 The time I have given is calculated for experiments made 
 with one pair of the nitric acid battery ; it is, however, by no 
 means necessary that this be employed, as probably any other 
 form of voltaic combination would be efficient. It seems 
 more advisable to employ a diaphragm battery, or one which 
 produces a constant current, as otherwise the time cannot be 
 accurately determined. It is very necessary that the silver 
 of plates subjected to this process be homogeneous. Striae, 
 imperceptible in the original Daguerreotype, are instantly 
 
ELECTRO NITRURETS. 247 
 
 brought out by the action of the nascent anion ; probably 
 silver, formed by voltaic precipitation, would be found the 
 most advantageous. I transmit with this paper some speci- 
 mens of the prints of the etched plates, and of electrotypes 
 taken from them ; and in conclusion would call attention to 
 the remarkable instance which these offer of the effects of the 
 imponderable upon the ponderable ; thus, instead of a plate 
 being inscribed, as ' Drawn by Landseer, and engraved by 
 Cousins/ it would be ' Drawn by Light, and engraved by 
 Electricity.' 
 
 [With this communication were sent plates etched by the 
 process detailed in the text ; electrotype copies from the 
 same ; and a considerable number of prints obtained from the 
 former, SEC. ELECT. Soc.] 
 
 Postscript by the Author, Nov. i. Few. of the readers of the 
 ' Philosophical Magazine ' will have an opportunity of seeing 
 any specimens of the process ; and as the etching is not deep 
 enough to produce impressions sufficient to accompany the 
 paper, I may give an idea of them by saying that in the print 
 of a portrait which I have now before me the whole expres- 
 sion of the features is distinct, the pupil of the eye and the 
 speck of light upon it clearly defined, the gloss of the hair and 
 of the satin stock very accurate. The microscopic details alone 
 appear incapable of transference to paper ; but these, as stated 
 above, being absolutely perfect upon the etched plate, I had 
 intended to have directed some experiments to the substitu- 
 tion of more delicate materials than paper and printing-ink 
 for receiving the impressions ; incessant occupations have pre- 
 vented me, and will, I fear, for some time. 
 
 I would suggest the employment of hyposulphite of soda 
 instead of ammonia to remove the oxychloride. 
 
 ON SOME ELECTRO NITRURETS. 
 
 Proceedings of the Royal Society, Feb. 4, 1841. 
 
 THE author states that he has made many attempts to render 
 permanent the ammoniacal amalgam, and that he has suc- 
 ceeded in freezing it by means of solid carbonic acid, during 
 
248 EXPERIMENTAL INVESTIGATIONS, 
 
 which solidification, and also while in its solid state, it under- 
 went no chemical change. He subsequently attempted to 
 procure a permanent compound by electrolysing a solution of 
 hydrochlorate of ammonia with an extremely fusible alloy at 
 the cathode ; but this attempt was unsuccessful. It then 
 occurred to him, that by using an oxidable metal at the anode, 
 which could be revived in conjunction with nascent hydrogen 
 and nitrogen at the cathode, one or both of these elements 
 might be combined with the solid metal and so form perma- 
 nent compounds. 
 
 The experiment made in this manner with the metals zinc, 
 cadmium, and copper was perfectly successful. A spongy 
 mass collected at the cathode, which floated upon the liquid, 
 and which, when washed and dried, was analysed by heating 
 in a tube retort ; five grains of the zinc compound gave 073 
 of a cubic inch of permanent gas, which on examination 
 proved to be nitrogen with one-fourth hydrogen. The same 
 quantity of the cadmium compound gave 0*207 cubic inch of 
 nitrogen, with no admixture of hydrogen. A like weight of 
 the copper compound gave 0*107 of nitrogen. No ammonia 
 was evolved from either ; and the author is inclined to think 
 that the hydrogen yielded by the zinc compound resulted from 
 the reaction of the metal upon combined water. The specific 
 gravity of specimens of these substances which the author 
 tried were respectively 4, 6, 4, 8 and 5, 9. A mixed solution 
 of chloride of gold and hydrochlorate of ammonia, electro- 
 lysed with platinum electrodes, gave a black powder of the 
 specific gravity 10*3, five grains of which, being heated, gave 
 only 0*05 cubic inch of gas. The author proceeds to observe, 
 that the similarity in appearance and mode of formation of 
 these compounds and of the mercurio-ammoniacal amalgam 
 is strong evidence of identity of constitution, and that the 
 non-permanence of the latter substance is due to the mobility 
 of the mercury ; for if we place the compounds in similar 
 circumstances, that is, solidify the mercurial one, or liquefy 
 those of the other metals, the phenomena are perfectly analo- 
 gous. The experiments also bear immediately upon those of 
 Thenard, Savart, and others, where ammonia, passed over 
 Jieated metals, was found to be decomposed more completely 
 
VOLTAIC SYNTHESIS OF WATER. 249 
 
 by the oxidable than by the inoxidable metals, and to alter 
 their physical characters without materially increasing their 
 weight. On examining papers connected with this subject 
 the author found that Mr. Daniell had cursorily noticed a 
 deposit somewhat analogous to those here treated of, which 
 was formed upon the negative plate of his constant battery, 
 when this was charged on the zinc side with hydrochlorate of 
 ammonia, and the nature of which that gentleman observed 
 was worthy of further examination, but had not had time to 
 investigate. 
 
 SYNTHESIS OF WATER BY VOLTAIC ACTION. 
 
 Philosophical Magazine, Feb. 1839. 
 
 Two strips of platinum, two inches long and three-eighths of 
 an inch wide, standing erect at a short distance from each 
 other, passed, hermetically sealed, through the bottom of a 
 bell glass ; the projecting ends were made to communicate 
 with a delicate galvanometer ; the glass was filled with water 
 acidulated with sulphuric acid, and both the platina strips 
 made the positive electrodes of a voltaic battery until perfectly 
 clean, &c. ; contact with the battery having been broken, over 
 each piece of platinum was inverted a tube of gas, four-tenths 
 of an inch in diameter, one of oxygen, the other of hydrogen, 
 acidulated water reaching a certain mark on the glass, so 
 that about half of the platina was exposed to the gas, and 
 half to the water. The instant the tubes were lowered so as 
 to expose part of the surfaces of platinum to the gases the 
 galvanometer needle was deflected so strongly as to turn more 
 than half round ; it remained stationary at 1 5, the platinum 
 in the hydrogen being similar to the zinc element of the pile. 
 When the tubes were raised so as to cover the plates with 
 water, the needle returned slowly to zero ; but the instant that 
 the tubes were lowered again, it was again deflected ; if the 
 tubes were changed with regard to the platina, the deflection 
 was to the contrary side. 
 
250 EXPERIMENTAL INVESTIGATIONS. 
 
 The action lowered considerably after the first few minutes, 
 but was in some degree restored every time the tubes were 
 raised so as to wash the surface of the platina, and again 
 lowered. After twenty-four hours the water had risen half an 
 inch in the tube containing hydrogen, and three-eighths of an 
 inch in that containing oxygen. In two other tubes, without 
 platina, but with the same gases, and immersed in acidulated 
 water for the same time, the water had scarcely perceptibly 
 risen ; the effect therefore could not have been due to solution. 
 The same sheets of platinum were exposed to atmospheres of 
 common air and of similar gases, i. e. both to oxygen or both 
 to hydrogen, &c., but without affecting the galvanometer. The 
 platinum in the hydrogen was made the positive, and that in 
 the oxygen the negative electrode of a single voltaic pair ; the 
 water now rose at the rate of three-eighths of an inch per hour 
 in the hydrogen tube, and proportionally in the oxygen ; when 
 the platina was not assisted by a pair of metals the oxygen 
 was absorbed in more than its relative proportion. I hope, 
 by repeating this experiment in series, to effect decomposition 
 of water by means of its composition. 
 
 EXPERIMENTS ON VOLTAIC REACTION. 
 
 ON the weekly evening meeting of the Royal Institution for 
 March 13, 1840, I communicated some experiments and ob- 
 servations on certain phenomena which I collated under the 
 general term Voltaic Reaction. I then stated, that in certain 
 (probably in all) cases of the development of a voltaic current 
 a reaction was induced by the voltaic force itself, and that 
 upon the cessation of the initial force the reacting force was 
 apparent in an opposed direction. I showed, moreover, that 
 the diminution or removal of this reaction was one means of 
 increasing the power of the initial current This*, reaction in 
 electrolytes (though it is by no means confined to electrolytes) 
 is what has been generally called polarisation. 
 
 It recently occurred to me, that as one method of increasing 
 the power of the initial current was to diminish (or, as it were, 
 
VOLTAIC REACTION. 
 
 Fig. i. 
 
 Fig. 2. 
 
 absorb) this reaction, so another method of effecting the same 
 object would be, to add the reacting to the initial force, which, 
 from the separable character of the former, did not appear 
 impracticable. After sundry devices the following experiments 
 realised my views on the subject 
 
 Experiment I, fig. I. db is a single cell 
 of the nitric acid battery, exposing six square 
 inches of each metal ; v is an ordinary volta- 
 meter, each electrode exposing half a square 
 inch, charged with dilute sulphuric acid ; de- 
 composition was allowed to proceed with this 
 arrangement for six hours ; the battery, for 
 greater assurance of constancy, being in this 
 and the two following experiments re-charged every two 
 hours ; the level of the liquid in the voltameter was carefully 
 marked on the tube. 
 
 Experiment 2, fig. 2, is the 
 same nitric acid battery, d b, the 
 same voltameter, v, but with an in- 
 terposed pair of large platinum 
 plates, a c, exposing each to each 
 forty-two square inches of surface, 
 and immersed in dilute sulphuric 
 acid ; this arrangement was also set 
 to work for six hours. A slight 
 evolution of gas had taken place 
 in the voltameter in this experi- 
 ment, and the water-level was also 
 marked. 
 
 Experiment 3. The same ap- 
 paratus as fig. 2 ; but my assistant 
 was directed to change at a certain 
 
 interval the wires dipping into the mercury cups,^^, so as to 
 reverse the plates, a c, with regard to the direction of the cur- 
 rent, making what was the anode the cathode, and vice versa, 
 as shown by the dotted lines ; and at the expiration of a 
 similar interval to restore them to their original positions, and 
 to continue thus alternating the position of these plates with 
 reference to the current during six hours. 
 
252 EXPERIMENTAL INVESTIGATIONS. 
 
 The interval was to be dependent upon the following ob- 
 servation : When first the circuit was completed a marked 
 evolution of gas was perceptible in the voltameter ; this gra- 
 dually subsided, and when it had become nearly imperceptible 
 the change was to be made, when a fresh burst of gas took 
 place ; as this again subsided the wires were to be again 
 changed, and so on. At the expiration of six hours the water- 
 level was marked, as in the previous experiments. . 
 
 The following is the quantity of gas evolved in the volta- 
 meter, deduced from a mean of several experiments : 
 
 Cubic inch. 
 
 Experiment i. = OT5 
 Experiment 2. =0*10 
 Experiment 3. = 0*23 
 
 In neither of the last two experiments was a bubble of gas 
 perceptible on the large plates, a c. 
 
 It appears from these experiments that the nitric acid 
 battery will decompose water across two pairs of interposed 
 inoxidable electrodes, provided one be of considerable size 
 with reference to the other parts of the circuit, so as to lessen 
 resistance. 
 
 Whether this diminished resistance be occasioned by the 
 mere increase of the sectional area of the electrolyte ; whether 
 by the increased facility for solution of the oxygen and hydro- 
 gen ; whether the oxygen and hydrogen be not eliminated, 
 but merely thrown into a state of polar tension, or made to 
 adhere in a liquid or gaseous form to the plates ; or whether 
 any of these effects take place conjointly, I will not stop to 
 enquire, but proceed to the more remarkable fact, viz. that the 
 quantity of gas evolved in Experiment 3 is greater for a given 
 time, not only than that evolved in Experiment 2, but even 
 than that evolved in Experiment I ; thus we get the seeming 
 paradox, that a battery performs more work with an interposed 
 resistance than without it. 
 
 While the battery is decomposing water in the voltameter 
 v, it is polarising the plates a c, or accumulating by its own 
 force an antagonist force ; when the wires are changed this 
 reacting force is united in direction with the initial force ; in 
 
GAS VOLTAIC BATTERY. 253 
 
 fact two voltaic pairs are constituted. The reaction being 
 exhausted, a new polarisation commences, to be added in its 
 turn to the initial current; and the reason why we get the 
 increased work at the voltameter is, that while the polarisa- 
 tion is proceeding at a c, water is decomposed in the volta- 
 meter ; and although this may be somewhat less than the 
 battery would produce without the interposed plates a c, still 
 this deficiency is more than made up by the action of the 
 double pair at each alternation of the wires. If the view I 
 have taken be correct, as reaction can never be greater than 
 the action which occasions it, we should never get, in Experi- 
 ment 3, beyond the quantity of gas given by two pairs of 
 the battery d b, but we may indefinitely approach that 
 maximum. 
 
 A commutator might be easily arranged instead of the 
 hand for effecting the alternation at the proper periods, which, 
 by a little contrivance, may be made to work by the battery 
 itself, but I prefer stating the experiment in its most simple 
 form, and free from mechanical complication. 
 
 Although the experiment as here described is merely in 
 illustration of a principle, it appears to me to promise re- 
 sults of some practical value. The economy of this method of 
 applying force is evident : we get all but a double product with 
 a single consumption. The principle in all probability is not 
 confined to the voltaic force, but may perhaps be applied to 
 mechanics. 
 
 ON THE GAS VOLTAIC BATTERY. 
 
 Philosophical Transactions of the Royal Society. Received March 27. Read 
 May n, 1843. 
 
 IN the 'Philosophical Magazine' for December 1842 I have 
 published an account of a voltaic battery in which the active 
 ingredients were gases, and by which the decomposition of 
 water was effected by means of its composition. 
 
 The battery described in that paper consisted of a series 
 
254 EXPERIMENTAL INVESTIGATIONS. 
 
 of tubes containing strips of platinum foil covered with a pul- 
 verulent deposit of the same metal ; the platinum passed 
 through the upper parts of the tubes, which were closed with 
 cement, the lower extremities were open ; they were arranged 
 in pairs in separate vessels of dilute sulphuric acid, and of each 
 pair one tube was charged with oxygen, the other with hy- 
 drogen gas, in quantities such as would allow the platinum to 
 touch the dilute acid ; the platinum in the oxygen of one pair 
 was metallically connected with the platinum in the hydrogen 
 of the next, and a voltaic series of 50 pairs was thus formed. 
 With this battery the following effects were produced : 
 
 r. A shock was given which could be felt by five persons 
 joining hands. 
 
 2. The needle of a moderately sensitive galvanometer was 
 whirled round and remained permanently deflected 60. 
 
 3. A gold-leaf electroscope was notably affected. 
 
 4. A brilliant spark visible in broad daylight was given 
 between charcoal points. 
 
 5. Iodide of potassium, hydrochloric acid, and water acidu- 
 lated with sulphuric acid were severally decomposed ; the gas 
 from the decomposed water was collected and detonated. The 
 gases were evolved in the direction which the chemical theory 
 would indicate, the hydrogen travelling in one direction 
 thoughout the circuit, and the oxygen in the reverse. 
 
 When distilled was substituted for acidulated water in the 
 battery cells the effects were similar but more feeble. 
 
 The effects, though clear and decisive in themselves, were 
 further tested by counter-experiments, such as reversing the 
 current by reversing the gases, &c. ; but these I need not here 
 detail, as the electrical effects of the gas battery, when charged 
 with oxygen and hydrogen, have since the publication of that 
 paper been repeatedly verified. I farther stated, that when 
 carbonic acid and nitrogen were substituted for oxygen and 
 hydrogen no voltaic effects were produced ; that oxygen and 
 nitrogen produced no effects, but that hydrogen and nitrogen 
 did produce a voltaic current, which I attributed to the com- 
 bination, with the hydrogen, of the oxygen of atmospheric air 
 in solution. This opinion will be further tested in the following 
 paper. 
 
GAS BATTERY. 255 
 
 The voltaic current generated by this battery I attributed 
 to chemical synthesis, of an equal but opposite kind, in the 
 alternate tubes, at the points where the liquid, gas, and pla- 
 tinum met, and the object of covering the platinum with the 
 pulverulent deposit * was to increase the number of these 
 points, the liquid being retained upon the surface of the 
 platinum by capillary attraction. 
 
 The point which appeared to me at that time as most 
 important was the beautiful instance of the correlation of 
 natural forces exhibited by the fifth effect, in which gases by 
 combining and becoming a liquid transfer a force which is 
 capable of decomposing a similar liquid, and causing its con- 
 stituents to become gases, heat, chemical action, and electricity 
 being all mutually dependent. 
 
 The apparatus with which I made the above experiments 
 being composed of some pieces of tubing which happened to 
 be in my laboratory, did not enable me to attain any pre- 
 cise accuracy of measure as to the volumes of gases absorbed, 
 or to prove that Faraday's law of definite electrolysis finds no 
 exception in the gas battery. Since that paper was written 
 I have, after some failures, constructed apparatus by which I 
 have been enabled to verify this law and to extend my re- 
 searches into the nature of gaseous voltaic action. I have felt 
 the more called on to multiply experiments on this subject, as 
 a letter has been published on the gas battery, written by an 
 electro-chemist for whose opinion I have much respect, which 
 attributes its action to a cause different from that to which I 
 assigned it. 
 
 Soon after my original publication I received a letter from 
 Dr. Schcenbein, the substance of which has since appeared 
 in print.-)- Dr. Schcenbein there expresses an opinion that in 
 the gas battery oxygen does not immediately contribute to 
 the production of the current, but that it is produced by the 
 combination of hydrogen with water. I have recently heard 
 a similar opinion to that of Dr. Schcenbein expressed by other 
 
 * For the method of effecting this see Mr. Smee's paper, Phil. Mag., April 
 1840. 
 
 t Phil. Mag., March 1843, p. 105. 
 
256 
 
 EXPERIMENTAL INVESTIGATIONS, 
 
 philosophers, but I must take the liberty of dissenting from it 
 and of adhering to that which I expressed in my original 
 paper. My grounds of dissent will be seen in some of the 
 ensuing pages. 
 
 In describing the apparatus used in the following ex- 
 periments I shall mention three forms of gas battery, with 
 the first two of which my experiments were all performed ; 
 the latter has only occurred to me while writing -this paper. 
 I have, therefore, not yet had an opportunity of trying it,* 
 but it appears to me by far the best of the three, though, 
 doubtless, superior modifications will shortly be discovered. 
 
 Fig. I. Fig. 2. 
 
 Fig. 3- 
 
 o Ti 
 
 Fig. I, represents one of 
 these forms ; a, b, c, d is 
 a wide-mouthed glass jar, 
 into which a wooden 
 plug, a, b, fits tightly by 
 means of attached pieces 
 of cork ; this wooden 
 cover is perforated to 
 receive the tubes o, h, 
 of which the size is such 
 that the content of h shall be double that of o, and which 
 are firmly cemented into it ; the wooden cover is shown in 
 plan in fig. 2 ; the piece f is capable of being detached at 
 pleasure, in order to introduce a tube for charging the appa- 
 ratus with gas ; p r, p f r' are strips of well-platinised platinum 
 foil, slightly curved like a cheese-scoop to keep them erect 
 and in the centre of the tube, and riveted or welded to stout 
 platinum wires, which are hermetically sealed into the glass, and 
 terminate in brass mercury cups at g, g. This form of battery 
 is charged by inverting it so as to fill the tubes with liquid ; 
 on re-inversion the tubes may be charged with gas from a 
 crooked tube and bladder. The apparatus (fig. I.) is repre- 
 sented as charged and ready for use ; and in fig. 3 is a battery 
 of five cells, also represented as just charged. 
 
 The advantage of this form over that which I shall next 
 describe is the facility with which the tubes are filled with 
 
 * See Postscript. 
 
GAS BATTERY. 
 
 257 
 
 liquid, and the absence of any necessity of touching the 
 electrolyte with the fingers. On the other hand, its disadvan- 
 tages are the difficulty of examining the gases after experi- 
 ment, and the impossibility of doing so during experiment 
 without changing the electrolyte, as in order to examine the 
 gases the whole apparatus must be immersed in a water- 
 trough, and the cover with the attached tubes taken off while 
 the jar and the ends of the tube are under water. 
 
 Fig. 4 represents a cell of the second form ; b, c, d, e is a 
 parallelepiped glass or stoneware vessel, such as is com- 
 monly used for the outer cells of nitric acid Fig. 4. Fig. 5. 
 batteries ; the tubes are cemented into 
 pieces of wood, a b, a c, and can with 
 the wood be separately detached from 
 the trough, as shown in fig. 5. At the 
 aperture or space, a a, between the tubes 
 there is just room for a finger to enter, 
 close the orifice of either tube, and thus 
 detach it from the apparatus. In this 
 figure the platinum foil is turned up 
 round the edge of the tube, instead of being attached to a 
 wire sealed into the glass, and instead of a mercury-cup there 
 is a binding-screw connexion ; but it is obvious that this part 
 of the arrangement may be interchanged with the other ap- 
 paratus, or varied ad libitum. This apparatus I have found 
 in practice to be very much more convenient than the former, 
 from the facility of detaching either tube so as to discharge 
 
 Fig. 6. 
 
 some of the gas, if it be desir- 
 able to alter the level of the 
 water-mark ; or to examine or 
 change the gas in any of the 
 tubes. On the other hand, it 
 has the disadvantage of requir- 
 ing the finger to be immersed 
 in the electrolyte, which, when 
 the latter is of an active chemi- 
 cal character, is unpleasant and 
 in some cases injurious. In fig. 6 a battery of five cells of this 
 construction is represented as when charged with oxygen and 
 
 S 
 
258 EXPERIMENTAL INVESTIGATIONS. 
 
 hydrogen, and having been for some time connected with a 
 voltameter (fig. 7), the tubes of which are of the same size as 
 those of the battery. 
 
 In the form last described (figs. 4 and 6) the tubes were 
 all as nearly of the same size as could be procured ; they con- 
 tained each about one and a half cubic inch ; in the first 
 form (figs, i and 3) the portion o y r of the narrow tube con- 
 tained one and a quarter cubic inch, and the portion h, / of 
 the wide tube contained two and a half cubic inches. A por- 
 tion of the apparatus with which I wrought was constructed 
 by my order for the London Institution, and another portion 
 belonged to Mr. Gassiot, and was by him very kindly placed 
 at my disposal for the purpose of these experiments. Had it 
 not been for this valuable addition, I should have been obliged 
 to make all my experiments on a much smaller scale ; they 
 would have taken more time and been by no means so 
 satisfactory. 
 
 As I have already stated, a third form has occurred to me 
 while writing this paper, which I think in many respects more 
 advantageous than either of the two preceding, and which, as 
 it may be some time before I can experiment with it myself, 
 I will here describe for the benefit of those who are differently 
 situated. One cell of it is shown in fig. 8 : a, a, is a Woulfe's 
 Fig. 8. Fig. 9. bottle with three necks ; in the centre neck is 
 fitted a glass stopper, b ; in the other two 
 tubes o, h fit accurately by means of glass 
 collars (c c, fig. 9.) welded to them, and 
 ground on the outside ; the platinum is her- 
 metically sealed into the tops of the tubes, 
 which may be charged in a similar manner 
 to fig. i. By immersing this apparatus in 
 the water-trough each tube with the gas it 
 contains may be detached and examined 
 separately ; but its principal advantage is, that by slightly 
 greasing the stopper and collars it may be made perfectly 
 air-tight, which, for reasons that will be apparent in the 
 course of this paper, is a most material point. This apparatus, 
 moreover, being entirely composed of glass and platinum, 
 concentrated acid, alkaline or other corrosive solutions may 
 
GAS BATTERY. 259 
 
 be used as the electrolyte, without damaging the apparatus or 
 introducing foreign matter. 
 
 In the experiments I am about to describe the results were 
 generally tested by chemical action, as manifested by the 
 electrolysis, either of iodide of potassium or of water. I had 
 at my disposal a highly sensitive astatic galvanometer ; but I 
 found such slight local actions disturb it, that a range of test 
 experiments was in each case necessary to eliminate the true 
 battery action from the accidental currents ; and, with all the 
 pains that could be bestowed upon it, the results were less 
 definite and trustworthy than those obtained with the iodide. 
 
 I may here state also that, although with the battery 
 described in my original paper, when charged with oxygen, 
 hydrogen, and dilute sulphuric acid, I could not succeed in 
 perceptibly decomposing water with less than twenty-six cells, 
 yet the new arrangements, from their superiority in size and 
 construction, were capable, when charged with the same 
 gases and electrolyte, of decomposing water with four cells ; 
 and a single cell would decompose iodide of potassium. 
 
 Experiment i. Ten cells charged to a given mark on the 
 tube with dilute sulphuric acid, sp. gr. 1*2, oxygen and hydro- 
 gen, were arranged in circuit with an interposed voltameter,* 
 as in figs. 6 and 7, and allowed to remain so for thirty-six hours. 
 At the end of that time 2'i cubic inches of mixed gas were 
 evolved in the voltameter; the liquid had risen in each of the 
 hydrogen tubes of the battery to the extent of 1*5 cubic inch, 
 and in the oxygen tubes 07 cubic inch, equalling altogether 
 2*2 cubic inches ; there was therefore O'l cubic inch more of 
 hydrogen absorbed in the battery tubes than was evolved in 
 the voltameter. 
 
 This experiment was several times repeated, with the same 
 general results. I give some of them in the annexed table. 
 
 * These experiments were made with the battery fig. i, though for more 
 cloorly showing the volumes of the gases the second form is represented in 
 figs. 6 and 7. The voltameter employed on this occasion had electrodes of fine 
 platinum wire a quarter of an inch long. From the nature of the gas battery it 
 is difficult to know the efficient surface of the plates. In ordinary batteries I have 
 found, and stated some time ago, that for quantitative effects the electrodes 
 should be of the same size as the battery plates. 
 
 S 2 
 
260 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 Cubic inch of 
 oxygen absorbed 
 in the battery cells 
 
 Cubic inch of 
 hydrogen 
 absorbed in the 
 battery cells 
 
 Cubic inch of 
 oxygen 
 evolved in 
 voltameter 
 
 Cubic inch of 
 hydrogen 
 evolved in 
 voltameter 
 
 Time 
 
 Number 
 of cells 
 
 07 
 
 i '4 
 
 07 
 
 i'4 
 
 
 
 o*5 
 0-6 
 0-6 
 0-6 
 
 1*2 
 
 I'4 
 
 i '3 
 
 i '4 
 
 '5 
 
 0-6 
 
 o'S 
 0-6 
 
 1*1 
 i'3 
 
 I'2 
 
 i'3 
 
 .36 
 
 hours. 
 
 IO 
 
 Mean 0-6 
 
 i'34 
 
 0*58 
 
 1-26 
 
 
 
 We may observe generally, in these experiments, that the 
 hydrogen evolved in the voltameter is somewhat more than 
 double the volume of the oxygen, and that a still extra 
 quantity of hydrogen is absorbed in the battery. With 
 regard to the excess of hydrogen in the voltameter, this, as 
 is well known to electricians, is always observable in the 
 electrolysis of water, and has been attributed by Faraday to 
 the more ready solubility of oxygen, and its tendency to form 
 oxygenated water ; * but we have in the above experiments a 
 still greater excess of hydrogen absorbed in the battery tubes. 
 This result previous experiments had led me to expect. In 
 one of these I found voltaic action produced by tubes charged 
 alternately with hydrogen and water, and attributed it to the 
 combination of hydrogen with the oxygen of atmospheric air 
 in solution.f Assuming for the moment this explanation to be 
 correct, in a gas battery charged with oxygen and hydrogen 
 we should have, upon completion of the circuit, three distinct 
 voltaic actions : First, the principal action occasioned by the 
 gases in the tubes reacting upon each other through the medium 
 of the electrolyte, i.e. reverting to fig. 4, an action in which the 
 portions of the platinum exposed to the gases / q, p f q' would 
 be the efficient plates. Secondly, an action between the 
 hydrogen at /' g[ and the air in solution in the neighbourhood 
 of the immersed portion of the plate, q, r ; this would add to 
 the general current, but would tend disproportionately to 
 diminish the hydrogen. Thirdly, a local action between the 
 
 * Experimental Researches, 716, 717. 
 
 t Phil. Mag,, Dec. 1842, p. 419, Exp. u. 
 
GAS BATTERY. 261 
 
 hydrogen at/', q f and the air in solution around the part cf y /; 
 this would add nothing to the general current, but would also 
 tend to diminish the hydrogen. As this last is totally inde- 
 pendent of the general action, it could be abstracted by 
 merely placing a cell charged similarly to the battery out of 
 the circuit with the terminals unconnected, as in fig. i. In a 
 cell so placed the hydrogen was found to be absorbed in the 
 ratio of rather less than O'l cubic inch in twenty-four hours. 
 
 On some occasions I found the rise of liquid in the 
 hydrogen cell to be unequal in different tubes of the battery, 
 and this I found more particularly the case in the battery 
 fig. 4. It was some time before I discovered the cause of this. 
 I will not enumerate my different conjectures, but state that 
 which proved to be the correct one. As, in using the two 
 forms of batteries (figs, i and 4), the chief difference consisted 
 in the introduction of the finger, it occurred to me that my 
 assistant's hands, which were employed in various manipula- 
 tions, might, in placing the tubes in the cells of fig. 4, intro- 
 duce into the electrolyte small portions of foreign matter, 
 particularly metals, and that thus a local action might be 
 occasioned ; this view was strengthened by my frequently 
 observing copper deposited upon some of the immersed por- 
 tions of the platinum, and where this happened an excess of 
 hydrogen was generally found to have been absorbed. To 
 examine the accuracy of this view, in 
 
 Experiment 2, 1 caused four cells to be charged with a solu- 
 tion of sulphate of copper, and connected in closed circuit ; 
 after twenty-four hours' work, the liquid in the oxygen and 
 hydrogen tubes had risen equally in three of the pairs, but in 
 the fourth the liquid in the hydrogen tube had risen rather more 
 than twice as high as in any of the others, and the whole of 
 the platinum in this tube, from the water-mark downwards, 
 was covered with metallic copper ; it was thus evident that a 
 slight precipitation having commenced on this platinum from 
 some local circumstance, which offered less resistance in this 
 cell than in the others, a separate local current had been 
 established ; the hydrogen and the copper acting as a voltaic 
 circuit, fresh copper had been constantly deoxidated at the 
 expense of the hydrogen. The phenomenon is analogous 
 
262 EXPERIMENTAL INVESTIGATIONS. 
 
 to that observable in an ordinary sulphate of copper battery, 
 when a slight portion of copper is deposited upon the zinc, 
 and a local current is established by which the zinc is worn 
 into a hole without contributing to the general current. 
 
 I have been thus particular in order to explain points in 
 the action of this battery which might seem exceptions to the 
 law of definite electrolysis, or what perhaps we should here 
 call electro-synthesis. As a general result the equivalent 
 action of the battery was very beautiful ; with fifty cells in 
 action there was but a trifling difference in the rise of liquid 
 in all the cells; and the rise of gas in the voltameter appeared 
 so directly proportional, that an observer unacquainted with 
 the rationale of a voltaic battery would have said the gases 
 from the exterior cells of the battery were conveyed through 
 the solid wires and evolved in the voltameter ; and had this 
 been the first voltaic battery ever invented, this probably 
 would have been the theory of its action. 
 
 In my original paper I considered the points of voltaic 
 action to be those where the liquid, gas, and platinum met ; 
 and it was to increase the number of these points that I 
 employed platinised or spongy platinum ; indeed, from what 
 I have since observed, I have much doubt whether I should 
 have obtained any success had I used smooth platinum. The 
 local action detailed in the last experiment, however, made 
 me anxious to ascertain whether the principal points of action 
 were those which I had originally believed, or whether the 
 gases entered into solution first, and were then electro- 
 Fig. 10. synthetically combined by the immersed por- 
 tion of the platinum ; whether, for instance, 
 the efficient parts of the plates were the parts 
 / ^, p f q' (fig- 4), or q r, q' r f . To ascertain 
 this the following experiment was made : 
 
 Experiment 3. A battery of five cells was 
 constructed, in which the platinum reached only 
 to half the height of the tubes (see fig. 10). 
 This was charged with oxygen and hydrogen, 
 so that the liquid just covered the extremities of 
 the platinum. In this case we have only the immersed por- 
 tions of the platinum, q r, q' r', and can examine the action 
 
GAS BATTERY. 263 
 
 of the gases which enter into solution, and are unaffected by 
 the platinum until in solution. This battery so charged gave 
 a very trifling action indeed ; it would not decompose iodide 
 of potassium, and but slightly affected a highly sensitive 
 galvanometer ; but when a little gas was added, so as to 
 expose the platinum to the gaseous atmosphere, a consider- 
 able current was developed, and a single pair decomposed the 
 iodide. 
 
 If, again, a battery of this description (fig. 10) be charged 
 so that the water-mark is below the upper edge of the 
 platinum, and the ends are connected in closed circuit, the 
 liquid rises in both tubes until that in the hydrogen tube has 
 reached the top of the platinum, and then there is no farther 
 rise. This experiment decides the question as to what is to 
 be considered the working portion of the battery, but it does 
 not positively decide whether solution and electrolysis are 
 contemporaneous or successive, as it may be said that even 
 what I have termed the exposed parts of the platinum are 
 covered with a film of liquid. I should myself hesitate for 
 the present to express a decided opinion on this point ; my 
 first impression was, that there would be, as it were, three sets 
 of points in contact, but I have not been able to devise an 
 experiment definitely to settle this point* 
 
 I aimed next at farther establishing the analogies of this 
 battery with the ordinary voltaic battery, i.e. regarding the 
 hydrogen tube as analogous to the plate of zinc or other 
 oxidable metal at the anode. I wished to see how far this 
 relation was borne out. It was beautifully shown in 
 
 Experiment 4, where a single pair was charged with 
 oxygen and hydrogen, and a second with hydrogen in one 
 tube, the other being filled with dilute sulphuric acid ; when 
 the hydrogen of the second was metallically connected with 
 the oxygen of the first, and the liquid of the second with the 
 hydrogen of the first, as in fig. n, bubbles of gas rose from 
 the platinum, which proved, as I anticipated, to be hydrogen. 
 In short, though it required four pairs to decompose water 
 
 * I have sometimes remarked, that when mixed oxygen and hydrogen have 
 been collected in one tube of the gas battery over distilled water, the addition of a 
 little sulphuric acid causes the gases rapidly to disappear. 
 
264 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 Fig. ii. 
 
 with immersed platinum electrodes, yet the platinum in the 
 atmosphere of hydrogen being analogous to an oxidable 
 
 anode, one pair was, with this as- 
 sistance, sufficient to decompose 
 water, just as one pair of an 
 ordinary battery will decompose 
 water with an anode of copper. 
 
 The nitric acid battery, an 
 account of which I originally 
 published in 1839, having shown 
 me the value of highly oxy- 
 genated acids and peroxides as 
 voltaic excitants,* I determined, 
 with a view of farther extending the analogy of the gaseous 
 and metallic voltaic batteries, to try the nitric acid as an elec- 
 trolyte with the gas battery. Therefore, in 
 
 Experiment 5, I charged a battery with hydrogen and 
 nitric acid in alternate cells, the nitric acid being only diluted 
 sufficiently to prevent injury to the wooden parts of the 
 battery. With this arrangement I found that three cells 
 were capable of decomposing water ; and thus here, also, the 
 analogy held good, the gaseous hydrogen deoxidating the 
 nitric acid in this arrangement, by nascent hydrogen as in the 
 metallic battery. 
 
 I now endeavoured to produce the converse effects, viz. 
 to form a battery in which oxygen should be the gaseous 
 element, and be absorbed by an electrolyte having an affinity 
 for it. To this end, in 
 
 Experiment 6, I caused a battery of ten cells to be 
 charged, the one set of tubes with oxygen and the alternate 
 tubes with solution of protosulphate of iron. This battery 
 decomposed iodide of potassium, but was not able to decom- 
 pose water. The tubes which contained the solution of proto- 
 sulphate represented the hydrogen tubes of the ordinary gas 
 battery. The voltaic action caused by oxygen and protoxide 
 of iron was, however, but temporary.f After a few hours it 
 
 * See Phil. Mag., May and Oct. 1839, pp. 389, 290. 
 
 f In Experiment 26 it will be seen that a continuous current is obtained from 
 oxygen and a liquid (solution of ammonia) ; oxygen likewise gives a current with 
 solution of cyanogen, and probably with many organic compounds. 
 
GAS BATTERY. 265 
 
 abated, the iodide was no longer decomposed, and the liquid 
 did not rise perceptibly in the tubes containing oxygen. The 
 solution when tested by ferrocyanide of potassium gave a 
 blue precipitate, indicating the presence of peroxide, but the 
 greater portion of this was probably formed at the expense of 
 the atmospheric air. 
 
 In the last experiments and others I had observed that a 
 more decided effect was obtained when free hydrogen alone 
 was present than when oxygen was alone. In my former 
 paper I attributed this to the atmospheric air in solution ; and 
 for convenience of arrangement I have hypothetically assumed 
 this explanation in the commencement of this paper, but the 
 recent letter of Dr. Schcenbein induced me to look farther 
 into this point. Therefore, in 
 
 Experiment 7, I charged two batteries, of two cells each, 
 with hydrogen and dilute sulphuric acid in the alternate cells. 
 When tested by iodide of potassium each battery gave notable 
 effects. One of these batteries was then placed, together with 
 a cup containing phosphorus, in a shallow vessel of water ; 
 the phosphorus was ignited and a large glass vessel inverted 
 over the whole ; the terminal wires of the battery, carefully 
 protected by thick coatings of cement, passed under the 
 edge of this vessel through the water, the exterior surface 
 of which was covered with oil, more effectually to prevent the 
 absorption of air. The terminal wires were then united and 
 left so. After two hours, when the oxygen of the surround- 
 ing atmosphere had been exhausted by the phosphorus, the 
 effect became more feeble, but continued throughout the 
 evening. The next morning, however, the enclosed battery 
 produced not the slightest effect upon the iodide ; the liquid 
 had risen in the hydrogen tubes about 0*2 cubic inch, but 
 no other effect was perceptible. On the other hand, in the 
 battery which had been placed by its side, charged in the 
 same way, and similar in every respect but in the fact of 
 being exposed to the atmospheric air, a very decided effect 
 was produced ; hydrogen had been evolved from one of the 
 platinums to the extent of 0*3 cubic inch in the cell contain- 
 ing liquid, and a decided effect was produced on the iodide. 
 The two batteries were left in this state for three more days ; 
 the decomposition and the evolution of hydrogen continued 
 
266 EXPERIMENTAL INVESTIGATIONS. 
 
 in the exposed battery, but none was perceptible in the 
 inclosed one, although the liquid had risen a little more, 
 viz. O'l cubic inch, in the hydrogen tubes of the latter. After 
 the four days above mentioned the jar of nitrogen which 
 covered the battery was taken away, and the action of the 
 battery was tested by iodide of potassium. At first there was 
 no action, but after about fifteen minutes a slight action was 
 perceptible ; this gradually increased, and in two hours the 
 action was equal to that of the battery which had been from 
 the first exposed to the atmosphere. I cannot but regard 
 this experiment as a conclusive negation of that view which 
 regards hydrogen and water as the efficient agents in the gas 
 battery. The opinion appears to me to have arisen from the 
 circumstance of our working always in an atmosphere con- 
 taining oxygen, and also from the fact of this latter gas being 
 more soluble than hydrogen.* If we lived in an atmosphere 
 of hydrogen, and if this gas were equally or more soluble 
 than oxygen, I have little doubt that the converse effects 
 would be observed. A battery charged with hydrogen in one 
 set of tubes and acidulated water in the alternate ones, at 
 first gives an effect nearly equal to an oxyhydrogen gas 
 battery, but the action rapidly declines in the former, while it 
 is constant in the latter. Even the ordinary action of the 
 gas battery when charged with oxygen and hydrogen appears 
 to me unanswerable as to the point I am now discussing. 
 When we see a battery of a number of cells at work, and the. 
 liquid gradually rising in the oxygen tubes, just in the pro- 
 portion in which oxygen gas is eliminated in the voltameter, 
 and when in a similar battery placed by its side, similarly 
 charged, but not connected in closed circuit, not the slightest 
 rise takes place in any tube, it seems impossible to adopt 
 the conclusion that the oxygen has nothing to do with the 
 current. Here we have no slight galvanoscopic effects, but 
 chemical effects capable of quantitative admeasurement, 
 capable of being continued to an extent only limited by the 
 size of the apparatus, and equivalent to the chemical effects 
 
 * The tendency of oxygen to combine with platinum may also have its 
 influence. See M. De La Rive's various experiments on this subject, Bibl. Univ, 
 passim. 
 
GAS BATTERY. 267 
 
 observable at the voltameter. If, on the other hand, hydro- 
 gen and water be the only active elements, what becomes of 
 the hydrogen ? If it combine with the water, we undoubtedly 
 should by this means be able to obtain a suboxide of hydro- 
 gen,* a result of which I have not seen the slightest symptom 
 in a long course of experiments on this subject. Even if we 
 assume the action of the oxygen to be a depolarising one, as 
 suggested by Dr. Schcenbein, this comes to the same thing, 
 as this depolarisation can only be accounted for as being 
 effected by the combination of the oxygen with hydrogen ; 
 and we might conversely assume this combination to be the 
 efficient cause of the current, and the depolarisation to take 
 place in the hydrogen tubes. It seems that the effects at 
 both anode and cathode are reciprocally dependent. The 
 matter appears to me so clear that I should not have entered 
 into detail upon it were it not for the published letter of 
 Dr. Schcenbein above mentioned, and that the superiority of 
 the hydrogen is prima facie very striking. Knowing also the 
 fondness with which we all adhere to preconceived opinions, 
 as the consideration of the action of spongy or clean platinum 
 on mixed gases led me to the discovery of the gas battery, I 
 felt that I might be too apt to measure the correctness of my 
 opinions by the success of the experiments to which they led, 
 and therefore hesitated too confidently to rest upon what 
 appeared to my mind positive demonstration. 
 
 Having verified the rationale of the action of the gas 
 battery, I now sought to extend it to other gases, and caused 
 arrangements of ten cells to be charged with such gases as 
 were sufficiently insoluble to remain in the tubes time enough 
 for experimental investigation. In all the following experi- 
 ments, besides the ten cells charged in series, a single cell 
 charged with similar gases and electrolyte was placed by the 
 side, but with the terminals unconnected : thus, when the 
 battery circuit had been closed for some time, by comparing 
 the changes which had taken place in the battery tubes with 
 those in the detached and unconnected pair, the effects due to 
 
 * I see by a recent paper of Dr. Schoenbein that he believes this to be the 
 case. Archives de V Electricite , No. 7, p. 73. 
 
268 EXPERIMENTAL INVESTIGATIONS. 
 
 solution, local currents, or other causes could be abstracted 
 from those due to circulating voltaic action. 
 
 I shall arrange the following experiments in the order in 
 which I instituted them, making such comments as may be 
 necessary to explain my own deductions from the resulting 
 phenomena. When not otherwise mentioned, the electrolyte 
 will be considered as dilute sulphuric acid, sp. gr. r2. 
 
 . Experiment 8. A battery charged with oxygen and pro- 
 toxide of nitrogen produced no effect upon iodide of potassium. 
 Examined next day, the liquid had not risen in the oxygen 
 tubes ; in the protoxide tubes it had risen to an average of 
 0*3 cubic inch, both in the battery and detached pair. 
 
 Experiment 9. Oxygen and deutoxide of nitrogen pro- 
 duced a slight effect upon the iodide ; the effect subsided 
 after the circuit had been complete for a few minutes. On 
 examining the battery after the circuit had been closed for 
 twenty-four hours the liquid in the oxygen tubes had not 
 risen ; in the tubes containing deutoxide of nitrogen the 
 liquid had risen somewhat unequally in the different tubes to 
 an amount averaging O'2 cubic inch ; in the detached pair it 
 had risen to the same amount. Not the slightest voltaic effect 
 was now produced by the terminal wires. 
 
 Experiment 10. Oxygen and olefiant gas decomposed the 
 iodide, but rather feebly ; after the circuit had been closed for 
 twenty-four hours there was still a decomposition, which con- 
 tinued, but the action was extremely feeble. Two cells were 
 allowed to remain arranged in closed circuit for fifteen days, 
 a third being placed by the side, but with the terminals un- 
 connected ; at the expiration of this time the rise of liquid in 
 the tubes was as follows : 
 
 Rise of liquid in cells of closed circuit, in tubes of 
 Oxygen . . . 0^05 cubic inch. 
 Olefiant gas . . 0*4 
 
 Rise of liquid in cells of detached pair, in tubes of 
 Oxygen . . . 0*02 cubic inch. 
 Olefiant gas . . O'3 
 
 Rise of liquid apparently due to voltaic action, 
 In oxygen tubes . 0^03 cubic inch. 
 In olefiant gas tubes . O'l 
 
GAS BATTERY. 269 
 
 These quantities are too small to enable any satisfactory 
 inference to be deduced as to the equivalents of these gases 
 which contributed to electrolysis ; the more so as the rise of 
 liquid was not quite uniform, and the action due to solution 
 was so much greater than that due to electrolysis. 
 
 I do not feel entitled to draw any other conclusion from 
 this experiment than that there was a very feeble voltaic cur- 
 rent produced by these gases ; both the remaining oxygen 
 and the olefiant gas were unaltered in character. 
 
 Experiment n. Oxygen and carbonic oxide produced 
 notable effects upon the iodide, and slight symptoms of de- 
 composing water ; a few bubbles gathered upon the electrodes 
 of an interposed voltameter ; the effects continued ; and at 
 the expiration of fifteen days the following was the state of 
 the tubes in two cells, put aside as in the last experiment : 
 
 Rise of liquid in cells of closed circuit, 
 In oxygen tubes . . o - i2 cubic inch. 
 In carbonic oxide tubes . 0*93 
 
 Rise of liquid in tubes of detached pair, 
 In oxygen tubes . . 0*02 cubic inch. 
 In carbonic oxide tubes . 07 
 
 Rise of liquid apparently due to voltaic action, 
 In oxygen tubes . . O'l cubic inch. 
 In carbonic oxide tubes . 0^23 
 
 Before the battery was charged for this experiment the 
 carbonic oxide had been carefully freed from carbonic acid 
 by caustic potash. After action the liquid gave a slight pre- 
 cipitate with lime-water, showing that carbonic acid had been 
 produced by the action. In this experiment the rise was 
 more uniform in the different tubes than in the last, and the 
 action more decided. The results, although on a small scale, 
 appear more definite ; thus we get the proportion as I : 2*3 ; 
 and as the combining volumes of oxygen and carbonic oxide 
 are as one to two, if we add the local action due to the oxygen 
 of the air in solution, i to 2*3 is as near an approximation as 
 can be expected. Though much superior to olefiant gas, the 
 action of carbonic oxide is, however, very feeble when com- 
 pared with that of hydrogen. 
 
270 EXPERIMENTAL INVESTIGATIONS. 
 
 Experiment 13. Oxygen and chlorine. Very considerable 
 action on the iodide at first, but not constant; it abated within 
 the first hour, and after twenty-four hours the action was 
 extremely feeble, scarcely perceptible: the water had risen 
 nearly to the top of the chlorine tubes, but the level in the 
 oxygen tubes was unaltered. The chlorine was negative to 
 oxygen, or, in other words, the oxygen was in its voltaic 
 bearing to chlorine as hydrogen to oxygen. 
 
 As in this experiment the water-level in the oxygen tubes 
 was unaltered, it appeared that this gas had little to do with 
 the action ; I therefore, in 
 
 Experiment 14, charged the alternate tubes of a battery 
 with chlorine and dilute sulphuric acid ; the amount of action 
 was much the same as in experiment 1 3, and equally transi- 
 tory ; a few gaseous bubbles were perceptible on the platinums 
 in the oxygen cells, but not in sufficient quantity for exami- 
 nation. It is well known that chlorine of itself will slightly 
 decompose water, forming hydrochloric acid, and evolving 
 oxygen, and there is little doubt that the voltaic action here 
 observed was due to this. There was no appearance of the 
 platinum having been attacked in several experiments which 
 I made with chlorine. So slight a chemical action will, how- 
 ever, give rise to voltaic effects, that the absence of any ap- 
 parent corrosion is not conclusive. It is stated by chemists 
 that gaseous chlorine will not attack platinum, but that it is 
 only when nascent it combines with this metal ; non constat, 
 however, that in the gas battery the chlorine at the initiatory 
 instant of its electro-synthesis may not be in a state analogous, 
 as to its chemical energies, to that converse state called 
 nascent, and therefore we cannot venture to negative the 
 possibility of the platinum being slightly attacked. This 
 circumstance, added to its extreme solubility and power 
 of decomposing water, makes chlorine rather an unsatisfac- 
 tory element for the class of actions developed by the gas 
 battery. 
 
 Solutions of bromine, chlorine, and iodine have been before 
 experimented on (I believe by Dr. Schcenbein and M. Bec- 
 querel) as to their voltaic relations, but in examining the vol- 
 taic relations of bodies in a gaseous state, or, to express myself 
 
GAS BATTERY, 271 
 
 with more caution, in a state passing from gaseous to liquid, 
 I tried 
 
 Experiment 15. One set of tubes charged with gaseous 
 chlorine, and the alternate tubes with solutions of bromine 
 and iodine. The chlorine was negative to both, i.e. was to 
 these as oxygen to hydrogen. 
 
 I now tried hydrogen with several gases ; but as it was 
 next to impossible (I found it quite impossible), in experiments 
 on a large scale, perfectly to exclude atmospheric air from the 
 solution,* voltaic action was produced in every case ; and as, 
 with one exception (chlorine), oxygen was the most powerful 
 electro-negative gas, the action of the atmospheric air entirely 
 masked any effect which might have been produced by the 
 other gases.-)- I shall, therefore, not go through these experi- 
 ments in detail, but mention one or two only which appear 
 interesting, for the reasons which I shall state. 
 
 Experiment 16. Chlorine and hydrogen gave very power- 
 ful effects, as was expected by Dr. Schcenbein ; { water was 
 decomposed between platinum electrodes by two cells. This 
 is the most powerful gas battery, but not very satisfactory, 
 for the reasons above stated (Experiment 13). 
 
 Chlorine, in its voltaic relations, may be considered as the 
 converse of zinc, both decomposing water, but the one libe- 
 rating oxygen, the other hydrogen ; thus, a tube of the 
 gas battery charged with chlorine, and having acidulated 
 water as an electrolyte, and zinc as a positive element, forms 
 a combination of which one pair will decompose water. I 
 have tried to render this combination practically useful, by 
 charging the negative cell of a diaphragm battery with per- 
 oxide of manganese and muriatic acid, but the supply of 
 chlorine thus obtained is insufficient for quantitative voltaic 
 effects, though the intensity is great. 
 
 Experiment 17. Hydrogen and carbonic oxide were tried 
 
 * Gases will creep by a species of endosmose through water. Some time ago 
 I kept inverted over water for two months a vessel divided by a diaphragm of 
 porous ware, on one side of which was oxygen gas, on the other hydrogen ; the 
 diaphragm was constantly wet from capillary attraction ; at the end of that period 
 the water had risen considerably, and the gases on each side detonated. 
 
 f See Postscript. 
 
 | See his letter, Phil. Mag., March 1843. 
 
272 EXPERIMENTAL INVESTIGATIONS. 
 
 in order to ascertain their voltaic relations. Hydrogen was 
 much more electro-positive than carbonic oxide, or rather 
 formed, with the oxygen of the atmospheric air in solution, a 
 combination which overpowered the opposite tendency of the 
 carbonic oxide and air. 
 
 Experiment 18. Chlorine and olefiant gas gave a very 
 feeble effect upon iodide of potassium. After four hours the 
 liquid in the olefiant gas tubes had not risen more ^ in the 
 closed circuit than in the detached pair ; the chlorine was 
 nearly all absorbed in solution. 
 
 Experiment 19. Chlorine and carbonic oxide gave very 
 notable effects ; ten cells decomposed water. From the ex- 
 treme solubility of the former gas the equivalent relationship 
 could not be ascertained. 
 
 It now occurred to me that as oxygen and hydrogen are 
 evolved from water by electrolysis, and conversely form water 
 by electro-synthesis, so some other gases which are evolved 
 from certain electrolytes by voltaic action might, when ar- 
 ranged as a gas battery with the electrolyte from which they 
 are evolved, give rise to a current, although they would not do 
 so when arranged in circuit with a different electrolyte. To 
 test this view I tried 
 
 Experiment 20. Oxygen and deutoxide of nitrogen, in 
 alternate tubes of the gas battery, with dilute nitric acid ; the 
 effects were, however, precisely similar to Experiment 9, viz. a 
 very feeble action for a few minutes, then a cessation, and no 
 continuous chemical action. 
 
 Experiment 21. For the same reason oxygen and nitrogen, 
 with solution of sulphate of ammonia, were tried ; this arrange- 
 ment produced at first a slight effect upon the iodide, which 
 soon ceased, and after several days there was no more rise of 
 liquid in any cell of the closed circuit than in the detached 
 cell ; the rise of liquid in both was very trifling indeed (about 
 O'Oi cubic inch), and had evidently nothing to do with voltaic 
 action. In this experiment, and in every experiment that I 
 have tried, I have perceived a trifling action for the first few 
 minutes. This I should have attributed to accidental causes, 
 such as slight impurities in the gases, slight metallic deposits 
 on the plates, &c., but that it is always in the direction which 
 
GAS BATTERY. 273 
 
 the theory of the gas battery would indicate. Thus, in the 
 present experiment, the appearance of iodine indicated oxygen 
 to have the same voltaic relation to nitrogen as it has to 
 hydrogen. This temporary effect, therefore, appears to me 
 analogous to that action called by Continental experimentalists 
 polarisation, an apparent tendency to action, i.e. an arrange- 
 ment of molecules preliminary to electrolysis, but incapable 
 of producing a continuous current. In this and many other 
 experiments with the gas battery I have observed this effect, 
 but have never been able to produce any chemical change or 
 electro-synthetic absorption of nitrogen. 
 
 Experiment 22. As oxalic acid when electrolysed evolves 
 at the anode a mixture of oxygen and carbonic acid, and at 
 the cathode hydrogen and carbonic oxide, for the reasons 
 above stated I charged a gas battery with carbonic acid and 
 carbonic oxide in the alternate tubes, and with oxalic acid as 
 an electrolyte ; a slight effect was produced, the carbonic 
 oxide being to the carbonic acid as hydrogen to oxygen ; but 
 the current was evidently due to the atmospheric air in solution 
 combining with the carbonic oxide ; this I proved by some 
 of the test experiments before-mentioned, which I need not 
 recapitulate. 
 
 Experiment 23. Hydrogen, nitrogen, and sulphate of am- 
 monia. This combination also gave effects with which the 
 nitrogen appeared to have nothing to do, this gas being per- 
 fectly unaffected. I tried other experiments on this point, but 
 they all led to the same conclusion, viz. that my idea of real- 
 ising a voltaic action by conversion of the ordinary effects 
 of electrolysis was erroneous. It may be that the above 
 gaseous products of electrolysis are secondary, and that water 
 is the only electrolyte in these cases ; but for this, as for many 
 other theoretical questions, there are so many arguments pro 
 and con, that it is not worth while to dilate on them unless 
 they can be shown to lead, or to be likely to lead, to some 
 new valuable facts or natural relations. 
 
 Reviewing the above experiments, it appears that chlorine 
 and oxygen, on the one hand, and hydrogen and carbonic 
 oxide, on the other, are the only gases which were decidedly 
 capable of electro-synthetically combining so as to produce a 
 
 T 
 
274 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 voltaic current* I should perhaps except olefiant gas, which 
 appears to give rise to a continuous though extremely feeble 
 current ; and the vapours of bromine and iodine, were they 
 less soluble, would probably also be found efficient as electro- 
 negative gases. 
 
 It now occurred to me that as several of these gases (take 
 as an instance nitrogen) were absolutely without effect in the 
 gas battery, this would form a valuable instrument for the 
 analysis of atmospheric air or other mixed gases. I therefore 
 procured, 
 
 Experiment 24, Two narrow cubic inch tubes of seven 
 inches long, carefully graduated into 100 parts. These were 
 immersed in separate vessels of dilute sulphuric acid, and filled 
 with atmospheric air exactly to the extreme graduation ; the 
 water-mark within the tube was examined when exactly at 
 the same level as the exterior surface of the liquid ; folds of 
 paper were used to protect them from the warmth of the 
 hands, and thus prevent expansion ; the barometer and ther- 
 mometer were examined, and every precaution taken for 
 accurate admeasurement. One of these tubes was left empty, 
 in order to ascertain, and eliminate from the result, the effect 
 of solubility. Into the other was placed a strip of platinised 
 platinum foil, one quarter of an inch wide. This strip of foil 
 was connected by a platinum wire with another strip placed 
 * n a tu ^ e f hydrogen and inserted in the same 
 vessel. The apparatus is shown in fig. 1 2. After 
 the circuit had been closed for two days, the 
 liquid was found to have risen in the tube a 
 twenty-two parts out of the 100; in the tube 
 placed by its side it had risen one division. The 
 tubes were allowed to remain several days 
 longer, but no farther alteration took place. This 
 analysis gives, therefore, twenty-one parts in 100 
 as the amount of oxygen in a given portion of air. 
 Experiment 25. The tube a (fig. 12) was 
 charged with nitrogen to a given mark, and 0*5 
 cubic inch of pure hydrogen added ; the tube h was then 
 charged with oxygen, and the circuit closed. Examined 
 after twenty-four hours, the water had risen in the tube a 
 
 * See Postscript. 
 
 Fig 12 
 
GAS BATTERY. 275 
 
 exactly 0*5 cubic inch. The apparatus was left in this state for 
 several days, but without any farther effect ; the voltaic action 
 had thus perfectly exhausted the hydrogen and there stopped. 
 
 These experiments are sufficient to prove the accurate 
 eudiometric action of the gas battery. Performed on a large 
 scale this method of eudiometry appears to me likely to possess 
 some advantages. In the eudiometer of Volta, when gases 
 containing oxygen are to be analysed, if the hydrogen added 
 for detonation be impure the result is of course erroneous. 
 The same may be said of the detonation by spongy platinum, 
 or by a wire heated by a voltaic current, which I formerly 
 proposed.* 
 
 If, on the other hand, gases containing hydrogen are to be 
 analysed, errors may result from any impurities in the oxygen 
 which is added, or from inaccuracy in the measurement of 
 either gas ; in the electrolytic method of eudiometry the quan- 
 tity or purity of the hydrogen, in the one case, is of no im- 
 portance ; and in the other the quantity or purity of the 
 oxygen ; that is, provided there be sufficient to exhaust the 
 equivalent to be abstracted from the mixed gas subjected to 
 examination. 
 
 It should be observed, that in these experiments only a 
 single pair of the gas battery can be used, as, if more be 
 employed, the electrolyte is likely to be decomposed, and gas 
 added to the compound.^ The process is rather slow, but I 
 think very sure. Another valuable application of this process 
 is, that it affords (in Experiment 24) a simple method of 
 obtaining nitrogen of unquestionable purity. I know of no 
 method which effects this object so perfectly. All the oxygen 
 of the air is abstracted, as well as that free oxygen which may 
 be contained in the liquid ; and by subsequently introducing 
 a little lime-water into the tube a y the trifling quantity of car- 
 bonic acid may be removed, or the same thing may be at 
 once effected by using caustic potash as the electrolyte in the 
 apparatus, fig. 12. 
 
 Probably many other applications of the gas battery may 
 suggest themselves to other experimentalists, and obviously 
 
 * Phil. Mag., August 1841, p. 99. 
 f See Postscript. 
 
 T 2 
 
276 EXPERIMENTAL INVESTIGATIONS. 
 
 many more changes may be rung upon the gases employed, 
 and curious and valuable results obtained ; I have, however, 
 in this paper given a sufficient number of experiments fairly 
 to open the subject ; each appears so suggestive of new ones, 
 that it is difficult to know where to stop. 
 
 The experiments on eudiometry, which I have last named, 
 induced me to refer to Dr. Henry's paper on Gaseous 
 Analysis,* and on reading it I was struck with a coincidence 
 between the action of spongy platinum on mixed gases and 
 the gas battery, a coincidence strongly confirmatory of the 
 views which led me to its discovery. I will endeavour briefly 
 to state these, and I state them, not as being absolutely correct, 
 for differences of opinion may exist on this as on every other 
 scientific matter, but as being those which existed in my mind 
 prior to the experiments, and which are considerably, and to 
 me unexpectedly, strengthened by the results embodied in 
 the above-mentioned paper of Dr. Henry. 
 
 My original deduction may be stated and exemplified as 
 follows : When pure or amalgamated zinc is immersed in 
 acidulated water the oxygen, as is well known, will not com- 
 bine with the zinc ; but touch both zinc and liquid with 
 platinum and combination ensues, the platinum being un- 
 altered. So, with a mixture of oxygen and hydrogen, the 
 gases, although in intimate contact, will not chemically unite ; 
 but touch them with clean platinum and more or less rapid 
 combination ensues. Here also the platinum is unaltered. 
 Leaving out of the case any purely hypothetical explanation, 
 why may not effects so similar in their character be related in 
 other respects ? In the voltaic combination the platinum is 
 heated during action ; and if the surfaces, and consequently 
 the quantity of electro-chemical action, be considerable, it is 
 ignited ; so in the catalytic combination, if the platinum be 
 thin and of large extent, or in the form of a sponge, which 
 still more increases its surface, it is ignited. Why, therefore, 
 may we not regard the detonation of gas by platinum as a 
 voltaic effect ? or the combination of oxygen and zinc by the 
 presence of platinum a catalytic effect ? The only difference 
 is, that gases do not admit of that interchangeable relation of 
 particles which we call electrolysis. The necessity for this 
 
 * Phil. Trans,, 1824. 
 
GAS BATTERY. 277 
 
 interchange is, however, removed when the gases are in a state 
 of such intimate admixture that it is not requisite to convey 
 the action through a chain of particles ; in the gas battery 
 this chain is supplied by the intervening electrolyte, and thus 
 the same action which is local in the experiments of Dobereiner 
 is circulating in the gas battery. The latter bears the same 
 relation to the former as the action of the ordinary voltaic 
 battery does to the normal phenomena of chemical affinity. 
 This relation is confirmed by the facts detailed in the paper 
 of Dr. Henry, as the gases which he there found would com- 
 bine by the presence of spongy platinum are precisely those 
 which will combine in the gas battery ; thus oxygen and 
 hydrogen combine rapidly, oxygen and carbonic oxide much 
 more slowly, and oxygen and olefiant gas very feebly, so much 
 so, as, in Henry's experiments, to require heat to induce com- 
 bination. Of course chlorine and hydrogen, which will unite 
 without platinum, will, a fortiori, unite with the aid of plati- 
 num, or they may in the gas battery occasion secondary 
 action ; the oxygen evolved by the decomposition of water by 
 the chlorine combining with the free hydrogen in the tube. 
 As oxygen and ammonia will, when at a slightly elevated 
 temperature, combine by the influence of spongy platinum, 
 forming water and leaving nitrogen, I now, in order farther to 
 test this relation, tried 
 
 Experiment 26. Ten cells of the gas battery were charged 
 with oxygen and solution of ammonia, with a little sulphate 
 of ammonia added to improve its conducting power. This 
 arrangement produced a moderate effect upon the iodide, which 
 was continuous ; the liquid rose slowly but uniformly in the 
 oxygen tubes ; a gas was evolved in the alternate tubes, 
 which proved to be pure nitrogen. After three weeks closed 
 circuit, the gases, collected, measured, and averaged, gave for 
 each tube 
 
 Nitrogen evolved . . . . = 0*07 cubic inch. 
 Oxygen absorbed . . . = O' 1 2 cubic inch. 
 
 Experiment 27. To examine whether the mere alkaline 
 character of ammonia had anything to do with the effect, ten 
 cells were charged with oxygen and solution of caustic potash, 
 but produced no effect 
 
278 EXPERIMENTAL INVESTIGATIONS. 
 
 These experiments are strongly corroborative, and seem 
 to me conclusive as to the relation between the action of the 
 gas battery and catalysis by spongy platinum. Experiment 26 
 is also remarkable in regard to the binary theory of electro- 
 lysis, but upon this point I will not here enter. 
 
 Applying the hypothesis of Grotthus to the gas battery, 
 we may suppose that when the circuit is completed, at each 
 point of contact of oxygen, water, and platinum, in the oxygen 
 tube, a molecule of hydrogen leaves its associated molecule of 
 oxygen to unite with one of the free gas ; the oxygen thus 
 thrown off unites with the hydrogen of the adjoining molecule 
 of water ; and so on until the last molecule of oxygen unites 
 with a molecule of the free hydrogen ; or we may conversely 
 assume that the action commences in the hydrogen tube. In 
 these hypotheses we should bear in mind that we proceed 
 by steps which nature, as hitherto tested by experiment, has 
 not recognised. All we can safely predicate of the actions at 
 anode and cathode is that they are correlations ; although 
 they take place at a distance, the one has no more been proved 
 to take place without the other, or before the other, than 
 height has been proved to exist without depth. I therefore 
 allude to this hypothesis, not as explicitly adhering to it, but 
 because it is generally received, and may tend to associate 
 the action of the gas battery with the ordinary phenomena of 
 electrolysis. 
 
 A number of hypotheses has been and may be proposed 
 to account for these and other mysterious phenomenal rela- 
 tions ; they all agree in being assimilations of what is un- 
 familiar to what is familiar. They are undoubtedly useful as 
 illustrations, and it is as such that they have hithero contri- 
 buted to advance science. It is, however, a curious circum- 
 stance, and worthy of some consideration, that the voltaic 
 hypothesis of Grotthus, the emissive and undulatory hypo- 
 theses of light and heat, and, as far as I am aware, all physical 
 hypotheses hitherto propounded, represent natural agencies as 
 effects of motion and matter. These two seem the most 
 distinct, if not the only conceptions of the mind, with regard 
 to natural phenomena ; and when we try to comprehend or 
 explain affections of matter which are not obviously modes of 
 
GAS BATTERY. 279 
 
 motion, we hypothetically or theoretically reduce -them to it : 
 the senses perceive the different effects of sound, light, heat, 
 electricity, &c., but the mind appears capable of distinctly 
 conceiving them only as modes of motion. Does not this supply 
 an argument that all physical agencies are reducible to these 
 elements of mental conception ? Or are we to look for new 
 powers of mind, in other words, will greater familiarity with 
 phenomena, at present recondite, enable the mind more clearly 
 to comprehend them, and avoid the necessity of referring 
 them theoretically to more familiar and apparently more 
 simple phenomena ? To pursue this curious enquiry \vould 
 involve me in a discussion foreign to the object of this paper 
 and to the general character of contributions to the Royal 
 Society, but the question arises so immediately out of the 
 subject, and is so necessary to explain my own view, that I 
 trust this brief statement of it will be considered sufficiently 
 pertinent. It touches upon that interesting, scarce definable 
 boundary, where physical merges into metaphysical science. 
 
 There are one or two other theoretical points as to which 
 the gas battery offers ground of interesting speculation ; the 
 contact theory is one. If my notion of that theory be correct, 
 I am at a loss to know how the action of this battery will be 
 found consistent with it. If, indeed, the contact theory assume 
 contact as the efficient cause of voltaic action, but admit that 
 this can only be circulated by chemical action, I see little 
 difference, save in the mere hypothetical expression, between 
 the contact and chemical theories ; any conclusion which 
 would flow from the one would likewise be deducible from the 
 other ; there is no observed sequence of time in the pheno- 
 mena : the contact or completion of the circuit and the electro- 
 lytical action are synchronous. If this be the view of contact 
 theorists, the rival theories are mere disputes about 'terms. If, 
 however, the contact theory connects with the term contact 
 an idea of force which does or may produce a voltaic current 
 independently of chemical action, a force without consump- 
 tion, I cannot but regard it as inconsistent with the whole tenor 
 of voltaic facts and with general experience. 
 
 Another point of theory suggested by the gas battery is 
 the relation of latent heat in the different cells of the battery 
 
280 EXPERIMENTAL INVESTIGATIONS. 
 
 and voltameter. According to our received theory of caloric, 
 oxygen and hydrogen cannot assume the gaseous from the 
 liquid state without rendering sensible heat latent. Now, as 
 in the gas battery the gases evolved from the liquid in the 
 voltameter must require and absorb precisely as much heat as 
 is set free by the gases becoming liquid in each cell, it may be 
 a curious subject of future enquiry (an enquiry which that 
 beautiful instrument, the thermo-multiplier, will materially 
 aid) to ascertain whether the heat absorbed in the voltameter 
 be exacted from surrounding bodies, or whether it be sup- 
 plied by the action of the battery itself, i.e. as the chemical 
 force in the voltameter is conversely equivalent to that in each 
 cell of the battery, and the calorific force at the voltameter is 
 also the converse equivalent of that in each battery cell, whe- 
 ther there is the same mutual dependence of the latter as of 
 the former forces. The action in the voltameter of ordinary 
 batteries would argue strongly against the proposition, that 
 the heat is exacted from surrounding bodies, as it is well known 
 that water when electrolysed has its temperature rather in- 
 creased than diminished ; and I have found, when decompos- 
 ing water with the nitric acid battery at a rate of 1 50 cubic 
 inches a minute, a very considerable augmentation of tempe- 
 rature in the liquid subjected to decomposition, so much so, 
 that, if the quantity was not considerable, it was heated to 
 ebullition. Much of this adventitious heat may have arisen 
 from the restriction of the circuit by the voltameter plates and 
 connecting wires ; but if the gas battery be supposed to supply 
 exactly sufficient heat, or (to use a license of expression) to 
 convert electricity into sufficient heat to satisfy the demands 
 of the expanding gases each battery cell being able by the 
 condensation of its respective gases to afford this supply a 
 rise of temperature ought to be perceptible in the whole battery 
 equal to the heat produced by the condensation of gases in all 
 the cells, minus that of one cell. I have not as yet been able 
 to detect any elevation of temperature due to the action of the 
 gas battery, not having in my possession any instrument 
 capable of detecting such delicate thermoscopic effects. I am, 
 therefore, the more anxious to offer the point for the consi- 
 deration of those who may have such instruments at their 
 
GAS BATTERY. 281 
 
 command, and here for the present I leave the gas battery 
 and its theory. 
 
 Postscript, July 7. The length of time which has elapsed 
 between the communication and printing of this paper, as it 
 has enabled me to procure the apparatus fig. 8, will, I trust, 
 be deemed a sufficient reason for my adding a postscript con- 
 taining a few experiments with this form of battery, some of 
 which I cannot but consider important. 
 
 Experiment 28. In order farther to test the opinion ex- 
 pressed, p. 274, six cells of this battery were charged with 
 pure hydrogen and dilute acid in the alternate tubes. When 
 first charged they decomposed water freely, but after the cir- 
 cuit had been closed for a short time, to, exhaust the oxygen 
 of the atmospheric air in solution, they produced no voltaic 
 effect ; the whole series of six would not decompose iodide of 
 potassium ; when, however, a little air was allowed to enter 
 any one of the tubes containing liquid that single cell instantly 
 decomposed the iodide ; three cells were put aside, each in 
 closed circuit ; at the expiration of a week these produced no 
 effect upon a galvanometer, nor was there any gas evolved in 
 the tubes containing liquid ; the stoppers were now taken out, 
 and the liquid in the hydrogen tubes rose to an average of 
 0*3 cubic inch ; each cell contained a pint, and we may there- 
 fore regard 0*15 cubic inch as the amount of oxygen held in 
 solution by this quantity of acidulated water. Were it not 
 for the extreme practical difficulty of perfectly excluding 
 atmospheric air for a long period, the above would furnish an 
 excellent method of examining the quantity of oxygen held 
 in solution by water, and by applying the proper calculus we 
 might read off on our galvanometer scale the infinitesimal 
 bubbles of gas contained in a given bulk of liquid ; if, how- 
 ever, the acid water or the hydrogen contain foreign ingre- 
 dients, a very different result follows, and the liquid, for reasons 
 which will now be obvious, frequently rises considerably in" 
 the hydrogen tubes. 
 
 Experiment 29. I repeated experiment 24 with the battery 
 fig. 8, expecting that as the external air was shut out I should 
 obtain the result more speedily ; I was indeed not without a 
 vague hope of producing some effect upon the nitrogen. The 
 
282 EXPERIMENTAL INVESTIGATIONS. 
 
 first result did follow : upon taking out the stoppers the morn- 
 ing after the battery had been charged, the liquid rose in the 
 air-tube one-fifth of the gaseous volume. I now closed it 
 again and examined it three days afterwards ; a very curious 
 effect had taken place ; the volume of the gas in the air- tube, 
 which had previously contracted, had now increased, and it 
 continued slowly increasing day after day. I at first believed 
 that the nitrogen was decomposed, but after many conjectures 
 and experiments found that the increase was due to the addi- 
 tion of hydrogen. On repeating the experiment with nitrogen 
 instead of air the same effect took place, but of course with- 
 out the previous contraction. I now returned to battery fig. 
 4; several of these .cells charged, some with atmospheric air 
 and hydrogen, and others with nitrogen and hydrogen, did not 
 exhibit the effect, though suffered to remain six weeks, each 
 in closed circuit, 
 
 To ascertain whether the vacuum formed by the abstrac- 
 tion of oxygen from the liquid had anything to do with the 
 above effect, a central narrow tube, open at both ends, was 
 substituted for the stopper in the battery fig. 8 ; the hydrogen 
 was still evolved. Not to detail a tedious set of test experi- 
 ments, I at length found that two points were essential to 
 obtaining the effect with certainty ; first, the exclusion of 
 any notable quantity of, atmospheric air from solution ; and 
 secondly, great purity in the hydrogen. In the former case, 
 when the hydrogen could find oxygen to combine with, it 
 was not evolved ; in the latter, there would be mixed or 
 rather diluted gas on both sides, and the forces would be ba- 
 lanced ; thus I have never succeeded in obtaining the effect 
 in the open battery, fig. 4 with hydrogen obtained in the 
 ordinary way from granulated zinc or iron filings, but have 
 sometimes succeeded with hydrogen procured by electro- 
 lysis. In the battery, fig. 8, I have succeeded in producing 
 the effect, but in a feeble degree, from hydrogen obtained 
 in the common way, but have never failed with hydrogen 
 obtained by electrolysis. Oxygen of the greatest purity, 
 voltaically associated with nitrogen, does not produce a 
 similar effect. The above unexpected results render it 
 necessary, in order to ensure accuracy in the eudiometric 
 
GAS BATTERY. 283 
 
 Experiment 24, either purposely to use common hydrogen in 
 the batteries figs. 4 and 12, or, what is more expeditious and 
 accurate, to use a battery similar to fig. 8, but with tubes 
 longer in proportion to their width ; and having first charged 
 the tubes with hydrogen and atmospheric air, to allow these 
 to remain in closed circuit until all the oxygen is abstracted 
 and a little hydrogen added, by the electrolytic effect, to the 
 residual nitrogen ; then to substitute oxygen for the original 
 hydrogen, which will in its turn abstract the hydrogen from 
 the nitrogen and leave only pure nitrogen. I have frequently 
 done this with perfect success. 
 
 Experiment 30. Hydrogen and carbonic acid in battery 
 fig. 8 produced the same effect. The volume of the carbonic 
 acid was increased, and hydrogen was found to have been 
 added to it. The effect therefore is not due to any peculiarity 
 of nitrogen, but yet some gas is necessary, for Experiment 28 
 proves that hydrogen alone will not decompose water. I need 
 scarcely say that when the above-mentioned effect took place 
 an interposed galvanometer was deflected, but the current was 
 much too feeble to decompose iodide of potassium. 
 
 I have tried, associated with hydrogen in battery fig. 8, 
 carbonic oxide, olefiant gas, protoxide of nitrogen, and deu- 
 toxide of nitrogen ; the two former produced no current or 
 chemical effect, the two latter gave a current and were decom- 
 posed. The volume of the deutoxide contracted one-half; the 
 residual gas was found to be nitrogen, which thenceforth was 
 gradually increased by hydrogen. The volume of the pro- 
 toxide did not undergo the previous contraction, except 
 slightly from solubility, but its change of state was denoted 
 by the absorption of hydrogen in the associated tube. 
 
 I likewise tried the effect of a vacuum and hydrogen, by 
 charging a battery (fig. 8) with I cubic inch oxygen and 3 
 cubic inches hydrogen ; the current was much enfeebled by 
 the resistance offered by the vacuum ; at first iodide of potas- 
 sium was decomposed and the galvanometer needle whirled 
 round ; after twenty-four hours the galvanometer needle was 
 only deflected 10; thus a physical was opposed to, and re- 
 sisted, a chemical force ; the current, however, continued, and 
 all the gas in the oxygen tube disappeared, except a minute 
 
284 EXPERIMENTAL INVESTIGATIONS. 
 
 bubble ; this was probably nitrogen from the atmospheric air 
 in solution, which had escaped to fill the vacuum. When the 
 stopper was taken out the liquid rose suddenly in the hydro- 
 gen tube 2'2 cubic inches, giving the equivalent of the oxygen 
 in the tube and in solution. It is very possible that this ex- 
 periment repeated might sometimes exhibit an evolution of 
 hydrogen in the oxygen tube arising from the escape of the 
 nitrogen of the atmospheric air in solution, and acting as in 
 Experiment 29, but I have not seen this effect take place. It 
 should be distinctly understood that in all the experiments 
 mentioned in this postscript, except the first part of Experi- 
 ment 28, single cells only were used. 
 
 Upon the theory of the Experiments 29 and 30 I will 
 venture no positive opinion. That gaseous hydrogen should 
 abstract oxygen from hydrogen, without the latter forming any 
 other combination, is a fact so novel, that any attempted ex- 
 planation is likely to prove premature. If, contrary to the 
 views of Dalton, we suppose that gases when mixed are held 
 together by a feeble chemical affinity, then we may say that 
 the affinity of the nitrogen or carbonic acid for hydrogen pro- 
 duces the effect ; the affinity of the oxygen of the water, 
 being balanced between the hydrogen in the liquid and that 
 in the tube, would enable the resultant feeble affinity of the 
 nitrogen for hydrogen to prevail ; but on this supposition why 
 does not oxygen produce an analogous effect ? Its tendency 
 directly to combine with platinum may indeed be regarded as 
 an opposing force, but this tendency is by many considered 
 hypothetical. On the other hand, it may be called an effect 
 of contact ; but this, unconnected with a chemical theory, 
 presents no other idea to the mind than the fact itself pre- 
 sents- it furnishes no link by which we may extend the phe- 
 nomena. I therefore, until a better theory be found, should 
 be inclined to adopt the former view, and to regard mixed 
 gas as in a state of feeble chemical union, the more especially 
 as throughout nature we find no absolute lines of demarcation, 
 though for conventional reasons we arc obliged to adopt them. 
 There must be many cases in which it is difficult, if not im- 
 possible, to draw the line between mechanical mixture and 
 chemical combination. 
 
VOLTAIC ACTION OF PHOSPHORUS, ETC. 285 
 
 In conclusion, I would say, with regard to the whole of 
 the experiments contained in this paper, that a longer time 
 and more experience may give positive results in cases where 
 I have only obtained negative ones ; it is far from impossible 
 that since curious solid combinations are formed by slow 
 electrical currents, as in the experiments of Crosse and Bec- 
 querel, so novel gaseous or liquid products may be obtained 
 by the long-continued voltaic action of gases and liquids. This 
 time alone can show. 
 
 For previous experiments and theories on the combination 
 of gases by platinum I may refer to Dobereiner's paper, 
 ' Phil. Mag./ Oct. 1823, in which I find he expresses an opinion 
 that it is a voltaic effect ; to the papers of Dulong and The- 
 nard, ' Annales de Chimie,' torn. 23 and 24 ; and to Faraday's 
 ' Experimental Researches/ series 6. The various experi- 
 ments on polarised electrodes of Ritter, Faraday, De la Rive, 
 Becquerel, Matteucci, and Schcenbein are also in point. 
 
 VOLTAIC ACTION OF PHOSPHORUS, SULPHUR, AND 
 HYDROCARBONS. 
 
 Phil. Trans., R.S. Received June 5. Read June 19, 1845. 
 
 IN a paper which was honoured by publication in the ' Philo- 
 sophical Transactions ' for 1 843 I described certain forms of 
 the gas voltaic battery, together with a series of experiments 
 in which different gases were employed as voltaic combi- 
 nations, and the consequent application of voltaism to eudio- 
 metry. 
 
 To ensure confidence in the accuracy of the eudiometric 
 experiments it was essential that the position which I laid 
 down as to the absence of all voltaic action in a combination 
 of oxygen and nitrogen should be rigidly true. I may state 
 with certainty that it is so, but an apparent exception noticed 
 (Experiment 21) in my last paper obtains during the first few 
 
286 EXPERIMENTAL INVESTIGATIONS. 
 
 minutes after the circuit is closed, and sometimes for a much 
 longer time. The examination of this temporary action in 
 the first instance, with the view of ascertaining whether it was 
 a specific action of the nitrogen or attributable to adventitious 
 circumstances, led me to the results which I have the honour 
 of laying before the Royal Society in this paper. 
 
 Before detailing these results I will, for convenience sake, 
 premise that they were all obtained by the form of battery 
 represented in fig. 8 of my last paper (which, with a slight 
 addition, to be referred to presently, is again represented at 
 fig. 2), p. 294, charged with distilled water slightly acidulated 
 with pure sulphuric acid. 
 
 I will also, when alluding to my last paper, to avoid the 
 needless repetition of the word experiment, refer to the number 
 of the experiments as though they were paragraphs, and con- 
 tinue those numbers in the paragraphs of this paper. 
 
 As the form of battery (fig. 2) by which the interfering 
 action of the atmosphere is entirely prevented was not devised 
 until the greater part of the experiments in my last paper had 
 been completed, I repeated some of them which seemed to 
 require such verification with this battery. To one of these 
 only is it essential that I should now refer. I should likwise 
 mention that in the experiments to be described the proper 
 reductions for temperature and pressure have been made 
 when necessary ; where it was practicable the experiments 
 were examined on days when the temperature and pres- 
 sure were, as nearly as may be, the same as when they were 
 set by. 
 
 (31.) Oxygen and deutoxide of nitrogen, which in the 
 open form of battery gave only a temporary action (9), when 
 employed in the closed form (fig. 2) gave a continuous 
 current. The following three sets of experiments were con- 
 tinued each for a month in closed circuit, during which time 
 they were constantly tested by the galvanometer, and evi- 
 denced a continuous voltaic action ; at the expiration of the 
 month the results were as follows : 
 
 Experiment I. Rise of liquid in tubes of Cubic in. 
 
 Oxygen =0*32 
 
 Deutoxide of nitrogen . . =1*26 
 
VOLTAIC ACTION OF PHOSPHORUS, ETC. 287 
 
 Cubic in. 
 
 Experiment 2. In oxygen tubes . . . . =O'5 
 Deutoxide of nitrogen . . = 2*5 
 
 Experiment 3. In oxygen tubes .... =O'2 
 Deutoxide of nitrogen . . =075 
 
 In oxygen tubes, rise . . . =0^34 
 Mean result In deutoxide tubeS) rise . . =r5 
 
 The slight excess being undoubtedly due to the greater 
 solubility of the deutoxide, it appears that four volumes of 
 deutoxide of nitrogen are absorbed in the gas battery for one 
 of the associated oxygen, and the result would be a compound 
 of I equiv. nitrogen + 3 oxygen, or hyponitrous acid, which is 
 exactly that formed by the slow combination of these two 
 gases in the ordinary chemical way. The difference of 
 amount of action in the three experiments depended on the 
 temperature, the second experiment being made towards the 
 close of last summer, the last experiment during the con- 
 tinuous cold weather of the present spring. 
 
 These experiments, coupled with the converse ones with 
 hydrogen and deutoxide of nitrogen (30), afford very satis- 
 factory instances of the illustration of the law of definite 
 combining volumes by the gas battery, exhibiting in one 
 result, and itself registering that result, the action of equivalent 
 chemical combination, catalysis and voltaism. 
 
 (32.) I now pass to the experiments which will form the 
 more immediate subject of this paper. The temporary action 
 to which I have alluded (21) being greater when nitrogen and 
 oxygen were the gases used, if the nitrogen were obtained by 
 burning phosphorus in atmospheric air, than if procured from 
 other sources, it naturally occurred to me that this action was 
 due either to some phosphorous acid remaining in a state of 
 vapour in the nitrogen, or to a slight portion of the phosphorus 
 itself being held in solution in the nitrogen, as believed by 
 Vauquelin and the older experimentalists. If this last suppo- 
 sition were the correct one, it seemed to offer a means of 
 rendering phosphorus, though a non-conductor and insoluble 
 in aqueous liquids, yet a permanent voltaic excitant analogous 
 to the oxidable metals. 
 
 (33.) A small piece of phosphorus having been carefully 
 
288 EXPERIMENTAL INVESTIGATIONS. 
 
 dried, and weighing when dry 9*6 grains, was passed up 
 through the liquid into the large tube of a gas battery by 
 means of a small loop of mica, which kept it separated both 
 from the glass and the platinum ; the tube was now charged 
 with pure nitrogen, and the associated tube with pure oxygen, 
 the level of the gases or water-mark being noted by a little 
 slip of paper pasted on the tube ; a check experiment of 
 oxygen and nitrogen without phosphorus was charged at the 
 same time ; the whole was carefully closed from the atmo- 
 sphere and set by for twenty-four hours in closed circuit, to 
 get rid of any current from adventitious circumstances ; the 
 next day, when tested by the galvanometer and iodide of 
 potassium, a very decided action was apparent in the phos- 
 phorus battery, the iodide being decomposed and the galva- 
 nometer needle swinging round to 30, the nitrogen with the 
 phosphorus representing the zinc of an ordinary voltaic com- 
 bination ; the check experiment gave not the least deflection 
 or decomposition. The experiments were suffered to remain 
 in closed circuit for four months, from August 10 to De- 
 cember 14, 1844, having been frequently tested in the in- 
 terim, and the galvanometer always evidencing a continuous 
 voltaic action in the phosphorus battery. On the I4th of 
 December the water in the oxygen tube having by its rise 
 denoted the absorption of a cubic inch of oxygen, plus the 
 slight quantity 0*05 cubic inch of oxygen due to solution, as 
 proved by comparison with the second battery, the experi- 
 ment was examined : the result was as follows : 
 
 Rise of liquid in oxygen tube, I cubic inch. 
 
 in nitrogen tube, o. 
 
 Original weight of phosphorus, 9-6 grains. 
 Present weight of phosphorus, 9'2 grains. 
 
 The battery was again charged in a similar manner, and 
 put by on December 19, 1844; the phosphorus weighed 
 2*8 grains. This, in consequence of the extremely cold 
 weather which has prevailed almost without intermission from 
 that time to the present period, proceeded much more slowly, 
 and was not examined until May 17, 1845, when the results 
 were as follows : 
 
VOLTAIC ACTION OF PHOSPHORUS, ETC. 289 
 
 Permanent deflection of galvanometer, 8. 
 
 Rise of liquid in oxygen tube, 0*35 cubic inch. 
 
 ,, in nitrogen tube, o. 
 
 Weight of phosphorus = 2 '6 5. 
 
 Taking a mean of these two experiments, which in their 
 relative results approximate more closely than I could have 
 anticipated under the circumstances, we get O'4i5 as the pro- 
 portional weight of phosphorus lost for a cubic inch of oxygen. 
 Now, as 24 31*4 :: 0*34 : 0^444. The result of these experi- 
 ments therefore leaves no doubt that phosphorous acid is the 
 product of the voltaic action, as it is of the slow combustion 
 of phosphorus in air. The experiment was repeated with dis- 
 tilled water ; the action was at first very trifling, but increased 
 every day, and the water gradually acquired an acid reaction. 
 
 No light was apparent in any part of the apparatus when 
 examined in the dark ; indeed, the action was much too slow 
 to render such an effect probable ; though if subsequently by 
 heat or other means I should succeed, as I hope, in producing 
 light, it will be curious to observe in what part of the circuit 
 the luminous effect in the voltaic combustion is perceptible. 
 
 A series of ten cells of phosphorus and nitrogen associated 
 with oxygen were charged, and perceptibly decomposed water 
 with platinum electrodes. 
 
 The result of the above experiments gives, I believe, the 
 first instance of the employment of a solid, insoluble non-con- 
 ductor as - the excitant of a continuous voltaic current ; it 
 proves that the existence of diffused phosphorus in nitrogen, 
 as noticed by the old experimentalists, is not a consequence, 
 as was once believed, of a partial combustion, but of an effusion 
 continuing as long as the previously diffused phosphorus is 
 abstracted, and it gives the very curious result of a true com- 
 bustion, the combustible and the ' comburant* being at a dis- 
 tance ; phosphorus burned by oxygen which is separated from 
 it by strata both of water and gas, of an indefinite length. 
 This result, arrived at by a progressive series of inductions, 
 scarcely now appears extraordinary, but would have been in 
 all probability listened to with incredulity if simply stated as 
 a fact a few years ago. By the galvanometer we may also 
 
 U 
 
290 EXPERIMENTAL INVESTIGATIONS. 
 
 ascertain the rate of this very slow and minute chemical 
 action ; thus, if by an apparatus as above described, my gal- 
 vanometer gives a deflection of 8 degrees, I know by the 
 above results that the phosphorus is being consumed at the 
 rate of the seven millionth part of a grain per minute. 
 
 (34.) The next step was to ascertain whether this action 
 was peculiar to nitrogen or common to other gases ; for this 
 purpose, a day or two after the first experiment was set aside, 
 the following were also made, and the dates and results were 
 as follows : 
 
 No. i. Phosphorus suspended in protoxide of nitrogen 
 associated with oxygen : weight of phosphorus 5*3 grains. 
 Charged August u, 1844. 
 
 No. 2. Similar experiment, but without phosphorus. 
 
 Tested occasionally by galvanometer, the first battery gave 
 invariably a small deflection, but less than in experiment (33) ; 
 the second gave no deflection. 
 
 Examined April 22, 1845. 
 
 No. I. Water risen in tube of oxygen 075 cubic inch. 
 
 In protoxide tube, 17 inch. 
 
 No. 2. Water risen in oxygen tube 0*1 cubic inch. 
 
 In protoxide tube, I '6 cubic inch. 
 
 Phosphorus weighed 5 grains, therefore loss = 0*3 grain. 
 
 In this experiment the rise of liquid in the tubes contain- 
 ing protoxide was evidently due to the solubility of that gas, 
 as it was very nearly equal in both the batteries, and the 
 second gave not the slightest galvanometric deflection ; the 
 result gives 0^65 cubic inch of oxygen consumed by 0*3 grain 
 of phosphorus, bearing nearly the same relative proportions 
 as experiment (33) ; the only difference between the action of 
 phosphorus in nitrogen and in protoxide of nitrogen is, that 
 in the former it is more rapid, as proved both by the galvano- 
 metric deflection and by the quantity of oxygen absorbed in 
 a given time. 
 
 (35.) Charged August n, 1844. 
 
 No. I. Phosphorus in carbonic acid gas associated with 
 oxygen ; weight of phosphorus 5-9 grains. 
 
 No. 2. Same, without phosphorus. 
 
 Tested by galvanometer, No. i. always gave a deflection, 
 No. 2. none. 
 
VOLTAIC ACTION OF PHOSPHORUS, ETC. 291 
 
 On the 3rd of December the carbonic acid gas in both 
 batteries had been absorbed, and the liquid had reached the 
 extremities of both tubes. 
 
 In the oxygen tube of 
 
 No. i, rise of liquid = O75 cubic inch. 
 
 In No. 2, o - O5 cubic inch. 
 
 Phosphorus weighed 5*6 grains; the proportional weight 
 was therefore 0*3 grain phosphorus to 0*7 cubic inch oxygen. 
 
 Here again the proportions came out just as in (33) and 
 (34), the intensity of action being intermediate, less than the 
 former and greater than the latter. 
 
 (36.) Charged December 18, 1844. 
 
 No. i. Phosphorus in pure oxygen associated with oxygen, 
 great care being taken to exclude atmospheric air : this ar- 
 rangement having been kept in closed circuit for twenty-four 
 hours, gave a very feeble deflection of the galvanometer. 
 
 Examined February 15, 1845. The rise of liquid in the 
 tube containing phosphorus was equal to 0*3 cubic inch, in that 
 containing the associated oxygen = 0*05. I find no note of the 
 phosphorus being re -weighed ; probably I considered it use- 
 less, as the consumption of oxygen in the associated tube was 
 so very trifling, scarcely sufficient to be distinguished from the 
 effect of its solubility. 
 
 (37.) Charged April 23, 1845. 
 
 No. r. Phosphorus suspended in deutoxide of nitrogen as- 
 sociated with oxygen ; weight of phosphorus =4*3 grains. 
 
 No. 2. Same, without phosphorus. 
 
 Examined May 27, 1845. Galvanometer gave 25 per- 
 manent deflection in No. i, and 10 in No. 2. 
 
 No. i. Rise of liquid in deutoxide tube = 07 cubic inch. 
 Rise of liquid in oxygen tube = O'6 cubic inch. 
 
 No. 2. Rise of liquid in deutoxide tube = 07 cubic inch. 
 In oxygen tube = 0*2 cubic inch. 
 
 Weight of phosphorus 4*17 grains. 
 Consequently had lost 0*13 grain for 0*4 cubic inch of oxygen. 
 
 In this and all the preceding experiments the residual 
 gases were unchanged in quality, and in this experiment it 
 appears that the action of the deutoxide of nitrogen and the 
 oxygen was perfectly unaffected by the phosphorus, the con- 
 
 u 2 
 
292 EXPERIMENTAL INVESTIGATIONS. 
 
 sumption of deutoxide of nitrogen being exactly the same in 
 both batteries. In another experiment, which I did not re- 
 cord, on account of a minute bubble of air having entered the 
 tubes containing the deutoxide, the phosphorus appeared to 
 have exercised a retarding influence on the voltaic combination 
 of deutoxide of nitrogen and oxygen ; this I attributed to a 
 slight deposit of phosphorous acid upon the platinum, by 
 which its catalytic power was deteriorated. 
 
 (38.) It thus appears that the effect we have been examin- 
 ing, of the diffusion of phosphorus in gas, is not due to any 
 peculiarity of nitrogen, and is not peculiar to any particular 
 gas, as once believed ; but being in all probability common to 
 all gases which do not exercise a specific action on the phos- 
 phorus, it may be more properly called a volatilisation of 
 phosphorus at ordinary temperatures than a solubility in gas ; 
 the ordinary slow combustion of phosphorus in air is, in fact, 
 a combustion of its vapour. I incline to think that the in- 
 feriority of its vaporisation in pure oxygen is due to a pro- 
 tecting film being formed, and that the phenomenon is in 
 some respects analogous to the inactivity of iron in nitric acid. 
 
 (39.) Phosphorus in nitrogen was associated with hydrogen 
 in the gas battery, to ascertain their voltaic relation ; the 
 hydrogen was positive to the phosphorus, i.e. represented the 
 zinc of an ordinary voltaic combination. 
 
 (40 ) To realise the curious novelty of two non-conducting 
 solids forming the elements of a voltaic battery, and producing 
 a continuous current, phosphorus suspended in nitrogen in one 
 tube of a gas battery was associated with iodine in nitrogen 
 in the other ; a very decided current was the result, which 
 continued for months, the nitrogen remaining unaltered in 
 volume, but the liquid becoming gradually tinged by the 
 excess of iodine vapour. The following is the result of the 
 experiment : 
 
 Charged January I, 1845. 
 
 Weight of iodine . . =5 '9 grains. 
 Weight of phosphorus . = 6*4 grains. 
 
 Examined May 17, 1845. 
 
 Weight of iodine . . =4*6 grains. 
 Weight of phosphorus . =6'28 grains. 
 
VOLTAIC ACTION OF PHOSPHORUS, ETC. 293 
 
 The phosphorus has consequently lost O'I2 grain, the 
 iodine 1-3. Assuming that the phosphorus consumed 3 equi- 
 valents of oxygen, as in experiments (33), (34), (35) (37), 
 we should have 3 equivalents of hydrogen eliminated, and 
 consequently 3 of iodine consumed, or 
 
 31-4 : 126-6 ::cri2 : 0-48 
 0-48x3= 1-44. 
 
 The result is tolerably near, but from the iodine vapour in 
 solution an excess and not a deficit in the consumption of this 
 was to have been expected. 
 
 (41.) It was necessary for my own satisfaction to make a 
 great number of other experiments for the purpose of check- 
 ing and eliminating any adventitious results which might pos- 
 sibly interfere with the actual voltaic action of the gas battery, 
 such as placing phosphorus in single tubes containing the 
 different gases, but with platinum foil and without associated 
 tubes, others without the platinum foil or associated tubes ; 
 but as these had no influence on the results, and were merely 
 used as tests for my own satisfaction, it would be useless and 
 tedious to detail them. 
 
 (42.) Having examined the action of phosphorus in the 
 gas battery under these various circumstances, my next step 
 was to ascertain if any other substance produced a similar 
 effect. Sulphur, the nearest analogue of phosphorus, was the 
 body which naturally presented itself, but from its different 
 characteristics required a different mode of manipu- Fig. i. 
 lation. The following was adopted. Into a little p\ 
 capsule of glass, having a long solid leg (see fig. i), 
 was placed a small piece of solid sulphur ; this was 
 held in the large aperture of a gas battery cell, while 
 the tube was passed carefully over it ; the platinum 
 in this tube was connected with the zinc of a single 
 cell of the nitric acid battery, while an anode of 
 platinum was placed in the liquid through the central 
 aperture ; by this means all the oxygen of the 
 atmospheric air was exhausted, and the surplus 
 hydrogen was in turn taken away by connection with the 
 associated tube charged with oxygen ; the same effect 
 might have been more slowly produced by the process 
 
294 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 described (29). The sulphur was now in an atmosphere of 
 pure nitrogen, and this could have been effected in no other 
 way that I am aware of without wetting or forming some 
 deposit on the sulphur. Having connected it in closed circuit 
 with the oxygen tube for twenty-four hours, the galvanometer 
 gave no deflection. A small hoop of iron with a handle was 
 now heated and passed over the tube 
 containing sulphur and nitrogen, the 
 wires being connected with the gal- 
 vanometer (see fig. 2). The result 
 was very striking. I had directed my 
 assistant to watch the galvanometer 
 while I attended to the manipulation. 
 At the same instant he exclaimed 
 that the galvanometer was deflected, 
 and I that the sulphur was melting ; 
 the galvanometer continued deflected 
 during the whole time that the sulphur 
 remained fused, and indeed some 
 time afterwards, until all the sulphur 
 vapour diffused in the nitrogen had become exhausted. The 
 sulphur represented the zinc of an ordinary voltaic com- 
 bination. It was of course impracticable in this case to 
 ascertain the equivalent consumption. This experiment 
 very strikingly exhibits the analogy of sulphur with phos- 
 phorus, and proves that the instant sulphur is fused it 
 becomes a volatile body, as phosphorus is when solid ; 
 the suddenness of its action, coupled with the insoluble 
 character of sulphur, leads to the conclusion that solution in 
 the electrolyte is not a necessary antecedent to voltaic action 
 in the gas battery. Indeed, this might have been deduced 
 from the experiments with phosphorus, as its vapour must 
 have been nearly, if not absolutely insoluble in the electrolyte, 
 or the equivalent results would not have come out so accu- 
 rately ; possibly solution and electrolysis are in these cases 
 synchronous. 
 
 (43.) I was now led to try in the gas battery other sub- 
 stances differing from phosphorus and sulphur, but possessing 
 characters which had hitherto prevented their being used as 
 
VOLTAIC ACTION OF PHOSPHORUS, ETC. 295 
 
 voltaic excitants ; as, if my view of the volatility of phos- 
 phorus and sulphur were correct, other volatile bodies ought 
 to act similarly. Camphor was the first substance I experi- 
 mented on. A piece of camphor weighing 12*9 grains was 
 placed in a similar manner to the phosphorus (33) in nitrogen, 
 and associated with oxygen ; tested by the galvanometer it 
 gave a feeble deflection, which, however, was continuous ; it 
 was allowed to remain four months in closed circuit ; at the 
 expiration of that time the liquid had risen in the oxygen 
 tube O'3 cubic inch ; the nitrogen in which the camphor was 
 suspended had increased in volume 0*15 of a cubic inch. 
 The camphor weighed 11*4 grains, but some minute crystals 
 of it were observed at the top of the tube, so that the loss of 
 weight was greater than that due to voltaic action. 
 
 (44.) The smallness of the quantity of the gas which had 
 been added to the nitrogen precluded an accurate analysis of 
 it ; enough was ascertained, however, to lead me to believe 
 that it was hydrocarbonous, and it then became my aim to 
 produce it in greater quantities. I attached a piece of cam- 
 phor to a platinum wire, and to the same wire I also attached 
 a piece of sponge platinum ; I passed these up into a tube of 
 nitrogen over distilled water, and at the expiration of three 
 months the gas had increased O'l cubic inch ; this proved 
 that the camphor vapour was decomposable by the catalytic 
 action of platinum at ordinary temperatures, and that the 
 effect in the nitrogen cell of the 
 battery was not due to its voltaic 
 association ; but the experiment 
 did not give me a sufficient quan- 
 tity of the gas for analysis. 
 
 (45.) I therefore had recourse 
 to the apparatus, fig. 3. a is an in- 
 verted cylindrical test-glass ; b a /jr 
 platinum capsule with a pin-hole 
 in the bottom for drainage, stand- 
 ing on an ivory pedestal ; c, c two 
 very stout platinum wires ; d a coil 
 
 of fine platinum wire. Into the capsule b was placed the cam- 
 phor, the glass a filled with distilled water was inverted over 
 
296 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 it and charged with pure nitrogen, to a level marked some- 
 where below the capsule ; the wires w w' are now connected 
 with a voltaic battery of sufficient power fully to ignite the 
 wire d. 
 
 (46.) After the wire was ignited the volume of the gas 
 gradually increased ; when the original volume was doubled 
 the gas was examined. It had a strong disagreeable odour, 
 very similar to that of coal-gas ; it burned with a blue flame, 
 slightly tinged with yellow : placed in an eudiometer, such as 
 I formerly described,* and mixed with hydrogen, it under- 
 went no alteration. Two volumes of it, mixed with one 
 volume of oxygen, contracted one-sixth of the whole volume, 
 and, subsequently agitated with lime-water, contracted two- 
 sixths more, lining the tube with a crust of carbonate of 
 lime. The residual gas was nitrogen. It was thus clear that 
 
 Fig. 4 . 
 
 the vapour of camphor was decomposed by the ignited wire 
 into carburetted hydrogen and carbonic oxide, and the ana- 
 logy is too direct to leave any doubt that these gases were 
 also formed in Experiments (43) and (44) by the influence of 
 the platinum foil and spongy platinum. 
 
 The apparatus (fig. 3) offers a most convenient means of 
 decomposing volatile hydrocarbons, and possibly other sub- 
 stances. 
 
 (47.) Portions of oil of turpentine and of cassia were 
 now placed in capsules (fig. i), weighed and exposed each to 
 an atmosphere of nitrogen in the large tube of a gas battery, 
 
 * Phil. Mag., Aug. 1841, p. 99 ; and Phil. Trans., 1843, P- IO 5- 
 
VOLTAIC ACTION OF PHOSPHORUS, ETC. 297 
 
 by the same means as described (42) ; they gave a very 
 decided deflection (the nitrogen representing zinc). This 
 deflection continues, the rise of liquid has been slow but 
 continuous, and the galvanometer feebly deflected. In the 
 turpentine experiment the rise is = 07 cubic inch, in the 
 cassia 0*5 ; the weights, however, from the irregularity of 
 absorption and evaporation, give no data as to the equivalent 
 consumption ; thus, the turpentine has lost 07 grain, the 
 cassia gained 0*05 grain. 
 
 (48.) Alcohol and ether were tried in a similar manner, 
 and produced notable voltaic effects ; alcohol the most power- 
 ful probably, on account of its greater solubility in water. 
 
 (49.) The rationale of the action in Experiments (43) and 
 (47) is curious. It seems that the platinum in the nitrogen 
 tube first decomposes the vapour of the hydrocarbons,* and 
 then the same platinum, with its associated plate, re-combines 
 the separated constituents with oxygen. In Experiment (43) 
 the decomposition takes place more quickly than the re-com- 
 position, as indeed would be expected from the absence of the 
 resistance of the electrolyte in the former case, and hence the 
 increase of gas in the nitrogen tube. 
 
 (50.) The analogy of the action of the above volatile sub- 
 stances strengthens the position advanced (38), that solid 
 phosphorus should be regarded as volatile at ordinary tempe- 
 ratures, and sulphur when fused ; the whole of these experi- 
 ments also serve to introduce the galvanometer as a new and 
 delicate test, and in some cases a measurer of volatilisation. 
 
 (51.) As the gas battery was shown in the former paper, 
 which I had the honour to communicate to the Society, to 
 give us the power of introducing gases which had been pre- 
 viously untried as voltaic excitants, and to ascertain their 
 electro-chemical relations, it has, by the means detailed in this 
 paper, opened a field for ascertaining the voltaic relations and 
 quantitative electro-chemical combinations of solid and liquid 
 substances, which from their physical characteristics had not 
 hitherto been recognised in lists of the voltaic relations of 
 
 * I use this word here and in the title to avoid periphrasis ; it is not quite 
 correct as applied to some of these bodies. 
 
298 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 different substances, and consequently formed to a certain 
 extent a blank in the chemical theory of the voltaic pile. 
 These results, coupled with the previously-arranged tables of 
 electro-chemical relations, and with the great improvements 
 in apparatus for measuring these relations recently made by 
 Mr. Wheatstone and others, offer every promise of the ultimate 
 establishment of accurate measures of affinity. I give the 
 following tables as an approximative list, without attempting 
 to give the degrees of intensity, which can only be filled up 
 by a careful series of researches exclusively devoted to this 
 object : 
 
 (52.) Chlorine. 
 
 Bromine. 
 
 Iodine. 
 
 Peroxides. 
 
 Oxygen. 
 
 Deutoxide of nitro- 
 gen. 
 
 Carbonic acid. 
 
 Nitrogen. 
 
 Metals which do not 
 decompose water 
 under ordinary cir- 
 stances. 
 
 Camphor. 
 Essential oils. 
 Olefiant gas. 
 Ether. 
 Alcohol. 
 Sulphur. 
 Phosphorus. 
 Carbonic oxide. 
 Hydrogen. 
 
 Metals which decompose 
 water. 
 
 Though carbonic acid and nitrogen appear to be neutral, 
 and consequently might be bracketed with the metals which 
 do not decompose water, as forming the nodal point or zero of 
 the table, yet, in consequence of the peculiar action exercised 
 by them, and detailed (29) and (30), I have placed them 
 above the metals. 
 
 (53.) The results embodied in my present and my former 
 paper I think sufficiently indicate the field of research opened 
 by the gas battery, a field which may of course be indefinitely 
 extended. I have never thought of the gas battery as a 
 practical means of generating voltaic power, though in conse- 
 quence of my earlier researches, which terminated in the nitric 
 acid battery, having had this object in view, I have been 
 deemed by some to have proposed the gas battery for the same 
 
NEW FORM OF GAS BATTERY. 
 
 299 
 
 purpose \ there is, however, a form of gas battery which I 
 may here describe, which, where continuous intensity or electro- 
 motive force is required, but the quantity of electricity is 
 altogether unimportant, appears to me to offer some advan- 
 tages over any form of battery hitherto constructed, and which, 
 independently of any practical result, is, from circumstances 
 peculiar to the gas battery, not without interest. It is shown 
 at figs. 4 and 5. A A' is a long glass tube, with a series of legs 
 or glass tubes attached to and opening into it ; the lower 
 extremities of these are open, and the main tube or channel 
 
 Fig. 5- 
 
 A A' terminates at the extremity A in a glass stopper, and at 
 A' opens out into a funnel, as shown in the figure. Into a 
 series of glasses B B' are cemented platinum wires having 
 attached to them strips of platinised platinum foil, two to each 
 glass, the one being four inches long and half an inch wide, 
 the other \\ inch long by one inch wide ; the former set are 
 placed lower than the latter, so that when the glasses are filled 
 with liquid the former set shall be just covered, and the latter 
 bisected by the water-mark ; the last glass B' has no platinum. 
 These platinum strips are connected metallically by external 
 wires, the narrow platinum of one cell with the wide one of the 
 next, and so on in series. The glasses having been filled to 
 the top of the narrow platinum with acidulated water, let a 
 piece of zinc be placed on a pedestal in the vessel B', and the 
 stopper being out of the extremity A, the apparatus A A 7 
 lowered into the glasses, the tubular legs covering each one of 
 
300 EXPERIMENTAL INVESTIGATIONS. 
 
 the narrow platinum plates. The tubes will of course be full 
 of water, and the main channel full of atmospheric air ; this 
 will gradually be displaced by the hydrogen ascending from 
 the zinc, which hydrogen, in consequence of the curve at A, 
 will retain its position. When it is judged that the greater 
 portion of air has been expelled, the stopper at A, covered 
 with a little grease, is to be inserted ; the hydrogen now will 
 rapidly descend in all the tubes until the zinc is laid bare, and 
 then remain stationary. 
 
 We have now a gas battery the terminal wires of which 
 will give the usual voltaic effects, the atmospheric air supply- 
 ing an inexhaustible source of oxygen, and the hydrogen being 
 renewed as required by the liquid rising to touch the zinc ; by 
 supplying a fresh piece of zinc when necessary, it thus becomes 
 a self-charging battery, which will give a continuous current ; 
 no new plates are ever needed, the electrolyte is never satu- 
 rated, and requires no renewal except the trifling loss from 
 evaporation, which indeed is lessened, if the battery be in 
 action, by the newly-composed water. There is an aperture 
 in the pedestal with a moveable slide, through which the 
 vessel B' can be removed, when necessary, to replace the zinc, 
 and the remaining part of the apparatus is never disturbed. 
 This battery would form an elegant substitute for the water 
 battery ; it would much exceed in intensity a similar number 
 of series of that apparatus ; it would be applicable to experi- 
 ments of slow crystallisation, and possibly to the telegraph. 
 Its construction is difficult, and makes its prime cost expen- 
 sive, but after that it is the most durable, the most easily 
 charged, and the most free from local action of any known 
 form. I have had one of ten cells constructed, shown at fig. 5 
 which succeeds perfectly, giving sparks, decomposing water, 
 &c., and is ever ready for use. Any number of such sets 
 might be united by adapter-tubes, or indeed it would be much 
 more economical, and reduce to a minimum the damage from 
 breakage, to have the main channels A A' made of varnished 
 wood or porcelain, with apertures into which separate glass 
 tubes might be cemented. 
 
THERMOGRAPHY AND VOLTAISM. 301 
 
 THERMOGRAPHY AND VOLTAISM. 
 
 Lecture at the London Institution. 
 Literary Gazette, Jan. 21, 1843. 
 
 THE following facts Mr. Grove had himself observed with 
 regard to metallic thermographs, i. When two portions of the 
 same metals are juxtaposed each being at the same temperature 
 little or no effect is produced. 2. When at a different tempe- 
 rature a slight effect is perceptible. 3. When the metals are 
 different at the same apparent temperature a greater effect is 
 perceptible. 4. When the metals are different, and at different 
 temperatures, a still greater effect is visible. Again, taking, 
 for example, a sovereign placed on a silver or copper plate, the 
 effect is increased if the coin be breathed on before being 
 brought into contact ; it is still more increased if the coin be 
 rubbed with oil and wiped apparently dry ; and it is still in- 
 creased if it be held for an instant over the vapour of a 
 substance capable of chemically acting on the juxtaposed plate ; 
 as, for instance, over ammonia, before being placed on copper. 
 These facts led him to believe that such impressions were 
 caused by a radiation and condensation of whatever vapour 
 exists between the metals, and which, by being condensed 
 unequally (in the case of different metals, by their different 
 conducting and radiating powers), produce an impression 
 according to the unequal distance of the different parts of the 
 stamp on the coin. In fact, if we rightly understood him, the 
 phenomenon was in some degree analogous to dew. Be this 
 as it may, whatever the nature of the radiation be, there is 
 unquestionably some radiation ; and this fact of inter-radiation 
 was applied by Mr. Grove to explain a fundamental experi- 
 ment in voltaic electricity, hitherto a subject of much contro- 
 versy, being the effects of Volta's original experiment on the 
 contact and severance of two discs of dissimilar metals. The 
 contact theory of electricity attributes the effects on the elec- 
 troscope produced by this instrument to simple contact ; the 
 
302 EXPERIMENTAL INVESTIGATIONS. 
 
 antagonistic theory asserts that no electrical disturbance can 
 take place without some chemical or physical change. Now, 
 as the facts of thermography prove a radiation of some sort 
 (for this argument, no matter of what sort) to take place on 
 the juxtaposition of different metals, a physical change is 
 produced, electricity ought to be developed on their separation, 
 caused either by their action on each other, or by the different 
 degrees of evaporation from each of their surfaces when sepa- 
 rated ; this physical or chemical change ought to produce, and 
 does produce, a disturbance of electric equilibrium ; and thus 
 recourse to the incomprehensible action of simple contact is 
 unnecessary. An experiment was shown strongly confirmatory 
 of this view : the discs of zinc and copper were juxtaposed, 
 surface to surface, but contact prevented by a rim of paper, 
 and yet when separated the electroscope was affected ; here, 
 then, was no metallic contact, and yet the electrical effects were 
 produced, which should be so by the mutual radiation, but 
 which should not be so by the contact theory.* 
 
 VOLTAIC PHENOMENA. 
 
 Electrical Mag., Sept. 1843. 
 
 IN the 'Philosophical Magazine,' for October 1839, I have 
 noticed the fact that when copper formed the anode of a 
 voltaic pile in nitro-sulphuric acid, it became inactive, and was 
 neither oxidated, nor was oxygen evolved from its surface ; 
 differing in the latter respect from inactive iron, it perfectly 
 arrested electrolytic action, and consequently the passage of 
 the voltaic current. Some facts which I subsequently ob- 
 served will throw some light on this anomaly, and, as far as I 
 am aware, are in themselves new. 
 
 * See Mr. Gassiot's confirmation of this, avoiding the rim of paper. Phil. 
 Mag., Oct. 1844. 
 
VOLTAIC PHENOMENA. 303 
 
 If a copper wire form the anode of a voltaic battery of 
 moderate power in dilute sulphuric acid, it is, as is well 
 known, gradually oxidated and dissolved. If the intensity of 
 the battery be increased, I have found that, at a certain point, 
 (varying with the voltaic combination employed), the action 
 is arrested, the copper is no longer dissolved, and a galvano- 
 meter interposed in the circuit gives little or no indication of 
 a current. Let the intensity be now farther increased, by 
 gradually adding to the voltaic series, and a very curious phe- 
 nomenon is exhibited : the copper is apparently disintegrated, 
 boiling off, as it were, in a very fine reddish-brown powder ; 
 the heat is intense, and the electrode rapidly worn away ; the 
 powder thus formed is partially dissolved, provided the acid 
 be not already saturated ; after saturation it subsides, and 
 may be collected and examined ; it proved to be the second 
 oxide, or copper 32 -f oxygen 8, called by some authors the 
 protoxide, by others the deutoxide. Besides employing the 
 usual chemical tests, I analysed it voltaically, by comparing 
 the weights lost from the anode with the hydrogen evolved 
 at the cathode; the following are three experiments taken 
 from my note-book, dated April 27, 1841 : 
 
 Thermometer, 60 Fahrenheit. 
 
 Exp. i. For 6 cub. in. of hydr. collected, anode lost 4*2 
 
 n 2- ,, ,, 4'2 
 
 > 3- 4'3 
 
 3)i27_ 
 
 4-23 
 Now, 47*2 : 32 ::6 : 4-07. 
 
 This result was obtained with a common pair of scales, 
 and although too coarse for a very accurate determination of 
 the equivalent, was quite sufficient to prove that it was the 
 full oxide, which was all I required. 
 
 The battery employed varied from I to 20 cells of the 
 nitric acid combination. I generally found that the intensity 
 at which the current was arrested was when from 5 to 10 cells 
 were employed 12 to 16 seldom failed to give the disinte- 
 grating effect. 
 
 The points worthy of notice in this experiment are 
 
304 EXPERIMENTAL INVESTIGATIONS. 
 
 1st. The purely mechanical effect of the voltaic current, 
 the oxide being forcibly detached without any farther chemical 
 combination. 
 
 2nd. The neutral point when the current is arrested, the 
 intensity being too great for the usual action of solution, and 
 too feeble for the disintegrating action. 
 
 3rd. The intense heat generated, which here, as in all 
 other voltaic experiments, appears to be proportionate to the 
 resistance. 
 
 When, with a very powerful voltaic battery, small wire 
 electrodes of platinum are dipped into water acidulated with 
 sulphuric acid, a brilliant combustion, attended with a decrepi- 
 tating noise, is observed at the cathode, provided the anode 
 be sufficiently immersed in the liquid ; on inverting the 
 arrangement, i.e. immersing the cathode and touching the 
 surface with the anode, a slight spark only is perceptible ; in 
 both cases the heat is very intense, and the liquid boils at the 
 point of contact, giving off sulphurous acid fumes ; it would 
 seem, at first, that this phenomenon was contrary to the usual 
 fact observed in the voltaic combinations, viz. that the com- 
 bustion is more active at the positive than at the negative 
 terminal ; but I found it to be in reality no anomaly, as it is 
 the sulphur of the dilute acid which is principally concerned 
 in producing the brilliant effect at the cathode, and which, as 
 it is evolved, forms itself the positive terminal. When the 
 experiment was continued for some time a very dark choco- 
 late powder was deposited in the liquid, and was found to be 
 precipitated when ingredients of the utmost purity were em- 
 ployed. After various tests had been employed, this precipi- 
 tate was ultimately found to be soluble in nitro-muriatic acid ; 
 the solution, treated with ammonia, gave a double salt of 
 platinum, it also gave a precipitate with chloride of barium ; 
 the original deposit was, in short, sulphuret of platinum. The 
 gas evolved from the negative electrode in this experiment 
 was hydrogen, mixed with sulphuretted hydrogen. 
 
 Entertaining a strong opinion that electricity is not a 
 specific fluid or fluids, but an affection of matter (possibly a 
 mode of motion) ; and, believing that all electrical phenomena 
 are intermolecular changes of the bodies, through which what 
 
VOLTAIC PHENOMENA. 305 
 
 is called the current of electricity passes, the following question 
 occurred to me in some experiments on the calorific effects of 
 voltaic electricity. If the ignition of a thin wire be the result 
 of a change in the arrangement of its own particles, caused by 
 an antagonist force to that which determines its aggregation, 
 what will be the effect of this force, pushed to the extreme ? 
 e.g. Voltaically ignite a given wire to fusion, arrange it so 
 that after fusion the wire shall retain its position in the 
 voltaic circuit, and what will be the result ? The experiment 
 was tried with platinum and lead wires, placed in a little 
 gutter of unglazed porcelain, which would retain the metal 
 after fusion. The effect upon the platinum wire was as 
 follows : after fusion the wire appeared to swell slightly, and 
 then burst asunder with a snapping noise ; one of the frustra 
 again ignited, again burst, and so on, until the portions 
 became too thick to be fused by the voltaic force employed. 
 The broken ends presented no very remarkable appearance. 
 My deduction from this experiment was, that the voltaic cur- 
 rent tended to expand the wire in the transverse, and to 
 contract it in the longitudinal direction. 
 
 With wires of lead the effects were more remarkable. 
 After fusion this metal evinced a tendency to contract in 
 length ; and, the direction of this contraction being followed 
 by gently approaching the terminal wires, which touched its 
 extremities, it appeared, as it were, to germinate ; it shot out 
 an irregular series of nodules. I may rudely compare the 
 effect to a membranous gullet, tied with a series of ligatures, 
 and then distended to the utmost. Each nodule pressed upon 
 its neighbour and formed dividing facets, which, when cold, 
 could be easily separated by the nail or by a blunt penknife. 
 
 The experiment, when carried on for some time, generally 
 ended either in the breaking asunder of the lead (though, 
 from its want of elasticity, unattended with the snapping 
 noise), or else the circuit was interrupted by the oxidation of 
 the lead. 
 
306 EXPERIMENTAL INVESTIGATIONS. 
 
 THE BAKERIAN LECTURE. ON CERTAIN PHE- 
 NOMENA OF VOLTAIC IGNITION AND THE DE- 
 COMPOSITION OF WATER INTO ITS CONSTITUENT 
 GASES BY HEAT. 
 
 Phil. Trans., R.S. Received September 3. Read November 19, 1846. 
 
 IN the 'Philosophical Magazine' for August 1841, I recom- 
 mended for eudiometrical purposes the use of a platinum 
 wire ignited by a voltaic battery. In fig. i, is represented 
 a form of apparatus for this purpose ; it consists of a tube of 
 Bohemian glass, with a loop of platinum wire -^th of an inch 
 diameter sealed into its upper end ; the size of the glass tube 
 may be adapted to the quantity of gas sought to be analysed, 
 and may, when necessary, be reduced to extremely small 
 dimensions, one-eighth of an inch being ample ; into this the 
 gas may readily be made to ascend, by the insertion of a wire 
 of copper, platinum, or glass, as may be suitable to the gas ; 
 two cells of the nitric-acid battery are sufficient fully to ignite 
 the wire, and the same battery supplies, by electrolysis, pure 
 oxygen and hydrogen for the analysis. Since the period 
 when I first proposed this I have seldom used any other 
 apparatus for such gaseous analyses as are performed by 
 combining the gas to be examined with oxygen or hydrogen. 
 This eudiometer possesses the advantage of enabling the 
 operator either to detonate or slowly to combine the gases, by 
 using different powers of battery, by interposing resisting 
 wires, or by manipulation alone a practised hand being able, 
 by changing the intervals of contact, to combine or detonate 
 the gas at will. My general practice has been to produce a 
 gentle heat in the wire until the gases contract, and then 
 gradually to increase the heat until a full ignition takes place, 
 by which means all the objects of the eudiometer of Volta are 
 fulfilled, without detonation, without dependence on the fickle 
 electric spark, and without thick tubes, danger of explosion, 
 or of the gases being projected from the eudiometer. 
 
 I have commenced with a description of this eudiometer, 
 
DECOMPOSITION OF WATER BY HEAT. 307 
 
 as it has been indirectly the means of my undertaking the 
 experiments detailed in this lecture ; and as its very great 
 convenience has never been generally understood, I think 
 that in strongly recommending it I shall be of service to 
 chemists. 
 
 In a paper honoured by insertion in the ' Philosophical 
 Transactions' for 1845, P a g e 358, I have shown another 
 method of eudiometry also performed by voltaic ignition ; in 
 that experiment the vapour of camphor was decomposed into 
 carbonic oxide and carburetted hydrogen ; it was an appli- 
 cation of voltaic ignition to effects analogous to those produced 
 by Priestley and others, by passing compound gases through 
 ignited tubes of porcelain. 
 
 But the voltaic process has this immense advantage, that 
 the heat can be rendered incomparably more intense ; that 
 the quantity of vapour or gas to be operated on may be inde- 
 finitely small ; that there are no joints, stop-cocks or ligatures ; 
 and that there is no chance of endosmose, which takes place 
 through all porcelain vessels. I therefore determined to ex- 
 amine by these means several gases, both with a view of 
 verifying, under different circumstances, known re- Fig. i. 
 suits, and seeking for new effects by this new and 
 advantageous application. I used an eudiometer 
 (fig. i) of 8 inches long and 0*4 inch internal dia- 
 meter, exposing the gases to intense heat, and subse- 
 quently analysed the residues in one of the same 
 length, but 0*2 inch diameter. 
 
 I will first consider the physical effects of different 
 gases on the ignition of the wire itself. 
 
 In a paper on the ' Application of Voltaic Ignition _ 
 to lighting Mines/ * I have mentioned the striking effects of 
 hydrogen in reducing the intensity of ignition of a platinum 
 wire, so much so that a wire voltaically ignited to incan- 
 descence in atmospheric air, is apparently extinguished by 
 inverting over it a jar of hydrogen ; with other gases the 
 effects are not so striking, and with them these differences are 
 
 * Phil. Mag., Dec. 1845. 
 
 X2 
 
308 EXPERIMENTAL INVESTIGATIONS. 
 
 best shown by including a voltameter in the circuit. Davy 
 found that the conducting power of a wire diminished in pro- 
 portion to the degree to which it was heated : assuming the 
 accuracy of this position, the amount of gas in the voltameter 
 would be inverse to the intensity of ignition in the wire. 
 The following is the result I obtained with different gases, 
 employing the same battery (the nitric acid combination at 
 its most constant period), the same wire, and the same 
 vessel : 
 
 Cubic inches of Gas evolved in 
 Gases surrounding the wire. the voltameter, per minute. 
 
 Hydrogen 77 
 
 Olefiant gas 7*0 
 
 Carbonic oxide 6'6 
 
 Carbonic acid 6'6 
 
 Oxygen 6*5 
 
 Compressed air, 2 atmospheres . 6*5 
 
 Nitrogen 6*4 
 
 Atmospheric air . . . ... . 6*4 
 
 Rarefied air. 6*3 
 
 Chlorine 6'i 
 
 To ascertain the relation between the amount of radiant 
 heat generated by the same battery and wire in gases which 
 presented striking differences as to the luminous effects of the 
 platinum wire, an apparatus was prepared in which the bulb 
 of a thermometer was retained at a certain distance from the 
 coil of wire ignited by a battery of four cells, and exposed, 
 first, to an atmosphere of hydrogen, and then to one of atmo- 
 spheric air, at the same temperature and pressure ; the ther- 
 mometer rose 7-2 m fi ye niinutes in the hydrogen, and 1 5 in 
 the air in the same time. Both the heating and luminous 
 effects appear therefore to be greater in atmospheric air than 
 in hydrogen. I cannot satisfactorily account for the differences 
 shown in the above table ; there appears a general tendency 
 to greater ignition in the electro-negative than- in the com- 
 bustible gases, but the facts are far too few to found a gene- 
 ralisation. I was at first inclined to regard the difference of 
 effect in hydrogen as analogous to the peculiarity mentioned 
 
DECOMPOSITION OF WATER BY HEAT. 309 
 
 by Leslie * respecting its convection of sound, but the parallel 
 does not hold : sound is transmitted imperfectly through 
 rarefied air, and also through hydrogen ; on the contrary, the 
 heat of the ignited wire is most intense in the former, and 
 least so in the latter ; the heat is also very much reduced in 
 intensity in the compounds of hydrogen, ammonia and olefiant 
 gas, or even by a small admixture of hydrogen with another 
 gas. such as nitrogen ; hydrogen, therefore, appears to have a 
 peculiar and specific action in this respect. 
 
 I now pass to the consideration of the effects of the ignited 
 wire on different gases. The ignition was in every case raised 
 to the fullest extent, and the gases after exposure to it were 
 carefully cooled down to their original temperature. 
 
 When the experiments were made over water, the whole 
 
 Fig. 2. 
 
 Fig. 3- 
 
 Library 
 
 eudiometer was immersed in a vessel of distilled water, occa- 
 sionally having an inch depth of oil on the surface (see fig. 2 f ) ; 
 when they were made over mercury, and a long-continued 
 exposure was required, a bent tube was employed, as at fig. 3, 
 the closed end being immersed in water or oil, to prevent the 
 fusion of the glass, which would otherwise have ensued. 
 
 The tubes are much more easily preserved from cracking, 
 and the ignition better kept up with oil on the exterior than 
 with water ; but, as in many of these experiments I might 
 have been considerably misled by a crack in the glass, or a 
 
 * Trans, of the Cambridge Phil. Sec., vol. i. p. 267. 
 
 f In this and in figs. 3 and 5 the lines leading from the platinum loop to the 
 mercury cups represent copper wires. 
 
3 io EXPERIMENTAL INVESTIGATIONS. 
 
 bad sealing of the wire, allowing a portion of oil to enter the 
 tube, I used water in the greater number of them until I was 
 assured of the phenomena.* 
 
 The apparatus, fig. 3, is superior in one respect to fig. 2, 
 even for experiments over water, as the wire being situate 
 Fig. 4. below the volume of gas, the circulation is more rapid. 
 This object may also be effected by employing the 
 form of eudiometer, fig. 4, in which the loop of wire 
 is near the centre of the tube, so as to be just above 
 the surface of water in the tube ; there are, however, 
 some difficulties of manipulation with this form, 
 which render it practically of less value than fig. I. 
 
 Binoxide of nitrogen over distilled water con- 
 tracted differently in proportion to the heat of the 
 wire ; in the best experiment it contracted to one- 
 ~~ third of its original volume ; the residual gas was nitro- 
 gen. Nitric acid was found in solution in the water. 
 
 Over mercury the effects were nearly the same: the mercury 
 was attacked, and the orange fumes of nitrous acid were visible. 
 Protoxide of nitrogen was decomposed into nitrogen and 
 oxygen ; the volume increased by 0*35 of the original volume ; 
 I could not get the full equivalent proportion, or 0*5 of oxygen. 
 Carbonic acid underwent no perceptible alteration. 
 Ammonia increased to double its original volume ; it was 
 now no longer absorbable by water, and gave 3 volumes of 
 hydrogen, plus I nitrogen. 
 
 Olefiant gas contracted slightly, deposited carbon, the 
 residue being hydrogen and olefiant gas, more of the former 
 in proportion to the heat, but I could not succeed in entirely 
 decomposing it. 
 
 Nitrogen suffered no change. 
 
 Oxygen gave a very slight contraction, amounting to ^th 
 of its volume ; the oxygen employed was very pure, obtained 
 from chlorate of potash and manganese, and also from water 
 by electrolysis : no change in properties was perceptible in 
 the oxygen after its exposure to the ignited wire. This con- 
 traction I incline to attribute to a slight portion of hydrogen 
 present, which view will, I think, be considered as strengthened 
 by the effect of the ignited wire on hydrogen, to be presently 
 
DECOMPOSITION OF WATER BY HEAT. 311 
 
 detailed. I at one time thought that the contraction might 
 be due to a slight oxidation of the wire, but it never went 
 beyond a very limited point ; nor was the wire altered in size 
 or weight, though it was kept ignited for many hours. 
 
 Chlorine over water gave dense white fumes ; a greyish 
 yellow insoluble powder accumulated on the sides of the tube 
 near the platinum wire, which appeared of the same nature as 
 the vapours ; the deposit was insoluble in cold nitric, sul- 
 phuric, or muriatic acid, but dissolved by the last when boiled. 
 The fumes did not, as far as I could judge, affect litmus- 
 paper ; a barely perceptible tinge of red was indeed communi- 
 cated to it, but this, I had every reason to believe, was attri- 
 butable to a slight portion of muriatic acid not absorbed by 
 the water. I have not yet worked out this result, as it is 
 probable, considering the number of experiments that have 
 been made on heated chlorine, that it is a known product, 
 though I cannot find, in several books to which I have 
 referred, any substance answering to it in description, and the 
 field opened by voltaic ignition is so new that each result 
 demands a separate and prolonged examination ; if I find 
 that this is an unknown compound I shall probably resume 
 its investigation. * 
 
 Cyanogen gave, though in very minute quantities, a some- 
 what similar deposit, but at its then very high temperature it 
 began to act rapidly on the mercury, and I was obliged to 
 give up the experiment after an hour's ignition. Both these 
 gases require peculiar and novel apparatus for examination by 
 voltaic ignition. It will presently be seen that my whole 
 attention and disposable time were necessarily occupied with 
 certain phenomena to which this class of experiments ulti- 
 mately led me. 
 
 Hydrogen gave a very notable contraction, amounting in 
 some cases to one-tenth of its volume. This was an unex- 
 pected result, and I examined it with care. It took place both 
 over water and over mercury ; rather more with the former 
 than with the latter. It obtained equally with hydrogen pro- 
 cured by electrolysis from carefully distilled water and pure 
 
 * See Supplemental Paper, p. 20. 
 
3 i2 EXPERIMENTAL INVESTIGATIONS. 
 
 sulphuric acid ; with that procured from common zinc and 
 pure sulphuric acid diluted with distilled water ; and with that 
 obtained from distilled zinc and pure diluted sulphuric acid. 
 The contraction was less when the water from which the 
 hydrogen was obtained was carefully purged of air by boiling 
 and the air-pump, but yet there was a notable contraction 
 even when the water had been freed from air to the utmost 
 practicable extent. In the numerous experiments which I 
 made on this subject the contraction varied from the T ^th to 
 the ^V tn f tne whole volume. 
 
 After many fruitless experiments I traced it to a small 
 quantity of oxygen which I found hydrogen to contain under 
 all circumstances in which I examined it. Phosphorus placed 
 in hydrogen, obtained with the utmost care, gives fumes of 
 phosphorous acid, shines in the dark and produces a slight 
 contraction, but there is after this a farther contraction by the 
 use of the ignited wire. 
 
 I may cite the following as an easy experiment and simple 
 illustration of the rapidity with which hydrogen appropriates 
 oxygen. Let hydrogen be collected over water well purged 
 of air ; let a piece of phosphorus remain in it until all com- 
 bustion has ceased, the hydrogen will then be full of phos- 
 phoric vapour ; fill another tube with water, and pass the 
 hydrogen rapidly into it, the second tube will instantly be 
 filled with a dense white fume of phosphorous acid ; the 
 hydrogen having instantly carried with it oxygen from the 
 stratum of water it has passed. 
 
 A very careful experiment was made as follows : Distilled 
 water was boiled for several hours, to this was added one- 
 fortieth part by measure of pure sulphuric acid, and it was 
 cooled under the receiver of an air-pump ; it was now placed 
 in two test-glasses, connected by a narrow inverted tube, full 
 of the same liquid : platinum electrodes were placed in each 
 glass, and the hydrogen caused to ascend immediately into 
 the eudiometer tubes ; the whole was completed within two 
 or three minutes after the water had been removed from the 
 air-pump. Here the ordinary sources of impurity in hydrogen 
 were avoided ; no zinc was used, the sulphuric acid was pure, 
 and the quantity was so small, that, had it not been pure, the 
 
DECOMPOSITION OF WATER BY HEAT. 313 
 
 error could have been but very trifling. The hydrogen so 
 obtained contracted in volume -^th ; hydrogen prepared in 
 the same way, and exposed to phosphorus, gave dense white 
 fumes ; the phosphorus was luminous in the dark for more 
 than an hour, and the contraction (temperature and pressure 
 being carefully examined) was g^th ; the amount of contrac- 
 tion by the wire would'of course equal three times the volume 
 of oxygen mixed with the hydrogen, consequently the oxygen 
 would be y^th of the whole volume ; the platinum wire 
 induces therefore a greater absorption of oxygen than the 
 phosphorus, unless the volume of hydrogen is increased by 
 the phosphoric vapour ; the sequel of this paper will render 
 it probable that even the ignited wire does not, and cannot 
 induce combination of all the oxygen existing in the hy- 
 drogen. 
 
 I have looked into the papers of MM. Berzelius and 
 Dulong, and of M. Dumas, on the equivalent weight of 
 hydrogen. The latter contains a most careful experimental 
 investigation, and is by far the best determination we have ; 
 although it is not there mentioned that hydrogen contains 
 oxygen, yet a correction is made for the air contained in the 
 sulphuric acid employed. M. Dumas does not state how the 
 quantity of that air is calculated. There can be no question 
 that nothing approaching in elaborate care to these experi- 
 ments has been yet performed on the subject ; but, with the 
 fullest consciousness of M. Dumas' skill, I have, in all my 
 experiments, perceived such an inveterate tendency of hy- 
 drogen to possess itself of oxygen, that I cannot help enter- 
 taining some doubts whether we have yet the real weight of 
 hydrogen within the assigned limits of error. 
 
 It is difficult to see how hydrogen can be absolutely 
 deprived of oxygen which has once existed in it ; neither an 
 oxidable metal, as zinc, or an ignited inoxidable metal, as 
 platinum, getting rid of all the oxygen, and phosphorus, if it 
 does so, replaces it by its own vapour. The near approach, 
 however, of the equivalent of hydrogen, as determined by M. 
 Dumas, to the ratio of whole numbers, renders it probable 
 that it is a very close approximation to the truth. 
 
 I have not been able to detect nitrogen in the hydrogen, 
 
3 i4 EXPERIMENTAL INVESTIGATIONS. 
 
 but the probability is that a slight quantity also exists in it. 
 Whether the oxygen proceeds from portions of air still 
 remaining in solution in the liquid from which the air is 
 exhausted, or whether it is a part of the water actually decom- 
 posed, but of which the oxygen is not absorbed by the zinc, 
 is a question to resolve which farther experiments are neces- 
 sary. 
 
 Hydrogen and carbonic acid mixed in equal volumes were 
 readily acted on by the ignited wire ; they contracted to 0*48 
 of the original volume ; the residue was carbonic oxide ; one 
 equivalent of oxygen had therefore united with the hydrogen ; 
 and the slight additional contraction was probably due to 
 the farther combination of hydrogen with oxygen as above 
 stated. 
 
 Carbonic oxide exhibited a remarkable effect, and one 
 which, coupled with the last experiment, -gave rise to con- 
 siderations which mainly led to the results to be detailed in 
 the body of this paper. Carbonic oxide, very pure and care- 
 fully freed from carbonic acid, was exposed to the ignited 
 wire over distilled water ; the gas increased in volume in one 
 experiment to one-third of its original volume, in the greater 
 number of instances to one-fifth : this increase depended upon 
 the intensity of ignition, which it was very difficult to main- 
 tain at its maximum on account of the frequent fusions of 
 the platinum wires. 
 
 Here again I hacj a long research and many erroneous 
 guesses, which I need not detail. The effect did not take 
 place with perfectly dry gas over mercury, and I thence was 
 led to attribute it to some combination with aqueous vapour ; 
 the increase turned out to be occasioned by the formation of 
 carbonic acid. By agitation with caustic potash or lime-water 
 the gas was reduced to exactly its original bulk, but it was 
 now found to be mixed with a volume of hydrogen equal to 
 the volume of carbonic acid by which it had been increased ; 
 it was thus perfectly clear that half a volume or one equivalent 
 of oxygen derived from the vapour of the water, had combined 
 with one volume or equivalent of carbonic oxide, and formed 
 one volume or equivalent of carbonic acid, leaving in place of 
 the carbonic oxide with which it had combined the one 
 
DECOMPOSITION OF WATER BY HEAT. 315 
 
 volume or equivalent of hydrogen with which it had been 
 originally associated. 
 
 Comparing the last experiment, viz. that of mixed carbonic 
 acid and hydrogen, with this, I was naturally struck with the 
 curious reversal of affinities under circumstances so nearly 
 similar ; in the one case, hydrogen taking oxygen from car- 
 bonic acid to form water and leaving carbonic oxide ; in the 
 other, carbonic oxide taking oxygen from water to form 
 carbonic acid and leaving hydrogen. 
 
 I thought much upon this experiment ; it appeared to me 
 ultimately that the ignited platinum had no specific effect in 
 producing either composition or decomposition of water, but 
 that it simply rendered the chemical equilibrium unstable, and 
 that the gases then restored themselves to a stable equilibrium 
 according to the circumstances in which they were placed 
 with regard to surrounding affinities ; that if the state of mixed 
 oxygen and hydrogen gas were, at a certain temperature, more 
 stable than that of water, ignited platinum would decompose 
 water as it does ammonia. 
 
 This is a very crude expression of my ideas, but we have 
 no language for such anticipatory notions, and I must adapt 
 existing terms as well as I am able. 
 
 It now appeared to me that it was possible to effect the 
 decomposition of water by ignited platinum ; that, supposing 
 the atmosphere of steam in the immediate vicinity of ignited 
 platinum were decomposed, or the affinities of its constituents 
 loosened, if there were any means of suddenly removing this 
 atmosphere I might get the mixed gases ; or secondly, if, as 
 appeared by the last two experiments, quantity had any in- 
 fluence, that it might be possible so to divide the mixed gases 
 by a quantity of a neutral ingredient as to obtain them by 
 subsequent separation (or as it were filtration) from the neutral 
 substance. Both these ideas were realised. 
 
 To effect the first object, after, as usual in such circum- 
 stances, much groping in the dark, I cemented a loop of 
 platinum wire in the end of a tube retort similar to fig. 3, and 
 covered it with asbestos, ramming this down so as to form a 
 plug at the closed extremity of the tube, the platinum wire 
 being in the centre. My object was, by igniting the platinum 
 
316 EXPERIMENTAL INVESTIGATIONS. 
 
 wire, to drain the water out of the asbestos, and the ignited 
 wire being then in an atmosphere of steam, I hoped the water 
 would by capillary attraction keep constantly oozing down to 
 the platinum wire as the steam or decomposed water ascended. 
 The experiment did not succeed : the water established a 
 current through the asbestos by washing away fine particles, 
 and the phenomena of ordinary ebullition took place, unless the 
 intensity of the battery was very much exalted, when a very 
 slight decomposition was perceptible, which I attributed to 
 electrolysis. This experiment, however, suggested another, 
 which did succeed. In one or two cases the asbestos plug 
 became compressed above the platinum and so choked up the 
 tube that the wire suddenly fused. It now occurred to me 
 that by narrowing the glass tube above the platinum wire I 
 had the result at my command, as the narrow neck might be 
 made of any diameter and length, so as just to allow the water 
 Fig. 5. to drip or run down 
 
 as the steam forced 
 its way up ; a tube 
 was so formed, and 
 is shown with its ac- 
 companiments at 
 
 fig- 5- 
 
 The result of this experiment was very striking : when 
 two cells of the nitric acid battery were applied the air was 
 first expanded and expelled, the water then soon boiled, and 
 at a certain period the wire became ignited in the steam. At 
 this instant a tremulous motion was perceptible, and separate 
 bubbles of permanent gas of the size of pin-heads* ascended 
 and formed a volume in the bend of the tube. It was not a 
 continuous discharge of gas, as in electrolysis, but appeared to 
 be a series of rapid jerks ; the water, returning through the 
 narrow neck, formed a natural valve which cut off by an inter- 
 mitting action portions of the atmosphere surrounding the 
 wire : the experiment presented a novel and indescribably 
 curious effect. The gas was oxyhydrogen. It will occur at the 
 first to many of those who hear this paper read that this effect 
 might be derived from electrolysis. No one seeing it would 
 think so for a moment ; and although I shall by my subse- 
 
DECOMPOSITION OF WATER BY HEAT. 317 
 
 quent experiments, I trust, abundantly negative this supposi- 
 tion, yet as this was my first successful experiment on this 
 subject, and is per se an interesting and striking method of 
 showing the phenomenon of decomposition by heat, I will 
 mention a few points to prove that the phenomenon could not 
 be occasioned by electrolysis. 
 
 To account for it by electrolysis, it must be supposed that 
 the wire offered such a resistance to the current that this 
 divided itself, and the excess of voltaic power passed by the 
 small portion of water which trickled down, instead of by the 
 wire. 
 
 In the first place, the experiment was performed with dis- 
 tilled water, and only two cells of the battery employed, which 
 will not perceptibly decompose distilled water. 
 
 2. No decomposition took place until the instant of ignition 
 of the wire, though there was a greater surface of boiling 
 water exposed to the wire before than after the period of 
 ignition. 
 
 3. A similar experiment was made, but with the wire 
 divided in the centre so as to form two electrodes, and the 
 water boiled by a spirit-lamp ; here the current had no wire to 
 conduct any part of it away, but the whole was obliged to 
 pass across the liquid, and yet no Fig. 6. 
 decomposition took place, or if there 
 
 were any it was microscopic. 
 
 4. When, instead of oil, distilled 
 water was used in the outer vessel,* 
 even the copper wires, one of which 
 would form an oxidable anode, gave 
 
 no decomposition across the boiling water outside, while the 
 ignited wire inside was freely yielding mixed gases. 
 
 5. To prevent the water from being the shortest line for the 
 current, I repeated the experiment with a perfectly straight wire 
 (fig. 6). The result was precisely the same ; but the experi- 
 ment is more difficult, as a certain length of wire is neces- 
 
 * Jan. 8. I have since found that the exterior tube of oil or water may be 
 dispensed with in this experiment, as the water which trickles down prevents the 
 fusion of the glass, 
 
3i8 ^EXPERIMENTAL INVESTIGATIONS. 
 
 sary, the sealing is more troublesome, and the size of the 
 bulb is much more difficult to adapt to the production of 
 steam in exactly the requisite quantity : the straight wire 
 being more suddenly extinguished and more easily fused : 
 with careful manipulation, however, it succeeds equally well 
 with the former experiment. 
 
 I might add other experiments and arguments, but I believe 
 when the remainder of this paper has been read, that the above 
 will be thought scarcely necessary. 
 
 I now directed all my efforts to produce the effects by heat 
 alone without the battery. I will mention a few of my un- 
 successful attempts, as it will save trouble to future experi- 
 menters. I sealed a platinum wire into the extremity of a 
 curved tube, filled the latter with water, and applied a strong 
 heat by the blowpipe to the projecting end of the wire, hoping 
 that the conducting power of the platinum, although inferior 
 to that of most other metals, was sufficiently superior to that 
 of glass to enable me to ignite the portion of the wire within 
 the tube, and thus surround it with an atmosphere of steam ; 
 the water, however, all boiled off from the glass ; nor could I 
 succeed in igniting the platinum by heat from without. A 
 similar failure occurred when, on account of its superior 
 conducting power, a gold wire was substituted for that of 
 platinum. 
 
 I sealed spongy platinum and bundles of platinum wire 
 into the ends of Bohemian glass tubes, closing the glass over 
 them, and then filling the tubes with water and heating the 
 whole extremity; but the water boiled off from the glass, and 
 the platinum could not be made to attain a full incandescence. 
 
 After many similar trials I returned to the battery, and 
 sought to apply it in a manner in which electrolysis could not 
 possibly take place. I had hoped, as I have above stated, to 
 obtain a residual decomposition of water by masking or dilut- 
 ing the gases by a neutral substance. I therefore tried the 
 following experiment : a tube similar to fig. I was filled with 
 water which had been carefully freed from air by long boiling 
 and the air-pump ; it was then inverted in a vessel of the same 
 water, and a spirit-lamp applied to its closed extremity, until 
 the upper half was filled with vapour (see fig. 7). The wire 
 
DECOMPOSITION OF WATER BY HEAT. 319 
 
 was brought to a full ignition by the battery, and kept ignited 
 for a few seconds ; connection was then broken and the lamp 
 removed, so that the water gradually ascended. A bubble of 
 the size of a large mustard-seed Fi 
 
 was left in the extremity of the 
 tube, and I was much gratified at 
 finding that when this was caught 
 by a lighted match at the surface 
 of the water trough it detonated. 
 The experiment was then repeated, 
 continuing the ignition for a longer 
 time, but the gas could not be in- 
 creased beyond a very limited quantity ; indeed it was not 
 to have been expected, as, supposing it to be mixed gas, 
 re-combination of the excess would have taken place, and 
 the fact of any uncombined gas existing when exposed to 
 incandescent platinum will doubtless surprise those who hear 
 it for the first time. 
 
 The experiment was repeated as at first and the bubble 
 transferred to another tube ; the wire was then again ignited 
 in vapour, another bubble was instantly formed and trans- 
 ferred, and so on, until after about ten hours' work sufficient 
 gas was collected for analysis ; this gas was now placed in an 
 eudiometer, it detonated and contracted to 0*35 of its original 
 volume ; the residue being nitrogen. The experiment was 
 repeated several times with the same general results, the residue 
 sometimes containing a trace of oxygen. 
 
 Here electrolysis was out of the question ; the wire was 
 ignited in (if I may use the expression) dry steam, the upper 
 part of the tube being far above the boiling-point, and of 
 course perfectly transparent ; if not an effect of heat it must 
 have been a new function of the electric current, at least one 
 hitherto unknown, 
 
 As the voltaic arc and electric spark afford heat of the 
 greatest intensity, I tried a succession of electric sparks from 
 platinum wires through steam in the apparatus fig. 8, the water, 
 as in all my experiments, having been previously purged of 
 air (to save circumlocution I will in future call it prepared 
 water). The sparks were taken from the hydro-electric 
 
320 EXPERIMENTAL INVESTIGATIONS. 
 
 machine of the London Institution ; they had in the steam a 
 beautiful crimson appearance ; on cooling the tube a bubble 
 was perceptible, which detonated by the match. 
 
 As in the previous experi- 
 Flg< 8f ments, a whole day's work did 
 
 not increase the bubble, but 
 when it was transferred another 
 instantly formed ; the gas was 
 similarly collected ; it detonated 
 and contracted to 0*4 of its ori- 
 ginal volume ; the residue was 
 nitrogen with a trace of oxygen. 
 
 This experiment will again surprise by its novelty ; the 
 very means used in every laboratory to combine the mixed 
 gases and form water here decompose water.* From a vast 
 number of experiments which I have made on the voltaic and 
 electric disruptive discharges (which are I believe similar phe- 
 nomena, differing only in quantity and intensity), I believe the 
 decompositions produced by them are the effects of heat alone, 
 and this experiment was therefore to my mind a repetition of 
 the last under different circumstances ; others however may 
 think differently. This experiment also I several times 
 repeated. 
 
 By counting the globules given off, and comparing a certain 
 number of them with the average volume of steam in the last 
 two experiments, an attempt was made to ascertain what pro- 
 portion of water could be decomposed by an ignited platinum 
 wire in aqueous vapour, or, which amounts to a corollary from 
 this proposition, what degree of dilution would enable mixed 
 gas to exist without combustion in an atmosphere of steam 
 exposed to an ignited platinum wire. The proportion in an 
 experiment in which the globules were so counted was I in 
 2,400 ; the probability is however that different temperatures 
 of the platinum wire would give different volumes of gas so 
 decomposed, the volume being greater as the wire is more 
 intensely ignited. 
 
 * I need scarcely point out the distinction, in fact, between this experiment 
 and those in which liquid water has been decomposed by the electric spark. See 
 Supplemental Paper. 
 
DECOMPOSITION OF WATER BY HEAT. 321 
 
 Although there was no known effect of electricity which 
 could produce the phenomenon exhibited by the last two ex- 
 periments, and it was in any event new, still, firmly convinced 
 that it was an effect of heat, I again determined to attempt 
 its production by heat alone, and without the use of the 
 battery. I procured a tube of silver 9 inches long and 0-4 inch 
 diameter ; at the extremity of this was a platinum cap, to 
 which a smaller tube, also of platinum, was soldered. This 
 platinum tube was closed at the end and soldered with gold 
 solder. The apparatus was filled with prepared water ; the 
 water was boiled in the tube to expel the air from the narrow 
 tube and any which might have adhered to the vessel ; the 
 tube was then, when full of hot FifT 
 
 water, inverted into water, and the 
 flame of a common blowpipe made 
 to play against the platinum tube 
 (see fig. 9) until a white heat was 
 obtained. Upon inverting it under 
 water a bubble of the size of a 
 mustard-seed rose to the surface, 
 which gave a very feeble detona- 
 tion with the match. Similar bubbles were collected as before, 
 and the gas in an eudiometer contracted to 0*7. On repetition 
 the experiment did not succeed so well, and upon several 
 repetitions it sometimes succeeded and sometimes failed, and 
 I should not mention it but that it was the first experiment 
 which gave me, although not very satisfactorily, the effect of 
 decomposition by heat alone. The reason of its uncertainty 
 I believe to have been the want of a sufficiently intense heat, 
 as I dared not venture, on account of the gold sojder, to push 
 the ignition very far ; in fact, I subsequently fused the ex- 
 tremity and spoiled the apparatus by applying the oxyhydro- 
 gen flame to it ; had the platinum tube been welded instead 
 of gold-soldered, it would doubtless have succeeded better. 
 I should state that the object of the silver tube was to prevent 
 the chance of re-composition by the catalytic effect of a large 
 platinum surface ; to have, in short, a small portion of plati- 
 num exposed to the steam, and that at a high temperature. 
 
 Y 
 
322 EXPERIMENTAL INVESTIGATIONS. 
 
 This experiment, although, coupled .with the previous ones, 
 tolerably conclusive, did not satisfy me, and I attacked the 
 difficulty in another manner. The experiment (fig. 5) in- 
 duced me to believe that if I could get platinum ignited 
 under water, so as to be in an atmosphere of steam, decompo- 
 sition would take place ; and M. Boutigny's experiments on 
 the spheroidal state of water led me to hope I might keep 
 platinum for some time under conditions suitable for my 
 purpose. 
 
 After a few failures I succeeded perfectly by the following 
 experiment. The extremity of a stout platinum wire was 
 fused into a globule of the size of a peppercorn by a nitric acid 
 battery of 30 cells ; prepared water was kept simmering by a 
 spirit-lamp, with a tube filled with water inverted in it ; char- 
 coal being the negative terminal, the voltaic arc was taken 
 between that and the platinum globule until the latter was at 
 the point of fusion ; the circuit was now broken, and the highly 
 incandescent platinum plunged into the prepared water : sepa- 
 rate pearly bubbles of gas rose into the tube, presenting a 
 somewhat similar effect to Experiment (fig. 5). The process 
 was repeated, the globule being frequently plunged into the 
 water in a state of partial fusion ; and when a sufficient quan- 
 tity of gas was collected it was examined ; it detonated, leaving 
 0*4 residue ; this was as usual nitrogen with a trace of oxygen. 
 A second experiment gave a still better result, the gas con- 
 tracting to 0*25 of its original volume. 
 
 On making the platinum negative and the charcoal posi- 
 tive a very different result followed; the carbon was, as is 
 known to electricians, projected upon the platinum ; and the 
 gas in this case was mixed with carburetted hydrogen and 
 carbonic oxide. I know no experiment which shows so 
 strikingly the different effects at the disruptive terminals as 
 this ; when the platinum is negative it gives much carbonic 
 gas, when it is positive, not a trace (the gas was delicately and 
 carefully tested for it) ; nay, more, by changing the platinum 
 from negative to positive the carbon is instantly removed, and 
 in a single experiment the platinum becomes perfectly clean. 
 
 Here then I produced very satisfactorily decomposition by 
 heat ; it is true the battery was used, but used only as a means of 
 
DECOMPOSITION OF WATER BY HEAT. 323 
 
 fusing the platinum, as this was, as soon as fused, entirely 
 separated from the circuit and could have no possible voltaic 
 action. Wishing, however, altogether to avoid the use of the 
 battery, I repeated this experiment, employing as my means 
 of fusing the platinum the oxyhydrogen blowpipe ; the ex- 
 periment was equally successful, indeed rather more so, as 
 the manipulation was more easy. 
 
 I could readily by this means collect half a cubic inch or 
 more of the 'gas ; when detonated, the residue of nitrogen 
 averaged 0*35 of the original volume. 
 
 In carefully watching this experiment I observed that at 
 first a rapid succession of bubbles ascended into the tube from 
 the incandescent platinum ; it then became quiescent ; the 
 spheroidal state was assumed by the water and no gas 
 ascended ; on losing the spheroidal state a sudden hiss was 
 heard, and a single bubble ascended into the tube. I deter- 
 mined to examine separately the gas from the platinum before 
 and after the quiescent state ; to effect this I placed two in- 
 verted tubes in the capsule, with the orifices near each other ; 
 the platinum at the point of fusion was immersed under one 
 tube, say tube A, and as soon as the ascent of bubbles ceased 
 it was removed across to tube B, and the last bubble then 
 entered that tube ; the gases from each tube were separately 
 analysed, and tube A gave nearly all detonating gas, the resi- 
 due being only 0*2 ; tube B gave none ; the gas collected in it 
 was nitrogen, with a trace of oxygen. 
 
 In order to examine the effect of an oxidable metal under 
 similar circumstances, I fused by the oxyhydrogen blowpipe 
 the end of a stout iron wire, plunged it into prepared water, 
 and collected the globules of gas ; no oxygen was given off, 
 or at least no more than I have always found to accompany 
 hydrogen, which, with a small residue of nitrogen, was the gas 
 given off in this experiment. 
 
 I was now anxious to produce a continuous development 
 of mixed gas from water subjected to heat alone, in other 
 words, to succeed in an experiment which should bear the 
 same relation to Experiment fig. 9 as fig. 5 did to fig. 7 ; for 
 this purpose the apparatus shown at fig. 10 was constructed : 
 
 Y 2 
 
3 2 4 EXPERIMENTAL INVESTIGATIONS. 
 
 a and b are two silver tubes four inches long by 0*3 inch dia- 
 meter ; they are joined by two platinum caps to a platinum 
 tube c t formed of a wire one-eighth of an inch diameter drilled 
 through its entire length, with a drill of the size of a large 
 pin ; a is closed at the extremity, and to the extremity of b 
 is fitted, by means of a coiled strip of bladder, the bent glass 
 tube d. The whole is filled with prepared water ; and having 
 expelled the air from a by heat, the extremity of the glass 
 tube is placed in a capsule of simmering water. Heat is now 
 applied by a spirit-lamp, first to b and then to a, until the 
 whole boils ; as soon as ebullition takes place the flame of an 
 oxyhydrogen blowpipe is made to play upon the middle part of 
 the platinum tube c, and when this has reached a high point 
 of ignition, which should be as nearly the fusing-point of 
 platinum as is practicable, gas is given off, which, mixed 
 
 Fig. 10. 
 
 with steam, very soon fills the whole apparatus and bub- 
 bles up from the open extremity either into the open air 
 or into a gas collector. Although by the time I had devised 
 this apparatus I was from my previous experiments tolerably 
 well assured of its success, yet I experienced a feeling of great 
 gratification when, on applying a match to one of the bubbles 
 which were ascending, it gave a sharp detonation ; I collected 
 and analysed some of it ; it was 07 oxyhydrogen gas, the resi- 
 due nitrogen, with a trace of oxygen. 
 
 Those who have endeavoured to deprive water of air will 
 have no difficulty in accounting for the residual nitrogen, or 
 nitrogen mixed with a small portion of oxygen, which has 
 occurred in all my experiments. De Luc pointed out the im- 
 possibility of practically depriving water of air, and Priestley, 
 
DECOMPOSITION OF WATER BY HEAT. 325 
 
 from observing the obstinacy with which water retained air, 
 was led to believe that water was convertible into nitrogen 
 (phlogisticated air). I have repeated several of Priestley's 
 experiments under much more stringent circumstances, and 
 have never been able to free water from air, or so to boil water 
 that for every ebullition of vapour a minute bubble of perma- 
 nent gas was not left, which appeared to have been an in- 
 dispensable nucleus to the vapour. 
 
 The difficulty of boiling water increases, as M. Donny has 
 proved, in proportion to its freedom from air, and at last the 
 bursts of vapour become so enormous that the vessels em- 
 ployed are generally broken. There appears to me a point 
 beyond which this resistance does not extend ; but even at 
 this point a minute bubble of air is left for each burst of va- 
 pour, though they are so few and distant that the aggregate 
 amount of gas is very trifling. I have produced from water 
 which had been previously carefully deprived of air by the ordi- 
 nary methods three-fourths of its own volume of permanent gas, 
 which proved to be nitrogen ; but as the water in this experi- 
 ment was boiled under a long column of oil, it is probable that 
 if any oxygen were present it might have been absorbed by the 
 oil ; I have, however, always found the proportion of oxygen 
 to decrease as the boiling was continued. It may be worth 
 noticing, as having had some influence on my mind, that many 
 months ago, when considering the experiments of Henry and 
 Donny on the cohesion of water, I mentioned to Mr. Gassiot, 
 and also to Mr. Bingham, my assistant (to whose assiduity I 
 am much indebted), that I was inclined to think if water could 
 be absolutely deprived of air it would be decomposed by heat, 
 a result which I have now attained by a totally different series 
 of inductions. It is a circumstance worthy of remark, that I 
 find the greater part of the air to be expelled at a compara- 
 tively low temperature, and when the water has come in con- 
 tact with the platinum, while the decomposition all takes 
 place when the platinum is surrounded by an atmosphere of 
 steam, if steam it may be called, for the state of this atmo- 
 sphere at the first immersion of the platinum is at present very 
 mysterious. 
 
 I think I may now safely regard it as proved that plati- 
 
326 EXPERIMENTAL INVESTIGATIONS. 
 
 num intensely ignited will decompose water, and several 
 considerations press on the mind in reflecting on this novel 
 phenomenon. 
 
 First of all, to those who are attached to the cui bono argu- 
 ment, and estimate physical science in proportion only to its 
 practical applications, I would say that these experiments 
 afford some promise of our being, at no distant period, able to 
 produce mixed gases for purposes of illumination, &c., by 
 simply boiling water and passing it through highly ignited 
 platinum tubes, or by other methods which may be devised ; 
 we in fact by this means, as it were, boil water into gas, and 
 there appears theoretically no more simple way of producing 
 chemical decomposition. 
 
 To pass however to more important considerations : the 
 spheroidal state, which has lately attracted the attention of 
 philosophers, appears to be closely connected with these 
 results, and is rendered more deeply interesting. The last 
 experiment but two which I have mentioned shows that the 
 spheroidal state is intermediate between ordinary ebullition and 
 the decomposing ebullition. It is probably therefore a state 
 of polar tension, co-ordinate in some respects with that which 
 takes place in the cell of a voltaic combination before decom- 
 position, or when the power employed not being of sufficient 
 intensity to produce actual decomposition, the state commonly 
 called polarisation of the electrodes, obtains. The phenome- 
 non brings out also a new relation between heat, electricity, 
 and chemical affinity ; hitherto many electrical phenomena 
 could be produced by heat and chemical action, the difference 
 being that in the effects produced by the last two forces there 
 was no polar chain, but every minute portion of the matter 
 acted on gave rise to the phenomena which in the electrical 
 effects are only observable at the polar extremities ; thus in 
 decomposing water by iron and sulphuric acid, or by passing 
 steam over heated tubes of iron, parallel results are obtained 
 to the electrolysis of water with an iron anode ; but in the 
 former cases every portion of the iron oxidated gives off its 
 equivalent of hydrogen ; in the latter the equivalent is evolved 
 from the cathode at a point distant from that v/here the oxi- 
 dation takes place. Hitherto electricity has been the only 
 
DECOMPOSITION OF WATER BY HEAT. 327 
 
 force by which many compounds, and particularly water, could 
 be resolved into their constituents without either of these 
 being absorbed by another affinity. The decomposition by 
 ignited platinum removes this exception, and presents the 
 parallel effect produced by heat alone. 
 
 Although there is no substance except platinum and some 
 of the more rare metals, such as iridium, which promise much 
 success in a laboratory experiment made for the purpose of 
 producing the effect I have described, as the greater number 
 of substances which will bear a sufficient heat are fragile, 
 oxidable, or affected by water, yet general considerations 
 from the nearest analogies in chemistry would lead us to 
 expect a similar effect from all matter in a state of intense 
 ignition ; even assuming the presence of solid matter to be 
 necessary, the catalytic effects of platinum are shared in 
 different degrees by other substances ; it therefore appears 
 probable that at a certain degree of heat water does not exist 
 as water or steam, but is resolved into its constituent elements. 
 If, therefore, there be planets whose physical condition is 
 consistent with an intense heat, the probability is, that their 
 atmosphere and the substances which compose them are in a 
 totally different chemical state from ours, and resolved into 
 what we call elements, but which by intense heat may be 
 again resolved into more subtle elements. The same may be 
 the case in the interior of our planet, subject, however, to the 
 counter-agency of pressure. 
 
 The experiments strongly tend to support the views of 
 Berthollet, that chemical and physical attraction are affinal, or 
 produced by the same mode of force. All calorific expansions 
 appear to consist in a mechanical repulsion of the molecules 
 of matter ; and if heat produce effects of decomposition merely 
 by increase of intensity, there seems no reason why we should 
 assign to it in this case a different mode of action from its 
 normal one. On this view physical division carried on inde- 
 finitely must ultimately produce decomposition, and chemical 
 affinity is only another mode of molecular attraction. Thus a 
 high degree of rarefaction, as at the bounds of the atmosphere, 
 or in the interplanetary spaces, may entirely change the 
 chemical condition of matter. 
 
328 EXPERIMENTAL INVESTIGATIONS. 
 
 In a paper published in the ' Philosophical Transactions ' 
 for 1843, p. 1 1 1, I have shown that we may oppose a chemical 
 action by a physical one (electrolysis by a vacuum), that 
 antagonising chemical by physical tension, they mutually 
 oppose each other. I believe the converse of this experiment 
 has been made by M. Babinet, who by physical compression 
 has prevented the development of chemical action. 
 
 I have also described in the ' Philosophical Magazine ' for 
 November 1845 certain phenomena which appear to me to 
 be irreconcileable with received chemical views ; and though 
 I then believed that the theory of Grotthus would be obliged 
 to give way, I now incline to think that some of our chemical 
 doctrines must ere long undergo a revision. 
 
 It is rather surprising that the valuable applications of 
 which the phenomena of voltaic ignition are capable, and the 
 fertile field which (as I believe) it presents for discoveries, 
 both physical and chemical, should have been so completely 
 neglected. It is true that, until a recent period, the imper- 
 fection of the voltaic battery rendered accurate and continued 
 experiment on this subject difficult of performance, but still 
 much might have been done. Davy made several experiments 
 on the voltaic disruptive discharge, which, in many points, 
 may be regarded simply as very intense ignition ; but I am 
 only aware of two experiments of his on voltaic ignition ; one, 
 in which he employed it in an exhausted receiver to examine 
 to what extent the radiation of heat was carried on in vacno ; 
 and another, already alluded to, in which, by immersing a 
 portion of an ignited wire in water, he observed that it con- 
 ducted in some inverse ratio to its heat. 
 
 I have made a vast number of experiments on the voltaic 
 arc or disruptive discharge, in various media ; * when this is 
 taken in a medium incapable of acting chemically on the elec- 
 trodes, the phenomena are those of intense ignition of the 
 terminals, which are dissipated in vapour and condensed upon 
 the interior of the vessel in which the discharge is taken. I 
 have examined some of these deposits, and they appear to 
 consist of the metal of the terminals in a finely-divided state ; 
 
 * Phil. Mag. t June 1840; Lit. Gaz. and Athenceum y Feb. 7, 184$, 
 
DECOMPOSITION OF WATER BY HEAT. 329 
 
 this is strikingly shown with zinc. If the arc be taken between 
 zinc points in an exhausted receiver, a fine dark powder, 
 nearly black, is deposited on the interior, which, when col- 
 lected, proves to be pure zinc, and on the application of a 
 gentle heat takes fire in the open air and burns into the white 
 oxide : to casual observation the zinc would appear to be 
 burned twice. The experiment appears to me to present an 
 argument in favour of the dynamic theory of heat. 
 
 With charcoal, on the other hand, there is little or no 
 deposit, but the charcoal continually yields carbonic oxide 
 and hydrogen, and this for hours after the presence of water 
 would be deemed impossible. I have taken the arc between 
 pieces of well-burned charcoal for eight or nine successive 
 hours, and there was still gas generated ; indeed, it appeared 
 to be given off as long as there was any charcoal remaining, 
 and a conversion of the carbon into inflammable gas might 
 have been supposed. Much still remains to be done with this 
 powerful agent, the voltaic arc : where, however, the object is 
 simply to expose gases to an intense heat, the ignition of a 
 conjunctive wire of platinum is more simple in its application, 
 more uniform in its action, and instead of requiring a powerful 
 battery, the effect can be satisfactorily produced by five or six 
 cells, in many cases by two. 
 
 The heat is not so intense as that of the arc, but as it can 
 be brought to within a few degrees of the fusing-point of pla- 
 tinum, it is far more intense than any heat usually employed 
 in laboratories, certainly than any which can be applied to 
 minute, I may say microscopic portions of gas or vapour. 
 
 In conclusion, I must express my sincere thanks to the 
 Managers of the London Institution, for having permitted me, 
 as an honorary member, to carry on these experiments in the 
 laboratory of the Institution. 
 
330 EXPERIMENTAL INVESTIGATIONS. 
 
 SUPPLEMENTARY PAPER ON CERTAIN PHENOMENA 
 OF VOLTAIC IGNITION, AND THE DECOMPOSI- 
 TION OF WATER INTO ITS CONSTITUENT GASES 
 BY HEAT. 
 
 Phil. Trans., R.S. Received November 26. Read November 26, 1846. 
 
 IN selecting the above title I endeavoured to give as clear an 
 enunciation of the phenomena to be described in the paper as 
 was consistent with the brevity usual in a title. 
 
 An exception has, however, been taken to it, that as the 
 effects of decomposition are produced by ignited platinum, 
 the phenomena may result from that obscure mode of action 
 called catalysis. That I did not intend to exclude from con- 
 sideration any possible action of the substance employed will 
 be evident from the paper itself, in which I have called atten- 
 tion to the general production of catalytic effects by solid 
 bodies. 
 
 Whatever value or novelty there may be in the facts I 
 have communicated is the same, whether they be regarded as 
 resulting from catalytic or from thermic actions. If the action 
 be catalytic, it is one absolutely the reverse of that usually 
 produced by platinum, and therefore just as much at variance 
 with received experience as decomposition of water by heat 
 would be, the effect of platinum, like that of heat, on the 
 elements of water having been hitherto known only as com- 
 bining them. With regard to any theoretic views I may have 
 advanced, I by no means attach the same importance to them 
 as I do to the facts themselves, though I consider it necessary 
 for the collation of facts, and desirable for the progress of 
 science, that an author pretending to communicate new results 
 should give with them the impressions which led to their dis- 
 covery, and the inferences which he regards as immediately 
 deducible from them. No expression can be given to facts 
 which does not involve some theory ; and admitting the diffi- 
 culty (perhaps insuperable) of correctly enunciating new phe- 
 nomena, and the probability of future discoveries entirely 
 
DECOMPOSITION OF WATER BY HEAT. 331 
 
 changing our views regarding them, I cannot at present see 
 that the title of my paper could be altered without being open 
 to greater objections. I am of this opinion, not so much 
 because other bodies than platinum will produce the effect, as 
 I shall presently show, nor from the fact that the electrical 
 spark will decompose aqueous vapour, though these are argu- 
 ments in its favour, but from the following considerations. 
 The catalytic action of platinum will induce or enable combi- 
 nation to take place where there is already a strong affinity 
 or tendency to combine, as with mixed oxygen and hydrogen 
 gases ; it will also induce decomposition where the affinities 
 are extremely weak, or in a state of unstable equilibrium, 
 as in Thenard's peroxide of hydrogen ; again, where there 
 are nicely-balanced compound affinities, it may change the 
 chemical arrangement of the constituents of a compound, but 
 I do not know of any case in which a powerful chemical 
 affinity can be overcome by catalytic action ; to effect this we 
 require some natural force of greater intensity than that to be 
 overcome. We might as well say that the platinum electrodes 
 of a voltaic battery decompose water, as to say that platinum 
 decomposes it in the case in question ; there the force of 
 electricity acts only by means of matter, and matter of a 
 peculiar description ; its action also is only perceptible at the 
 surface of this matter. I seek to use the expression in my 
 title with reference to heat in a similar sense to that in which 
 we use similar terms with reference to electricity, i.e. to regard 
 heat as the immediate dynamic force which overcomes the 
 affinity ; thus, as we say when employing the voltaic battery, 
 that we decompose water by electricity, so here we should say 
 that we decompose it by heat. 
 
 If it be said that heat so weakens or antagonises the 
 affinity of the elements of water as to enable catalytic action 
 to separate them, this amounts to the same theory, as heat is 
 then regarded as the antagonising force, and in this case the 
 action, both thermic and catalytic, is the reverse of the normal 
 action. I have thought it desirable shortly to discuss this 
 question, as likely to lead to farther investigation, though I 
 have been somewhat embarrassed by the want of definite 
 meaning in the term catalysis ; I must plead guilty to have 
 
332 EXPERIMENTAL INVESTIGATIONS. 
 
 frequently used the term ; but notwithstanding, or perhaps on 
 account of, its convenience, it has I fear had an injurious effect 
 on scientific perspicuity. 
 
 The following experiments were made to ascertain whe- 
 ther platinum was the only substance by which the effect 
 could be produced. A knob or button of the native alloy of 
 indium and osmium of the size of a small pea was formed by 
 the voltaic battery ; to this was attached by fusion another 
 smaller knob of the same metal one-fourth the size of the 
 former, and to this smaller one was attached a stout platinum 
 wire ; the object of the second knob was both to prevent the 
 fusion of the platinum wire, and also to avoid the possibility 
 of any of the platinum being exposed under the recipient 
 tube or alloyed with the metal to be heated. The preparation 
 of this simple instrument was very troublesome, but when 
 made it answered the purpose well ; the larger button could 
 be fully ignited to an intense glow, while, on account of the 
 narrow neck which united them, the smaller was barely red- 
 hot, and the platinum wire not perceptibly ignited. An ex- 
 periment having been made with this metallic button and 
 prepared water, similar to that previously made with platinum, 
 gas was given off which averaged 0*3 of mixed gas ; the 
 residue was nitrogen mixed with varying small quantities of 
 oxygen. The effect, upon the whole, was decidedly inferior 
 to that of the platinum. Indeed, as platinum is the most 
 dense and unalterable of all known substances, it would be 
 likely, upon any received theory of heat, to produce the 
 greatest effects. 
 
 I tried palladium in the same manner ; the gas yielded 
 was hydrogen with small quantities of oxygen, and the water 
 was stained with the oxide of the metal. 
 
 I now tried silica and other oxides, but the results were 
 not very satisfactory. A spheroid of silica was formed by 
 fusing pulverised silica on to a platinum wire, so as to cover 
 it for the length of 0*4 of an inch ; when this was plunged 
 into the hot water and again fused in the oxyhydrogen blow- 
 pipe, it constantly became frothed with small bubbles of 
 vapour, and after a few experiments generally separated in 
 fissures ; in the experiment which was continued for the 
 
DECOMPOSITION OF WATER BY HEAT. 333 
 
 longest time without disintegration, the gas given off contained 
 0*15 of oxyhydrogen gas; from the whole result I beljeve 
 there is an action of the water on the silica (probably forming 
 a hydrate decomposable by heat) which is a bar to satisfactory 
 results. With other oxides, at least such as would bear an in- 
 tense heat, the difficulties were still more insuperable. Priestley 
 has shown that water will corrode glass, and if I mistake not, 
 others have shown the same effect produced on silica. 
 
 Although, as applied to the facts detailed, I attached no 
 further meaning to the title of my paper than that which I 
 have above stated, yet in one or two theoretical inferences I 
 have certainly gone farther ; for instance, when I suppose the 
 possibility or probability of mechanical rarefaction producing 
 the same effects as heat, here (although I do not, indeed I 
 cannot conceive the existence of heat without matter) I cer- 
 tainly abstract from the proposition any consideration of solid 
 matter. In order to ascertain how far this view might be 
 founded on truth, I had thought of making a few experiments 
 on the effect of mechanical rarefaction on the tendency of 
 gases to combine, but (in addition to the interference of neces- 
 sary occupations) I find that M. de Grotthus has already 
 experimented on the point. His experiments, as far as they 
 go, corroborate the views I have put forth. 
 
 He finds * that mixed gases, such as chlorine and hydrogen, 
 or oxygen and hydrogen, when rarefied either by slow incre- 
 ments of heat or by the air-pump, do not take fire (' ne s'enflam- 
 ment pas') by the electric spark. From the context he evidently 
 means that the gases will not detonate or unite in volumes, 
 as he states that a partial combination ensues. Grotthus 
 appears to have considered the combination of gases by the 
 electric spark as an effect of sudden compression or molecular 
 approximation, certain particles being brought within the 
 range of their affinities by the sudden dilatation of others. 
 Although he did not pursue the subject far enough to ascer- 
 tain whether a degree of rarefaction could be reached which 
 would be an actual bar to combination, still his experiments 
 strengthen those views which assimilate mechanical and 
 
 * Annaks de Chitnie^ vol. Ixxxii. 
 
334 EXPERIMENTAL INVESTIGATIONS. 
 
 thermic molecular repulsion, and regard chemical affinity as 
 being antagonised by physical repulsion. 
 
 Pursuing the series of analogies from the decomposition of 
 euchlorine at a low temperature, that of ammonia at a higher, 
 that of metallic oxides at a higher, and so on to oxide of 
 hydrogen, there appears to be an extensive series of facts 
 which afford strong hope of a generalised antagonism be- 
 tween thermic repulsion and chemical affinity, and a con- 
 sequent establishment of the law of continuity in reference to 
 physical and chemical attraction. 
 
 The deposit from chlorine, to which I have alluded in my 
 paper, I have since examined, and though it differs in colour 
 from that described in books, I find it is a protochloride of 
 platinum, formed at the expense of the platinum wire. The 
 larger portion of the chlorine in the tube combines with the 
 hydrogen of the aqueous vapour, and the muriatic acid is 
 absorbed by the water ; when the experiment terminates the 
 gaseous volume is reduced to nearly one-half, and this residue 
 is oxygen; 
 
 This effect induced me to try an ignited wire on other 
 analogues of chlorine, and I tried bromine and chloride of 
 iodine in the apparatus (fig. 5). The tube was filled with the 
 liquid, and its extremity was in the first experiments im- 
 mersed in another narrow tube of the same liquid as that 
 which filled it. When the platinum wire was ignited perma- 
 nent gas was given off, both from the bromine and from the 
 chloride of iodine, which gas on examination proved, to my 
 surprise, to be oxygen. In one experiment I collected half a 
 cubic inch of gas from an equal volume of chloride of iodine. 
 As the experiment in this form required too large a quantity 
 of the liquid to enable me to observe any change which might 
 take place in its character, I repeated it with a tube five feet 
 long, bent in two angular curves. A small quantity of the 
 liquid was placed in the extremity of the tube containing the 
 wire, which was so arranged as to be the lowest point ; the 
 angles were placed in cold water and the experiment pro- 
 ceeded with ; my object was to enable the dense vapour of 
 the liquids to shelter them from the atmosphere, there being 
 no satisfactory method of shutting them in and yet allowing 
 
DECOMPOSITION OF WATER BY HEAT. 335 
 
 room for the elimination of the liberated gas, or of absorbing 
 the latter by combination without also absorbing the vapours. 
 
 I had hoped by the above means to proceed with the 
 experiments until all the oxygen was liberated that could be 
 driven off, and then to have examined the residue ; but I 
 found that, after experimenting for a short time, both the 
 platinum wire and the glass in proximity to it were attacked 
 by the liquids ; this difficulty, similar to those which have 
 hitherto prevented the isolation of fluorine, I have not yet 
 been able to conquer, though I hope to resume the experi- 
 ments. 
 
 As chloride of iodine is decomposed by water, it cannot 
 contain any notable quantity of the latter, but, until the expe- 
 riments are carried farther, it must remain a question whether 
 the oxygen results from a small quantity of water contained 
 in the liquid, the hydrogen combining with the liquid itself, 
 or from a decomposition similar to that of the peroxides. 
 The experiments certainly add a new and striking analogy to 
 those already known to exist between the peroxides and the 
 halogens, but they do not, as far as I have hitherto carried 
 them, necessarily prove analogy of composition. 
 
 In conclusion, I would call attention to a point which I 
 omitted to notice in my original paper, viz. the explanation 
 afforded by the results contained in it of the hitherto myste- 
 rious phenomena of the non-polar decomposition of water by 
 electrical discharges, as in the experiments of Pearson and 
 Wollaston. This class of decompositions may now be carried 
 much farther. With the exception of fused metals, I know 
 of no liquid which, when exposed to intense heat, such as that 
 given by the electric spark, the voltaic arc, or incandescent 
 platinum, does not give off permanent gas ; phosphorus, sul- 
 phur, acids, hydrocarbons, water, salts, bromine, and chloride 
 of iodine, all yield gaseous matter. 
 
 Viewing these effects simply as facts, and without entering 
 on any theoretical explanations or speculations, I cannot but 
 think that there is a remarkable generality pertaining to them 
 worthy of the most careful attention. 
 
 The apparatus I have described, particularly that repre- 
 sented by fig. 5, and the numerous applications of voltaic 
 
336 EXPERIMENTAL INVESTIGATIONS. 
 
 ignition which will occur to those who duly consider the sub- 
 ject, promise, I venture to believe, new methods and powers 
 of investigating the molecular constitution of matter, and 
 will, I trust, lead to novel and important results. 
 
 ON THE EFFECT OF SURROUNDING MEDIA ON 
 VOLTAIC IGNITION. 
 
 Phil. Trans. R. S. Received August 10. Read December 14, 1848. 
 
 IN the 'Philosophical Magazine' for -December 1845 I 
 pointed out a striking difference between the heat generated 
 in a platinum wire by a voltaic current, according as the wire 
 is immersed in atmospheric air or in hydrogen gas, and in the 
 Bakerian Lecture for 1847 I have given some farther experi- 
 ments on this subject, in which the wire was ignited in atmo- 
 spheres of various gases, while a voltameter enclosed in the 
 circuit yielded an amount of gas in some inverse ratio to the 
 heat developed in the wire. It was also shown, by a thermo- 
 meter placed at a given distance, that the radiated heat was in 
 a direct ratio with the visible heat. 
 
 Although the phenomena were apparently abnormal, there 
 were many known physical agencies by which they might 
 possibly be explained, such as the different specific heats of 
 the surrounding media, their different conducting powers for 
 electricity, or the varying fluency or mobility of their par- 
 ticles which would carry off the heat by molecular currents 
 with different degrees of rapidity. 
 
 The investigation of these questions will form the subject 
 of this paper. 
 
 An apparatus was arranged (see fig. i). Two glass tubes, 
 A and B, of 0*3 inch internal diameter and i'5 inch length, 
 were closed with corks at each extremity ; through the corks 
 the ends of copper wires penetrated, and joining these were 
 coils of fine platinum wire, one-eightieth of an inch diameter 
 and 37 inches long when uncoiled. Tube A was filled with 
 oxygen, tube B with hydrogen, and the tubes thus prepared 
 
VOLTAIC IGNITION. 
 
 337 
 
 were immersed in two separate vessels, in all respects similar 
 to each other, and containing each three ounces of water. 
 A thermometer was placed in the water in each vessel ; the 
 copper wires were connected, so as to form a continued 
 circuit, with a nitric acid battery of eight cells, each plate 
 exposing eight square inches of surface. Upon the circuit 
 being completed the wire in the tube containing oxygen 
 rose to a white heat, while that in the hydrogen was not 
 
 Fig. i. 
 
 visibly ignited ; the temperature of the water, which at the 
 commencement of the experiment was 60 Fahr. in each 
 vessel, rose in five minutes in the water surrounding the tube 
 of hydrogen from 60 to 70, and in that containing oxygen 
 from 60 to 8 1.* 
 
 Before I enter into a farther detail of experiments I would 
 remark upon the extraordinary character of this result. The 
 same current or quantity of electricity passes through two 
 similar portions of wire immersed in the same quantity of 
 
 * After the publication of the Bakerian Lecture my experiment on the pecu- 
 liar effect of hydrogen on the ignited wire was noticed in a paper by M. Matteucci, 
 which, though I had it in my hand shortly after its publication, I regret to say I 
 did "not read with the attention it deserved. I have read it since the experi- 
 ments in this paper were commenced, and I see that I am now executing a task 
 assigned to me by my friend. ' M. Matteucci, for a different object, makes a 
 somewhat similar experiment to the one given above, which, however, differs from 
 mine in the material point, that he operated first on one gas and then on the other, 
 and thus did not compare the effects produced by the same quantity of electricity. 
 I cannot quite agree in the conclusions deduced by him from this and the other 
 experiments he cites, but I will not here contest them, as it would lead me away 
 rom the main point of this paper. 
 
 Z 
 
338 EXPERIMENTAL INVESTIGATIONS. 
 
 liquid, and yet, in consequence of their being surrounded by 
 a thin envelope of different gases, a large portion of the heat 
 which is developed in the one portion appears to have been 
 annihilated in the other. Similar experiments, varying the 
 gas in one tube while hydrogen was retained in the other, 
 gave the following results. In five minutes the thermometer 
 rose 
 
 In hydrogen. In associated nitrogen. 
 
 1st. From 60 to 69-5. From 60 to 81-5. 
 
 In hydrogen. In carbonic acid. 
 
 2nd. From 60 to 70-5. . From 60 to 80. 
 
 In hydrogen. In carbonic oxide. 
 
 3rd. From 60 to 70. From 60 to 79-5. 
 
 In hydrogen. In olefiant gas. 
 
 4th. From 60 to 70-5. From 60 to 76-5.* 
 
 On a different day I tried the following experiments ; all 
 the circumstances were the same, excepting that the battery 
 was in more energetic action, for which reason I have not 
 tabulated them with the others. 
 
 In oxygen associated with coal gas the thermometer rose 
 in five minutes 
 
 In oxygen. In coal gas. 
 
 From 60 to 82. From 60 to 76. 
 
 In hydrogen associated with coal gas the thermometer 
 rose in five minutes 
 
 In hydrogen. In coal gas. 
 
 From 60 to 77, From 60 to 82-5. 
 
 From this it would appear that coal gas should be placed, as 
 to its cooling effect on the ignited wire, between hydrogen 
 and olefiant gas. 
 
 On another day sulphuretted hydrogen associated respect- 
 ively with oxygen and hydrogen was tried ; the wire in the 
 
 * I should perhaps remark, that several test experiments were tried to ascer- 
 tain the working of the apparatus ; thus, the same gas was placed in both tubes, 
 and the results given by the thermometer were found to be accurately the same in 
 both vessels. The tubes were also changed with reference to the containing 
 vessels and to the contained gases. The water was always agitated to render its 
 temperature uniform previously to reading off, c. 
 
VOLTAIC IGNITION. 339 
 
 sulphuretted hydrogen was at first ignited to a degree some- 
 what inferior to that in oxygen, but the gas was rapidly 
 decomposed ; sulphur being deposited on the interior of the 
 vessel and the intensity of ignition gradually decreased, so as 
 ultimately to be scarcely superior to the ignition in hydrogen ; 
 indeed the gas by this time had become nearly pure hydrogen. 
 The following were the effects on the thermometer in five 
 minutes, all being arranged as before : 
 
 In oxygen. In sulphuretted hydrogen. 
 
 From 60 to 86. From 60 to 76. 
 
 In hydrogen. In sulphuretted hydrogen. 
 
 From 60 to 79. From 60 to 81-5. 
 
 This result would place sulphuretted hydrogen between 
 hydrogen and coal gas ; but as the gas was rapidly decom- 
 posed, the greater part of the experiment was made with 
 hydrogen containing small quantities of sulphur combined, 
 and not with sulphuretted hydrogen. I therefore think that 
 proto-sulphuret of hydrogen, or the gas which consists of 
 equivalent ratios of the two elements, would be much farther 
 removed from pure hydrogen ; probably it would be about 
 equal in its cooling effect to carbonic acid or carbonic oxide. 
 
 In phosphuretted hydrogen the platinum wire is destroyed 
 by combining with the phosphorus the instant it reaches 
 ignition, so that its relation to the other gases could not be 
 ascertained. 
 
 Protoxide and deutoxide of nitrogen are, as I have 
 observed in the Bakerian Lecture, decomposed by the ignited 
 wire ; they, as well as atmospheric air, are, as nearly as may 
 be, equal in their effect to their elements separately. 
 
 In the vapour of ether the ignited wire is extinguished 
 nearly as completely as in hydrogen; I have not yet tried its 
 comparative effect, but should judge it to be nearly the same 
 as coal gas or olefiant gas. 
 
 In my former experiments* the following was the order of 
 the gases, testing the intensity of ignition by the inverse con- 
 
 * Phil. Trans., 1847, p. 2. 
 
 7. 2 
 
340 EXPERIMENTAL INVESTIGATIONS. 
 
 ducting power of the wire, as measured by the amount of gas 
 in a voltameter included in the circuit : 
 
 Cubic inches of gas evolved in 
 Gases surrounding the wire. the voltameter per minute. 
 
 Hydrogen . . . . . 77 
 
 Olefiant gas .... 7*0 
 
 Carbonic oxide .... 6*6 
 
 Carbonic acid .... 6*6 
 
 Oxygen ..... 6*5 
 
 Nitrogen ..... 6*4 
 
 Assuming that in the present experiments the heat in the 
 water is a correct indication of the intensity of ignition in the 
 wire, the order is the same in both series of experiments. 
 Hydrogen is, however, so far removed from both oxygen and 
 nitrogen in its effects upon the ignited wire, that in order more 
 accurately to ascertain the relative position of the latter two 
 gases, I made a few farther experiments on them as contrasted 
 with each other, and not with hydrogen. I first repeated my 
 former experiment on these two gases, varying it only by 
 changing the circumstances in the manner suggested by the 
 present experiments, which, on account of the vessel contain- 
 ing the wire being immersed in a given quantity of water, 
 instead of being exposed to the external atmosphere, would 
 occasion greater equality in the surrounding cooling effects, 
 and would give me the opportunity of combining both 
 methods in one experiment. 
 
 I filled both tubes A and B with oxygen, and included a 
 voltameter in the circuit ; in two minutes 3-43 cubic inches of 
 hydrogen were evolved in the voltameter, and the thermo- 
 meter in each cell had risen from 60 to 63. A similar ex- 
 periment with nitrogen gave in two minutes 3*4 cubic inches 
 of hydrogen, and the thermometer rose from 60 to 63. 
 
 This experiment accords with my previous one as to the 
 voltameter test, but indicates no difference in oxygen and 
 nitrogen with the thermometer test ; I therefore in the follow- 
 ing three experiments associated nitrogen with oxygen in the 
 apparatus (fig. i). All things being disposed as with the ex- 
 periments on hydrogen associated with other gases, in five 
 minutes the thermometer rose 
 
VOLTAIC IGNITION. 341 
 
 In oxygen. In associated nitrogen. 
 
 Exp. ist. From 60 to 71-5. From 60 to 73. 
 
 2nd. 60 to 77. 60 to 76. 
 
 3rd. 60 to 75. 60 to 76. 
 
 Mean . . 60 to 74-5. 60 to 75. 
 
 The battery had increased somewhat in power after the 
 first experiment ; but as both wires formed part of the same 
 circuit in each experiment, the variations in battery power do 
 not affect the comparative results. The second experiment 
 gives a variation in the position of oxygen and nitrogen with 
 reference to the first and third experiments, but the gases so 
 nearly approach in their cooling effects, that these slight 
 differences are not much to be relied upon ; however, I 
 applied a farther test. I associated in turn oxygen and 
 nitrogen with carbonic acid ; the following were the results. 
 In five minutes the thermometer rose 
 
 In oxygen. In carbonic acid. 
 
 Exp. ist. From 60 to 75. From 60 to 75. 
 
 2nd. 60 to 76. 60 to 75. 
 
 In nitrogen. In carbonic acid. 
 
 Exp. ist. From 60 to 74. From 60 to 73. 
 
 2nd. 60 to 73. 60 to 72-5. 
 
 The battery had in the last experiment a little decreased 
 in power ; the oxygen and nitrogen both produced a less cool- 
 ing effect than the carbonic acid, but the oxygen came nearer 
 to it than the nitrogen, thus according with the previous ex- 
 periments. Upon the whole it would appear that oxygen 
 produces a somewhat greater cooling effect on the ignited wire 
 than nitrogen, but these gases may, for the purposes of this 
 paper, be fairly regarded as equal. Atmospheric air produces 
 a similar effect to oxygen and nitrogen separately, though I 
 am inclined to think that a slight chemical change takes place 
 when atmospheric air is exposed to the ignited wire, and that 
 nitrous acid is formed ; for if litmus-paper be held over a vol- 
 taically ignited platinum wire in the air, a slight but very per- 
 ceptible tinge of red marks the portion of it immediately over 
 the wire. 
 
 With the view of ascertaining whether the specific heat of 
 
342 EXPERIMENTAL INVESTIGATIONS. 
 
 the surrounding media were the cause of the phenomenon, I 
 proceeded to try the effect of the wire carrying a voltaic current 
 on different liquids ; all things being disposed as in the pre- 
 vious experiments, and three ounces of water being associated 
 respectively with the same quantity of the following liquids. 
 The thermometer rose in five minutes 
 
 o o o o 
 
 In water, from 60 to 70*3. In spirit of turpentine 60 to 88. 
 
 In water, from 60 to 70-3. In sulphuret of carbon 60 to 87*1. 
 
 In water, from 60 to 69. In olive oil . . . . 60 to 85. 
 
 In water, from 60 to 70' I. In naphtha . . . . 60 to 78*8. 
 
 In water, from 60 to 70-5. In alcohol sp. gr. 0*84, 60 to 77. 
 
 In water, from 60 to 68*5. In ether 60 to 76-1. 
 
 I do not much rely on the last experiment the battery 
 was in more feeble action ; and though each of the above re- 
 sults is the mean of three experiments, yet the variations in 
 the results of the different experiments with ether being con- 
 siderable (while in the others they were very trifling), lead me 
 to place no great dependence on it. The rapidity of evapo- 
 ration and the readiness of ebullition of the ether require that 
 a larger quantity should be used ; but as this for the purpose 
 of comparison would have required all the experiments to be 
 repeated with different quantities of liquid, I have not thought 
 it worth while to go through the series a second time. It will 
 be observed that the effects with the above liquids are by no 
 means in direct relation with their respective specific heats ; 
 but in order to bring the results of the experiments with 
 liquids into comparison with those with gases, I now associ- 
 ated a gas with a liquid, viz. hydrogen with water. All things 
 being disposed as before, the tube A was filled with hydrogen 
 gas, the tube B with water, both being immersed in three 
 ounces of water. The thermometer rose in five minutes 
 
 In hydrogen. In water. 
 
 From 60 to 75-5. From 60 to 72. 
 
 This experiment of itself conclusively negatives the pos- 
 sibility of specific heat alone accounting for the phenomenon 
 under consideration ; and though, doubtless, specific heat 
 must have some influence on the cooling effects of different 
 
VOLTAIC IGNITION. 
 
 343 
 
 Fig. 2. 
 
 gases and liquids, yet in the former it is apparently of very 
 trifling import in comparison with the real physical cause of 
 the differences, whatever that may be. 
 
 Supposing, as is stated by Faraday,* that gases possess 
 feeble conducting powers for voltaic electricity, and suppos- 
 ing hydrogen, from its close analogy in chemical character 
 to the metals, to possess a greater conducting power than 
 the other gases, this would account for its peculiar effect on 
 the ignited wire, as a certain portion of the current, instead 
 of forcing its way through the wire, would be carried off by 
 the surrounding gas. In order to ascertain this I arranged 
 the following experiments. 
 
 ist. Into the closed end of a bent tube, fig. 2, a loop 
 of platinum wire, A B, and 
 two separate platinum wires, 
 C D, were hermetically sealed, 
 the extremities of the latter 
 being approximated as 
 closely as possible, and the 
 interval between them being 
 close to and immediately 
 over the apex of the loop. 
 The tube was filled with 
 hydrogen, and the wire A B 
 connected with a voltaic battery of sufficient power to raise it 
 to as high a degree of ignition as it would bear without fusion ; 
 C and D were now connected with the poles of another bat- 
 tery, a delicate galvanometer being interposed in the circuit. 
 Not the slightest effect on the galvanometer needle could be 
 detected, and a similar negative effect took place when the tube 
 was filled with atmospheric air. 
 
 2nd. Parallel portions of platinum wire were now arranged 
 in close proximity (see fig. 3), and so that each might be 
 ignited to a full incandescence by separate insulated batteries. 
 When surrounded by atmospheres, both of atmospheric air 
 and of hydrogen, and fully ignited, not the slightest conduc- 
 tion could be detected across the interval between the wires, 
 with ten cells of the nitric acid battery ; and being enabled by 
 * Experimental Researches , 272, 441, and 444. 
 
344 EXPERIMENTAL INVESTIGATIONS. 
 
 the kindness of Mr. Gassiot to repeat this experiment with 
 his battery of 500 well-insulated cells of the nitric acid com- 
 bination, air did not conduct when the ignited wires were ap- 
 proximated to the g-V tn f an mcn > on approaching them 
 nearer they came within striking distance, were instantly fused, 
 and the galvanometer needle, which had up to this time been 
 perfectly stationary, was whirled rapidly round. 
 
 Fig- 3- 
 
 
 I think I am entitled to conclude from this that we have 
 no experimental evidence that matter in the gaseous state 
 conducts voltaic electricity ; probably gases do not conduct 
 Franklinic electricity, as the experiments which would seem 
 primd facie to lead to that conclusion are explicable as 
 resulting from the disruptive discharge. 
 
 In Faraday's experiment two wires were approximated in 
 the flame of a spirit-lamp, and a slight conduction across 
 the interval in the flame was observed. This conduction 
 might have been due to certain unconsumed particles of car- 
 bon existing in the flame, or possibly to the flame itself. 
 According to Dr. Andrews, flame, even that of pure hydrogen 
 gas, conducts voltaic electricity.* 
 
 I now endeavoured to ascertain whether any specific in- 
 ductive effect of the hydrogen might have an influence : paral- 
 lel wires of platinum and parallel coiled copper wires were 
 placed in atmospheres of hydrogen and of atmospheric air, one 
 of which parallel wires conveyed the curre.it, and the other 
 wire was connected with a delicate galvanometer.f I could 
 detect no difference in the arcs of deflection of the needle at 
 the instant of meeting or breaking contact, whether the wires 
 were in atmospheres of hydrogen or of atmospheric air ; nor 
 when parallel platinum wires, with their surrounding atmo- 
 spheres of gas, were immersed in a given quantity of water, 
 
 * Phil. Mag., vol. ix., p. 176. 
 
 f I have obtained a slight effect on the galvanometer in a circuit with interposed 
 parallel wires ignited in the vacuum of a good air-pump. Quare, whether this is 
 true conduction or a disruptive effect. W.R.G., 1874. 
 
VOLTAIC IGNITION. 
 
 345 
 
 Fig. 4 . 
 
 B 
 
 could I detect any difference in the resulting heat, whether 
 the current passed in the same or in a different direction 
 through each wire respectively. 
 
 My next object was to ascertain whether, in cases of ordi- 
 nary ignition, the same apparent annihilation of heat took 
 place in hydrogen gas as with voltaic ignition. Two iron cy- 
 linders, A B, fig. 4, each weighing 390 grains, were attached 
 to long iron wires bent back in the form shown in the figure. 
 The cylinders were placed together 
 in a crucible of fine sand, and the 
 whole heated to a uniform white heat. 
 The cylinders were now taken out of 
 the sand, placed at the surface of 
 equal portions of water in the vessels 
 C and D ; two inverted tubes, e, f, the 
 one of hydrogen, the other of atmo- 
 spheric air, were placed over them, 
 and the whole quickly immersed in 
 the water, and retained by a little 
 contrivance, which I need not parti- 
 ticularise, in the position shown in the figure. The tempe- 
 rature of the water at the commencement of the experiment 
 was 60 Fahr. In four minutes the water surrounding the 
 hydrogen had risen to 94, and became stationary there, while 
 that surrounding the air had only reached 87 ; in ten minutes 
 the water surrounding the hydrogen had sunk to 92^5, while 
 that surrounding the air had risen to 93, which was the highest 
 temperature it reached ; thus the respective maxima were 94 
 and 93 ; but considering the greater time which the water 
 surrounding the air required to attain its maximum tempera- 
 ; '. nd t hat, being during this time at a temperature above 
 that of the surrounding atmosphere, it must have lost some- 
 thing of its acquired heat, we may fairly consider the maxima 
 to be the same, and that the difference of effect in the two 
 gases had reference solely to the time occupied in the trans- 
 ference of the heat. In a second experiment the results were 
 similar, the maximum being in this experiment 92*5 in 
 hydrogen, and 91 in air.* 
 
 * Iron wire produces a similar effect to platinum wire in the voltaic experi- 
 ments. 
 
346 EXPERIMENTAL INVESTIGATIONS. 
 
 As far as ordinary ignition is concerned, hydrogen has been 
 shown by the experiments of Leslie and Davy to produce a 
 more rapid cooling effect than air ; and the above experiment 
 having shown that it does not alter or convert into any other 
 force the actual amount of heat given off, my next step was to 
 enquire whether this rapidity of cooling effect of the hydrogen 
 would account for the effects observed with voltaic ignition. 
 Although the two classes of effects were apparently very dif- 
 ferent, it might be that the improved power of conduction in 
 the wire arising from the rapid cooling effect of the hydrogen 
 might, by enabling the current to pass more readily, carry off 
 the force in the form of electricity, which if the wire offered more 
 resistance (as it would when more highly ignited) would be 
 developed in the form of heat. By employing the same me- 
 dium, but impeding the circulation of the heated currents in 
 one case, while their circulation was free in the other, some 
 light might be expected to be thrown on the inverse relation 
 of the conducting power to the heat developed. The follow- 
 ing experiment was therefore tried : 
 
 In the apparatus represented in fig. I, tube A was uncorked, 
 so as to allow free passage for the water, while tube B was 
 filled up with fine sand soaked with water, and then corked at 
 both ends ; the current was passed, and the following was the 
 result. In the vessel containing tube A the thermometer rose 
 in five minutes from 52 to 60, and in that containing 
 tube B from 52 to 60 also; during a second five mi- 
 nutes the thermometer rose in the vessel containing A from 
 60 to 67, and in the vessel containing B from 60 to 67 also. 
 
 I tried another analogous experiment : a coil of platinum 
 wire was placed in a very narrow glass tube one-sixth of an 
 inch diameter ; this was hermetically sealed at one end, and 
 the other drawn into a very narrow aperture, little more than 
 sufficient to allow the platinum wire to pass, and filled with 
 water (it was necessary to leave a small aperture, to prevent 
 the bursting of the tube by the expansion of the heated 
 water) ; in the other vessel a similar coil of platinum wire was 
 placed, but without any glass tube at all. The circuit having 
 been completed as before, the thermometer rose in five 
 minutes 
 
VOLTAIC IGNITION. 347 
 
 In the water without the tube, from . . 60 to 87. 
 In the water containing the tube, from . 60 to 86. 
 
 Here the difference, slight as it was, was against what theory 
 would have led one to anticipate ; the exact equality, however, 
 of the previous experiment, and the close approximation of 
 the results in this one, afford no conclusive information as to 
 the point under consideration, though the negative result 
 rather tends against the view which would assimilate the 
 effects of voltaic to those of ordinary ignition. 
 
 As another method of attaining the object before men- 
 tioned, viz. the inverse relation of the conducting power of the 
 wire to the heat developed in it, I tried the following experi- 
 ment. A platinum wire of one foot long and -^th of an inch 
 diameter was ignited in air by ten cells of the battery, a volta- 
 meter being included in the circuit ; the amount of hydrogen 
 given off by the voltameter was one cubic inch in forty-four 
 seconds : half the wire was now immersed in water of the tem- 
 perature of 60 Fahrenheit ; by this means the intensity of 
 ignition of the other half was notably increased ; the volta- 
 meter now yielded one cubic inch in forty seconds : two- 
 thirds of the wire immersed gave one cubic inch in thirty-seven 
 seconds; and five-sixths immersed gave one cubic inch in 
 thirty-five seconds. The heat of the portion of wire not im- 
 mersed in water had, in the last experiment, nearly reached 
 the point of fusion of the platinum. By this result it appears 
 that the increased resistance to conduction of the ignited por- 
 tion is not equal to the increased conducting power of the 
 cooled portion of the same wire. 
 
 With a view of seeing how far the cooling effect upon the 
 ignited wire might be due to the greater or less fluency or 
 mobility of the particles of the different media surrounding it, 
 I have looked into the papers of Faraday * and of Graham, -f- 
 In the experiments of the former it appears that the escape 
 of different gases at a certain pressure through capillary tubes, 
 or the velocities of revolution of vanes or floats surrounded by 
 different gases, was in some inverse ratio to the density of 
 
 * Quarterly Journal of Science, vol. iii., p. 354. 
 f Phil. Trans., 1846, p. 573. 
 
348 EXPERIMENTAL INVESTIGATIONS. 
 
 such gases ; and the experiments of the latter show that the 
 effusion or escape of gases through a minute aperture in a 
 plate takes place with velocities inversely as the square root 
 of their specific gravities. In Graham's experiments, however, 
 when the escape took place through capillary tubes, the results 
 seemed subject to no ascertained law, though the compounds 
 of carbon with hydrogen passed through with greater facility 
 than other gases. 
 
 The cooling effects of gases on the ignited wire are de- 
 cidedly not in any ratio with their specific gravities ; thus, 
 carbonic acid, on the one hand, and hydrogen, on the other, 
 produce greater cooling effects than atmospheric air ; and 
 olefiant gas, which closely approximates air, and is far removed 
 from hydrogen in specific gravity, much more nearly approxi- 
 mates hydrogen, and is far removed from air in its cooling 
 effect 
 
 Upon the whole we may conclude, from the experiments 
 detailed in this paper, that the cooling effect of different gases, 
 or rather the difference in the cooling effect of hydrogen and 
 its compounds from that of other gases, is not due to differences 
 of specific heat ; it is not due to differences of specific gravity ; 
 it is not due to differences of conducting powers for electricity ; 
 it is not due to the character of hydrogen in relation to its 
 transmission of sound, noticed by Leslie, for reasons which I 
 have before given ; * it is not due to the same physical 
 characters of mobility which occasion one gas to escape from 
 a small aperture with greater facility than another ; but it 
 may be, and probably is, affected by the mobile or vibratory 
 character of the particles by which heat is more rapidly 
 abstracted. I at one time thought that the effect might have 
 relation to the combustible character of the gas, and that the 
 electro-negative gases were in respect to it contra-distinguished 
 from the electro-positive or neutral gases, but the experience 
 I have obtained from the experiments detailed here induces 
 me to abandon that supposition. 
 
 I incline to think that, although influenced by the fluency 
 of the gas, the phenomenon is mainly due to a molecular action 
 
 * Phil. Trans., 1847. 
 
VOLTAIC IGNITION. 349 
 
 at the surfaces of the ignited body and of the gas. We know 
 that in the recognised effects of radiant heat the physical state 
 of the surface of the radiating or absorbing body exercises a 
 most important influence on the relative velocities of radiation 
 or absorption ; thus, black and white surfaces are, as every 
 one knows, strikingly contra-distinguished in this respect : 
 why may not the surface of the gaseous medium contiguous 
 to the radiating substance exercise a reciprocal influence ? 
 Why may not the surface of hydrogen be as black and that of 
 nitrogen as white to the ignited wire ? This notion seems 
 to me the more worthy of consideration, as it may establish 
 a link of continuity between the cooling effects of different 
 gaseous media and the mysterious effects of surface in catalytic 
 combinations and decompositions by solids, such as platinum. 
 Epipolic actions will, I feel convinced, gradually assume a 
 much more important place in physics than they have hitherto 
 done ; and the farther development of them appears to me 
 the most probable guide to the connection by definite con- 
 ceptions of physical and chemical actions.* 
 
 The difference of the cooling effect of hydrogen, and of 
 those of its compounds, where it is not neutralised by a 
 powerful electro-negative gas, from all other gases, is perhaps 
 the most striking peculiarity of the phenomena I have de- 
 scribed. The differences of effect of all gases other than 
 hydrogen and such compounds are quite insignificant when 
 compared with the differences between the hydrogenous and 
 the other gases. There are some phenomena which I have 
 before observed, and which were, at the time I noticed them, 
 inexplicable to me ; but they now appear dependent on this 
 physical peculiarity of hydrogen. Thus, if a jet of oxygen 
 gas be kindled in an atmosphere of carburetted hydrogen, the 
 flame is smaller than when the converse effect takes place. 
 The voltaic arc between metallic terminals is also much 
 smaller in hydrogen gas than in nitrogen, though both these 
 gases are incapable of combining with the terminals ; indeed, 
 to obtain an arc at all in hydrogen is scarcely practicable. 
 
 Davy has, in his ' Researches on Flame,' given several 
 
 * See Dr. Tyndall's ' Researches on the Absorption and Radiation of Heat 
 by Gaseous Matter.' /V7. Trans. t 1861, 1862, and 1864. W.R.G., 1874. 
 
350 EXPERIMENTAL INVESTIGATIONS. 
 
 experiments which are similarly explicable ; but, though 
 noting the results, he nowhere, as far as I am aware, attributes 
 them to any specific peculiarity of hydrogen. 
 
 Of the phenomenon which I have examined in this paper 
 I first published an account in connection with some experi- 
 ments on the application of voltaic ignition to lighting mines, 
 and it does not appear impossible that the experiments now 
 detailed may ultimately find some beneficial application in 
 solving the problem of a safety-light for mines. A light which 
 is just able to support itself under the cooling effect of ordinary 
 atmospheric air would be extinguished by air mixed with 
 hydrogenous gas. 
 
 I am far from pretending to have devised any means of 
 fulfilling these conditions, and yet supplying an efficient light ; 
 I merely throw it out as a suggestion for consideration, 
 knowing that there are no additions to our knowledge which 
 are not ultimately valuable in their practical application ; and 
 that a suggestion, however vague a new point to those 
 whose minds may be occupied with the subject may lead 
 them to results which he who makes the suggestion is unable 
 to attain. 
 
 P.S. Since this paper was communicated I have received 
 a paper from Dr. Andrews, of Belfast, who published as early 
 as 1840, in the ' Proceedings of the Royal Irish Academy,' 
 experiments similar to those of mine first published in 1845. 
 My experiments were made in the same year as those of Dr. 
 Andrews, but as I withheld their publication, Dr. Andrews is 
 fully entitled to priority. Had I known of his experiments 
 earlier, I should have recited them in the first part of this 
 paper. 
 
 ON THE PROGRESS MADE IN THE APPLICATION OF 
 ELECTRICITY AS A MOTIVE POWER. 
 
 Royal Institution, February 9. Literary Gazette, February 17, 1844. 
 
 THE object of this lecture was to review the various applica- 
 tions of the electro-magnetic force which have been made since 
 
ELECTRICITY AS A MOTIVE POWER. 351 
 
 its first discovery J}y Oersted, in 1819, and first application to 
 actual rotary motion by Faraday. After explaining and illus- 
 trating by many novel experiments the character and direction 
 of the force exercised by voltaic currents upon magnets, Mr. 
 Grove pointed out thr.ee definite ramifications of the force which 
 different experimentalists had sought to apply to the practical 
 production of mechanical motion : I. The immediate tangen- 
 tial or deflective power of the current, which is shown in the 
 ordinary galvanometer, where a magnet is deflected by a 
 stationary wire affected by a voltaic current, and in the re- 
 volving wheel of Barlow, where the radii are deflected and 
 revolve by the influence of a stationary magnet. 2. What is 
 called the suspension principle, where powerful stationary 
 electro-magnets are made to attract pieces of soft iron placed 
 in the periphery of a wheel. As each piece arrives opposite 
 the magnet, the voltaic current is interrupted by the revolution 
 of the wheel itself, and this passes on until the next piece 
 comes within the influence of the magnet, which, being re- 
 electrised, again attracts, and so on in rotation. 3. The prin- 
 ciple of inversion of polarity, first introduced by Dr. Ritchie. 
 In this two sets of magnets are employed, the one set 
 stationary, and the other rotary ; the poles of the last being 
 alternately changed by inversion of the voltaic current, the 
 attractions of the two opposite poles of powerful magnets are 
 rendered available. 
 
 Beautiful working models of the application of the several 
 principles were exhibited, most of them representing patented 
 machinery. 
 
 Mr. Grove then entered upon the statistics of expense of 
 electro-magnetic machines. It appears from the experiments 
 of Dr. Botto, of Turin, that the consumption of 45 Ibs. of zinc 
 is necessary to work a one-horse power magnetic machine for 
 twenty-four hours. Dr. Botto worked with Mr. Grove's bat- 
 tery, against which, although admitted to be by far the most 
 powerful and equal in constancy to any other form, a general 
 notion prevails that it is expensive. This Mr. Grove says is 
 not the case, and gave his reasons, as follows : To contrast 
 the current cost of the nitric acid or Grove's battery with the 
 apparently cheapest form namely, a battery charged with 
 
352 EXPERIMENTAL INVESTIGATIONS. 
 
 dilute sulphuric acid only this latter, to overcome a given 
 resistance, say to decompose an equivalent of water, requires 
 a series of at least three cells, while only one of the nitric acid 
 battery is requisite. Hence we have in the first, three equi- 
 valents of zinc and three of sulphuric acid consumed ; in the 
 second, one of zinc, one of sulphuric acid, and one-third of 
 nitric acid; for nitric acid having five equivalents of oxygen, 
 three of which are actually available, does three units of work. 
 The calculation then becomes as given in the annexed table : 
 
 Chemical equivalent of zinc, 32. 
 
 Chemical equivalent of nitric acid, 54. 
 
 54-3=18. 
 
 32 : 181:45 : 25-3. 
 
 45 Ibs. of zinc at $d.= iis. $d. 
 
 25*3 Ibs. of real (i.e. 5o - 6 of commercial) nitric acid, at 6d. 
 = i/. sj. ftd. 
 
 us. 3</. + i/. $s. 7^</.= i/. i6s. ioj</., the expense of one 
 horse-power for twenty-four hours. 
 
 In this the small expense of the sulphuric acid and minute 
 waste of mercury are thrown out of the equation, as more than 
 balanced by the salts formed during the action of the battery. 
 It is evident from this that the expense of electro-magnetic 
 machines far exceeds that of steam ; indeed, it could hardly 
 be expected to be otherwise, as with the one we use for fuel 
 manufactured materials, in the production of which coals, 
 labour, &c., have been expended ; in the other, coals and 
 water are used directly. This is rather discouraging ; but at 
 the same time we must recollect that in magnetic motion 
 nothing is wasted ; when the machine stops the consumption 
 stops also ; and Mr. Grove thinks that we should not so much 
 direct our endeavours to rival steam, as to find out methods 
 of application where steam cannot be applied. The telegraph 
 is one of these, and probably some others will be found ; at 
 all events, the idea that so much ingenuity, labour, and per- 
 severance should have been applied, and should have marched 
 with rapid strides in vain, is contradicted by the whole 
 history of science ; and though it may not be very easy to 
 point out the exact where, when, and how of its success, still, 
 
APPLICATIONS OF ELECTRIC LIGHT. 353 
 
 considering the enormous progress of electro-magnetic appli- 
 cations made within the last few years, there is every ground 
 for hopefulness as to its ultimate success. 
 
 In a lecture at the Royal Institution, in 1849,* Mr. Grove dis- 
 cussed certain practical applications of voltaic ignition and the 
 voltaic arc, such as eudiometry, lighting mines, street-lighting, 
 and light-houses. He had made some experiments six years 
 ago on the subject, and then on one occasion delivered a lecture 
 at the London Institution, the theatre being illuminated by 
 the voltaic arc. In preparing the lecture he had made a rough 
 calculation as to expense, and the matter appeared to him 
 (though attended with many practical difficulties) to be hopeful 
 and promising. By interposing a voltameter in the circuit 
 while the arc was produced the consumption in the battery 
 could be calculated ; for every chemical equivalent of hydrogen 
 evolved in the voltameter an equivalent of zinc, of sulphuric 
 acid, and one-third of an equivalent of nitric acid, would be 
 consumed in each cell of the battery. Supplying these data 
 for calculation, and making proper allowance for the amount 
 of water contained in the commercial acids, &c., the theore- 
 tical expense of a battery such as he was exhibiting (fifty cells 
 of the nitric acid combination, each platinum plate two inches 
 by four) would be about two shillings an hour. He had tested 
 by the photometric method of equality of shadows the inten- 
 sity of the light as compared with a common wax-candle, and 
 found that after the battery had been an hour at work the 
 voltaic light was to the candle as 1,444 to l - He did not take 
 this comparison of intensities as an absolutely fair practical 
 comparison, nor did he give the above as a practical calcula- 
 tion, but thought it would be safe if twice that expense, or four 
 shillings per hour, were assumed ; the actual expense of charg- 
 ing the battery for a given time of action bore this out. He 
 showed the inferiority of central as compared with separate 
 lights for street illumination, but for lighthouses, particularly 
 for an intermittent light at regular intervals, or for signal 
 lights, the application appeared to him to be reasonably ap- 
 proximate, and for more general purposes far from hopeless ; 
 
 * Lit. Gaz., Feb. 24, 1849, 
 A A 
 
354 EXPERIMENTAL INVESTIGATIONS. 
 
 the practical difficulties, though undoubtedly not small, being, 
 in his opinion, by no means insurmountable. 
 
 Electrical Mag.) p. 601. Jan. 1845. 
 
 Mr. Grove communicated to the proprietors of the London 
 Institution, at this their first soiree for the season, some of the 
 leading discoveries in physical science during the past year. 
 
 Of electrical subjects, M. Matteucci's researches were de- 
 scribed, with experimental illustrations ; as also the magnetic 
 note. In reference to the latter Mr. Grove detailed a curious 
 experiment that had occurred to him, and which bore greatly 
 on the subject. A glass tube, with plate glass at the ends, and 
 protected along its length with a copper jacket, was filled with 
 water, in which was suspended powdered magnetic oxide of 
 iron. On looking through the tube at distant objects a con- 
 siderable portion of the light was intercepted by the hetero- 
 geneous arrangement of the particles of the oxide ; but, on 
 passing a current through a coil placed round the tube, these 
 particles assumed a symmetrical character, and much more 
 light was transmitted. This experiment was alluded to as an 
 illustration of the molecular polarisation and vibration that is 
 attributed to the particles of an iron rod, when a musical note 
 is obtained under the influence of the current. 
 
 [This result was the small outcome of a series of experi- 
 ments which ought to have led me to the discovery afterwards 
 made by Faraday. At the time I made the above experiment I 
 tried a tube having a coil of coated wire round it, and a Nichol's 
 prism at each end. The tube was filled successively with 
 various solutions, and I hoped to divert the plane of polarisa- 
 tion by passing a voltaic current through the coil. I failed 
 only because my coil was a single one, and not several times 
 reduplicated, as it should have been. I mentioned the experi- 
 ment at the time, to Mr. Gassiot, Faraday, and Pereira, and the 
 latter communicated it to the Managers of the London Insti- 
 tution. I subsequently observed that a plate of mica in vacuo 
 placed itself transversely to the line joining the poles of a 
 powerful electro-magnet, and was engaged on this when 
 Faraday published his paper on the magnetisation of all 
 matter, which included and explained the single case I was 
 working at.] W.R.G., 1874. 
 
MAGNETISM AND HEAT. 355 
 
 ON THE DIRECT PRODUCTION OF HEAT BY 
 MAGNETISM. 
 
 Ptoc. R. S. May 24, 1849. 
 
 THE author recites the experiments of Messrs. Marrian, Beat- 
 son, Wertheim, and De la Rive on the phenomenon made 
 known some years ago, that soft iron when magnetised emitted 
 a sound or musical note. 
 
 He also mentions an experiment of his own, published in 
 January 1845, where a tube was filled with the liquid in which 
 magnetic oxide had been prepared, and surrounded by a coil ; 
 this showed to a spectator looking through it a flash of light 
 when the coil was electrised. 
 
 These experiments led to the supposition that when iron 
 is magnetised a molecular change is produced throughout its 
 mass ; if this be the case, a species of molecular friction might 
 be expected to obtain, and by such molecular friction heat 
 might be produced. 
 
 Difficulties present themselves in proving this result, the 
 principal of which is that with electro-magnets the heat pro- 
 duced by the electrised coil surrounding them, might be 
 expected to mask any heat developed by the magnetism. 
 
 This interference, after several experiments, the author con- 
 siders he entirely eliminated by surrounding the poles of an 
 electro-magnet with cisterns of water, and by this means, and 
 by covering the keeper with flannel and other expedients, he 
 was enabled to produce in a cylindrical soft iron keeper, when 
 rapidly magnetised and demagnetised in opposite directions, 
 a rise of temperature several degrees beyond that which ob- 
 tained in the electro-magnet, and which therefore could not 
 have been due to conduction or radiation of heat from such 
 magnet. A series of experiments is given with this apparatus. 
 
 By filling the cisterns with water colder than the electro- 
 magnet, the latter could be cooled by the water while the 
 keeper was being heated by the magnetisation. 
 
 The author subsequently obtained distinct thermic effects 
 in a bar of soft iron placed opposite to a rotating permanent 
 
 A A 2 
 
356 EXPERIMENTAL INVESTIGATIONS. 
 
 steel magnet, using a delicate thermo- electrical apparatus 
 placed at his disposal by Mr. Gassiot. 
 
 To eliminate the effects of magneto-electrical currents, the 
 author then made similar experiments with non-magnetic 
 metals and with silico-borate of lead, substituted for the iron 
 keepers, but no thermic effects were developed. 
 
 He then tried the magnetic metals nickel and cobalt, and 
 obtained thermic effects with both, and in proportion to their 
 magnetic intensity. 
 
 Some questions of theory suggested by the above experi- 
 ments, and relating to the rationale of the action of what are 
 termed ' the imponderables ' and to terrestrial magnetism, are 
 then briefly discussed, and the author concludes by stating 
 that he considers his experiments prove satisfactorily that 
 whenever a bar of iron or other magnetic metal is magnetised, 
 its temperature is raised. 
 
 ON THE ELECTRO-CHEMICAL POLARITY OF GASES. 
 
 Phil. Trans. R. S. Received January 7. Read April i, 1852. 
 
 THE different effects of electricity upon gases and liquids has 
 long been a subject of interest to physical enquirers. There 
 are, as far as I am aware, no experiments which show any 
 analogy in the electrisation of gases to those effects now com- 
 monly comprehended under the term electrolysis. Whether 
 gases at all conduct electricity, properly speaking, or whether 
 its transmission is not always by the disruptive discharge, the 
 discharge by convection, or something closely analogous, is 
 perhaps a doubtful question,* but I feel strongly convinced 
 that gases do not conduct in any similar manner to metals or 
 electrolytes. 
 
 In a paper published in the year 1 849! I have shown that 
 
 * See M. Edmond Becquerel's experiments and one by myself by which I 
 detected a slight transmission of electricity between two highly ignited platinum 
 wires in close proximity and in the vacuum of a good air-pump. Notices, R. /.,. 
 Feb. 3, 1854. See note to p. 344. 
 
 t Phil. Trans., 1849, P- 55- 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 357 
 
 hydrogen or atmospheric air intensely heated showed no sign 
 of conduction for voltaic electricity, even when a battery of 
 very high intensity was employed. 
 
 In the Eleventh, Twelfth, and Thirteenth Series of Fara- 
 day's Experimental Researches the line of demarcation be- 
 tween induction across a dielectric and electrolytic discharge 
 is repeatedly adverted to ; induction is regarded as an action 
 of contiguous particles, and as a state of polarisation anterior 
 to discharge, whether disruptive, as in the case of dielectrics, 
 or electrolytic, as in electrolytes. See 1164, 1298, 1345? 
 1368, &c. 
 
 Mr. Gassiot, in a paper published in the year 1844,* has 
 shown that the static effects, or effects of tension, produced by 
 a voltaic battery, are in some direct ratio with the chemical 
 energies of the substances of which the battery is composed ; 
 in other words, that in a voltaic series, whatever increases the 
 decomposing power of the battery when the terminals are 
 united by an electrolyte, also increases the effects of tension 
 produced by it, when its terminals are separated by a di- 
 electric. 
 
 In none of the above papers, and in no researches on elec- 
 tricity of which I am aware, is there any experimental evidence 
 that the polarisation of the dielectric is or may be chemical 
 in its nature ; that, assuming a dielectric to consist of two sub- 
 stances having antagonistic chemical relations, as for instance 
 oxygen and hydrogen, the particles of the oxygen would be 
 determined in one direction, and those of the hydrogen in the 
 other ; the only experimental result bearing on this point 
 with which I am acquainted is the curious fact which was 
 observed by Mr. Gassiot and some other electricians who ex- 
 perimented with him in the year 1838, viz. that when two 
 wires forming the terminals of a powerful battery were placed 
 across each other, and the voltaic arc taken between them, the 
 extremity of the wire proceeding from the positive end of the 
 battery was rendered incandescent, while the negative wire 
 remained comparatively cool ; it was at that time believed 
 that there was some effect exhibited here extra the voltaic 
 
 * Phil. Trans., 1844, p. 39, 
 
358 EXPERIMENTAL INVESTIGATIONS. 
 
 circuit. Shortly afterwards I showed that with all, or at all 
 events a great number of metals, the positive terminal was more 
 heated than the negative, and that the portion of the crossed 
 wire which was positive became more incandescent than that of 
 the negative, from the greater heating effect developed at the 
 positive point when the disruptive discharge took place. I 
 suggested as an explanation of this phenomenon the possi- 
 bility that in air, as in water, or other electrolyte, the oxygen 
 or electro- negative element was determined to the positive 
 terminal, and that from the union of the metal with that of 
 oxygen a greater heating effect was developed. This, with 
 some other impressions, which turned out to be erroneous, I 
 mentioned in a letter to my friend Dr. Schonbein, not intended 
 for publication, but which shortly afterwards found its way 
 into print* 
 
 Though by no means thinking that this explanation was 
 in every respect satisfactory, there were many arguments in 
 its favour, and the fact strongly impressed my mind as evin- 
 cing a very striking difference in character between the effect 
 of the discharge at the positive and negative terminals, and as 
 presenting, as far as it went, a distant analogy to the effect of 
 electrolysis. 
 
 In the year 1 848, while experimenting with Mr. Gassiot 
 with a nitric acid battery consisting of 500 well-insulated cells, 
 I made the following experiment : Two wires of platinum 
 -jo-th of an inch in diameter, forming the terminals of the bat- 
 tery, were immersed in distilled water ; the negative wire was 
 then gradually withdrawn until it reached a point a quarter of 
 an inch distant from the surface of the water. A cone of blue 
 flame was now perceptible, the water forming its base, and 
 the point of the wire its apex ; the wire rapidly fused, and 
 became so brilliant that the cone of flame could be no longer 
 perceived, and the globule of fused platinum was apparently 
 suspended in air and hanging from the wire ; it appeared sus- 
 tained by a repulsive action like a cork ballon -&jet d'eau, and 
 threw out scintillations in a direction away from the water. 
 The surface of the water at the base of the cone was de- 
 pressed, and divided into little concave cups, which were i,n a 
 * Phil. Mag., 1840, p. 478. 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 359 
 
 state of continual agitation. When the conditions were re- 
 versed and the negative wire immersed, the positive wire being 
 at the surface, similar phenomena ensued, but not nearly in so 
 marked a manner ; the cone was smaller, and its base much 
 more narrow in proportion to its height. 
 
 This experiment, the beautiful effect of which requires to 
 be seen to be appreciated, indicates a new mode of transmis- 
 sion of electricity partaking of the electrolytic and disruptive 
 discharges. Not possessing a battery of this enormous inten- 
 sity, I have not been able to examine this phenomenon more 
 in detail ; but I have from time to time made many other 
 experiments on the voltaic arc taken in various gaseous 
 media, with the view of ascertaining the state of the interven- 
 ing media anterior to, during, and after the discharge ; these 
 experiments have hitherto given me no results of any value. 
 In the voltaic arc the intense heat developed so affects the 
 terminals and so masks the proper electrical effect, that the 
 difficulty of isolating the latter is extreme ; and I have latterly 
 sought for some modified form of electric discharge which 
 should be intermediate between the voltaic arc and the ordi- 
 nary Franklinic discharge or that from the prime conductor 
 of a frictional machine ; for something, in short, which should 
 yield greater quantitative effects than the electrical machine, 
 but not dissipate the terminals, as is done by the voltaic 
 arc. 
 
 An apparatus, to which M. Despretz was kind enough to 
 call my attention recently at Paris, seemed to promise me 
 some aid in this respect. It was constructed by M. Ruhm- 
 korff, on the ordinary plan for producing an induced current, 
 viz. a coil of stout wire round a soft iron core, with a secondary 
 coil of fine wire exterior to it, having an ingenious self-work- 
 ing contact breaker attached ; from the attention paid to insu- 
 lation in the construction of this apparatus very exalted effects 
 of induction could be procured. Thus, in air rarefied by the 
 air-pump an aurora or discharge of five or six inches long 
 could be obtained from the secondary coil, and in air of ordi- 
 nary density a spark of one-eighth of an inch long. 
 
 I procured one of these apparatus from M. Ruhmkorff; 
 the size of the coil portion of the apparatus is 6-5 inches long, 
 
360 EXPERIMENTAL INVESTIGATIONS. 
 
 4 inches diameter ; the length of the wires forming the coils 
 are (I give M. Ruhmkorff's measurements), stout wire, 30 
 metres long, 2 millimetres diameter, 200 convolutions ; fine 
 wire, 2,500 metres long, \ millimetre diameter, 10,000 con- 
 volutions. These measurements will only be taken as approxi- 
 mative, and indeed the exact size is immaterial to the 
 consideration of the experiments which I am about to detail. 
 I will not give my experiments in the order in which I made 
 them, as I should have to describe many fruitless ones, but I 
 will place first that which I consider the most important and 
 fundamental. 
 
 ist. On the plate of a good air-pump was placed a silvered 
 copper plate, such as is ordinarily used for Daguerreotypes, the 
 polished silver surface being uppermost. A receiver, with a 
 rod passing through a collar of leathers, was used, and to the 
 lower extremity of this rod was affixed a steel needle, which 
 could thus be brought to any required distance from the silver 
 surface ; a vessel containing potassa fusa was suspended in 
 the receiver, and a bladder of hydrogen gas was attached to a 
 stop-cock, another orifice enabling me to pass atmospheric air 
 into the receiver in such quantities as might be required.* A 
 vacuum being made, hydrogen gas and air were allowed to 
 enter the receiver in very small quantities, so as to form an- 
 attenuated atmosphere of the mixed gas. There was no baro- 
 meter attached to my air-pump, but from separate experiments 
 I found the most efficient extent of rarefaction for my purpose 
 was that indicated by a barometric height of from half to 
 three-quarters of an inch of mercury ; and, except where other- 
 wise stated, a similarly attenuated medium was employed for 
 all the following experiments. 
 
 Two small cells of the nitric acid battery, each plate 
 exposing four square inches of surface, were used to excite 
 the coil machine, and the discharge from the secondary coil 
 was taken between the steel point and the silver plate. The 
 distance between these was generally = O'l of an inch, but 
 this may be considerably varied. When the plate formed the 
 positive terminal, a dark circular stain of oxide rapidly formed 
 
 * See a figure, and description of the apparatus at th$ end of this Paper, 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 361 
 
 on the silver, presenting in succession yellow, orange, and blue 
 tints, very similar to the successive tints given by iodising in 
 the ordinary manner a Daguerreotype plate. Upon the poles 
 being reversed and the plate made negative, this spot was 
 entirely removed, and the plate became perfectly clean, leav- 
 ing, however, a dark, polished spot occasioned by molecular 
 disintegration, and therefore distinguishable from the re- 
 mainder of the plate. 
 
 The experiment was repeated a great many times, and 
 with varying proportions of gas, and I found that with pro- 
 portions varying from equal volumes of hydrogen and air to 
 those of one volume of the former to two and a half of the 
 latter, the experiments succeeded ; better, I should say, when 
 there was rather an excess of hydrogen as compared with the 
 equivalent of oxygen in the atmospheric air ; about one 
 volume of hydrogen to one and a half of air succeeded well ; 
 when excess of air was present, oxidation took place, whether 
 the plate was positive or negative, and when excess of 
 hydrogen was present no oxidation took place. 
 
 2nd. I experimented with an air vacuum (to borrow an 
 expression of Dr. Faraday), and found that oxidation took 
 place whether the plates were positive or negative, but in dif- 
 ferent degrees ; when the plate was positive, a small circular 
 spot was rapidly formed, quickly deepening in colour, and 
 apparently eating into the plate ; when the plate was negative, 
 a large diffuse spot was formed, the oxidation was more slow, 
 and the plate not so rapidly corroded. 
 
 3rd. I now operated with a hydrogen vacuum ; when the 
 plate was clean no discolouration took place, the plate retained 
 its polish, though after a long continuance of the discharge a 
 molecular change was perceptible, producing a frosted appear- 
 ance similar to the mercurialised portions of a Daguerreotype. 
 
 When the plate had been previously oxidated by the dis- 
 charge in an air vacuum the oxidation was rapidly and beau- 
 tifully cleared off by the discharge in the hydrogen vacuum, 
 and this whether the plate was positive or negative, the effect 
 being, however, better and more rapidly produced in the 
 latter case. 
 
 4th. I substituted respectively for the steel needle wires 
 
362 EXPERIMENTAL INVESTIGATIONS. 
 
 of copper, silver, and platinum, and found the effect produced 
 by all and with nearly equal facility ; if there were any dif- 
 ference, the platinum point was the least efficient ; this may 
 be due to the peculiar effect of platinum in itself combining 
 the gases, or to its inoxidable character, the oxygen being 
 thrown off from its surface, and not uniting with it, as with the 
 more oxidable metals ; the flame or luminous appearance 
 which surrounded the wire when the platinum was negative 
 was larger and more diffuse than with the other metals. 
 
 5th. As air, notwithstanding its containing a great excess 
 of nitrogen, gave an effect of oxidation at both electrodes, 
 though different in degree, I increased the proportion of 
 nitrogen by passing into the receiver nitrogen which had been 
 formed by the slow combustion of phosphorus, the phospho- 
 rous acid having been well washed away, and potash being 
 always in the receiver ; no more air was allowed to be present 
 than the very small quantity released from the apertures of the 
 stopcock ; with this mixture, viz. a maximum of nitrogen and 
 a minimum of oxygen, and rarefied as before, a similar effect 
 was produced to that shown in the mixture of air and hydrogen, 
 the positive plate being oxidated by the discharge, and the 
 spot when made negative being reduced. The effect of reduc- 
 tion was not so rapid, or so readily produced, as when hydrogen 
 was used, but was very decided. 
 
 6th. With nitrogen, as much deprived of oxygen as I could 
 procure, the colours of oxidation were not exhibited, but a 
 dark spot, apparently due to disintegration, was produced, 
 which was not removed by the plate being made negative ; if, 
 however, the coloured spot was produced by the plate being 
 made positive in an air vacuum, they were removed by the 
 plate being made negative in a nitrogen vacuum, leaving, 
 however, a darker spot than that which was exhibited when 
 they were reduced in hydrogen. Even when produced in an 
 air vacuum, and then a very perfect exhaustion effected, such 
 as would reduce the mercury in the barometer to the height 
 of -^ih of an inch, the spot was partially reduced when the 
 plate was made negative. 
 
 7th. An oxyhydrogen vacuum was formed, the gases being 
 in the proportion in which they form water ; and, thanks to 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 363 
 
 the attenuated atmosphere, it was easy to take the discharge 
 in this mixture without producing detonation or any sudden 
 combination of the gases, a possibility pointed out by Grotthus.* 
 With this mixture the effect took place as with the mixture 
 of atmospheric air and hydrogen. I expected it to have been 
 more efficient, but it was rather less so than the mixture of 
 air and hydrogen ; whether it be that the presence of nitrogen 
 lessens the tendency to combine of the gases oxygen and 
 hydrogen, and thus enables the electrical polarisation and dis- 
 charge to operate more efficiently, whether the nitrogen has a 
 specific effect in aiding the electro- chemical effect, as I have 
 shown it has in one peculiar case, f or whether any unknown 
 effect of nitrogen is concerned, I do not undertake to pro- 
 nounce ; I can only say that in several repetitions of the expe- 
 riment it appeared to me that the mixture of atmospheric air 
 and hydrogen was more efficient in exhibiting this phenomenon 
 that that of oxygen and hydrogen. 
 
 8th. Different proportions of oxygen and hydrogen were 
 employed, and here also I found that within a tolerably wide 
 margin I could vary the proportion of the gases. Three 
 volumes of hydrogen to one volume of oxygen I found to be 
 a very efficient mixture. 
 
 Qth. I now substituted for the silver plate plates of the 
 following metals : bismuth, lead, tin, zinc, copper, iron, and 
 platinum, the former three metals being burnished, the latter 
 polished. 
 
 Bismuth showed the effect nearly, if not quite as well as 
 silver ; it was oxidated in an air vacuum, reduced in a hydrogen 
 vacuum, and oxidated or reduced in the mixed gas, according 
 as it formed the positive or negative terminal. 
 
 Lead oxidated easily, but the spot of oxide could with 
 difficulty be reduced. Tin, zinc, and copper required the 
 admission of a great quantity of air to produce oxidation ; and 
 I could not succeed in reducing the oxide by the electrical 
 discharge, at least so as to restore the polish of the plate ; a 
 blackening effect was in some degree produced. Iron was not 
 oxidated until the receiver was nearly filled with air, and then 
 
 * Annales de Chimie, vol. kxxii. f Phil. Trans. , 1843, pp. no, III. 
 
364 EXPERIMENTAL INVESTIGATIONS. 
 
 a small spot of rust was formed, which I could not reduce. 
 With all the metals a slight whitish film, like the mercurialised 
 portion of a Daguerreotype, was visible beyond the circle 
 marked by the discharge when the plate was rendered positive, 
 which film was removed by negative electrisation in a hydrogen 
 vacuum ; it seemed to me that this film, as well as others 
 among those I have described, was affected by light, but I did 
 not turn aside to examine this effect. Platinum showed no 
 effect either of oxidation or reduction. 
 
 loth. As it was impossible to operate with an atmosphere 
 of chlorine with the apparatus which I possessed, and wishing 
 to vary the electro-negative element, I iodised a silver plate 
 by the vapour of iodine to a deep blue colour, and then made 
 it negative in an atmosphere of hydrogen : the iodine was 
 beautifully removed in a circle or disc opposite the point 
 which formed the positive terminal. 
 
 nth. I now substituted for the coil apparatus a very good 
 electrical machine, the cylinder of which was 16 inches dia- 
 meter, and the prime conductor of which, when the machine 
 was properly excited, gave a spark of 8 inches long. With 
 this machine, and in an attenuated atmosphere of one volume 
 hydrogen plus two of atmospheric air, I produced the effects 
 of oxidation and reduction very distinctly, the plate being in 
 turn connected with the conductor and with the ground ; but 
 the comparative minuteness of the spot, after many turns of 
 the machine, showed the great superiority of the coil machine 
 for producing quantitative effects over the ordinary electrical 
 machine ; and I question whether I should have detected the 
 phenomenon with the latter, had I not become previously well 
 acquainted with it by the former apparatus. Probably an ex- 
 tensive series of the water battery or a steam hydro-electric 
 machine would succeed equally well or better than the coil 
 machine. 
 
 1 2th. A solution of hyposulphite of soda removed the 
 spots formed by electrisation from the silver plate, just as it 
 removes the iodine from an iodised plate. 
 
 1 3th. In some of the above experiments I remarked a ten- 
 dency in the spots produced by the discharge to show circles 
 or zones of oxidation in different degrees, and in a more 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 365 
 
 marked manner than would be accounted for by the different 
 colours of the thin films of oxide formed. I determined to 
 examine this effect, and selected, after some experiments, an 
 atmosphere of one volume oxygen mixed with four volumes 
 of hydrogen, and attenuated by the air-pump, as in the pre- 
 vious experiments. The plate was made positive, and the 
 point was placed successively opposite different portions of 
 the silver plate, at distances of -^th, s^ths, 5-V ns > s^ths, 
 and y^-ths of an inch. The results are given, as- nearly as I 
 can copy them, in the accompanying plate, figs. I to 5. 
 
 The colour of the central spot was a yellow-green in the 
 centre, surrounded by a blue-green, then a clear ring of 
 polished silver, then an outer ring of crimson, with a slightly 
 orange tint on the inner side, and deep purple on the outer ; 
 the exterior portion of the spot was, as far as my eye could 
 judge, of a colour complementary to the interior of the external 
 ring, and the central portion of the spot of a colour comple- 
 mentary to the exterior portion of the ring. The colours 
 varied with the time, density of gas and other conditions, but 
 generally showed this complementary tendency. Symptoms 
 of a faint polished ring were visible beyond the outer ring, and 
 could be rendered more distinct by breathing on the plate. 
 As the distance between the point and the plate was increased 
 the colours became fainter, and the rings more diffuse, and 
 beyond the distance I have given nearly lost their defined 
 character ; but the first three distances, or those of 5-V tn > 
 gyhs, and ^ths of an inch gave very beautifully defined rings. 
 The luminous appearance on the needle in these experiments 
 extended from three-fourths of an inch to an inch from the 
 point. Frequently a small polished speck was visible, exactly 
 opposite the point of the needle. See fig. 6. 
 
 When the plate was made negative, the other conditions 
 being the same, a polished space appeared opposite the point 
 of the needle, surrounded by a dusky and ill-defined areola ; 
 its colour, when regarded from a point opposite the incident 
 light, was brown tinged with purple ; and when in the same 
 direction as the light, a greenish white, similar to the t tint 
 seen on mildew or on some of the lichens : these spots were 
 very different from the positive spots, and in some degree the 
 
366 EXPERIMENTAL INVESTIGATIONS. 
 
 converse of them ; but they were not nearly so well defined or 
 capable of being produced with the same uniformity. I have 
 endeavoured to represent one of them at fig. 7. 
 
 I4th. In order to ascertain whether the polished ring 
 intervening between the oxidated central spot and oxidated 
 external ring were a mere negation of effect or an antithetic 
 polar effect, such as would occasion reduction, I formed in an 
 air vacuum two large spots on a silver plate, with one the plate 
 being made negative, and with the other positive, oxidating 
 them until they began to pass from deep orange to purple. I 
 then perfectly exhausted the receiver, swept it with the gas 
 employed in the last experiment, and then took the discharge 
 in a vacuum of that gas, viz. one volume oxygen + four 
 hydrogen ; the plate being positive, and the needle ^-ths of 
 an inch over the centre of each spot in turn, a ring of clear 
 polish was formed rapidly in both the dark discs, just at the 
 distance where the ring of polish appeared in the last experi- 
 ment. I then exposed a clean portion of the plate to the 
 needle without any other change, and, on allowing the dis- 
 charges to pass, formed the rings, just as in the last experiment. 
 
 1 5th. I examined some of the spots with an achromatic 
 microscope, magnifying 200 diameters ; I could not, however, 
 discover any feature which the naked eye did not show, or 
 any peculiar molecular state ; the polishing scratches on the 
 plate were highly magnified, but the electrised spots only 
 showed more dimly the colours or the lights and shadows 
 which they exhibited to the naked eye. 
 
 1 6th. I took the discharge on a silver plate in vacua of 
 the following gases respectively : Oxygen, protoxide of 
 nitrogen, deutoxide of nitrogen, carbonic acid, carbonic oxide, 
 and olefiant gas. 
 
 The first four gases presented nothing remarkable ; the 
 plate was oxidated, whether positive or negative, as in a vacuum 
 of atmospheric air. In the protoxide of nitrogen the colour 
 of the discharge was a beautiful crimson on both terminals. 
 
 In deutoxide of nitrogen a greater tendency to reduction 
 was shown when the plate was negative than in the other 
 three gases, and there was also a tendency to the formation of 
 rings. In carbonic oxide the plate was oxidated when positive, 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 367 
 
 and the oxide reduced when negative, just as with a vacuum 
 of air and hydrogen, but rather more slowly ; with a mixture 
 of five volumes of carbonic oxide and one volume of oxygen, 
 the rings were formed very distinctly, particularly if the plate 
 was made negative first, and then positive. The luminous 
 spot on the plate, when positive in this gas, was coloured 
 green. 
 
 When the plate was negative in olefiant gas it darkened, 
 showing the rings of colour produced by thin plates, and very 
 distinct from the other rings of which I have spoken. After a 
 short time a pulverulent deposit was formed on the plate, 
 giving brilliant sparks or stars of light, which were not shown 
 by any other gas. 
 
 This deposit was too minute for analysis, but I have no 
 doubt, from the gas used and the appearances presented, it 
 was carbon. 
 
 I have given in the above experiments the conditions 
 under which they succeeded best ; but, upon repetition, al- 
 though the exact volumes of gases and other conditions were 
 carefully attended to, they sometimes required a slight altera- 
 tion to succeed, variations taking place from causes which I 
 could not detect ; thus it was sometimes necessary to add a 
 little more hydrogen, sometimes a little more oxygen or air, 
 to alter slightly the state of attenuation in the gas, &c. 
 
 The necessarily varying condition of the battery, and the 
 state of the contact breaker, slight impurities in the gases or 
 on the surface of the plates, would be quite sufficient to account 
 for these irregularities. I mention them for the guidance of 
 anyone who may wish to repeat the experiments ; a very 
 little practice will enable any electrician to have the results at 
 his command. When there is too great a proportion of air or 
 oxygen, oxidation takes place at both poles ; when too much 
 hydrogen, reduction takes place at both ; and to effect oxida- 
 tion or reduction by reversing the direction of the discharge, 
 an intermediate condition is requisite ; so if the gas be not 
 sufficiently attenuated the oxidation is too rapid, and the 
 plate too much corroded to bring out the effects clearly ; if 
 too much attenuated too long a time is required, and the effect 
 is feeble and indistinct. 
 
368 EXPERIMENTAL INVESTIGATIONS. 
 
 I have above selected all the experiments which I con- 
 sider material in this, I believe, new class of phenomena. 
 The spots produced by electrical discharges, both on con- 
 ducting bodies and on electrics, have been before noticed and 
 experimented on, one class by Priestley,* and another class 
 by Karstenf and others, but as far as I am aware no distinct 
 electro-chemical action in dry gases, depending upon the 
 antithetic state of the terminals, and presenting a definite 
 relation of the chemical to the electrical actions in gaseous 
 media, has been pointed out. I now proceed to consider the 
 relation which these results bear to other electrical phenomena. 
 
 As may be gathered from my opening remarks, the ex- 
 periments above detailed appear to me to furnish a previously 
 deficient link in the chain of analogy connecting dielectric 
 induction with electrolysis. The only satisfactory rationale 
 which I can present to my own mind of these phenomena is 
 the following : The discharges being interrupted (as is evident 
 from the nature of the apparatus, and may be easily proved 
 by agitating a mirror near them and regarding their reflected 
 images in the moving mirror), the gaseous medium is polarised 
 anterior to each discharge, and polarised not merely physically, 
 as is generally admitted, but chemically, the oxygen or anion 
 being determined to the positive terminal or anode, and the 
 hydrogen or cation being determined to the negative terminal 
 or cathode ; at the instant preceding discharge there would 
 then be a molecule or superficial layer of oxygen or of electro- 
 negative molecules in contact with the anode, and a similar 
 layer of hydrogen or of electro-positive molecules in contact 
 with the cathode ; in other words, the electrodes in gas would 
 be polarised, as the electrodes in liquid are. The discharge 
 now takes place, by which the superficial termini of metal or 
 of oxide, as the case may be, are highly ignited or brought 
 into a state of chemical exaltation at which their affinities can 
 act ; the anode thus becomes oxidated, and the cathode, if an 
 oxide, reduced. I have elsewhere { shown strong reasons for 
 
 * History of Electricity, 2nd edit., p. 624. 
 f Archives de V Electricite , vol. ii. p. 647 ; vol. ill. p. 310. 
 \ Phil. Trans., 1847, pp. 10, 16, 21. Correlation of Physical Forces, p. 50, 
 2nd edit. 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 369 
 
 assuming that the electric or voltaic discharge, the moment 
 polarity is subverted, may be regarded as an intensely heated 
 state of the particles of the electrodes, and of the inter- 
 medium across which it passes ; and my present explanation is 
 perfectly consistent with and derivable from my previous views 
 of the disruptive discharge. 
 
 Two other theories might be proposed to account for the 
 phenomena I am considering ; the one, that the disruptive 
 discharge itself is analogous to the electrolytic, and that the 
 oxygen and hydrogen are reciprocally transferred by the 
 discharge itself. This would not, I think, be consistent with 
 the generally known facts connected with the discharge, and 
 is entirely ineffectual in explaining the Experiments 2nd and 
 3rd, where either the positive or negative terminal can be 
 made either to oxidate or reduce, according to the nature of 
 the chemical medium present, while these experiments are 
 entirely in accordance with, and the results of them flow as a 
 necessary consequence of, the view first advanced. The other 
 theory which may be advanced is, that by dielectric induction 
 the gases may be bodily separated, a layer, not molecular, but 
 corporeal or voluminous, if I may be allowed these expres- 
 sions, of oxygen being developed on the side next the anode, 
 and one of hydrogen next the cathode, the gas intervening 
 between the terminals being thus divided, as it were, into two 
 halves : this would certainly be a most curious phenomenon, 
 but I believe it to be so inconsistent with the vast mass of 
 accumulated facts in electrical science, and likely to have 
 produced in cosmical phenomena so many results which, if 
 existing, must long ere this have been detected, that I will not 
 do more than advert to it. 
 
 I have adopted the view which I have first stated as being 
 the least removed from ordinary theories or modes of regard- 
 ing electrical phenomena, and because in the present instance 
 I can present the phenomena in no other way which is in the 
 least degree satisfactory to my own mind, while this view to 
 me well accounts for them. Assuming, then, for the present 
 this view, we get a close approximation, I may say an identity 
 of the state of polarisation in gaseous non-conducting dielec- 
 
 B B 
 
370 EXPERIMENTAL INVESTIGATIONS. 
 
 tries, and in electrolytes anterior respectively to discharge or 
 to electrolysis. 
 
 Faraday observes, ' Experimental Researches/ 1164, 'In 
 an electrolyte induction is the first state, and decomposition 
 the second.' My present experiments show, I believe, that 
 in induction across gaseous dielectrics there is a commence- 
 ment, so to speak, of decomposition, a polar arrangement 
 not merely of the molecules, irrespective of their chemical 
 characters, but a chemical alternation of their forces, the 
 electro-negative element being determined or directed, though 
 not travelling in one direction, and the electro-positive in the 
 opposite direction. 
 
 This arrangement is only evidenced at present, as it is in 
 electrolysis, by the action at the polar extremities or termini 
 of the dielectric ; possibly future researches may show, by 
 the action of polarised light, by magnetism or some other 
 means of analysis, that the polarity extends, as we theoreti- 
 cally believe it does, through the whole intervening matter. 
 
 In the Experiment No. 5, with oxygen and excess of 
 nitrogen, reduction takes place by the effect of negative 
 electricity and heat ; at least there seems every reason from 
 analogy to believe that the effect of the nitrogen is only 
 negative, protecting the plate from oxygen, or at farthest 
 catalytic, aiding the reduction as sulphuric acid aids the elec- 
 trolysis of water. Upon the state of association of the gases 
 in what is generally called mixture I venture an opinion with 
 the greatest diffidence. I have always inclined to the opinion 
 that the difference between physical admixture, as it is termed, 
 of gases and chemical union, is one of degree, and the views 
 of Dalton ever presented to my mind grave difficulties. 1 * 
 My present results seem to me in favour of the chemical view, 
 as otherwise we can scarcely imagine electricity as effecting in 
 the instances given a merely physical separation ; it may, 
 indeed be said that there is composition and decomposition 
 produced by the same discharge, but this is very difficult to 
 conceive, and can hardly apply to the cases of oxygen with 
 nitrogen and of carbonic oxide. 
 
 * Phil. Trans., 1843, p. 112. 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 371 
 
 In the experiments I have detailed, the flame or visible 
 effect of the electric discharge coincided with the chemical 
 effect ; when the plate was positive, a small globule of flame 
 of a purple colour was visible on the part of the plate 
 attacked, and a bluish flame extended over an inch or more 
 of the needle. When the plate was -negative, a wider and less 
 defined disc of blue flame extended over the part of the plate 
 opposed to the positive point, like a splash of liquid thrown 
 upon it, and a pencil of light appeared on the point. Some- 
 times, but not always, this flame avoided the oxidated por- 
 tion, probably from its inferior conducting power ; and when 
 this was the case reduction took place in a much slighter 
 degree, or not at all ; sometimes, and I observed this particu- 
 larly with bismuth, the flame attached itself to the oxidated 
 portion, and then reduction immediately followed. Here, as 
 in all the electrical phenomena that I can call to mind, we 
 get the visible effects of electricity associated with physical 
 changes in the matter acting, changes of state in the terminals, 
 polarisation of the intervening medium, or both.* These ex- 
 periments furnish additional arguments for the view which I 
 have long advocated, which regards electricity as force or 
 motion, and not as matter or a specific fluid.f 
 
 The chemical polarity of gases shown, as I believe, in this 
 paper, associates itself with an experiment which I made 
 known in a lecture at the London Institution in the year 
 1843,^ an d which was subsequently verified by Mr. Gassiot 
 with more perfect apparatus than I possessed, viz. that when 
 discs of zinc and copper are closely approximated, but not 
 brought in contact, and then suddenly separated, effects of 
 electrical tension are exhibited, the one disc making the elec- 
 troscope diverge with positive, and the other with negative 
 electricity, showing that the effects ascribed by Volta. to 
 
 * Gases at present believed to be elementary probably undergo a quasi 
 chemical polarisation by electricity ; thus portions of oxygen are changed to 
 ozone, &c. See a recent paper by MM. Fremy and E. Becquerel, Comptes 
 Rendus, Paris, March 15. Note added to the proof, W. R. G., 1852. 
 
 f Printed Lecture at the London Institution, 1842, p. 28. Correlation of 
 Physical Forces, p. 48. 
 
 \ Lit. Gaz,, 1843, p. 39. 
 
 Phil. Mag., October 1844. 
 
 B B 2 
 
372 EXPERIMENTAL INVESTIGATIONS. 
 
 contact can be produced without contact, and by mere ap- 
 proximation, the intermediate dielectric being polarised, or a 
 radiation analogous, if not identical, with that which produces 
 the images of Moser taking place from plate to plate. 
 
 The present experiments also associate themselves with 
 the gas battery, where, though an electrolyte is used as the 
 means of making the action continuous, or producing what is 
 called current electricity, the initiating effect is gaseous polar- 
 ity, the films of gas in contact with the respective plates of 
 platinum having antithetic chemical and electrical states. 
 
 The results detailed in Experiment 13 appear to open a 
 new field of research. Priestley observed concentric circles 
 produced by the electrical discharge from a powerful Leyden 
 battery, which he describes as consisting of minute cavities 
 and globules of fused metal* In my experiments there is an 
 alternation of oxidation and reduction, a medium capable of 
 producing both being present ; the lateral effect and comple- 
 mentary colours have to my mind something closely resembling 
 the phenomena of interference in light, although, from the 
 polar character of the force, it is difficult to imagine any pre- 
 cisely analogous condition of electricity. The discharge 
 taking place from different parts of the needle, and extending 
 from its point to a considerable distance over its surface, 
 would give different lengths for the lines of polarisation and 
 discharge to the different parts of the disc on the silver plate 
 affected by the discharge ; and assuming electricity to be pro- 
 pagated by undulations, there would be interference ; but 
 instead of alternations of light and darkness we get alterna- 
 tions of positive and negative electricity. The ring of polished 
 metal between the central spot and the exterior ring quite 
 distinguishes these rings from the ordinary colours of thin 
 plates, i.e. colours the annular succession of which depends 
 only on the different thicknesses of the film ; here doubtless 
 the colours of the oxidated portions are colours of thin plates. 
 Experiment 14 shows clearly that the action by which the 
 polished ring is formed is a polar action of the discharge, and 
 not a mere absence of action. 
 
 * History of Electricity, 2nd edition, p. 624. 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 373 
 
 When the plate is negative the effect is, as I have observed, 
 less marked and more uncertain ; but in this case it should be 
 recollected that the visible discharge issues from the point, 
 and does not extend, or extends to a very small degree, over 
 the surface of the needle. 
 
 If the phenomena were such that the central portion were 
 always clear, while around it was one, and one only, circle of 
 oxide, it might be accounted for by the hypothesis that the 
 lines of polarisation and discharge between a point and flat 
 surface assume the form of a hollow cone ; but a cone of 
 negative bounded by cones of positive action still gives the 
 idea of some lateral fits or phases of undulation. 
 
 The high rarefaction of the medium by the discharge, and 
 its intermitting character, might occasion pulsations by the 
 in-rushing of the surrounding gas, and thus vacua in circles 
 might be formed at the places where the action of oxidation 
 is rendered null ; but this view is, I think, inadmissible ; it 
 does not account for the effects obtaining only in certain mix- 
 tures, it does not account for the reducing action, the plate 
 being positive, and presents other difficulties. The point 
 involved in Experiments 13 and 14 presents apparently a 
 wide field of enquiry ; I therefore will not farther dilate on it 
 at present, and hope to make it the subject of future investi- 
 gation. 
 
 Postscript, April 24. I may, I trust, be permitted to add 
 to this paper one or two experiments on the subject last dis- 
 cussed. Assuming that the alternations of oxidation and re- 
 duction were produced by interference in consequence of the 
 discharge proceeding from successive points of the terminal 
 or terminals, a difference of effect might be anticipated if the 
 electricity passed from a point only, and not from a line, as 
 was the case in Experiment 1 3. I therefore sealed a platinum 
 wire -g^th of an inch in diameter into a piece of glass tubing, 
 and then ground the extremity to a flat surface, so that the 
 section only of the wire was exposed ; this wire was placed 
 opposite, and at 0*07 of an inch distance from the polished 
 silver plate, in a mixture of one volume of oxygen with five 
 volumes of hydrogen attenuated until the barometer stood at 
 half an inch ; discharges from the secondary coil were then 
 
374 EXPERIMENTAL INVESTIGATIONS. 
 
 passed, the plate being positive, and a round dark spot of 
 oxide formed, represented at fig. 8 ; the platinum sealed in 
 glass was then removed and the steel needle substituted for it, 
 all else, viz. plate, gas, barometer height, &c., being the same. 
 The system of rings represented at fig. 9 was now produced. 
 
 Another experiment was made, directed to the same point : 
 a wire of copper 0*04 inch diameter, and a thread of glass of 
 the same diameter, were attached by sealing-wax at their 
 extremities in a horizontal position 0^025 of an inch from dif- 
 ferent parts of a silver plate, being insulated from the silver 
 by the wax interposed at the extremities. The gaseous mix- 
 ture and barometric height being the same as in the last ex- 
 periment, and the silver plate made positive, when the platinum 
 wire sealed in glass was brought near the plate, and the dis- 
 charges passed, a spot similar to fig. 8 was formed ; but when 
 the coated point of platinum was brought over the copper 
 wire at 0*02 inch distance, a figure consisting of two separated 
 semicircles was formed, having spots in the bisection of the 
 chords, as shown at fig. 10, the portion between the spots and 
 the semicircular line of oxide being of polished silver. With 
 the glass thread the effect was the same, but produced with 
 greater difficulty and not so well defined. 
 
 In many repetitions of these experiments which I have 
 made I have invariably produced the alternately polished 
 and oxidated rings from "the bare wire, and have not procured 
 them from the coated wire, except to a very slight degree, 
 and under certain circumstances, which, as far as I could trace, 
 were as follows : 
 
 ist. When the extremity of the wire was very near the 
 plate, so that it had a sensible magnitude with reference to the 
 intervening space, a slight formation of minute rings could be 
 detected at the commencement of the experiment. 
 
 2nd. When the experiment was long-continued, or when 
 the coated platinum wire had been used for previous experi- 
 ments, a set of rings, not consisting of an alternation of 
 oxidated and polished rings, but of annuli of different degrees 
 of oxidation, were formed. 
 
 When the experiment is continued for some time a dark 
 deposit is formed on the glass around the extremity of the 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 375 
 
 platinum wire, giving an extended conducting surface ; and 
 this may be the reason why such rings are formed, though 
 these rings, in all the cases which I have observed, differ 
 broadly from the rings formed by the bare needle or wire, not 
 having the interposed spaces of perfectly bright silver ; and 
 in all the cases the difference of effect produced by the coated 
 and the bare wire is very marked ; in by far the greater 
 number of experiments, when proper precautions are taken, 
 not the slightest formation of rings takes place with the 
 coated wire ; with the bare wire, in the gaseous mixture last 
 mentioned, I have always seen them formed. 
 
 Thus there are three systems of rings which may be 
 formed by the discharge. First, rings such as those seen in 
 the ordinary cases of thin plates ; these I have only observed 
 with olefiant gas, though probably there are many other con- 
 ditions in which they may be produced. Secondly, rings 
 formed by the superposition of layers of oxides, possibly 
 arising from the fact that at certain definite periods portions 
 of the plate become by oxidation inferior conductors, and 
 other portions are attacked, and being at a different distance 
 undergo a different molecular change by oxidation. Thirdly, 
 and to me far the most interesting set of phenomena are pre- 
 sented by the rings alternately bright and oxidated, showing 
 effects of oxidation and reduction by the same current on the 
 same plate, and which only take place in certain gaseous mix- 
 tures, of which, up to this time, one volume oxygen + five 
 volumes hydrogen is the most efficient which I have obtained. 
 
 I cannot at present see any better mode of explaining 
 these phenomena than by regarding them as analogous to the 
 phenomena of interference in light, though doubtless, if this 
 be a right view, the very different modes of action of light 
 and electricity would present very numerous phenomenal dis- 
 tinctions. Alternations of opposite polar electrical actions in 
 the discharges passing in the same direction are, I think, very 
 clearly shown in these experiments, and this appears to me a 
 result worthy of attention. 
 
 Though acquainted with Nobili's beautiful experiments on 
 the formation of coloured rings by deposition in electrolysed 
 liquids, yet as I was working on gases it did not occur to me 
 
376 EXPERIMENTAL INVESTIGATIONS. 
 
 to refer to his memoirs ;* I have done so since making the 
 experiments given in this Postscript, and find that with regard 
 to the rings so formed by electrolysis he suggests interference 
 as a possible explanation. 
 
 The dark space in the discharge to which Faraday has 
 called attention may possibly be connected with these pheno- 
 mena. I have observed that in a well-exhausted receiver, 
 containing a small piece of phosphorus, the discharge is 
 throughout its course striated by transverse non-luminous 
 bands, presenting a very beautiful effect, and a yellow deposit, 
 which, as far as I have yet examined it, seems to be allotropic 
 phosphorus, is deposited on the plate of the air-pump and on 
 the neighbouring substances ; to show this effect well the 
 needle should be positive and the plate negative, and the 
 distance between them about an inch. 
 
 I could dilate much farther on these experiments, but have 
 already trespassed perhaps too far for a postscript. Variations 
 in the form of the terminals, in the nature of the gas, vapour, 
 or gaseous mixture, in the density of the gas, in the intensity 
 and quantity of the discharge, in the nature of the plate, &c., 
 will occur to those who may feel inclined to pursue these ex- 
 periments, and, if I am not over-sanguine, promise results of 
 much interest. 
 
 ADDITIONAL NOTE ON THE DARK DISCHARGE. 
 Phil. Mag., Dec. 1852. 
 
 I find the transverse dark bands can be produced in other 
 gases when very much attenuated, probably in all ; and I 
 rather think the reason why they are more easily seen in the 
 phosphorus vapour is, that all the oxygen having been 
 consumed, a better vacuum is formed. 
 
 In addition to these bands, and under circumstances where 
 they are barely visible, there is always seen a well-defined dark 
 space intervening between the glow surrounding the negative, 
 and the stream of light proceeding from the positive terminal ; 
 it appears independently of the length of the discharge, though 
 
 * Ann, de Ch. d de Phys., vol. xxxiv. 
 
ELECTRO-CHEMICAL POLARITY OF GASES. 377 
 
 a space of an inch is a convenient distance for exhibiting the 
 effect well. 
 
 This dark discharge is elaborately described by Faraday 
 as produced by the ordinary electrical machine, Experimental 
 Researches, 1,544 ^ se< 2- 
 
 Having in my mind the analogy of interference, it seemed 
 to me that this dark space might be due to the crossing 
 of the lines of discharge from the successive points of the 
 needle, the knob, or plate from which the negative discharge 
 issues. 
 
 As the positive discharge appears to issue from a point, and 
 not to surround the wire, as does the negative, the position of 
 the dark space in close approximation to the negative termi- 
 nal was in favour of this view ; if correct, it should follow that 
 if the terminals were coated points instead of wires, knobs, or 
 plates, this dark space would not be observed, or its position 
 would be changed. Experiment verified this expectation : 
 when platinum wires sealed in glass were employed and a 
 good vacuum formed, the line of luminous discharge was con- 
 tinuous when the platinum points were brought to a distance 
 of half an inch. 
 
 When these terminals are so far separated as not to give a 
 continuous line of discharge, a pencil appears on each termi- 
 nal, which gradually becomes fainter and fainter towards the 
 middle of the intervening space ; and if the distance be great, 
 the discharge ceases to be luminous towards the middle of 
 the intervening space, from excessive diffusion ; but this will 
 be seen to be a very different effect from the abrupt and well- 
 defined dark space which appears in close approximation to 
 the negative terminal when the coated wires are not em- 
 ployed. 
 
 When the positive terminal is coated and the negative one 
 bare, the dark space appears on the point of the bare wire, the 
 wire itself being surrounded by a lambent flame ; but with the 
 converse arrangement there is no such dark space. All this is 
 much in favour of interference taking place, the coincidence of 
 positive and negative phases of the discharge producing at 
 certain points mutual neutralisation. 
 
378 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 
 
 ,5" 
 ffi 
 
 
 
 DESCRIPTION OF PLATE. 
 
 Figs. I to 10 show the spots and rings in the order referred to : it should be 
 
 observed that printed figures give but a very imperfect notion of the 
 
 actual effects. 
 
 Fig. 1 1 is the coil apparatus, the contact-breaker being in front. 
 Fig. 12. The air-pump, of a construction which I proposed many years ago, 
 
 and have found most useful for electrical or chemical experiments on 
 
 gases. 
 
 P. An imperforate piston, with a conical end, which, when pressed 
 down, fits accurately the end of the tube, the apex touching the valve 
 v, which opens outwards. 
 
 A. Aperture for the air to rush from the receiver when the piston has 
 been drawn beyond it. 
 
 B. Bladder containing the gas to be experimented on. 
 
 The piston-rod works air-tight in a collar of leathers, and the opera- 
 tion of the pump will be easily understood without further description. 
 
 If it be required to examine the gas after experiment, a bladder or 
 tube leading to a pneumatic trough can be attached at the extremity 
 over the valve v. 
 
STRIDE IN THE ELECTRIC DISCHARGE. 379 
 
 ON THE STRI^ SEEN IN THE ELECTRICAL 
 DISCHARGE IN VACUO. 
 
 Phil. Mag., July 1858. 
 
 IN a paper communicated to the Royal Society in January 
 1852 I published the first notice of transverse bands or striae 
 which are seen in the discharge from a Ruhmkorff coil in the 
 vacuum of an air-pump, wherein is placed a piece of phos- 
 phorus ; and in a postscript to that paper (' Phil. Mag./ vol. 
 iv.) I stated that the ^striae could be seen in several attenuated 
 gases, probably in all. Some time after the publication of my 
 paper, but without having seen it, M. Ruhmkorff observed the 
 same phenomenon in the vapour of alcohol ; and it has subse- 
 quently been experimented on by Messrs. Masson, Quet, Du 
 Moncel, Robinson, and Gassiot, the last of whom has elabo- 
 rately examined it in the Torricellian vacuum. 
 
 No satisfactory rationale of the phenomenon has been given. 
 In my first paper I gave no opinion as to its cause ; but subse- 
 quently, in a communication to the British Association at Hull, 
 I stated my belief that it was closely connected with the inter- 
 ruptions of the contact-breaker. The following is the notice of 
 my communication in the ' Athenaeum,' August 30, 1856 : ' Mr. 
 Grove has observed that the mode of breaking contact has a 
 marked influence on the phenomenon. If, for instance, the 
 arm of the contact-breaker be made to rest on a slight spring 
 placed underneath it, the bands become narrower. If a 
 single breach of contact be effected, most observers have 
 remarked that the effect is still perceptible ; but it is very 
 difficult to effect a single breach of contact ; the fusion of the 
 metals at the point of contact, with the vibration accompany- 
 ing the movement, occasions a double or triple disruption. The 
 best mode is to place two stout copper wires across each other, 
 and with a firm hand draw one over the other until the end of 
 the former parts company with the latter ; when this is well 
 done the striae are, in the majority of cases, not observed. Of 
 all the substances which have been tried, the vapour of phos- 
 
380 EXPERIMENTAL INVESTIGATIONS. 
 
 phorus succeeds best ; and with this is seen a remarkable 
 effect on the powder or smoke of allotropic phosphorus 
 (which is always formed when the striae are observed) : this 
 smoke traverses from pole to pole from the negative to the 
 positive side, showing, unless there be some latent optical 
 deception, a mechanical effect of the discharge under the cir- 
 cumstances.' 
 
 Dr. Robinson, who has a little misunderstood my meaning 
 in this passage, says (' Proc. Royal Irish Acad./ December 
 1856) : ' Mr. Grove appears to think it arises from some vibra- 
 tion in the metal of a contact-breaker, which produces a fluc- 
 tuation in the inducing current.' I did not mean to say that 
 the effect was due to any peculiar vibration of the metal of the 
 contact-breaker, but to the interrupted and successive dis- 
 charges of the apparatus itself that the changes in the 
 character of the discharge attendant upon changes in the 
 action of the contact-breaker, as well as the frequent absence 
 of striae when only a single disruption was effected, afforded 
 evidence that the striae were connected with the multi- 
 plied discharges. My notice was, however, short and some- 
 what obscure, as, although I entered fully on the mattei 
 in the Section-room, I had prepared no memoir on the subject. 
 I still retain the opinion I then expressed, though I do not 
 assert it as a positive conviction ; the difficulty of proving it 
 arose from the circumstance that it was next to impossible to 
 produce, by the ordinary modes, a single discharge from the 
 induced coil, for the following reasons. When one extremity 
 of the wire of the primary coil is separated from the other, an 
 induced current is produced at the moment of disruption in the 
 secondary wire and a consequent discharge in the vacuum. But 
 at this same moment of disruption, the extra or induced current 
 in the primary wire itself finds a passage in the form of a spark 
 immediately the contact is broken, and this extra current 
 occasions necessarily a second induced current in the secondary 
 wire, which, having a ready path opened to it by the discharge 
 of the previous current, would be discharged through that 
 path, though it might not have tension enough to overcome 
 any great resistance. Although, therefore, these discharges 
 cannot be separated to the eye, it by no means follows that 
 
STRIDE IN THE ELECTRIC DISCHARGE. 381 
 
 there are not two discharges when an apparently single dis- 
 ruption of the primary circuit takes place. 
 
 I had repeatedly and in vain attempted to get rid of this 
 difficulty, and to produce what I could feel assured was a single 
 induced discharge, when a very simple plan occurred to me, 
 which I am surprised did not more readily present itself ; it 
 is to make an interruption in the secondary circuit, besides 
 that formed by the vacuum, such second interruption to be in 
 air, and of the extreme length of the spark, so that not more 
 than one spark should pass at a time across the intervening 
 air, and consequently not more than one discharge through 
 the vacuous space. 
 
 Using the air-pump with phosphorus and a stick of potash 
 in the receiver, I connected by a wire one of the secondary 
 terminals of a Ruhmkorff coil with the rod which entered the 
 receiver at the top. The wire from the second terminal was 
 attached to a glass stand or pillar, so that its end projected ; 
 to another glass stand was attached a wire with one end also 
 projecting, and the other end was fixed to the brass-work of 
 the air-pump. The two projecting ends could be made to 
 touch, or to remain at any desired distance from each other. 
 
 The ends of the wires being in contact, the coil apparatus 
 was set at work, and the striae very beautifully exhibited in 
 the receiver ; the projecting wires were now gradually sepa- 
 rated, and the striae for some time continued visible and until 
 the points of the wires were so far apart that an occasional 
 spark only passed from point to point ; the striae now disap- 
 peared, and a uniform luminous cloud was produced in the 
 receiver. Even in this extreme case, however, there is apt to 
 be a double discharge, which anyone who has not watched 
 closely the phenomenon with eye and ear would not detect, 
 but would pronounce the discharge to be single. 
 
 From unavoidable minute differences in the action of the 
 contact-breaker, discharges will, all conditions being appa- 
 rently the same, vary in strength and length ; those accus- 
 tomed to the apparatus will know that, by separating the ter- 
 minals beyond the normal distance of the discharge, sparks 
 will still pass at occasional, distant, and irregular intervals, 
 proving the slightly varying intensity of the apparatus. 
 
382 EXPERIMENTAL INVESTIGATIONS. 
 
 Now, when the points are separated to the maximum for 
 the experiment which I have detailed above, two classes of 
 sparks will be perceived at different times in the air the one 
 a thin blue spark, giving to the ear a single sharp sound, and 
 the other a burred yellow spark, with a sound not so clear and 
 metallic, but attended with a slight whizz : this contrast of 
 sound may be roughly compared to that presented by a whole 
 or cracked piece of porcelain when struck ; it requires attention 
 and habit to distinguish these two classes ; but when the 
 power is attained, it will be seen that with the first or single 
 discharge there are no striae in the receiver ; with the second, 
 which I believe to be a double discharge, striae are visible. 
 
 By careful adjustment I have been able to obtain sparks 
 of the former class only, and then the experiment may be 
 continued indefinitely with no striae visible ; but on the slightest 
 alteration in the distance the striae appear. It is, however, 
 more frequently the case that, notwithstanding all the care 
 used, sparks of both characters will pass in irregular succes- 
 sion ; and then the discharges will be without striae in some 
 cases, and with them in others. 
 
 It is necessary to mention the above distinctions, as other- 
 wise any person repeating the experiments might be deceived, 
 and not succeeding in obtaining discharges without striae, 
 though the discharges were apparently single, would ima- 
 gine the results I have described to be illusions arising from 
 an error of vision. 
 
 I repeated the above experiments in one of Mr. Gassiot's 
 vacuum tubes, which showed the striae beautifully under ordi- 
 nary circumstances ; but when the division in the circuit was 
 carefully made and carried to its fullest extent, the discharges 
 passed without any striae, the tube being filled at each dis- 
 charge with a uniform glow. 
 
 From the above experiments I am satisfied that in atten- 
 uated media wherein the striae are well seen when the ordinary 
 interrupted currents are used, discharges may be made to pass 
 which exhibit no striae, and under such circumstances that 
 there is every reason to believe single discharges only pass. 
 Join to this the fact that the appearances of the striaa vary 
 with any variation of the contact-breaker, that the striae are 
 
STRIDE IN THE ELECTRIC DISCHARGE. 383 
 
 more numerous and narrower in proportion as the intermis- 
 sions of the contact-breaker are more rapid, which I have 
 observed to be very notably the case, the evidence seems 
 strong, if not conclusive, that the striae or bands are due to 
 the mechanical interference or reciprocal impulsions of two 
 or more discharges, or rather of the impulsions on the medium 
 affected.* 
 
 I will take this opportunity of mentioning one experiment 
 which I believe to be new and not devoid of interest. M. 
 Plucker and Mr. Gassiot have published several interesting 
 experiments on the effects produced by magnets on the striae 
 or on the electric discharge in highly attenuated media, com- 
 monly called vacuua. 
 
 In repeating some of these experiments with a vacuum- 
 tube kindly given me by Mr. Gassiot, two feet nine inches long 
 between the platinum wires, and one inch internal diameter, 
 I found that when the pole of a powerful bar magnet was 
 brought close to the positive platinum in a certain direction, 
 the visible discharges only passed occasionally and at intervals 
 through the tube, notwithstanding the apparatus went on 
 working, and the contact-breaker vibrating as usual. After 
 many experiments, I found that I could stop the discharge 
 entirely, by placing the poles of two magnets opposite each 
 other and in a particular direction with reference to the line of 
 discharge. The two platinum wires in this tube are inserted 
 perpendicularly to the axis, and penetrate half an inch into 
 the tube. If, now, the tube be placed horizontally, the plati- 
 num wires uppermost, and the observer looking along the tube 
 from the positive to the negative wire, the north pole of the 
 magnet should be placed on the right-hand side, and as nearly 
 as can be to the positive platinum, i.e. touching, or nearly so, 
 the glass tube, while the south pole of another magnet is 
 similarly placed on the opposite or left-handed side. The dis- 
 charges cease entirely, and there is no conduction through 
 the tube, showing that it is not a case of dark discharge, 
 but an arresting of the passage of the discharge through the 
 
 * This conclusion is too limited. Mr. Gassiot subsequently to the publication 
 of this paper obtained the strise with the voltaic arc. This is, however, regarded by 
 some as intermittent, and is at all events not a single discharge. W.R.G., 1874. 
 
384 EXPERIMENTAL INVESTIGATIONS. 
 
 attenuated medium. If the tube be turned one-fourth round 
 its axis in either direction, the discharges reappear, as they also 
 do if the magnets be reversed. The experiment requires care, 
 and it seems to be necessary to make it with a power of coil 
 just sufficient to enable the discharge to traverse the distance 
 between the platinum terminals and with a certain magnetic 
 power : if made with a powerful electro-magnet, the discharge 
 is stopped, whichever side the poles be placed with reference to 
 the platinum wires, but the discharge is only made intermit- 
 tent when the platinum wires are placed in a line joining the 
 poles of the magnet. In the above-mentioned tube the por- 
 tion of one end of the glass around one of the platinum wires 
 was covered with a dark deposit of platinum, from the terminal 
 at this end having been constantly used as the negative pole ; 
 when this was made positive, instead of the clean end, the 
 effect of arresting the current could not be produced : this 
 circumstance, and the delicacy of the conditions necessary to 
 ensure success in this experiment, lead me to think that the 
 stoppage of the discharge is due to the diversion in direction 
 of the electric line of force produced by the magnet ; the dis- 
 charge is, so to speak, blown out by the magnet ; but I hesitate 
 to express a positive opinion on this point. 
 
 When magnets are made to approach the negative pla- 
 tinum there is no stoppage of the discharge, but attractions 
 and repulsions are observed on the blue flame, which have 
 been described by Mr. Gassiot, and which I need not therefore 
 repeat. 
 
 I tried the effect of magnets on the spark from Ruhm- 
 korff's coil in air, but could not arrest them, though they 
 appeared to pass with somewhat greater difficulty and longer 
 intermissions ; the difference was, however, very slight. 
 
 ON SOME ANOMALOUS CASES OF ELECTRICAL 
 DECOMPOSITION. 
 
 Philosophical Magazine, March 1853. 
 
 WOLLASTON showed, In a well-known experiment, * that 
 when Franklinic electricity from the prime conductor of an ordi- 
 
 * Phil. Trans. 1801, p. 431. 
 
ANOMALOUS ELECTRO-CHEMICAL EFFECTS. 385 
 
 nary electrical machine was made to pass from fine points of 
 gold wire sealed into glass tubes and immersed in water, both 
 the positive and negative pole yielded mixed oxyhydrogen 
 gas, the exact proportions of which are not given. This expe- 
 riment has been subsequently discussed by Ritchie,* Faraday, f 
 and others ; but differing, as the decomposition in this case 
 does from electrolysis by the voltaic battery, I am not aware 
 that it has been repeated on a large scale, or that anything 
 more than a mere verification of the fact has been attempted. 
 
 The apparatus constructed by M. Ruhmkorff, which I have 
 described in a paper in the ' Philosophical Magazine ' for De- 
 cember last, p. 500, having given me the means of procuring 
 electricity of tension in quantity far exceeding that of the best 
 electrical machine, and having my attention directed to Wol- 
 laston's experiment by the wires sealed in glass tubes which I 
 used for my recent experiments, I determined to make some 
 experiments similar to Wollaston's, but with the spark from 
 the secondary coil instead of that from the electrical machine. 
 
 In the experiments which I am about to detail the ter- 
 minals of the secondary coil consisted of two wires of pla- 
 tinum, -g^th of an inch in diameter, sealed into glass tubes. 
 One of these coated wires was prepared on purpose for these 
 experiments ; and, having been carefully sealed into a glass 
 tube, the extremity was ground on a hone until the section of 
 wire formed one surface with the glass. 
 
 The other coated wire had been similarly prepared, but 
 had been used for some time for the experiments in attenuated 
 gases, given in the paper to which I have alluded ; and the 
 extremity of the wire was worn beneath the surface of the 
 glass, a circumstance which proved of some little importance. J 
 
 The tubes containing the platinum wires were curved in 
 form, something resembling the letter Z (see fig. i), the ex- 
 tremities immersed in a porcelain capsule containing the liquid 
 
 * P/iil. Trans. 1832, p. 282. f Ibid. 1833, p. 23. 
 
 | This result is important in another point of view than that relating to the 
 experiments in this paper, viz. it shows that even in the aurora borealis experi- 
 ment, or passage of the spark in highly rarefied gas, solid matter, even such as the 
 dense substance platinum, is given off ; thus favouring the theory that the spark 
 is ignited matter, and rendering the hypothesis of a fluid unnecessary for its 
 explanation. 
 
 C C 
 
3 86 
 
 EXPERIMENTAL INVESTIGATIONS. 
 
 to be experimented on, a graduated test-tube filled with the 
 liquid being inverted on the extremity of each. 
 
 From the Z form of the terminals the extremities pro- 
 jected jths of an inch into the test-tubes, so that no gas could 
 possibly escape. At first I tried distilled water, which had 
 been boiled immediately before the experiment to deprive it 
 of air ; I found, however, that from its want of conducting 
 power, only an infinitesimal decomposition was produced ; a 
 few drops of sulphuric acid were added, and immediately a 
 notable evolution of gas took place, a spark being visible at 
 the end of the wire which was worn beneath the surface of 
 
 Fig. i. 
 
 the glass, whether this wire were positive or negative, and 
 none on that which was even with the glass. This spark had 
 a material influence on the relative quantity of gas given off 
 by each terminal, the gas being generally less from the spark- 
 giving terminal than from that which did not yield a spark ; 
 and when both were constructed so as to give sparks, the 
 quantities of gas were irregular, and bore no definite relation 
 to each other. Thus, in one experiment the gas from the 
 negative was to that from the positive as 2-3 : i ; in another 
 experiment the gas from the positive was to that from the 
 negative as 2 '5 : I ; frequently the quantities from each were 
 nearly equal. It is obvious that when the wire was depressed 
 below the surface of the glass a film of gas must intervene 
 between the wire and the liquid, across which film the spark 
 passes. When the wire is even with the glass the gas escapes 
 more readily, and there are fewer or no sparks. 
 
ANOMALOUS ELECTRO-CHEMICAL EFFECTS. 387 
 
 There is no doubt that as the bubble of gas forms on the 
 point of platinum the spark recombines a portion of it, and 
 the number of sparks, the size of the bubble preceding the 
 spark, and the quantity recombined, would vary indefinitely 
 in each different experiment, and even in the course of the 
 same experiment. 
 
 I now give a selection of experiments from my note-book, 
 with the volumes and analyses of the gases evolved. The 
 decomposition was arrested in each case when the larger 
 volume of gas had reached to 0*3 c. i. The analyses were 
 made by an eudiometer, which I have formerly described,* 
 consisting of a loop of fine platinum wire sealed into a 
 graduated glass tube, and heated or ignited by the voltaic 
 battery : 
 
 ist. The gas from the positive pole contracted to 0*4 of its 
 original volume ; the residue, mixed with half its volume of 
 oxygen, detonated, leaving a minute residue, apparently nitro- 
 gen. The gas from negative pole, 2*3 times the volume of 
 the positive, contracted to 0*15 of its volume ; the residue con- 
 tracted very slightly on the addition of hydrogen. 
 
 2nd. Gas from positive, 2*5 volumes contracted to 07 of 
 its volume. 
 
 From negative, i volume contracted to 0*4 ; the residues 
 mixed contracted again to 07 ; and this residue, mixed with 
 half its volume of oxygen, detonated, leaving only a very 
 minute bubble. 
 
 3rd. Instead of two coated wires one coated wire was em- 
 ployed, and the opposite terminal consisted of a strip of 
 platinum foil, \ inch width and 4 inches length, exposing con- 
 sequently, reckoning both sides, 2 square inches of surface. 
 
 This strip was attached to a platinum wire, which was 
 sealed into a glass tube, the extremity of the platinum foil 
 being \ an inch within the orifice of the tube, so that no gas 
 could escape. (See fig. 2.) 
 
 With the coated point positive, the platinum plate nega- 
 tive, 0*3 c. i. of gas was collected from the point ; a minute 
 bubble only appeared in the tube containing the strip or plate ; 
 
 * Phil Mag. Aug. ^841. Phil. Tram. 1847, p. i. 
 > CC2 
 
3 88 EXPERIMENTAL INVESTIGATIONS. 
 
 the gas contracted in the eudiometer to 07 of its volume ; the 
 residue, mixed with half its volume of oxygen, detonated, 
 leaving a very small residuum. 
 
 4th. In the converse experiment, i.e. coated wire negative, 
 plate positive, o - 3 c. i. of gas was collected from the negative 
 coated wire, and, as in the former case, only a minute bubble 
 appeared in the tube containing the plate, and far too minute 
 for analysis ; the gas from the negative point contracted to 
 half its volume in the eudiometer ; the residue was hydrogen, 
 as before. 
 
 5th. In the above experiments I had used from 2 to 3 oz. 
 of distilled water, with a few drops of sulphuric acid added to 
 
 Fig. 2. 
 
 improve its conducting power, without paying any particular 
 attention to the exact quantity of acid which was mixed. 
 
 I now determined to try different degrees of dilution of 
 the acid, and to note more accurately its proportion to the 
 water. I first took the extreme case of dilution, using 3 oz. 
 of distilled water, and merely touching it by a narrow glass 
 rod which had been dipped into sulphuric acid. 
 
 The coated wire was the positive pole, the plate of pla- 
 tinum foil the negative ; O'3 c. i. was collected from the coated 
 wire, and not a trace of gas was perceptible in the tube con- 
 taining the plate. The gas contracted to one-half its volume 
 in the eudiometer, and the residue, mixed with one-half its 
 volume of oxygen, contracted to 0*3 of its original volume ; 
 the residue of this second contraction appeared to be nitrogen, 
 and its greater quantity in this than in the previous experi- 
 ment was doubtless due to the greater time which, from the 
 inferior conducting power of the solution, was necessary for 
 the decomposition, more air having thus entered into solution. 
 
ANOMALOUS ELECTRO-CHEMICAL EFFECTS. 389 
 
 In this experiment the liquid was exposed for twenty-four 
 hours, during ten of which the discharges were kept up, while 
 in the previous experiments less than an hour was sufficient, 
 and the water had not cooled from its preliminary boiling 
 before the experiment was completed. 
 
 6th. I next tried a strong solution of sulphuric acid, specific 
 gravity 1*23 ; the gas from the coated point, which was positive, 
 contracted to O'2 of its volume ; the residue was hydrogen. 
 
 7th. I used a still stronger solution, specific gravity 1*45, 
 both points being coated ; the volume from the negative pole 
 was to that from the positive as 4*5 to 3*25 ; the gas from the 
 negative pole contracted in the eudiometer to only 0*95 of its 
 volume ; the residue was pure oxygen ; the gas from the posi- 
 tive pole contracted to 0-5 ; the residue was hydrogen. Here, 
 to my surprise, there was, after the first contraction, a resi- 
 duum of oxygen in the ratio of more than 2 volumes to I of 
 the residuum of hydrogen. The experiment was repeated 
 with acid of specific gravity 1*5 ; from the positive point 3*3 
 volumes were collected, from the negative 2*6 ; the gas from 
 the negative pole contracted to 0*9 of its volume ; the resi- 
 duum was pure oxygen ; that from the positive contracted to 
 0*4 of its volume, the residue being hydrogen. 
 
 8th. I now tried a saturated solution of sulphate of cop- 
 per with a few drops of sulphuric acid added ; a remarkable 
 difference was shown in the conducting power of this solution 
 as compared with the dilute acid ; the contact-breaker of the 
 primary coil gave very minute sparks, unattended with the 
 usual snapping noise ; it was much as if the terminals of the 
 secondary coil had been united by a metallic connection ; and 
 the superiority of the conducting power of the solution of sul- 
 phate of copper in this experiment over dilute sulphuric acid 
 was far greater than is shown in ordinary electrolysis by the 
 voltaic battery. The coated point was positive, the plate nega- 
 tive ; the gas from the positive point detonated, leaving a 
 very slight residue ; but on the interior of the collecting tube, 
 near the point, was a deposit of pulverulent metallic copper. 
 On the negative plate there was no deposit, and no gas was 
 given off from it. Here the copper deposit was evidently 
 a reduction occasioned by the excess of hydrogen, but there 
 
390 EXPERIMENTAL INVESTIGATIONS. 
 
 was no symptom of any effect produced by the missing equi- 
 valent of oxygen. 
 
 9th. With a solution of caustic potash the coated point, 
 which was positive, gave a gas which contracted to 0*32 of its 
 volume, the residue being hydrogen ; the plate, which was 
 negative, gave off a mere trace of gas. 
 
 loth. Two plates were used as the terminals of the secon- 
 dary coil in dilute sulphuric acid, instead of a plate and a point. 
 No gas was given off from either, though the experiment was 
 continued for several hours. 
 
 nth. I could detect no difference in the solutions employed 
 before and after the experiment ; but as the sulphuric acid 
 might be expected to mask the effect of any oxygenated com- 
 pound which might result from the absorption of oxygen, 
 which took place in all these experiments, except those where 
 the strong solutions of sulphuric acid were employed, and as 
 pure distilled water would give no effect, I tried the effect of 
 the spark passed through aqueous vapour. Some distilled 
 water was boiled and placed under the receiver of an air- 
 pump, with a coated point and plate of platinum enclosed in 
 a tube filled with the liquid, and inverted; the point and plate 
 communicating respectively with the upper and under plate 
 of the receiver, and thence brought into metallic communica- 
 tion with the ends of the secondary coil. 
 
 The receiver was then exhausted, and the vapour which 
 had formed at the top of the tube soon dilated sufficiently to 
 expose the point and plate ; the spark was then passed across 
 the vapour, and a permanent increase in volume of the vapour 
 was soon detected. The experiment was continued for a week, 
 the apparatus working five hours each day ; at the end of that 
 time much of the gas formed had bubbled out into the re- 
 ceiver, and on letting in the air the water rose in the tube until 
 a bubble of gas of 0*03 c. i. remained in the top ; this was 
 analysed, and proved to be pure hydrogen. 
 
 On the interior of the tube near the platinum point was a 
 dark pulverulent deposit, far too minute in quantity for analysis, 
 but which had evidently proceeded from the platinum. 
 
 The only possible mode in which I can account for this 
 experiment is, that this deposit consisted of an oxide or per- 
 
ANOMALOUS ELECTRO-CHEMICAL EFFECTS. 391 
 
 oxide of platinum, and to account for it thus it must be 
 assumed that platinum, like zinc or iron, will decompose 
 water by abstracting its oxygen. 
 
 1 2th. I endeavoured, by working for a long time upon a 
 limited quantity of water very slightly acidulated with sul- 
 phuric acid, to detect some compound which might be formed 
 by the oxygen which disappeared. 
 
 I placed in a small tube OT5 c. i. of distilled water, 
 touched with a rod which had been smeared with sulphuric 
 acid. I continued working on this for a fortnight, averaging 
 five hours a day ; and as the liquid diminished by evaporation 
 I added distilled water which had been recently boiled. No 
 change could be detected in the liquid at the expiration of 
 this period. It gave an acid reaction with test-paper, preci- 
 pitated chloride of barium, and showed no bleaching proper- 
 ties, which I looked for as a result of absorbed oxygen. 
 
 I have applied the expressions positive and negative to 
 the terminals of the secondary coil which bore this relation 
 to each other upon breaking contact of the primary coil : as far 
 as I could ascertain by direct experiment, there was, on mak- 
 ing contact, no spark or decomposition from the secondary 
 coil ; but although from the time which the magnetic coil takes 
 to acquire its magnetism no spark is visible with this appa- 
 ratus on making contact, possibly some slight decomposition 
 might then take place ; this, however, though it is well to 
 notice the point, is immaterial to the consideration of the 
 results detailed in this paper, as, if the currents were in alter- 
 nate directions, the proportions of the gases would be equally 
 exceptional and anomalous. 
 
 I have made many variations of the above experiments, 
 but it would be a needless waste of space to detail them. The 
 following are the general results : 
 
 ist. With distilled water, containing a small quantity of 
 sulphuric acid, there is always a notable excess of hydrogen ; 
 in one case the oxygen was only one-fifteenth of the whole 
 volume of gas evolved. With solution of potash a similar 
 result takes place. 
 
 2nd. When the quantity of acid is increased to the point 
 at which the electrolytic power of the solution is the best, the 
 
392 EXPERIMENTAL INVESTIGATIONS. 
 
 excess of hydrogen is less ; and probably if the exact point of 
 the solution were attained, there would be no excess. 
 
 3rd. When the solution is much stronger than that which 
 makes the best electrolyte, there is a notable excess of oxygen. 
 
 4th. When the electrodes exceed a certain size, water con- 
 ducts without any apparent signs of decomposition. 
 
 On making a search for any previous experiments on this 
 subject, I find that Faraday ('Phil. Trans.' 1834, p. 91) has 
 observed that when a strong solution of sulphuric acid, formed 
 by mixing two measures of oil of vitriol with one of water, was 
 subjected to ordinary electrolysis, a remarkable disappearance 
 of oxygen took place. The strength of the acid he then em- 
 ployed would be very nearly that employed by me in experi- 
 ment 7 ; but in my experiment, when the solution is subjected 
 to decomposition by the spark from the secondary coil, ex- 
 actly the converse effect takes place, and there is a notable 
 disappearance of hydrogen. 
 
 Faraday attributes the disappearance of the oxygen to 
 the formation of peroxide of hydrogen ; but in those of my 
 experiments where oxygen disappeared, this could hardly be 
 the case, as the gases are eliminated at a point of ignition at 
 which this compound would be decomposed, and in many of 
 my experiments the liquid was kept very nearly at the 
 boiling-point. 
 
 I abstain at present from expressing any opinion as to the 
 cause of these singular anomalies, and hope to make a farther 
 communication on the subject, when I can resume and com- 
 plete some experiments which I have in view, but which it 
 will be some time before I can undertake. 
 
 THE ELECTRICITY OF THE BLOWPIPE FLAME. 
 
 Phil. Mag., January 1854. 
 
 VOLTA and Erman made known the first indications of the 
 production of electricity by flame. Pouillet and Becquerel 
 have experimented and reasoned on the statical effects of such 
 electricity, while Andrews, and more recently Hankel and 
 
ELECTRICITY OF THE BLOWPIPE FLAME. 393 
 
 Buff, have published very interesting results on the effects of 
 flame as to conduction and production of voltaic electricity. 
 
 The experiments of which I am about to give a notice 
 were for the greater part made before I had read the papers 
 of the two last authors ; and while they differ in results from 
 theirs, they give a means of producing a voltaic current from 
 flame far more distinct and powerful than any which I have 
 tried or read of. 
 
 The flame I have worked with is that of naphtha or spirit 
 of wine, urged by an ordinary glass-worker's blowpipe ;' and 
 with a galvanometer, the needles of which are barely deflected 
 to 2 by any current which I can procure by the flame of a 
 common spirit-lamp, I can by the blowpipe flame procure de- 
 flections of 20 or even 30, and with great certainty and 
 uniformity of direction. 
 
 I am not aware that the blowpipe flame has ever been used 
 for the production - of electricity, though I see by M. E. Bec- 
 querel's recent paper that M. Becquerel, sen., has used it as a 
 means of adventitiously heating a spiral placed in another 
 flame. 
 
 Two platinum wires of six inches long and -^th of an inch 
 diameter have their ends formed into coils of th of an inch 
 long and wide ; these wires are attached to copper wires in- 
 sulated by glass stands, and having their farther extremities 
 connected with a galvanometer. My galvanometer is by 
 Ruhmkorff ; the wire is not as long as those now constructed, 
 being only 544 feet, but I have so magnetised the needles as 
 to render them highly astatic ; they take four minutes to make 
 one oscillation. 
 
 An additional binding screw is connected with the wire at 
 54 feet of its length, so that I can ascertain by the same 
 instrument the effect of a slighter resistance. 
 
 When the flame of a spirit-lamp is urged by the blowpipe 
 one of the above-mentioned coils is placed in the full yellow 
 flame just beyond the apex of the blue cone, and the other 
 near the orifice of the brass jet, or at what may be called the 
 root of the flame, just beyond the base of the blue cone, the 
 distance ^between the two coils being 2\ inches. The coil in 
 the full flame is at a white heat and brilliantly incandescent, the 
 
394 EXPERIMENTAL INVESTIGATIONS. 
 
 coil near the orifice or at the commencement of the flame is 
 cherry-red. The galvanometer is deflected to an average of 
 6, the coil near the orifice or at the root of the flame being 
 positive, or related to the farther coil as zinc to platinum in 
 the voltaic trough. On reversing the position of the wires 
 the galvanometer is deflected 6 in the opposite direction. 
 
 This current is not due to thermo-electricity excited in the 
 wires at the point of junction of the platinum and copper, for 
 it is unaltered in direction by a powerful thermo-current ex- 
 cited in these by heating the points of junction by another 
 spirit-lamp. 
 
 By making this thermo-current aid or counteract the flame- 
 current a slight difference in degree is perceptible in the de- 
 flection, according as the point of junction of the one or other 
 wire is heated, but no difference in direction. 
 
 The flame-current, moreover, scarcely affects the short- 
 wired galvanometer, while the thermo-current of copper and 
 platinum whirls the needle to 90. 
 
 It is not a thermo-electric current arising from the unequal 
 heating of the two coils, for it is in the same direction when 
 the farther coil is removed from the full flame, so as to be less 
 heated than the coil at the root of the flame. It is 'also dif- 
 ferent in direction from the thermo-current produced by un- 
 equally heating the coils in similar parts of the flame, or the 
 current described by M. Buff", and to which he ascribes the 
 electricity of flame. The flame-current proper in my experi- 
 ments is opposed to and conquers the thermo-current. 
 
 On advancing the coil from the root of the flame towards 
 the farther coil the deflection lessens, but still preserves its 
 direction until the wires get very closely approximated, when 
 the deflection of the flame-current yields to that of the 
 thermo-current, and the direction of the needle depends on 
 the relative heat of the coils. 
 
 A wire of zinc near the root of the flame with the platinum 
 coil in the full flame gave a much smaller deflection, only 2 ; 
 when the direction of these wires was reversed the current 
 was stronger, the galvanometer marking 5 ; the direction of 
 the deviation was in both instances the same as with platinum 
 and platinum. 
 
ELECTRICITY OF THE BLOWPIPE FLAME. 395 
 
 Iron and copper acted as zinc, but rather more feebly. 
 As in these experiments the wires of zinc, iron, and copper 
 respectively were stouter than the platinum wire, I attribute 
 the superior amount of deflection when the oxidable metal 
 was in full flame to the greater cooling effect of the thicker 
 wire reducing the antagonising thermic current. 
 
 As the above experiments seemed to show that there was a 
 proper flame-current irrespective of, and even overcoming the 
 thermic flame-current, I was led to expect that by uniting in 
 direction these two currents I might get more marked results. 
 The following experiment, it will be seen, realised this expec- 
 tation. 
 
 I formed a little cone of platinum foil of ^ths of an inch 
 in depth, and the same width at the widest part ; I suspended 
 this in a ring of platinum wire and substituted it for one of 
 the coils. 
 
 Being placed in the full flame, the coil being at the root, 
 it was filled with water, and water dropped into it from a 
 pipette to supply that which was boiled away. I now readily 
 obtained a deflection of 20 in the same direction as in my 
 original experiment, and frequently the needles deviated to 
 30. When the cone filled with water was placed at the root 
 and the coil in the full flame the deviation was only 5. 
 
 In all the above experiments with the blowpipe it will be 
 seen that the direction of the current was, as far as a com- 
 parison can be instituted between the blowpipe flame and ordi- 
 nary flame, the reverse of that indicated in the experiments of 
 Hankel, excepting his experiment with the flame of hydrogen ; 
 and also the reverse of the greater part of the experiments of 
 Buff, which he rightly attributes to thermo-electricity. 
 
 One result of M. Buff ('Archives d'Electricite,' vol. xvii. 
 p. 275), when he places one wire in the centre and the other 
 at the outer margin of the flame, is probably dependent on a 
 similar cause to mine ; though when he places the second wire 
 in the full flame the current is in a contrary direction to that 
 which I obtain. 
 
 My experiments prove, I think, distinctly that there is a 
 voltaic current, and that of no mean intensity, due to flame, 
 and not dependent on thermo-electricity. 
 
396 EXPERIMENTAL INVESTIGATIONS. 
 
 I know of no better theory to account for these results 
 than that which Pouillet applied to the effects on the con- 
 denser, viz. that it is the result of combustion, the platinum 
 at the commencement of chemical action, or where the 
 elements are entering into combination, being as the zinc of 
 the voltaic battery ; and that at the termination of combus- 
 tion, or at the points where the chemical action is completed, 
 being as the platinum of the voltaic combination. 
 
 Although there is a distinct thermo-current produced by 
 the contact of two unequally heated bodies with flame, yet 
 when we see, as in the above experiments, a marked current, 
 in a contrary direction to, and overcoming that which is ex- 
 cited by the thermo-current in the flame, and also that at the 
 points of junction of the wires away from the flame, I see no 
 means of viewing the resulting current as a thermo current. 
 The blowpipe flame, from its definiteness of direction, brings 
 out most distinctly this current ; in other flames, from the 
 more confused circulation of the heated and burning particles, 
 the results are less significant ; and the various flame-currents 
 counteracting each other, the thermic current obtains a pre- 
 dominance. 
 
 The current from the blowpipe flame, when the platinum 
 in the full flame is cooled, is so marked, that I have little 
 doubt, by attaching to a powerful pair of bellows a tube from 
 which a row of jets proceeds, and alternating pairs of platinum 
 in flames urged by the jets, a flame-battery might be con- 
 structed which would produce chemical decomposition and all 
 the usual effects of the voltaic pile.* 
 
 ON A METHOD OF INCREASING CERTAIN EFFECTS 
 OF INDUCED ELECTRICITY. 
 
 Phil. Mag., Jan. 1855. 
 
 IN the course of last year I observed that, by connecting the 
 coatings of a Leyden phial with the extremities of the secon- 
 
 * This was subsequently done, and the effects of the flame battery shown 
 at an evening meeting of the Royal Institution. See Proceedings of the Royal 
 Institution^ Feb. 3, 1854. 
 
NEW EFFECTS OF INDUCED ELECTRICITY. 397 
 
 dary coil of Ruhmkorff's apparatus, a great increase in the 
 brilliancy of the discharge could be obtained. Circumstances 
 diverted my attention from it at the time, and I did not pub- 
 lish the experiment, though I believe Mr. Gassiot mentioned 
 it in one of his papers. I have since heard that M. Sinsteden, 
 in France, had made the same observation, though I do not 
 know when, nor whether he has published his experiments. 
 
 The point which I now think may be worth insertion in 
 the ' Phil. Mag.' is the conversion, by means of a Ruhmkorff 
 coil, of an indefinite amount of voltaic into static electricity. 
 If a small Leyden phial have its coatings connected respec- 
 tively with the extremities of the secondary wire of a Ruhm- 
 korfT coil (the primary being, as usual, connected with the 
 condenser of M. Fizeau, and two wires being attached to 
 the terminals and brought within striking distance), the 
 noise and brilliancy of the discharges are greatly increased, 
 with generally a slight diminution in their length. If 
 now the voltaic series be increased, the coil and Leyden 
 phial remaining the same, but little increase in the length or 
 brilliancy of the sparks will ensue, that is, provided the bat- 
 tery was in the first instance sufficiently powerful to give the 
 maximum effect of the coil without the phial. For instance, 
 if with a Ruhmkorff" coil of the size now usually made, ten 
 inches long by four diameter, four cells of two inches by four 
 of the nitric acid battery be used, and a pint Leyden phial, but 
 little increase of effect will be obtained by using eight or more 
 cells, and the platinum at the contact-breaker would be 
 rapidly destroyed by the sparks. 
 
 But substitute for the pint Leyden phial one of double 
 the capacity, and it will be found that though this second 
 phial was inferior to the first with a battery of four cells (giv- 
 ing shorter sparks, and fewer in a given time, though some- 
 what denser), yet it is far superior to the first with the battery 
 of eight cells, and the sparks at the contact-breaker are no 
 longer injurious. 
 
 By adding more coated surface, for instance, another phial, 
 four more cells may be added, and increased effects will be 
 obtained, and thus with the same coil the brilliancy of the dis- 
 charge may be increased to an extent to which I have not yet 
 found a limit. I obtained this result some months back ; but 
 
398 EXPERIMENTAL INVESTIGATIONS. 
 
 not having a large battery, I did not go beyond ten cells, 
 which I found would well bear a jar of one square foot coated 
 surface. 
 
 By the aid of Mr. Gassiot's more powerful apparatus, I 
 have, with him, used thirty cells of the nitric acid battery, two 
 inches by four, and five square feet of coated surface ; the 
 effects were very striking a roar of voluminous discharge of 
 O'6 of an inch long increased to i'5 inch when the flame of a 
 spirit-lamp was placed between the terminals. I have never 
 witnessed such a torrent of electrical discharges ; the noise 
 could not be borne long without great discomfort. 
 
 With the same voltaic battery, and an additional square 
 foot of coated surface, the effect was somewhat diminished. 
 Mr. Gassiot had not more than thirty cells available at the 
 time of our experiments, so that I have not yet ascertained 
 the limit to which this increase of power can be carried. I 
 presume, however, there is a limit, for reasons which will be 
 presently apparent. 
 
 The following precautions are essential to the success of 
 the experiment : 
 
 ist. The wire proceeding from the outer extremity of the 
 secondary coil must be connected with the inside or insulated 
 coating of the Leyden battery, if the battery is not wholly 
 insulated. The reason of this is, that the outer extremity of 
 the coil is the better insulated portion, and also that to which 
 electricity of tension flies off; a good spark can, under ordi- 
 nary circumstances, be obtained from the outer, but scarcely 
 any from the inner terminal of the coil. 
 
 2nd. The distance between the hammer of the contact- 
 breaker and the soft iron core should be made as great as 
 practicable, at least one-eighth of an inch ; this is an impor- 
 tant point as to the theory and experimental results of the 
 Ruhmkorff coil. Time, as is well known, is necessary for the 
 development of electro-magnetism ; and M. Matteucci, in his 
 recent valuable book on induced electricity, has shown some 
 remarkable results flowing from this fact. If the hammer be 
 too near the core, the former is raised before the latter has 
 time to be fully magnetised ; and when a Leyden condenser is 
 used ; farther time is required for this to be charged. This 
 
NEW EFFECTS OF INDUCED ELECTRICITY. 399 
 
 demand of time indicates the probable limit to the increase of 
 power to which I have above alluded. 
 
 It is very curious to see the absorption, so to speak, of 
 voltaic power by the Leyden battery; when the maximum 
 effect for a given Leyden jar has been passed, the contact- 
 breaker shows by its sparks the unabsorbed induced electricity 
 which now appears in the primary wire ; an additional jar 
 acts as a safety-valve to the contact-breaker, and utilises the 
 power, and so on. 
 
 It is a question of some interest why a jar charged in the 
 ordinary way by temporary contact of the terminals of a 
 secondary coil will only receive a very slight charge, and give 
 a discharge of scarcely measurable length, yet, when perma- 
 nently connected with the terminals, will give a long and 
 powerful discharge. The following is the best theory I can 
 offer : At the moment of the inductive action or wave of elec- 
 tricity the same wire which is affected by the electric impulse 
 is unable to conduct it back again, and thus to discharge the 
 jar ; while, when the jar is attempted to be charged in the 
 ordinary way, the contact, however apparently of short dura- 
 tion, lasts longer than the single impulse of electricity, and so 
 the coil in great part discharges the jar. Some such state of 
 the wire as that I have suggested must exist at the moment of 
 an induced current, as otherwise the wire would discharge 
 itself, or, in other words, would never receive a charge or state 
 of opposite electricity of great tension at its extremities. At one 
 time I considered the explanation to be, that at the moment 
 of breaking contact a portion of the induced electricity flies off 
 across the discharging interval in the form of a spark, and thus 
 enables the jar to discharge itself, just as the voltaic arc will 
 pass across the path of an electric spark, though it will not 
 pass through a measurable distance of interposed air without 
 the spark. This theory, however, does not satisfactorily explain 
 the great increase in the charge of the Leyden phial in the 
 above experiments, as compared with the charge by contact. 
 
 3rd. It must be borne in mjnd that each coating of the 
 Leyden phial must be connected with each terminal ; the jar 
 is not, as many have tried the experiment, to be interposed in 
 the secondary circuit. 
 
400 EXPERIMENTAL INVESTIGATIONS. 
 
 The number of discharges in a given time will depend upon 
 the intensity of the battery, and its relation to the amount 
 of coated surface ; the eye cannot estimate this, but a rough 
 measurement of the rapidity of succession may be made in 
 the following way : Move across the line of discharge with a 
 steady hand a strip of writing-paper : it will be punctured with 
 a row of holes, which will be the more closely approximated 
 in proportion as the succession of discharges is more rapid. 
 By a disc of paper attached to an axis moving with a given 
 rate of revolution this measurement may be made very accu- 
 rate and useful. 
 
 Those who possess the coil apparatus will find it very con- 
 venient to have a plate of glass, coated on each side with tin- 
 foil, placed in the base of the machine, and having strips 
 leading from each coating to binding-screws, with which the 
 terminals of the secondary coil can be connected at will. 
 
 EXPERIMENTS ON THE APPARENT CONVERSION OF 
 ELECTRICITY INTO MECHANICAL FORCE. 
 
 Phil. Mag., January 1856. 
 
 IN a communication made to the members of the Royal 
 Institution, on Friday, January 25, entitled ' Inferences from 
 the Negation of Perpetual Motion,' I showed an experiment 
 which, with one or two others now added, may be thought 
 worth recording in the * Philosophical Magazine.' They are 
 hardly, I think, likely to be deduced from our received elec- 
 trical theories, though possibly not inconsistent with some of 
 them. 
 
 My object was to show that when electricity performs any 
 mechanical work which does not return to its source, electrical 
 power is lost, The first experiment was made in the following 
 manner : A Leyden jar of one square foot coated surface has 
 its interior connected with a Cuthbertson's electrometer, be- 
 tween which and the outer coating of the jar are a pair of dis- 
 charging balls fixed at a certain distance (about -J an inch 
 apart). Between the Leyden jar and the prime conductor is 
 
ELECTRICITY AND MECHANICAL FORCE. 401 
 
 inserted a small unit-jar of 9 square inches surface, the knobs 
 of which are 0*2 inch apart. 
 
 The balance of the electrometer is now fixed by a stiff wire 
 inserted between the attracting knobs, and the Leyden jar 
 charged by discharges from the unit-jar. After a certain 
 number of these (twenty-two in the experiment performed in 
 the theatre of the Institution), the discharge of the large jar 
 takes place across the ^-inch interval ; this may be viewed as 
 the expression of electrical power received from the unit-jar. 
 The experiment is now repeated, the wire between the balls 
 having been removed, and therefore the ' tip ' or the raising of 
 the weight is performed by the electrical repulsion and attrac- 
 tion of the two pairs of balls ; at twenty-two discharges of 
 the unit-jar the balance is subverted, and one knob drops 
 upon the other, but no discharge takes place, showing that some 
 electricity has been lost, or converted into the mechanical 
 power which raises the balance. By another mode of expres- 
 sion the electricity may be supposed to be masked or ana- 
 logous to latent heat, and would be restored if the ball were 
 brought back, without discharge, by extraneous force. 
 
 The experiment I believe to be new, and to be suggestive 
 of others of a similar character, which may be indefinitely 
 varied. Thus, two balls made to diverge by electricity should 
 not give to an electrometer the same amount of electricity as if 
 they were, whilst electrified, kept forcibly together. An experi- 
 ment of this sort I have made since my lecture in the following 
 manner: To a thick brass wire, two feet long, insulated and 
 terminated by knobs, are suspended, by fine platina wires, 
 two pairs of discs of paper coated with tinfoil, and four inches 
 in diameter. The apparatus is electrised in a dry atmosphere 
 by sparks from a machine, and the discs of each pair respect- 
 ively diverge. To one of the pairs a silk thread is attached, 
 by which the discs can be forcibly approximated, and as rfften 
 as this is done the divergence of the other pair increases. 
 
 Another mode of showing the same effect is the following : 
 On the top of an ordinary gold-leaf electroscope place two 
 brass plates, such as those commonly used for a condenser ; 
 connect them by a long fine wire, and electrify them by a 
 rubbed rod of glass or sealing-wax, so that the gold-leaves 
 
 D D 
 
402 EXPERIMENTAL INVESTIGATIONS. 
 
 diverge. Now raise the upper plate by a glass handle : the 
 leaves collapse in proportion as it is raised and again diverge 
 as it is depressed. It should be recollected that the plates 
 are electrified by the same electricity, and are always metalli- 
 cally connected by the fine wire, in which respect this differs 
 from ordinary induction experiments. It may be said that 
 here the mechanical force is given by the hand ; but this is 
 only in part : the repellent effect of electricity does part of the 
 work, and would be therefore expended ; it is analogically as 
 though a man were to add his force to the piston-rod of a 
 steam-engine, which would not prevent the loss of heat by the 
 dilating steam. I had hoped to have carried the experiments 
 farther and examined the relative quantities, but unfortunately 
 I have no time for such enquiry, and must leave it to others 
 who have more leisure. 
 
 NEW METHODS OF PRODUCING AND FIXING 
 ELECTRICAL FIGURES. 
 
 Phil. Mag., January 1857. 
 
 A CLASS of figures produced on polished surfaces by elec- 
 trising a metallic bas-relief, such as a coin or medal placed 
 on glass, mica, or polished metal, was made known by M. 
 Karsten,* who refers in his memoir to the previous results of 
 Moser and Riess, the latter having given the name of roric 
 figures to those produced by electrical discharges, on account 
 of their becoming visible, as Karsten's did, when breathed on. 
 M. Karsten states that he had but imperfectly succeeded in 
 fixing these figures by exposing them to the vapours of iodine 
 or mercury, f and that when an insulating substance was inter- 
 posed between the object and the recipient plate the figures 
 were not formed. J 
 
 This class of experiments possesses much interest, as 
 showing the molecular changes accompanying electrical phe- 
 
 * Archives de VElectridte, vol. ii. p. 647 ; iii. p. 310 ; iv. p. 457. 
 f Ibid.) vol. ii. p. 651. 
 \ Ibid.) vol. iv, p. 464. 
 
FIXING ELECTRICAL FIGURES. 4 3 
 
 nomena ; and believing, as I have for many years, that electri- 
 city is nothing else but motion or change in matter, or a force 
 and not a fluid, I have recently made some experiments to 
 ascertain whether similar effects took place in cases where 
 electrical light is visible upon insulated surfaces only, a great 
 number of experiments having already shown that the par- 
 ticles of metals or conducting bodies are projected when the 
 electrical spark proceeds from them. 
 
 M. Du Moncel has shown that when two plates of glass, 
 coated respectively on their exteriors with metallic plates, are 
 kept separate and electrised, a brilliant electrical light is seen 
 between the plates.* I thought I might render evident the 
 molecular change which I believed to be taking place on the 
 opposed surfaces of glass in such cases, and the following 
 experiments, selected from many others, will, I think, prove 
 that this is the fact : 
 
 1. Two plates of window-glass, 3 by 3^ inches, were im- 
 mersed in nitric acid, then washed, and dried by a clean silk- 
 handkerchief until their surfaces gave a uniform flush when 
 breathed on. Between these plates was then placed a piece 
 of hand-bill printed on one side only ; pieces of tinfoil rather 
 smaller than the glasses were placed on the outside of each, 
 and these coatings were connected with the secondary termi- 
 nals of a Ruhmkorff coil After a few minutes' electrisation 
 the coatings were carefully removed, and the interior surface 
 of the glass, when breathed on, showed with great beauty the 
 printed words which had been opposite it, these appearing 
 as though etched on the glass, or having a frosted ap- 
 pearance; even the fibres of the paper were beautifully 
 brought out by the breath, but nothing beyond the margin 
 of the tinfoil. 
 
 2. It now occurred to me that I might render these im- 
 pressions permanent by the use of hydrofluoric acid, and a 
 similar experiment was made, the naked plate of glass after 
 electrisation being exposed over a leaden dish containing 
 powdered fluor spar and sulphuric acid, and slightly warmed ; 
 the letters came out rather imperfectly, but some creases in 
 the paper were beautifully reproduced. 
 
 * Notices sur Vappareil de Ruhmkorff^ p. 46. 
 DD2 
 
404 EXPERIMENTAL INVESTIGATIONS. 
 
 3. I now cut out of thin white letter-paper the word ' Volta,' 
 and placed it between the plates of glass ; they were submitted 
 to electrisation as before, and the interior surface of one of 
 them, without the paper letters, was subsequently exposed to 
 the hydrofluoric acid vapour ; the previously invisible figures 
 came out perfectly, and formed a permanent and perfectly 
 accurate etching of the word * Volta,' as complete as if it had 
 been done in the usual mode by an etching-ground. This, 
 of course, could be washed and rubbed to any extent without 
 alteration, and the results I have obtained give every promise 
 for those who may pursue this as an art of producing very 
 beautiful effects, enabling silhouette designs, or even fine 
 engravings, to be copied on glass, &c. 
 
 4. I again electrised a plate in the same manner, and then 
 covered the surface having the invisible image, with iodised 
 collodion, and immersed it in a bath of nitrate of silver (40 
 grains to the ounce) in a room lighted by a candle, in the 
 usual mariner as for a photograph. It was then held opposite 
 a window for a few seconds, and taken back into the darkened 
 room, and on pouring over it a solution of pyrogallic acid the 
 word ' Volta/ and the border of the glass beyond the limits of 
 the tinfoil, were darkened, and came out with perfect distinct- 
 ness, the other parts of the glass having been, as it were, pro- 
 tected by electrisation from the action of light ; the figures 
 were permanently fixed by a strong solution of hyposulphite 
 of soda. 
 
 5. A similar experiment to the last was made, but after 
 fixing the impression the collodion film was floated off; this 
 contained the impression, as it does with an ordinary photo- 
 graph ; and the glass plates being washed with distilled water 
 and dried, showed no impression when breathed on. 
 
 6. An electric impression of the word ' Volta ' was well 
 rubbed with a handkerchief, then washed with water and 
 alcohol, then dried ; the impression still came out by breathing 
 upon it. Some one of the chemicals used in the collodion 
 process had probably had the effect of removing the figure in 
 Experiment 5, but I have not yet ascertained to which this 
 removal is due. 
 
 7. Letters cut in tinfoil were substituted for those of 
 
FIXING ELECTRICAL FIGURES. 405 
 
 paper ; the effect was the same, but it seemed to me more 
 feeble. 
 
 8. A solution of nitrate of silver was poured over the sur- 
 face of an electrised plate, so as to form a bath on its surface ; 
 a rake, formed of ten common pins, was made to touch the 
 glass with its points along the course of the invisible image ; 
 the silver was of course precipitated in an arborescent form, 
 and I thought it probable that the lines of deposition might 
 follow the course of the invisible image, but I could not be 
 certain of any such effect, though in one experiment there 
 seemed to be some slight indication of it. 
 
 The above experiments were repeated many times, both 
 with positive and negative electricity from the coil. At first 
 I believed I had found a remarkable difference in the effect of 
 the change of direction of the discharge in the cases where 
 hydrofluoric acid was employed ; and in two experiments, in 
 which the tinfoil of the upper glass plate was connected with 
 the positive terminal, the impression of the word on the lower 
 side of the upper plate after exposure to the vapour was 
 polished, while the residue was frosted ; and in two subsequent 
 cases, the upper tinfoil being connected with the negative 
 terminal, the reverse was the case. In subsequent experi- 
 ments, however, great irregularities took place in this relation, 
 and it seemed to depend on the time of exposure and on 
 slight differences in the distance between the paper letters 
 and the glass, the latter not being ^brought into perfectly 
 uniform contact with the surfaces of the glass. 
 
 After the first few experiments I placed a marble paper- 
 weight on the upper glass, and found the effects more uniform 
 and perfect. 
 
 An electrisation for periods of from five to ten minutes 
 produced the sharpest and clearest effects ; when the electri- 
 sation was prolonged, a blur or second margin gradually 
 appeared, and increased in extent around the outline of the 
 letters, having somewhat the appearance which would have 
 been presented had the paper letters been moist, and the 
 liquid slightly extended itself from their edges over the glass. 
 
 When the electrisation was thus prolonged the figures 
 were visible on inclining the glass to the light, without 
 
4 o6 EXPERIMENTAL INVESTIGATIONS. 
 
 breathing on them, and gave a strong impression of the glass 
 having suffered a superficial disintegration or decomposition, 
 but I could not by the microscope or by polarised light 
 detect a difference in its structure. I hope, however, to do 
 so by a more prolonged and varied examination. 
 
 It may be well to state that counter-experiments were 
 made, such as allowing the plates to remain with the letters 
 between them, but without electrisation, in which case no 
 effect was produced ; but it is probable, by analogy with 
 Moser's class of phenomena, that a very long period of con- 
 tact would produce some effect. 
 
 ON THE INFLUENCE OF LIGHT ON THE POLARISED 
 ELECTRODE.* 
 
 Phil. Mag., December 1858. 
 
 SOON after the experiments of Daguerre were published it 
 occurred to me that the galvanometer might be used as a test 
 for the chemical effects of light ; and I succeeded in obtaining 
 a deflection of the needle by allowing a beam of light sud- 
 denly to impinge on a daguerreotype plate in a trough of 
 water the plate being connected with one extremity of a 
 galvanometer, and a gridiron of silver wire placed in front of 
 the plate with the other. This experiment I showed at a 
 lecture at the London Institution in 1843 J an ^ it was subse- 
 quently used as an illustration of the convertibility of force, in 
 my essay on the ' Correlation of Physical Forces/ 
 
 I tried some farther experiments at the time, without ob- 
 taining results of any importance, but, as galvanometers at 
 that period had not reached the degree of delicacy they have 
 since attained in the hands of M. Ruhmkorff, I determined 
 this summer to resume the enquiry ; and the results which I 
 have obtained I now proceed to describe. The galvanometer 
 used in the following experiments is by Ruhmkorff, formed 
 of 544 feet of fine copper wire, and though not as delicate as 
 
 * Read at the meeting of the British Association, Leeds, Sept. 1858. 
 
INFLUENCE OF LIGHT ON ELECTRODES. 407 
 
 the very long-wired instruments used by M. DuBois Reymond 
 and others, it has proved sufficiently delicate for most of the 
 effects I aimed at. 
 
 The idea with which I started was to arrange two plates 
 of platinum in an electrolyte in such manner that a bright 
 beam of light should impinge on one while the other was in 
 darkness, and yet to allow free electrolytic communication. 
 After making a somewhat complex apparatus, which did not 
 answer the purpose, the following simple means of effecting 
 my object was adopted : In a cell similar to those used for 
 the nitric acid battery the outer cell being of thin glass and 
 the inner one of porous ware, two platinum plates were placed, 
 each of six inches by two four inches by two, or the immersed 
 portions of the plates, being platinised or coated with a de- 
 posit of black platinum. Both the outer and inner cell were 
 filled with distilled water slightly acidulated with sulphuric 
 acid ; and some tow steeped in the same solution was stuffed 
 into the upper part of the porous cell around the platinum, 
 so that this latter plate was perfectly excluded from light. 
 The extremities of the two plates were metallically connected. 
 A brass cylinder, covered at the top, was placed over the 
 whole, its lower circumference resting on a circular pad of 
 paper, so as to exclude light. 
 
 The apparatus, thus disposed, was set aside for ten days, 
 so as to allow the local currents to subside. At the expiration 
 of this period the apparatus was taken into bright sunlight, the 
 position of the plates so arranged that the one in the outer or 
 glass cell should be opposite the sun, the terminals connected 
 with the galvanometer, and the temporary deflection occasioned 
 by polarisation allowed to subside, or rather to reach a fixed 
 point, for there was always a slight deflection. 
 
 The brass cylinder which excluded light from the ap- 
 paratus was now removed ; and the galvanometer needle 
 instantly deviated to 10, the platinum exposed to sunlight 
 being positive to that in the dark, or as zinc to copper. The 
 platinum plates were now reversed, that which had been in 
 the outer cell placed in the porous cell, and vice versa, and the 
 apparatus again set aside for ten days ; at the end of this period 
 it was again taken out, the experiment repeated, and the same 
 
408 EXPERIMENTAL INVESTIGATIONS. 
 
 result obtained ; i.e. on removing the brass cylinder there was 
 a deflection of 12, the platinum exposed to light being posi- 
 tive to the sheltered one. 
 
 This identity of electrical effect taking place with the re- 
 versed plates seemed so strongly in favour of the impact of the 
 solar rays having an initiatory effect in producing a voltaic 
 current, that the only remaining point seemed to be to ascer- 
 tain whether it was due to light or heat, to the chemical or 
 calorific rays of the sun ; yet the conclusion I then came to 
 was erroneous, as will presently be seen. 
 
 In order to ascertain how far the effect was due to heat, I 
 arranged, in a room lighted by a small candle, the same appa- 
 ratus over a fire of asbetos heated by coal-gas, so that both 
 radiant heat and an ascending current of hot air impinged on 
 the side of the glass in which was the exposed platinum, while 
 the opposite side was entirely sheltered from the heat by a 
 metallic shelf, on which the cell rested ; this experiment was 
 continued until one side of the cell was uncomfortably hot to 
 the hand, while the other side was quite cool ; but not the 
 slightest deviation of the galvanometer took place. 
 
 I now repeated the former experiment with sunlight, 
 changing the liquid each time. In three successive experi- 
 ments the deflections on the impact of light were in the same 
 direction, the exposed platinum being positive ; but in a fourth 
 the deflection was in the reverse direction, the exposed platinum 
 being negative. In several subsequent experiments there was 
 always a notable deflection which ensued on the impact of 
 light ; but it was sometimes in one direction and sometimes 
 in the other. I ultimately discovered that, in the deflection 
 produced by light, the needle of the galvanometer deviated 
 in the same direction which it took upon the first contact of 
 the wires connected with the platinum plates. The effect of 
 light was therefore to increase the deflection occasioned by 
 the polarisation of the platinum plates ; and this my subse- 
 quent experiments have, I think, fully established. Although 
 the experiment on the impact of heat seemed to show that the 
 heating effect of the solar rays was not the cause of the 
 phenomena, yet it might well be that the solar rays absorbed 
 by the platinum black, would produce a greater heating effect 
 
INFLUENCE OF LIGHT ON ELECTRODES. 409 
 
 at the actual point of contact of the platinum and liquid than 
 any non-luminous heat would produce ; and I was therefore 
 anxious to ascertain whether the different coloured rays of 
 light showed any difference in their effects. To this end I 
 procured three plates of coloured glass, one blue, the second 
 yellow, and the third red ; a strip of thick brown paper was 
 pasted to the opposite sides of each of these plates of glass, so 
 as to form a nearly cylindrical chamber cut by the plane of the 
 glass. A cover was placed over each of them ; and the cham- 
 bers so formed could be placed over the cell containing the 
 platinum plates, the coloured glass plates intervening between 
 the sun and the platinum in the outer cell. A great number 
 of experiments were made with these apparatus ; and in all 
 the deviations of the galvanometer were notably greater with 
 the blue glass than with the yellow or red, and, of the latter 
 two, the yellow gave slightly greater deflections than the red 
 glass. 
 
 This result is, I think, conclusive in favour of the effect 
 being due to the chemical, not to the calorific rays of the sun, 
 the more so when we consider that the yellow allowed a far 
 larger quantity of light to pass than the blue glass. I may 
 also add that I have obtained a slight galvanometric deflec- 
 tion when diffused daylight was allowed to impinge on the 
 platinum plate, and when there was no perceptible difference 
 of temperature between the illuminated and the non-illumi- 
 nated plates. 
 
 The superiority of the yellow over the red was not so 
 strongly marked; and, considering that the yellow glass 
 allowed much more light to pass than the red, I am not dis- 
 posed to think that there was any actual superiority in the 
 former ; the effects observed with these two colours are, how- 
 ever, corroborative of the effects not being due to the red or 
 heating rays of the sun. 
 
 I substituted for the water acidulated with sulphuric acid 
 (which may be regarded electrically as pure water with its con- 
 ducting power improved), muriatic and nitric acids; the effects 
 were the same, but less marked with the nitric acid, probably 
 from its more completely depolarising the plates. 
 
 In a small number of experiments the following effect took 
 
410 EXPERIMENTAL INVESTIGATIONS. 
 
 place : After a certain time of connexion with the galvano- 
 meter, the cover being over the apparatus, the sign of polari- 
 sation changed ; i.e. supposing the needle of the galvanometer 
 to have deviated to the left, and indicated that the exposed 
 platinum was positive, the needle would gradually return, pass 
 the zero-point, and be deviated to the right ; when this was the 
 case, on removing the cover, the effect occasioned by the impact 
 of light was a return of the needle towards the zero-point, indi- 
 cating an influence in the direction of the original polarisation. 
 On setting aside the apparatus for twenty-four hours with the 
 plates in metallic connection, and then repeating the experi- 
 ment, the deviation of the galvanometer was in the direction 
 of the final polarisation. This apparent anomaly may have 
 arisen from a conflict of two classes of currents, the one arising 
 from imperfect mixing or want of homogeneity in the liquid, 
 and the other from the state of surface of the platinum ; 
 the latter would most probably be the current affected by light. 
 
 As the general effect of light was to increase the deflections 
 occasioned by polarisation, whatever direction this assumed, 
 it seemed probable that the exclusion of one of the plates from 
 the light, which I had commenced with in the hope of obtain- 
 ing currents initiated by light, was unnecessary, and that the 
 observed effects would be rather increased by exposing both 
 plates to light. I therefore arranged two platinised plates in 
 a cell without a porous diaphragm, inclined to each other like 
 the letter V, but without contact, and allowed the light to im- 
 pinge on the interior surfaces, or those opposed to each other ; 
 but, to my surprise, the effect was very trifling, the needle de- 
 viating only one or two degrees, and that in a sluggish and 
 irregular manner. 
 
 When, however, the two plates were arranged parallel, the 
 one shading the other from the ligh 1 -, as good deflections were 
 produced as with the porous cell the more so if the back or 
 shaded plate was of polished and the front plate of platinised 
 platinum. 
 
 Why light should produce a greater augmentation of the 
 current when impinging on one than on the two plates I can- 
 not explain on the theory of polarized plates, and will leave 
 for farther experiment. 
 
INFLUENCE OF LIGHT ON ELECTRODES. 411 
 
 In all the experiments I have made on the subject of this 
 paper the most marked effect upon the galvanometer is pro- 
 duced when the polarisation causes a small permanent deflec- 
 tion of from 5 to 10 ; when the polarisation of the plates is 
 extremely slight, the effect of light is very feeble ; and when 
 the polarisation is considerable, so as to deflect the galvano- 
 meter to 20 or 30, the increased force required to produce a 
 small increase of deflection is too great to afford notable 
 results. 
 
 I have used the term polarisation, having no better word 
 to indicate the feeble currents which are always observed 
 when two platinum plates immersed in a liquid are connected 
 with a delicate galvanometer. The electrical currents which 
 would ensue if the plates were polarised by connecting them 
 with a voltaic battery and then detaching them, would be far 
 too powerful for the delicate indications which I have been 
 examining. There can be no doubt, at least to those who 
 adopt the chemical theory of the voltaic pile, that both these 
 classes of polarisation are due, when one homogeneous liquid 
 is employed, to slight deposits on the plates, either of films of 
 gas or of some substance which acts chemically on the liquids, 
 and the effect of light would seem simply to be an augmen- 
 tation of the chemical action taking place at the surface of the 
 electrodes, which is the locus where the chemical changes pro- 
 ducing or produced by voltaic currents are always observable. 
 
 With more sensitive galvanometers, and with a greater 
 variety of solutions, this class of experiments may, I venture 
 to hope, be found important in farther investigating the effects 
 of light on chemical actions ; and the pure coloured rays of 
 the spectrum may be employed. 
 
 There can, I think, from analogy, be little doubt that light 
 would influence those actions of surface which are com- 
 prehended among the various effects to which the term cata- 
 lysis is applied. In an experiment I made in the month of 
 September 1851 two similar glass tubes, containing each 15 
 grains of water, were placed, the one under an opaque porcelain, 
 and the other under a glass vessel of the same size, with cap- 
 sules of sulphuric acid by their sides ; I found that evaporation 
 took place much more rapidly in the one exposed to light, 
 
4i2 EXPERIMENTAL INVESTIGATIONS. 
 
 though it was in a room with a northern aspect, on which the 
 sun never shone. In twelve days the water under the glass 
 vessel had lost 6*3 grains, that under the porcelain 5*4, show- 
 ing a difference of nearly a sixth part in the evaporation in 
 favour of the tube exposed to light. I mention this experi- 
 ment here as showing a probability that the liberation of 
 vapour or gas may be accelerated by light, as M. Donny's 
 remarkable experiments seem to show that evaporation is a 
 surface action ; and the effect of light on polarised plates may 
 be somewhat of the same nature. I will not, however, enter 
 into theories, which must be necessarily vague, on such a novel 
 subject, but conclude by giving a table of some of the most 
 trustworthy results which I have obtained when the sunlight 
 has been most steady, in order to show the extent of deflec- 
 tions and differences with coloured light : they were all made 
 between the hours of 10 a.m. and 12, and on the finest days I 
 could select. My first experiments were made in London, in 
 the months of June and July. I was then absent for some 
 time on circuit ; and I have resumed and continued them 
 during my vacation in the months of August and September. 
 
 June 21, apparatus having been arranged on June 12. 
 Solution, water with a few drops of sulphuric acid. 
 
 Deflection by sunlight 8, i.e. from 12 the deflection by 
 polarisation, to 20. 
 
 Exposed platinum positive. 
 
 July 4. Same solution, platinum having been changed on 
 the 24th. 
 
 Deflection by sunlight of 9, i.e. from i to 10. 
 Exposed platinum positive. 
 
 August 24. Solution, water with sulphuric acid. 
 
 Deflection by polarisation . i 
 
 Sunlight .*..... 5 
 
 Blue glass 3 
 
 Yellow glass ...... 2 
 
 Red glass 2 
 
 Exposed platinum positive. 
 
INFLUENCE OF LIGHT ON ELECTRODES. 413 
 
 August 25. Solution, water with sulphuric acid. 
 
 o 
 
 Deflection by polarisation ... .2 
 
 Sunlight . 7 
 
 Blue glass . . . . . 4'5 
 
 Yellow glass 3 
 
 Red glass 3 
 
 Exposed platinum positive.* 
 
 September 4. Very dilute muriatic acid. 
 
 o 
 
 Deflection by polarisation .... 6 
 
 Sunlight 14 
 
 Blue glass ....... 9 
 
 Yellow glass ...... 7*5 
 
 Red glass . 7'5 
 
 Diffuse daylight 8 
 
 Exposed platinum negative. 
 September 6. Very dilute nitric acid. 
 
 Deflection by polarisation . i 
 
 Sunlight 7 
 
 Blue glass 3 
 
 Yellow glass ...... 2 
 
 Red glass i 
 
 Exposed platinum negative. 
 September 8. Dilute nitric acid. 
 
 o 
 
 Deflection by polarisation .... 3 
 
 Sunlight ....... 6 
 
 Exposed platinum negative. 
 
 * In a second cell with the same solution the exposed plate was negative, and 
 the deflection increased by sunlight in the same direction as the polarisation, from 
 2 to 10. 
 
4H EXPERIMENTAL INVESTIGATIONS. 
 
 Dilute muriatic acid. 
 
 o 
 
 Deflection by polarisation . ... 4 
 
 Sunlight . . . . . . .10 
 
 Blue glass ....... 7 
 
 Yellow glass ...... 6 
 
 Red glass 5'5 
 
 Exposed platinum negative. 
 
 September 10. Dilute nitric acid. 
 
 o 
 
 Deflection by polarisation i 
 
 Sunlight ....... 5 
 
 Exposed platinum positive. 
 
 Dilute muriatic acid. Polished platinum plates. 
 
 o 
 
 Deflection by polarisation . . i 
 
 Sunlight ....... 2 
 
 Exposed platinum negative. 
 
 September 14. Two sets, one of polished, the other of 
 platinised platinum plates, in dilute sulphuric acid, without 
 porous diaphragm. 
 
 The polished plates gave : 
 
 o 
 
 Deflection by polarisation ... 2 
 With sunlight 3 
 
 Platinised plates gave : 
 
 o 
 
 By polarisation ..... 3 
 With sunlight 4 
 
 The pair with black platinum was then tried with a porous 
 cell ; and as it was not set aside for more than a few minutes, 
 to allow the polarisation to subside, the 
 
 O 
 
 Deflection by polarisation was , . 15 
 Increased by sunlight to ... 20 
 
 Exposed platinum positive. 
 
ELECTROLYSIS ACROSS GLASS. 4*5 
 
 This being covered, but kept in connection with the gal- 
 vanometer, the polarisation gradually diminished, passed the 
 zero-point, and took up an opposite deflection of 7'5 > the 
 outer platinum being now negative, the effect of sunlight was 
 now to reduce the 
 
 Deflection to 3 
 
 Red glass to ...... 6*5 
 
 Yellow glass to 6*5 
 
 Blue glass to 5'5 
 
 On September 16, this same cell having been set aside in 
 the interim, with the plates metallically connected, the deflec- 
 tion by polarisation was increased by sunlight, the exposed 
 platinum being negative. 
 
 September 1 5. Black and polished platinum plates in glass 
 cell without porous cell, the black platinum being towards the 
 light, and the plain platinum shaded by it ; the black platinum 
 was positive to the plain : 
 
 o 
 
 Deflection by polarisation . . . 10 
 
 Sunlight 1 8 
 
 Blue glass . . . . . . 1 2 
 
 Yellow glass n 
 
 After half an hour in connexion with a galvanometer : 
 
 o 
 
 Deflection by polarisation had sunk to . 4 
 
 Sunlight . . . . . . 12 
 
 Blue glass . . . - . . . 6 
 
 Yellow glass ...... 4 
 
 Red glass ...... 4 
 
 ON THE TRANSMISSION OF ELECTROLYSIS ACROSS 
 
 GLASS. 
 
 Phil, Mag., Aug. 1 860. Read at the Chemical Section of the British 
 Association, June 28, 1860. 
 
 IF glass or an equally non-conducting substance be interposed 
 between electrodes in an electrolyte, so that there be no liquid 
 
416 EXPERIMENTAL INVESTIGATIONS. 
 
 communication around the edges, it is hardly necessary to 
 say that, according to received opinions and experiments, no 
 current passes and no electrical effects appear. I was led 
 by some theoretic considerations to thiiik that this rule might 
 not be without an exception, and the following experiment 
 realised my views : 
 
 A Florence flask, well cleaned and dried, was filled two- 
 thirds full of distilled water, with a few drops of sulphuric acid 
 added to it, and placed in an outer vessel containing similar 
 acidulated water, which reached to the same height as the 
 liquid in the interior. A platinum wire was passed through a 
 glass tube, one end of which was hermetically sealed to the 
 platinum, but so as to allow a small portion of the wire (about 
 ^th of an inch) to project ; the other end of the wire was 
 passed through a cork, and the cork fitted to the mouth of 
 the flask ; when the cork was introduced the projecting end 
 of the platinum wire was three-quarters of an inch below the 
 surface of the interior liquid ; a similar coated wire was dipped 
 into the outer liquid, and this and the wire which passed 
 through the cork were brought respectively into connection 
 with the extremities of the secondary coil of a Ruhmkorff 
 apparatus. Upon the latter being excited by the battery a 
 stream of minute bubbles arose from both the platinum 
 points, proving clearly that electrolysis took place, notwith- 
 standing the interposition of the glass. The portions of the 
 flask above the liquid, both outside and inside, were perfectly 
 dry, so that there could have been no communication of the 
 current over the surface of the glass. This was farther proved 
 by removing the outer wire a short distance from the surface 
 of the water, when sparks passed nearly equal in length to 
 those which took place between wires from the terminals. As 
 the outer wire was farther removed, keeping it near the glass, 
 the sparks passed along the surface of the latter for a short 
 distance ; and as it was farther removed they ceased, thus 
 showing conclusively that there was no passage of electricity 
 over the upper and unwetted surface of the glass. 
 
 With distilled water unacidulated I could observe no effect 
 of electrolysis. 
 
 With acidulated water and the same arrangement I could 
 
ELECTROLYSIS ACROSS GLASS. 4*7 
 
 detect no signs of electrolysis, when instead of the Ruhmkorff 
 coil a nitric acid battery of thirty cells was employed. 
 
 In the first experiment the evolution of gas gradually 
 diminished, and ceased after about twenty minutes' experi- 
 ment ; but upon intercepting communication with the battery 
 for ten minutes, and then re-connecting it, the evolution took 
 place again ; or a recurrence of electrolysis could be produced 
 by reversing the direction of the current. 
 
 When the flask, after twenty minutes' experiment, was 
 removed from the outer vessel and tapped, minute bubbles 
 rose from the interior surface. When a tolerably thick test- 
 tube was used instead of the Florence flask, a very slight effect 
 of electrolysis could be detected ; and when the outer wire 
 was removed to a short distance from the surface, sparks 
 passed, but not of half the length of those with the Florence 
 flask. When, however, a large phial of somewhat greater 
 contents than the Florence flask was used, the effects were 
 the same as with the latter, showing, as I expected, that 
 surface is an important element in the success of the expe- 
 riment. 
 
 There seems little doubt from the above experiments that 
 the electrolysis was effected by induction across the thin glass 
 of the Florence flask ; and its cessation after a time and recur- 
 rence after interruption of the current would seem to indicate 
 something like a state of charge or of polarisation of the sur- 
 face of the glass. 
 
 Whether the bubbles which arose from the interior surface 
 of the glass were the effect of electrolytic action or mere air- 
 bubbles, cannot be affirmed with certainty ; but as there was 
 distinct evolution from the platinum wires, the corresponding 
 elements must have been either dissolved, evolved, or depo- 
 sited somewhere, and the most probable place of evolution 
 would be the surface of the glass. If this be so, the glass 
 would act in effect just as an interposed plate*of inoxidable 
 metal, though the one acts by induction, the other by con- 
 duction. 
 
 The oxygen and hydrogen may, however, be spread over 
 the surface of the glass without evolution in the form of gas ; 
 
 E E 
 
4i8 EXPERIMENTAL INVESTIGATIONS. 
 
 and when the glass is, so to speak, coated by the elements, 
 decomposition would cease. 
 
 The quantity of gas in the above experiment was too small 
 for analysis ; and in all probability, could it have been 
 examined, some mixed gas would have been found to have 
 been eliminated from each electrode. When, however, there 
 is a small interruption in the secondary circuit of RuhmkorfFs 
 coil, producing a rapid succession of sparks, I have found that 
 between electrodes in the same circuit true polar decompo- 
 sition takes place, and a galvanometer is steadily deflected 
 according to the direction of the current, while without such 
 interruption the movements of its needle are most irregular. 
 I therefore repeated the experiment of interposed glass with 
 an interruption in the circuit, and found that electrolysis took 
 place apparently as before ; and in this case there was pro- 
 bably true polar decomposition. 
 
 Although I failed with thirty cells of the nitric acid bat- 
 tery, I should expect that a battery of very high intensity, 
 such as the 500 cells nitric acid, or the water-battery of Mr. 
 Gassiot, would produce effects of electrolysis across glass 
 without the use of the coil. 
 
 AN EXPERIMENT IN MAGNETO-ELECTRIC 
 INDUCTION. 
 
 Philosophical Magazine, March 1868. 
 
 SHORTLY after the publication of Mr. Wilde's experiments on 
 magneto-electric induction, it occurred to me that some of the 
 ordinary effects of the Ruhmkorff coil might be produced by 
 applying to it a magneto-electric machine. I tried an ordi- 
 nary medical machine with a small coil, made by Mr. Apps, of 
 3^ inches length by 2 inches diameter, and having about f of 
 a mile of fine secondary wire. 
 
 The result was very unexpected. The terminals of the 
 magneto-electric coils being connected with the primary coil 
 of the Ruhmkorff, and the contact-breaker being kept closed, 
 
EFFECTS OF HEAT ON FLUIDS. 419 
 
 so as to make a completed circuit of the primary wire (a 
 condition which would have appeared a priori essential to 
 success), no effect was produced ; while if the circuit was in- 
 terrupted by keeping the contact-breaker open, sparks of 0*3 
 of an inch passed between the terminals of the secondary 
 coil of the Ruhmkorff, and vacuum-tubes were readily illumi- 
 nated. Here there was in effect no primary coil, no metallic 
 connexion for the primary current ; and yet a notable effect 
 was produced. 
 
 I did not at the time publish this experiment farther than 
 by communicating it to a few friends, hoping to be able to 
 find a satisfactory explanation of it. All I have observed 
 since is that the effect is dependent upon the condenser, for 
 when that is removed no result is produced. 
 
 It would appear, then, to depend on an electrical wave or 
 impulse, shot, so to speak, into the uncompleted primary coil, 
 similar to the wave which will deflect in succession magnetic 
 needles placed at different distances on a telegraphic cable, 
 without the current passing through the whole length of wire, 
 as shown in the experiments of Mr. Latimer Clark and others. 
 But why there should be no effect, or an inappreciable one, 
 when the primary circuit is completed, the current being alter- 
 nated by the rotation of the coils of the magneto-electric 
 machine, I cannot satisfactorily explain. 
 
 ON SOME EFFECTS OF HEAT ON FLUIDS. 
 
 Journal of the Chemical Society -, May 21, 1863. 
 
 HAVING been honoured by an invitation from the Council of 
 the Chemical Society to give a lecture on one of the evening 
 meetings of the Society, it was not without hesitation that I 
 accepted the task. My professional occupation having of late 
 years so much prevented any continuous devotion of mind to 
 scientific investigation, it was only by reverting to researches 
 made several years ago that there was a chance of my com- 
 municating anything worthy the attention of the Society. I 
 
 E E 2 
 
420 EXPERIMENTAL INVESTIGATIONS. 
 
 had, however, at one period made a rather careful investiga- 
 tion into some phenomena, suggested partly by a paper of 
 M. Donny's, published in 1843, and partly by my own experi- 
 ments on the decomposition of water by heat ; and although 
 this investigation did not lead to results as important as I at 
 the time anticipated, yet, as they seemed to add something to 
 our stock of knowledge, and may, it is hoped, form a start- 
 ing-point for important discoveries, I have ventured to bring 
 them forward on this occasion. 
 
 The paper of M. Donny (' Memoires de 1' Academic Royale 
 de Bruxelles,' 1843) makes known the fact that in proportion 
 as water is deprived of air, the character of its ebullition 
 changes, becoming more and more abrupt, and boiling like 
 sulphuric acid with soubresauts, and that between each burst 
 of vapour the water reaches a temperature above its boil- 
 ing-point. To effect this, it is necessary that the water be 
 boiled in a tube with a narrow orifice, through which the 
 vapour issues ; if it be boiled in an open vessel, it continually 
 re-absorbs air and boils in the ordinary way. 
 
 In my experiments on the decomposition of water by heat 
 I found that with the oxyhydrogen gas given off from ignited 
 platinum plunged into water, there was always a greater or less 
 quantity of nitrogen mixed ; this I could never entirely get 
 rid of, and I was thus led into a more careful examination of 
 the phenomenon of boiling water, and set before myself this 
 problem what will be the effect of heat on water perfectly 
 deprived of air or gas ? 
 
 Two copper wires were placed parallel to each other 
 through the neck of a Florence flask, so as nearly to touch 
 the bottom ; joining the lower ends of these was a fine 
 platinum wire, ij inch long, and bent horizontally into 
 a curve. Distilled water, which had been well boiled, and 
 cooled under the receiver of an air-pump, was poured into 
 this flask, so as to fill about one-fourth of its capacity. It was 
 then placed under the receiver of an air-pump, and one of the 
 copper wires brought in contact with a metallic plate covering 
 the receiver, the other bent backwards over the neck of the 
 flask, and its end made to rest on the pump plate. By 
 this means, when the terminal wires from a voltaic battery 
 
EFFECTS OF HEAT ON FLUIDS. 421 
 
 were made to touch, respectively the upper and the lower 
 plate, the platinum wire would be heated, and the boiling 
 continued indefinitely in the vacuum of a very excellent air- 
 pump. The effect was very curious : the water did not boil, 
 but at intervals a burst of vapour took place, dashing the 
 water against the sides of the flask, some escaping into the 
 receiver. (There was a projection at the central orifice of 
 the pump plate, to prevent this overflow getting into the 
 exhausting tube.) 
 
 After each sudden burst of vapour the water became per- 
 fectly tranquil, without a symptom of ebullition until the next 
 burst took place. These sudden bursts occurred at measured 
 intervals, so nearly equal in time that, had it not been for the 
 escape from the flask at each burst of a certain portion of 
 water, the apparatus might have served as a timepiece. 
 
 I showed this experiment at one of my lectures at the 
 time, as affording an illustration of the action of the geysers, 
 of which it seemed to me to afford a rational explanation. 
 Supposing a source of heat at a certain depth beneath the 
 earth's surface, and subterranean wells whose only communi- 
 cation with the air was a narrow tube, probably formed by the 
 issue of vapour, the air, if any, dissolved in the water of these 
 wells would be expelled, and the boiling would take place 
 at intervals and by sudden bursts, instead of in the ordinary 
 way. 
 
 This experiment, though instructive, did not definitively 
 answer the question I had proposed, as I could not of course 
 ascertain whether there was some minute residuum of gas 
 which would form the nucleus for each ebullition, and I pro- 
 ceeded with others. A tube of glass, five feet long, and -f^ths 
 inch internal diameter, was bent into a V shape ; into one end 
 a loop of platinum wire was hermetically sealed with great 
 care, and the portion of it in the interior of the tube was pla- 
 tinised. When the tube had been well washed, distilled water, 
 which had been purged of air as before, was poured into it to 
 the depth of eight inehes, and the rest of the tube filled with 
 olive-oil ; when the V was inverted the open end of the tube 
 was placed in a vessel of olive-oil, so that there would be 
 eight inches of water resting on the platinum wire, separated 
 
422 EXPERIMENTAL INVESTIGATIONS. 
 
 from the external air by a column of four feet four inches of 
 oil. The projecting extremities of the platinum wire were 
 now connected with the terminals of a voltaic battery and the 
 water heated by the wire ; some air was freed, and ascended 
 to the level of the tube this was made to escape by carefully 
 inverting the tube, so as not to let the oil mix with the water, 
 and the experiment continued. After a certain time the 
 boiling assumed an uniform character, not by such sudden 
 bursts as in the Florence flask experiment, but with larger 
 and more distinct bursts of ebullition than in its first boiling. 
 
 The object of platinising the wire was to present more 
 points for the ebullition, and to prevent soubresauts as much 
 as possible. 
 
 The experiment was continued for many hours, and, in some 
 repetitions of it, for days. After the boiling had assumed a 
 uniform character the progress of the vapour was carefully 
 watched, and as each burst of vapour condensed in the oil, 
 which was kept cool, it left a minute bead of gas, which 
 ascended through the oil to the bend of the tube : a bubble was 
 gradually formed here which did not seem at all absorbed by 
 the oil. This was analysed by a eudiometer, which I will 
 presently describe, and proved to be nitrogen. The beads of 
 gas, when viewed through a lens and micrometer scale at the 
 same height in the tube, appeared as nearly as may be of the 
 same size. No bubble of vapour was condensed completely, 
 or without leaving this residual bubble. The experiment was 
 frequently repeated, and continued until the water was so 
 nearly boiled away, that the oil, when disturbed by the boil- 
 ing, nearly touched the platinum wire ; here it was necessarily 
 stopped. 
 
 To avoid any question about the boiling being by electri- 
 cal means, similar experiments were made with a tube, without 
 a platinum wire, closed at its extremity, and the boiling was 
 produced by a spirit-lamp. The effects were the same, but the 
 experiment was more difficult and imperfect, as the bursts of 
 vapour were more sudden, and the duration of the intervals 
 more irregular. 
 
 The beads of gas were extremely minute, just visible to the 
 naked eye. I cannot find any record of their exact measure. 
 
"EFFECTS OF HEAT ON FLUIDS, 423 
 
 In these experiments there was no pure boiling of water, 
 i.e. no rupture of cohesion of the molecules of water itself, but 
 the water was boiled, to use M. Donny's expression, by eva- 
 poration against a surface of gas. 
 
 It is hardly conceivable that air could penetrate through 
 such a column of oil, the more so as the oil did not perceptibly 
 absorb the nitrogen freed by the boiling water and resting in 
 the bend of the tube ; but to meet this conjectural difficulty, 
 the following experiment was made : A tube, one foot long and 
 T %ths inch internal diameter, bent into a slight obtuse angle, 
 had a bulb of f inch diameter blown on it at the angle ; this 
 angle was about three inches from one end and nine from 
 the other ; a loop of platinum wire was sealed into the 
 shorter leg, and the whole tube and bulb filled with and 
 immersed into mercury ; water, distilled and purged of air as 
 before, was allowed to fill the short leg, and by carefully ad- 
 justing the inclination, the water could be boiled so as to allow 
 bubbles to ascend into the bulb and displace the mercury. 
 The effect was the same as with the oil experiment, no 
 ebullition without leaving a bead of gas, the gas collected 
 in the bulb, and was cut off by what may be termed a 
 valve of mercury, from the boiling water, then allowed to 
 escape, and so on ; the experiment was continued for many 
 days, and the bubbles analysed from time to time ; they 
 proved, as before, to be nitrogen, and, as before, continued 
 indefinitely. 
 
 A similar experiment was made without the platinum 
 wire, and though, from the greater difficulties, the experiment 
 was not so satisfactory, the result was the same. 
 
 As the mercury of the common barometer will keep air 
 out of its vacuum for years, if not for centuries, there could 
 be no absorption here from the external atmosphere, and I 
 think I am fairly entitled to conclude from the above experi- 
 ments which I believe went far beyond any that have been 
 recorded that no one has yet seen the phenomenon of pure 
 waterboiling, I .^ToTthe disruption of the liquid particles of 
 the" oxyhydrogen compound, so as to produce vapour which 
 will, when condensed, become water, leaving no permanent 
 gas. Possibly, in my experiment of the decomposition of 
 
4 2 4 EXPERIMENTAL INVESTIGATIONS, 
 
 water by ignited platinum, it may be that the sudden appli- 
 cation of intense heat, and in some quantity, so forces asunder 
 the molecules that, not having sufficient nitrogen dissolved to 
 supply them with a nucleus for evaporation, the integral 
 molecules are severed, and decomposition takes place. If 
 this be so, and it seems to me by no means a far-fetched 
 theory, there is probably no such thing as boiling, properly so 
 called, and the effect of heat on liquids in which there is no 
 dissolved gas may be to decompose them. 
 
 Considerations such as these led me to try the effect of 
 boiling on an elementary liquid, and bromine occurred as the 
 most promising one to work upon. As bromine could not be 
 boiled in contact with water, oil, or mercury, the following 
 plan was ultimately devised : A tube, four feet long and T %t ns 
 inch diameter, had a platinum loop sealed into one closed ex- 
 tremity ; bromine was poured into the tube to the height of 
 four inches ; the open end of the tube was then drawn out to 
 a fine point by the blow-pipe, leaving a small orifice ; the bro- 
 mine was then heated by a spirit-lamp ; and when all the air 
 was expelled, and a jet of bromine vapour issued from the 
 point of the tube, it was sealed by the blow-pipe. There was 
 then, when the bromine vapour had condensed, a vacuum in 
 the tube above the bromine. The platinum loop was now 
 heated by a voltaic battery and the bromine boiled ; this was 
 continued for some time, care being taken that the boiling 
 should not be too violent. At the end of a certain period 
 from half-an-hour to an hour the platinum loop gave way, 
 being corroded by the bromine : the quantity of this had 
 slightly decreased. On breaking off, under water, the point 
 of the tube, the water mounted and showed a notable quantity 
 of permanent gas, which on analysis proved to be pure oxygen. 
 As much as a quarter of a cubic inch was collected at one ex- 
 periment. The platinum wire, which had severed at the 
 middle, was covered with a slight black crust, which, suspect- 
 ing it to be carbon, I ignited by a voltaic spark in oxygen in a 
 small tube over lime-water ; it seemed to give a slight opa- 
 lescence to the liquid, but the quantity was so small that the 
 experiment was not to be relied on. No definite change was 
 perceptible in the bromine ; it seemed to be a little darker in 
 
EFFECTS OF HEAT ON FLUIDS. 425 
 
 colour, and had a few black specks floating in it, which I judged 
 to be minute portions of the same crust which had formed on 
 the platinum wire, and which had become detached. 
 
 The experiment was repeated with chloride of iodine, and 
 with the same result, except that the quantity of oxygen was 
 greater ; I collected as much as half a cubic inch in some 
 experiments, from an equal quantity of chloride of iodine ; the 
 platinum wire, however, was more quickly acted on than with 
 the bromine, and also to some extent the glass of the tube 
 around it. 
 
 Melted phosphorus was exposed to the heat of the voltaic 
 disruptive discharge by taking this between platinum points 
 in a tube of phosphorus, similarly to an experiment of 
 Davy's, but with better means of experimenting; a consi- 
 derable quantity of phosphuretted hydrogen was given off, 
 amounting in several experiments to more than a cubic inch. 
 
 A similar experiment was made with melted sulphur and 
 sulphuretted hydrogen given off, but not in such quantities as 
 the phosphuretted hydrogen. I tried in vain to carry on these 
 experiments beyond a certain point ; the substance became 
 pasty, mixed with platinum from the arc, and from the diffi- 
 culty of working with the same freedom as when they were 
 fresh, the glass tubes were always broken after a certain time. 
 Had I time for working on the subject now, I should use the 
 discharge from the Ruhmkorff coil, [which had not been in- 
 vented at the period of these experiments. At a subsequent 
 period, when this discharge was taken in the vacuous receiver 
 of an air-pump from a metallic point to a metallic capsule 
 containing phosphorus, a considerable yellow deposit lined 
 the receiver, which, on testing, turned out to be allotropic 
 phosphorus. No gas is, however, given off. I had an air- 
 pump (described 'Phil. Trans.' 1852, p. 101) which enabled 
 me to detect very small quantities of gas, but I could get 
 none. It was in making these experiments that I first de- 
 tected the striae in the electric discharge, which have since 
 become a subject of such interesting observations, which are 
 seen, perhaps, more beautifully in this phosphorus vapour 
 than in any other medium, and which cease, or become very 
 feeble, where the allotropic phosphorus is not produced. 
 
426 EXPERIMENTAL INVESTIGATIONS, 
 
 I tried also phosphorus highly heated by a burning-glass 
 in an atmosphere of nitrogen, but could eliminate no percep- 
 tible quantity of gas, though the phosphorus was changed 
 into the allotropic form. 
 
 It is not difficult to understand why gas is not perceptibly 
 eliminated in the last two experiments ; the effect is probably 
 similar to that described in my paper on ' The Decomposition 
 of Water by Heat/ where, when the arc or electric spark is 
 taken in aqueous vapour, a minute bubble of oxyhydrogen 
 gas is freed and disseminated through the vapour, recombina- 
 tion being probably prevented by this dilution ; but, however 
 long the experiment may be continued, no increased quantity 
 of the gas is obtained, all beyond this minute quantity being 
 recombined. If, however, the bubble of gas be collected, by 
 allowing the vapour to cool, and then expelled, a fresh portion 
 is decomposed, and so on. 
 
 So with the phosphorus in the experiments in the air-pump 
 and with the burning-glass, if any gas is liberated it is probably 
 immediately recombined with the phosphorus ; possibly a 
 minute residuum might escape recombination, but the circum- 
 stances of the experiment did not admit of this being collected, 
 as was the case with the gas in aqueous vapour. 
 
 When, on the other hand, the gas freed is immediately cut 
 off from the source of heat, as when the spark is taken in 
 liquids, an indefinite quantity can be obtained. 
 
 Decomposition and the elimination of gas may thus take 
 place by the application of intense heat to a point in a liquid, 
 or also in gas or vapours, but in the latter case it is more likely 
 to be masked by the quantity of gas or vapour through which 
 it is disseminated. 
 
 I believe there are very few gases in which some alteration 
 does not take place by the application of the intense heat of 
 the voltaic arc or electric spark. If the arc be taken between 
 platinum points in dry oxygen-gas over mercury, the gas 
 diminishes indefinitely, until the mercury rises, and by reaching 
 the point where the arc takes place puts an end to the expe- 
 riment. I have caused as much as a cubic inch of oxygen to 
 disappear by this means. I at one time thought this was due 
 to the oxidation of the platinum, but the high heat renders 
 
EFFECTS OF HEAT ON FLUIDS. 427 
 
 this improbable, and the deposit formed on the interior of the 
 glass tube in which the experiment is made has all the pro- 
 perties of platinum-black; so if the spark from a Ruhmkorffcoil 
 be taken in the vapour of water for several days, a portion of 
 gas is freed which is pure hydrogen, the oxygen freed being 
 probably changed into ozone and combined with the mercury 
 or dissolved by the water. 
 
 I have alluded to the eudiometer by which I analysed the 
 gases obtained in these experiments ; it was formed simply of 
 a tube of glass frequently not above 2\ millemetres in dia- 
 meter, with a loop of wire hermetically sealed into one end, 
 the other having an open bell-mouth. By a platinum wire a 
 small bubble of the gas to be examined could be got up 
 through water or mercury into the closed end of the tube, and 
 by the addition of a bubble of oxygen or hydrogen gas a very 
 accurate analysis of very minute quantities of gas could be 
 made ; I have analysed by this means quantities no larger 
 than a partridge-shot. 
 
 I need hardly allude to results on the compound liquids, 
 such as oils and hydrocarbons, as the fact that permanent gas 
 is given off in boiling such liquids would not be unexpected ; 
 but the above experiments seem to show that boiling is by no 
 means necessarily the phenomenon that has generally been 
 supposed, viz. a separation of cohesion in the molecules of a 
 liquid from distension by heat. I believe, from the close 
 investigation I made into the subject, that (except with the 
 metals, on which there is no evidence) no one has seen the 
 phenomenon of pure boiling without permanent gas being 
 freed, and that what is ordinarily termed boiling arises from 
 the extrication of a bubble of permanent gas, either by 
 chemical decomposition of the liquid, or by the separation of 
 some gas associated in minute quantity with the liquid, and 
 from which human means have hitherto failed to purge it ; 
 this bubble once extricated, the vapour of the liquid expands 
 it, or, to use the appropriate phrase of M. Donny, the liquid 
 evaporates against the surface of the gas. 
 
 My experiments are, in a certain sense, the complement of 
 his. He showed that the temperature of the boiling-point 
 was raised in some proportion as water was deprived of air. 
 
428 EXPERIMENTAL INVESTIGATIONS. 
 
 and that under such circumstances, the boiling took place by 
 soubresauts. I have, I trust, shown that when the vapour 
 liberated by boiling is allowed to condense, it does not alto- 
 gether collapse into a liquid, but leaves a residual bubble of 
 permanent gas, and that at a certain point this evolution be- 
 comes uniform. 
 
 Boiling, then, is not the result of merely raising a liquid to 
 a given temperature, it is something much more complex. 
 
 One might suppose that with a compound liquid the 
 initial bubble by which evaporation is enabled to take place 
 might, if all foreign gas were or could be extracted, be formed 
 by decomposition of the liquid, but this could not be the case 
 with an elementary liquid whence then comes the oxygen 
 from bromine or the hydrogen from phosphorus and sulphur ? 
 as with the nitrogen in water, it may be that a minute 
 portion of oxygen, hydrogen, or of water is inseparable from 
 these substances, and that if boiled away to absolute dryness, 
 a minute portion of gas would be left for each ebullition. 
 
 With water there seems a point at which the temperature 
 of ebullition and the quantity of nitrogen yielded become 
 uniform, though the latter is excessively minute. 
 
 The circumstances of the experiments with bromine, phos- 
 phorus, and sulphur, did not permit me to push the experi- 
 ment so far as was done with water, but, as far as it went, the 
 result was similar. 
 
 When an intense heat, such as that from the electric spark 
 or voltaic arc, is applied to permanent gas, there are, in the 
 greater number of cases, signs either of chemical decomposition 
 or of molecular change ; thus compound gases, such as hydro- 
 carbons, ammonia, the oxides of nitrogen, and many others 
 are decomposed. Phosphorus in vapour is changed to allo- 
 tropic phosphorus, oxygen to ozone, which, according to 
 present experience, may be viewed as allotropic oxygen ; 
 there may be many cases where, as with aqueous vapour, a 
 small portion only is decomposed, and this may be so masked 
 by the volume of undecomposed gas as to escape detection ; 
 if, for instance, the vapour of water were incondensable, the 
 fact that a portion of it is decomposed by the electric spark or 
 ignited platinum would not have been observed. 
 
EFFECTS OF HEAT ON FLUIDS. 429 
 
 All these facts show that the effect of intense heat applied 
 to liquids and gases is far more complex, and presents greater 
 interest to the chemist, than has generally been supposed. In 
 far the greater number of cases, possibly in all, it is not mere 
 expansion into vapour which is produced by intense heat, but 
 there is a chemical or molecular change. Had circumstances 
 permitted I should have carried these experiments farther, and 
 endeavoured to find an experimentum crucis on the subject ; 
 there are difficulties with such substances as bromine, phos- 
 phorus, &c., arising from their action on the substances used 
 to contain and heat them, which are not easy to vanquish, 
 and those who may feel inclined to repeat my experiments 
 will find these difficulties greater than they appear in narra- 
 tion ; but I do not think they are insuperable, and hope that, 
 in the hands of those who are fortunate enough to have time 
 at their disposal they may be overcome. 
 
 To completely isolate a substance from the surrounding 
 air and yet be able to experiment on it, is far more difficult 
 than is generally supposed. The air-pump is but a rude in- 
 strument for such experiments as are here detailed. 
 
 Caoutchouc joints are out of the question ; even platinum 
 wires carefully sealed into glass, though, as far as I have been 
 able to observe, forming a joint which will not allow gas to 
 pass, yet it is one through which liquids will effect a passage, 
 at all events when the wires are repeatedly heated. 
 
 In some experiments with the ignited platinum wire her- 
 metically sealed into a tube of glass, the end of the tube 
 containing the platinum wire was placed in a larger tube of 
 oil to lessen the risk of cracking the glass. After some days' 
 experimenting, though the sealing remained perfect, a slight 
 portion of carbon was found in the interior liquid. This does 
 not affect the results of my experiments, as I repeated them 
 with glass tubes closed at the end and without platinum wires, 
 and also without the oil-bath, -but it shows how difficult it is 
 to exclude sources of error. When water has been deprived 
 of air to the greatest practicable extent it becomes very avid 
 for air. The following experiment is an instance of this : A 
 single pair of the gas-battery, the liquid in which was cut off 
 from the external air by a greased glass stopper, had one 
 
430 EXPERIMENTAL INVESTIGATIONS, 
 
 tube filled with water, the other with hydrogen, the platinised 
 platinum plates in each of these tubes were connected with a 
 galvanometer, and a deflection took place from the reaction of 
 the hydrogen on the air dissolved in the water. After a time 
 the deflection abated, and the needle returned to zero, all the 
 oxygen of the air having become combined with the hydrogen. 
 If now the stopper were taken out, a deflection of the galvano- 
 metric needle immediately took place, showing that the air 
 rapidly enters the water, as water would a sponge. Absolute 
 chemical purity in the ingredients is a matter- for refined 
 experiments, almost unattainable ; the more delicate the test, 
 the more some minute residual product is detected ; it would 
 seem (to put the proposition in a somewhat exaggerated form) 
 that in nature everything is to be found in anything if we 
 carefully look for it. 
 
 I have indicated the above sources of error to show the 
 close pursuit that is necessary when looking for these minute 
 residual phenomena. Enough has, I trust, been shown in the 
 above experiments to lead to the conclusion that, hitherto, 
 simple boiling, in the sense of a liquid being expanded by 
 heat into its vapour without being decomposed or having per- 
 manent gas eliminated from it, is a thing unknown. Whether 
 such boiling can take place may be regarded as an open ques- 
 tion, though I incline to think it cannot ; that if water, for 
 instance, could be absolutely deprived of nitrogen, it would 
 not boil until some portion of it was decomposed ; that the 
 physical severance of the molecules by heat is also a chemical 
 severance. If there be anything in this theoretic view, there 
 is great promise of important results on elementary liquids, if 
 the difficulties to which I have alluded can be got over. 
 
 The constant appearance of nitrogen in water, when boiled 
 off out of contact with the air almost to the last drop, is a 
 matter well worthy of investigation. I will not speculate on 
 what possible chemical connexion there may be between air 
 and water ; the preponderance of these two substances on the 
 surface of our planet, and the probability that nitrogen is not 
 the inert diluent that is generally supposed, might give rise 
 to not irrational conjectures on some unknown bond between 
 air and water. But it would be rash to announce any theory 
 
NEW CLASS OF APLANATIC TELESCOPES. 431 
 
 on such a subject ; better to test any guess one may make, 
 by experiment, than to mislead by theory without sufficient 
 data, or to lessen the value of facts by connecting them with 
 erroneous hypotheses. 
 
 ON APLANATIC TELESCOPES. 
 
 Phil Mag., March 1867. 
 
 IN my address as President of the British Association at Not- 
 tingham, last August, I suggested ' oily or resinous substances, 
 such as castor-oil, Canada balsam, &c.,' as materials to be 
 used, in combination with glass lenses, to reduce or annihilate 
 the defect in the achromatic telescope arising from the irra- 
 tionality of spectra. Since the delivery of that address, the 
 specification of a patent of Mr. Wray has been published, in 
 which these, among other substances of similar optical cha- 
 racter, the principal one being oil of cassia, are named for the 
 like purpose. Although provisional protection was obtained 
 previously, the public could know nothing of the invention 
 until the publication of the specification : this was shown to 
 me by a friend last week. 
 
 In it the author does not, to my mind, get rid of the 
 greatest difficulty which is experienced in the use of such 
 substances, viz. the want of permanency in the telescope, 
 arising from the unequal shrinking or drying of the substances 
 when used between lenses not having touching curves. He 
 says that the glasses, when the edges are chamfered, are her- 
 metically sealed by the correcting substance ; but I think he 
 will find that this will not bear the test of time. It seems to 
 me desirable that I should publish experiments I have made 
 on this subject at different periods during many years, and 
 which I have communicated to several persons, among whom 
 I may name Dr. Frankland and Mr. Cooke, the well-known 
 optician. Such publication may assist others in promoting 
 this important object. Though at work on the subject for 
 some time previously, the first published record on the sub- 
 ject I can lay my hand on is in the Monthly Astronomical 
 
432 EXPERIMENTAL INVESTIGATIONS. 
 
 Notices for January 14, 1853, in which occurs the following 
 paragraph : ' In some cases, where the inner curves of the 
 flint- and crown-glasses approximate, Mr. Grove had employed 
 with success a highly refracting cement made of very clear 
 rosin and castor-oil, which, acting as a third lens or convex 
 meniscus of a medium dispersing the coloured spaces differ- 
 ently from the other two lenses, corrected to a very great 
 degree the chromatic without increasing the spherical 
 aberration.' 
 
 Before the date of the above communication I had used 
 for a similar purpose copal varnish between the lenses ; and a 
 three-inch object-glass, of four feet focus, so corrected by me 
 was used for several years by Mr. B. Hill, of Swansea. 
 Scarcely any secondary colour was perceptible ; but in time, 
 beginning at the second year, small bubbles appeared between 
 the lenses, arising from the drying of the varnish, though the 
 meniscus of it was so thin that it only shortened the focus 07 
 of an inch. It was this defect that led me to use the rosin and 
 castor-oil, which formed a tough cement practically unalterable. 
 The telescope alluded to in the above extract from the 
 Monthly Notices was of three inches aperture and forty inches 
 focus, the focus without the cement being forty-two inches ; 
 this telescope was a very good one : it divided y Leonis, 7 
 Virginis, and other stars of that class ; the secondary spec- 
 trum was extremely slight, and capable of being detected only 
 by a practised eye on a bright spot, such us the planet Venus. 
 I used it for some time ; but wishing to make it more perfect, 
 I broke it, as I have done many others. I was convinced from 
 the above and other results that oils or liquids which dry or 
 shrink more or less in time, were open to objection similarly 
 to Blair's object-glasses, where the correction is by a concave 
 lens of liquid instead of a convex meniscus, and the substances 
 for correction used with glass are therefore necessarily chosen 
 from towards the bottom instead of the top of Brewster's list 
 (' Optics,' Lardner's Cycl., pp. 374 and 375), in which oil of 
 cassia stands first and sulphuric acid last. I was satisfied 
 that the substance to be aimed at should be one which lique- 
 fied by heat and became solid when cold ; Canada balsam 
 in its ordinary state was therefore objectionable, but when 
 
NEW CLASS OF A PLAN ATI C TELESCOPES. 433 
 
 obtained hard and then tempered by admixture with castor- 
 oil, so as to form a substance just taking, when cold, the im- 
 pression of the nail, it succeeded very well, though, from my 
 experience of the accidents with large glasses, I only tried it 
 on small ones, two inches and under. Viscous Canada balsam 
 and castor-oil, mixed and heated and cooled repeatedly, be- 
 came ultimately solid. 
 
 I have nothing better to propose as a substance actually 
 experimented on than the above two cements, viz. rosin and 
 castor-oil, and fused Canada balsam and castor-oil. Oil of 
 cassia I abandoned after an experiment or two, on account of 
 its limpidity. Doubtless this and other substances might be 
 used in combination with a fused resinous substance, but I 
 have not tried it. Sulphuret of carbon, the third liquid in 
 Brewster's list, mentioned by Mr. Wray, seems to me wholly 
 impracticable, from its great volatility. 
 
 Although the substances at the head of Brewster's list, 
 speaking generally, have a greater dispersive action on the 
 more refrangible rays of the spectrum, and those at the bot- 
 tom of it on the less refrangible rays (see Brewster's diagram 
 of oil of cassia and sulphuric acid spectra, ' Optics,' p. 78), yet 
 each substance has peculiarities, and must be experimentally 
 tried by the old plan of two prisms or similar methods before 
 it is used. None in theory absolutely correct the irrationality ; 
 but several do so practically, and the problem is for each 
 specimen of flint and crown glass used to select a substance 
 which sufficiently corrects the secondary spectrum and can be 
 used by heat-fusion, so as to form a tough solid when cold. To 
 this point, in my opinion the most important, Mr. Wray does 
 not allude. Of liquids I have found castor-oil the most durable ; 
 and if the lenses be in an iron well-fitting cell (for it acts on 
 brass rapidly), I am not sure that such material might not be 
 used alone with advantage ; but with large object-glasses it is 
 excessively difficult to prevent the meniscus of castor-oil from 
 being wedge-shaped, and so giving a tail to stars and planets. 
 When the size of the glasses is notable it is extremely difficult 
 to remedy this defect. I proceed to mention some modes of 
 overcoming difficulties of manipulation which cost me much 
 labour ; I omit my failures. 
 
 F F 
 
434 EXPERIMENTAL INVESTIGATIONS. 
 
 ist. To apply solid transparent cements which melt by 
 heat to object-glasses without danger of breaking or unanneal- 
 ing the glass. An iron plate, about one-quarter of an inch in 
 thickness, and at least an inch greater in diameter than the 
 glass, is placed on a ring of metal and well levelled ; on it 
 place a piece of soft filtering-paper, and on this the object- 
 glass, the flint glass being next the paper. Have the cement 
 melted in a porcelain vessel placed in a sand-bath, the tempe- 
 rature being just high enough to fully melt it but not to car- 
 bonise it, and keep it melted as short a time as practicable ; 
 then heat uniformly, by a spirit-lamp underneath the iron 
 plate, until the object-glass above is so hot that the hand can 
 just bear it ; remove the crown glass, pour the cement on to 
 the centre of the convexity in the flint and replace the crown 
 on it ; press with the finger on the centre of the crown until the 
 cement exudes all round from between the glasses ; remove 
 the lamp and support the plate of iron by a firm prop ; then 
 place a thick annular pad of filtering-paper, two-thirds of the 
 diameter of the glass, on the crown glass, and on this a mo- 
 derate weight, such as will press the glasses very close without 
 being so heavy as to distort them ; the weight had better be 
 as warm as the glasses. Props should be arranged so as to 
 keep the glasses in situ ; and if they move on each other, a 
 little adjustment may be made by the hands. Allow the 
 whole to cool slowly ; when cold, wash off the overflow of 
 cement with naphtha or other solvent, and the operation is 
 complete. 
 
 2nd. To filter rosin or similar substances when not quite 
 clear. Procure the most transparent rosin ; I have obtained 
 some at drysalters' in Long Acre, which in films of O'l to 0*2 
 of an inch has scarcely perceptible colour. 
 
 Make a single conical filter of white filtering-paper, sus- 
 pend it by the ring of a retort-holder placed near the top, 
 using no external funnel ; put the rosin, roughly broken, into 
 this filter, and place it a few inches in front of a bright par- 
 lour fire, turning it occasionally : the rosin will melt and drop 
 from the filter beautifully clear. It may be collected in a 
 vessel, or dropped at once into castor-oil or other liquid used ; 
 they should then be warmed and well stirred together, and 
 may be put aside for use when required. 
 
DORMANT IMPRESSIONS ON THE RETINA. 435 
 
 3rd. Levelling. If the glasses thus prepared have not the 
 meniscus of cement of uniform thickness for the same dis- 
 tances from the centre (a defect ascertainable in a moment by 
 looking through the telescope at a real or artificial star), warm 
 them again until the cement is melted, and no more, and with 
 the two hands gently pinch the edges together, turning the 
 glasses constantly round until cold, or nearly so. I have never 
 found this fail, if neatly executed. 
 
 If air-bubbles appear in the cement, slide the one glass 
 when hot over the other until the bubbles escape, and then 
 gently slide it back. 
 
 Many details can only be learned by practice. I have 
 given all the points as to materials and manipulation which 
 occur to me as essential. I strongly urged on Mr. Cooke some 
 years since to take up the subject, as I am, of course, not suf- 
 ficiently a practical optician to make good telescopes. I hope 
 it will now be taken up in earnest. 
 
 ON A MODE OF REVIVING DORMANT IMPRESSIONS 
 ON THE RETINA. 
 
 Philosophical Magazine ', May 4, 1852. 
 
 FIRST. Look steadily at a luminous object, sufficiently bright 
 to be borne by the eyes without great inconvenience, then 
 turn the eyes upon a dark body or dark space : an image of 
 the object previously looked at will be seen, a fact familiar to. 
 everybody. When the image has completely faded away and 
 is no longer visible, pass backwards and forwards between the 
 eye and the dark body a white substance, say a sheet of paper : 
 the image will be immediately revived, and may be thus in- 
 definitely reproduced. 
 
 If the light is, in the first instance, not sufficiently vivid to 
 produce the continued impression on the retina, but is nearly 
 so, the invisible image may be brought out, or first rendered 
 visible, by moving the white object between the eye and the 
 dark body or dark space looked at. The white substance 
 should be in a situation exposed to light, so that its whiteness 
 affects the eye, and not held in shadow. After a little prac- 
 
 FF 2 
 
436 EXPERIMENTAL INVESTIGATIONS. 
 
 tice, it is astonishing to what an extent and for how long a 
 time images may be thus reproduced. 
 
 2nd. Reverse the experiment, looking from the bright 
 object at white paper, and a dark image of the object will be 
 seen ; when this has faded away, move between the eye and 
 the paper a dark substance, held so as to reflect as little light 
 as possible to the eye, and the image is reproduced on the 
 white paper ; or may be, in the first instance, produced, as with 
 the converse experiment. 
 
 The explanation which occurs to me is, that the effect is 
 one of contrast between the portions of the retina which have 
 not been strongly affected and those which have. 
 
 The white paper dulls or deadens the sensitive portion of 
 the retina for an instant, more than the part which has been 
 previously rendered non-sensitive to other impressions than 
 that which it has received by the bright light, and the black 
 supervenes as a contrast to the parts affected by the white, 
 but not to those unaffected. In the converse experiment the 
 black relieves or renders more sensitive the comparatively un- 
 affected portions of the retina, but has little or no operation 
 on the non-sensitive parts ; thus, at the moment of removing 
 the black body, the unimpressed portions of the eye are 
 affected by the white substance, but the impressed portion is 
 comparatively dead to it. 
 
 PHOTOGRAPHIE NATURELLE. 
 
 Cosmos ; Janvier 12, 1858. 
 
 Aux faits si inte"ressants et si curieux enumeres par M. Millot- 
 Brule, vient se joindre une observation curieuse de M. Grove. 
 II s'agit cette fois de photographic naturelle des truites. M. 
 Grove avait pris une grande truite, et n'ayant pas de baquet 
 ou il put la deposer, il 1'avait jetee au pied d'un arbre. Quand 
 il revint une heure apres, il s'apergut qu'elle etait couverte de 
 larges taches blanches. En examinant avec grand soin, il 
 remarqua que ces taches reproduisaient 1'image de feuilles et 
 d'herbes qui avaient etc en contact avec la truite, ou placees 
 
NATURAL PHOTOGRAPHY. 437 
 
 dans son voisinage. Cette remarque 1'amena a faire 1'experi- 
 ence suivante. II plaga de chaque cote de la truite deux 
 feuilles dentelees sur leurs bords, et la de*posa doucement sur 
 le sol, de telle sorte que 1'un des flancs, celui qui touchait la 
 terre, fut tout a fait a 1'abri de la lumiere, tandis que 1'autre 
 flanc, au contraire, restait en pleine lumiere. Au bout d'une 
 heure, il enleva les feuilles, et vit sur le flanc superieure une 
 image nette et tres-bien defmie de la feuille, absolument 
 comme si, dans les premiers ages de la photographic, on Tavait 
 placee sur une feuille de papier sensibilise. Sur Fautre flanc, 
 au contraire, celui qui etait a 1'abri de la lumiere, il n'y avait 
 aucun changement produit. L'image de la feuille sur la peau 
 de poisson etait blanche ou blanchatre sur un fond obscur. 
 II semblait que la lumiere avait tout noirci a 1'entour de la 
 partie recouverte par la feuille. M. Grove avait conclu tout 
 aussitot a une action photogenique ; il lui reste cependant un 
 doute qu'il n'as pas encore eclairci. La difference de couleur 
 ne pourrait-elle pas etre attribute a un effet de secheresse ou 
 d'oxygenation ? II ne le pense pas, parce que le cote infe- 
 rieur etait lui-meme expose" a 1'air, quoique moins librement, 
 et que la feuille simplement placee sur le flanc superieur la 
 defendait assez mal du contact de Fair. M. Grove ajoute : 
 ' Je crois avoir entendu dire que la truite change rapidement 
 de nuance lorsqu'elle passe d'un ruisseau a 1'autre, ou meme 
 d'une region a 1'autre dans un meme ruisseau. L'effet que 
 j'ai constate a de 1'analogie avec la modification de teinte qui 
 la lumiere solaire determine sur la peau humaine ; mais chez 
 la truite, en la supposant de meme nature, elle est plus rapide. 
 Les truites sur lesquelles j'ai experimente pesaient un kilo- 
 gramme, elles'etaient de la variete ceillet-violaceV 
 
 ON THE REFLEXION AND INFLEXION OF LIGHT 
 BY INCANDESCENT SURFACES. 
 
 Phil. Mag., March 1859. 
 
 ON putting in order some old papers I found a manuscript 
 in my own handwriting, and the subject of which I had en- 
 
438 EXPERIMENTAL INVESTIGATIONS. 
 
 tirely forgotten ; and it was not until some time had elapsed 
 that I could recollect anything about the experiments con- 
 tained in it. I now remember that they were made at the 
 London Institution, and it must have been from ten to fifteen 
 years ago. I have no recollection of the reason why I did 
 not publish them, and can only guess that it was in accord- 
 ance with my general habit of not publishing negative results. 
 The results here, however, though negative, seem to me in- 
 teresting, as positive results would a priori be expected ; and 
 may be worth publishing in the ' Philosophical Magazine.' 
 
 The difference in appearance to an observer of a polished 
 surface when at ordinary temperatures and when ignited is 
 sufficiently marked. The self-luminous character of the ignited 
 body apparently removes the impressions of surrounding 
 objects, and would lead to the belief that reflexion, at least 
 that of the character yielded by polished surfaces, was de- 
 stroyed. Such has been the a priori impression of those whose 
 opinions I have asked on the subject. My own belief was, that 
 if polished surfaces when ignited reflected light, they at all 
 events broke up or scattered the reflected rays, and would 
 cease to have the character of a polished surface ; and that if 
 they reflected light at all or notably, they would reflect it as 
 paper or snow does, dispersing the rays so as to produce a 
 general impression of luminosity, instead of throwing them 
 back in a parallel beam, or one in which they preserved their 
 original relative inclination. 
 
 The subject appeared worth investigation ; and as I could 
 not find that it had been attempted, I determined to make 
 a few experiments upon it. The difficulty which immediately 
 presented itself was, that the surfaces which are mainly em- 
 ployed for polished reflexion being oxidable metals, their phy- 
 sical structure would be changed by the oxidation consequent 
 on incandescence. Gold or platinum, therefore, were the 
 only substances which promised any success ; and the latter, 
 from its reflecting white light and more ready capability of 
 retaining a high temperature, was selected. A strip of plati- 
 num-foil, 2 inches long by 0*2 broad, was firmly stretched on a 
 piece of plate glass, polished with putty-powder and tripoli 
 until it had reached as high a lustre as it could be made to 
 
REFLEXION OF LIGHT. 439 
 
 attain. One extremity was then fixed in a clamp attached to 
 a wooden frame ; and to the other extremity was attached, by 
 a similar clamp, a metal weight, from which weight a wire 
 extended and dipped into a vessel of mercury. The whole was 
 arranged with care, so as not to bend or disturb the plane 
 surface of the platinum. The foil thus suspended was brought 
 opposite a vertical cleft in a window-shutter facing the meri- 
 dian, which cleft could be made of any convenient size by 
 horizontally moveable boards. The platinum-foil was placed 
 opposite the cleft, so as to receive a sunbeam ; a sheet of 
 white paper was arranged directly in the path of the reflected 
 beam ; and the distance of the paper from the platinum was 
 indefinitely varied during the experiment. Having accurately 
 marked the boundaries of the reflected beam and its intensity, 
 as far as the eye could judge, the platinum was made part of 
 the circuit of a voltaic battery, the intensity of which was 
 varied so as to produce effects on the foil varying from a 
 heat scarcely visible in the dark to incandescence up to the 
 point of fusion, or rather to the point at which the foil broke, 
 from its diminished cohesion ; for with a weight suspended, 
 although not more than barely sufficient to keep the foil 
 stretched, it always broke off at a temperature short of its 
 point of fusion. In none of these variations, however, was 
 there the slightest apparent difference in the reflected light on 
 the paper. Or if, as occasionally happened, the shape a little 
 changed during the progress of the experiment, it was fully 
 explained by the elongation dependent upon the heat, or by 
 the consequent removal of slight curvatures. 
 
 A similar experiment was made with diffused daylight, and 
 with similar effects, also with the light from an argand 
 lamp. In the latter case, when the reflected beam was so 
 dim as to be interfered with by the light afforded by the in- 
 candescent platinum, the image was proportionately affected. 
 Still it preserved its character, and, as far as could be judged, 
 its intensity ; and it was only by a very high degree of incan- 
 descence and a very feeble incident light that the reflected 
 image seemed to merge in the direct light from the incandes- 
 cent body. 
 
 I now brought my eye into the position where the paper 
 
440 EXPERIMENTAL INVESTIGATIONS. 
 
 had been placed, so as to catch the reflected beam, while my 
 assistant alternately made and broke contact with the battery. 
 When the incident sunlight was sufficiently intense to mask 
 the emitted light of incandescence I could not, in the slightest 
 degree, distinguish whether the platinum was ignited or cold. 
 When I first tried it I two or three times complained wrongly 
 to my assistant that he had not made contact when I told him 
 to do so ; when the incident light was very dim, the emitted 
 light was, of course, also distinguishable. I now caused the 
 spectrum from a flint-glass prism to fall, on the platinum, and 
 with similar effect ; i.e. when the reflected spectrum was very 
 intense, no difference could be detected between the light from 
 the platinum, whether cold or ignited, or whether received 
 upon paper or upon the eye ; when less intense, the red por- 
 tion of the spectrum was elongated by the light of incandes- 
 cence, and the other portions partook of the character of the 
 spectrum superposed upon, or blended with, the light of incan- 
 descence. 
 
 The prism was also arranged so as to intercept the reflected 
 instead of the incident beam ; the effects were similar. 
 
 A beam of light polarised by reflection at the proper angle 
 from a plate of glass was made to fall on the platinum surface, 
 and then analysed by a tourmaline ; no difference was per- 
 ceptible in the plane of polarisation, whether the platinum 
 was ignited or not. 
 
 The light reflected from the platinum was similarly polar- 
 ised and analysed, but no difference dependent upon incan- 
 descence was detected. 
 
 A wire of platinum, 6 inches long and -^th of an inch in 
 diameter, was vertically suspended in the narrow fissure of the 
 shutter ; the bands of interference were received on paper 
 placed at different distances from the wire, and examined 
 both by the eye and by a lens ; no difference could be de- 
 tected in these bands when the wire was ignited by a voltaic 
 battery. 
 
 In all the above experiments the foil or wire was ignited 
 by the battery previously to the commencement of each class 
 of experiment, so as to avoid any effect arising from the altera- 
 tions caused by the platinum having been subjected to heat, 
 
REFLEXION OF LIGHT. 441 
 
 such, for instance, as the effect of annealing might produce, 
 or the burning off from it of films of moisture, or of oxidable 
 substances. 
 
 The general result of these experiments is, that no differ- 
 ence is perceptible by the eye in light reflected by a polished 
 surface, whether that surface is ignited or not ; that the 
 superficial molecular uniformity, which causes a bundle of 
 parallel rays to preserve their parallelism of direction when 
 reflected, is, if the ignited substance be inoxidable, not broken 
 up by ignition. 
 
 I know of no photometer which would be suitable for indi- 
 cating their effects with greater accuracy than the eye ; but, 
 although these results lead to a conclusion different, I believe, 
 from that which would have been arrived at a priori, they by 
 no means exclude the possibility or even probability of some 
 difference being produced in the direction or character of light 
 reflected from ignited surfaces as compared with that reflected 
 from unignited surfaces. 
 
 The fixed lines in the spectrum, for instance, differ mate- 
 rially according to the source of light ; and even supposing 
 the ignited surface to make no difference in the character or 
 position of the fixed lines of reflected solar light, a point which 
 I have no apparatus sufficiently delicate to detect, yet there 
 is every probability of novel and valuable results being attained 
 by the interference of this light of incandescence with that of 
 solar or other light reflected from the incandescent body, the 
 same body being then, in some sense, the source of two dif- 
 ferent descriptions of light, which differences are capable of 
 detection by the different position and character of the fixed 
 lines in their respective spectra. Such experiments, and many 
 others which they obviously suggest, appear to me to offer 
 an Interesting field of experiments in physical optics ; and to 
 those who are more practically acquainted with this science 
 than I can pretend to be, and who may possess more delicate 
 means of detecting minute effects, I therefore leave them. 
 
442 EXPERIMENTAL INVESTIGATIONS. 
 
 NOTE ON THE OCCULTATION OF JUPITER, 
 NOVEMBER 8, 1856. 
 
 Proc. Astronomical Society, Nov. 14, 1856. 
 
 I HAD a good position for witnessing the partial occultation of 
 Jupiter from an upper room of my house, situate opposite 
 Clarence Gate, Regent's Park. I used a small but unexcep- 
 tionably good achromatic telescope, if -inch aperture and 22 
 inches focus ; the magnifying power was 43. 
 
 It seemed to me (but I attach little value to the obser- 
 vation, as the effect might have been due to an agitation in 
 the earth's atmosphere) that at the moment before immersion 
 the margin of Jiipiter was slightly elongated, or, so to speak, 
 stretched itself out to meet the moon ; this was followed im- 
 mediately by a flattening of the same portion of the disk. 
 
 Throughout the whole period a dark line was plainly per- 
 ceptible between Jupiter and the moon. This line appeared 
 to me straight, as though cutting off the convexity both of 
 Jupiter and the moon. The light of Jupiter was notably less 
 brilliant than that of the moon ; I should say hardly equal 
 to half the intensity of the latter. It was of a much bluer 
 tint than the moon, probably an effect of contrast arising 
 from the different intensities of their respective lights. The belts 
 of Jupiter were plainly seen throughout the occultation, and, 
 as far as I could observe, were not in any respect distorted. 
 
 The dark line I thought at first arose from the intervention 
 of a fine rim of the dark body of the moon, but this could 
 hardly be the case towards the termination of the occultation. 
 About a fifth part of the planet appeared to me to be occulted 
 at the maximum period. 
 
 NOTE ON THE OCCULTATION OF JUPITER OF 
 JANUARY 2, 1857. 
 
 Proc. Astronomical Society, Jan. 4, 1857- 
 
 OWING to the kindness of Mr. Gillett, I was enabled to ob- 
 serve this occultation through his binocular telescope. This 
 
OCCULTATION OF JUPITER, 1857. 443 
 
 instrument is by Ross, the object-glasses 3f inches diameter, 
 focal length 5 1 inches.; portions (about j inch) of the glasses 
 are ground away to enable them to be approximated suffi- 
 ciently for the distance between the eyes of the observer. The 
 power used was 60, which gave a greater idea of magnitude 
 than double that power in a monocular telescope. 
 
 The dark limb of the moon was faintly visible throughout 
 the time of the occultation. The disappearance of the satel- 
 lites was not instantaneous, as with a fixed star, but (as indeed 
 might have been expected) they gradually faded away. The 
 light of the moon's edge was too feeble for me to ascertain 
 whether they disappeared at the exact edge or not. I should 
 say that they seemed to be a little within it, but I could not 
 positively assert this. Jupiter was cut off with perfect sharp- 
 ness, and I could not detect the slightest distortion. When 
 about one-third was eclipsed the light of the planet appeared 
 brighter along the edge of the moon, and this apparently in- 
 creased brightness continued until it was about three-fourths 
 eclipsed. Two spots in the neighbourhood of the upper belt 
 (as viewed by inverting telescope) were notably bright, so as 
 to attract attention, and they continued so during the period 
 above-mentioned, each looking somewhat like an electric glow 
 coming off the dark edge of the moon. This effect may be 
 optical, and caused by the immediate proximity of darkness 
 to central and more luminous portions of the planet, but it 
 was very marked. I observed no other effect during the 
 period of immersion. I looked for an effect said to have been 
 observed on previous occasions, viz. a faint illumination of the 
 dark edge of the moon upon contact, but could not detect the 
 slightest trace of it. On emersion I could detect no distortion 
 of any sort, nor did I see the dark line which I noticed in the 
 occultation of Nov. 8, 1856. The difference of light and 
 colour between the planet and the moon was the same as then 
 observed. 
 
 After emersion there was a decided stereoscopic effect, 
 seen both by Mr. Gillett (who up to this time had left the 
 telescope entirely to me) and myself ; Jupiter in the binocular 
 appearing as at a distance behind the moon. Mr. Gillett tells 
 me that with terrestrial objects the effect of the binocular, if 
 
444 EXPERIMENTAL INVESTIGATIONS. 
 
 inverting eye-glasses are used, is in many instances pseudo- 
 scopic, the more distant objects appearing nearer than the 
 more proximate, which is the effect that theory would 
 indicate. 
 
 The satellite, last in order on emersion, seemed to cling 
 for a second or so to the moon's edge, probably an effect due 
 to irradiation. I did not observe this with the three others, 
 but I did not catch the moments of emersion so well with 
 them. I was greatly struck by the beautiful impression con- 
 veyed to the mind by the binocular telescope ; and judging 
 from what I had seen with a monocular telescope of my own, 
 of 4*3 inches aperture, and therefore not far differing in area 
 from the compound area of the two object-glasses in the 
 binocular, the latter was decidedly superior, and the ease and, 
 if I may be allowed the expression, the naturalness of the 
 vision, was very remarkable. 
 
 NOTE ON COMET V. 1858. 
 Proc. Astronomical Society ', Nov. 12, 1858. 
 
 THE following extract from a letter adressed by Mr. Grove to 
 Mr. De La Rue refers to the phenomena observed during the 
 transit of the tail of the comet over Arcturus on the evening 
 of October 5. The telescope employed by Mr. Grove was 
 a small instrument, of only two inches aperture, and his re- 
 marks are offered under an impression that the weather else- 
 where was not generally favourable for observing the comet on 
 the interesting occasion to which they refer : 
 
 ' When the comet had entered well within the margin of 
 the tail a dark notch was formed, cutting out a portion of the 
 tail round the star ; and as the star got farther in, this became 
 a dark areola surrounding the star, and in diameter equal to 
 about one-tenth of the line of transit. This continued until 
 the star reached the middle ; at this part there is a broad dark 
 
OCCULTATION OF SATURN, 1859. 445 
 
 line which extends from the nucleus to a distance considerably 
 above the point where the star crossed. When Arcturus 
 arrived here this dark space was perfect up to the star, but on 
 the other side the white light of the tail appeared to come 
 quite up to the star ; in short, the bright part of the tail was 
 darkened in the vicinity of the star, and the dark part was 
 brightened, at least so much of it as was on the side farthest 
 from the nucleus. 
 
 1 1 saw the notch again on the opposite side previous to 
 emersion, and then lost it by clouds. 
 
 1 The effects I have described are, doubtless, optical, and 
 the notch and areola evidently due to the bright light of this 
 star ; the effect on the dark central part is not so easy to 
 explain/ 
 
 NOTE ON OCCULTATION OF SATURN, 1859. 
 
 Proc. Astronomical Society, May 8, 1859. 
 
 I OBSERVED the occultation of Saturn with a six-foot tele- 
 scope, 4*3 inches aperture, power 165. I imagined that upon 
 the moment of first contact the ring was a little drawn out 
 into a point, succeeded by a knot or apparent excrescence ; 
 but on thinking it over since it has occurred to me that, as I 
 could not see the edge of the moon's dark limb, and had no 
 clock or means to advise me of the moment of impact, the 
 effects I noticed might well have been due to the cutting 
 off, or, as it were, sharpening the point of the ring by the 
 oblique incidence. I mention the apparent effect, however, as 
 if better observers have noticed it there may be something 
 in it as a corroboration. Excepting this, from the impact 
 until the total disappearance, I could not notice the slightest 
 distortion. 
 
 On the emergence the extremely faint light of Saturn, con- 
 trasted with the moon, was very remarkable ; Saturn was a 
 mere ghost of himself, a faint grey-blue shadow. The line of 
 the moon's edge was perfect, and no dark band was percep- 
 
446 EXPERIMENTAL INVESTIGATIONS. 
 
 tible, as I and others noticed in the occultation of Jupiter. 
 The part of the ring last emergent appeared flattened or 
 shorter than the opposite portion, doubtless from the greater 
 contrast of light or from irradiation. I think the former was 
 the cause, as some disproportion continued after Saturn was 
 clear of the moon, and distant from the edge about the dia- 
 meter of its (Saturn's) own orb. The atmosphere was better 
 than the average for the early time of the night, and I doubt 
 not some good observations will have been made. 
 
 NOTE ON APPEARANCES OF MARS, 1862. 
 Proc. Astronomical Society, Dec. 12, 1862. 
 
 MR. GROVE, in a letter dated November 5, 1862, gives an 
 account (illustrated by three pencil sketches) of the appear- 
 ances presented by the planet Mars on October 26 and 31, 
 and November 3. The telescope used was 4^ inches aperture ; 
 focal length, 6 feet 2 inches ; object-glass by Cooke, of York, 
 capable of dividing 77 Corona. He is perfectly satisfied of 
 there being notable changes in the distribution of the lights 
 and shadows, inconsistent, it appears to him, with their being 
 due to land and water, or, as he should perhaps say, land and 
 water only ; clouds condensed over large aqueous districts 
 might possibly account for the changes observed ; the changes 
 of position by axial revolution would not explain them.* 
 
 THE AURORA BOREALIS OF 1870. 
 
 Ftom 'Nature,' Nov. 1870. 
 
 I SHALL be obliged if you will put on record a few scattered 
 notes which I took of the splendid Aurora Borealis of Oct. 25, 
 seen from Arthingworth, Northamptonshire. When I first 
 observed it, at half-past five P.M., a crimson glow extended in 
 
 * This notice requires the sketches, and I cannot find them. W.R.G., 1874. 
 
AURORA BOREALIS, 1870. 447 
 
 an irregular band from NNE. to W., most prominent at 
 about 20 to 30 above the horizon. This increased in height 
 and breadth until it nearly reached a point SW. of the zenith, 
 and about 15 WNW. of the star Vega. At this time the 
 northern part of the sky was perfectly free from aurora ; 
 gradually that part and the whole dome of the heavens, with 
 the exception of a section from W. to nearly S., became filled 
 with luminous streamers. These, for about 20 on each side 
 of N., were white, the others crimson striped with white or 
 rather greenish light ; but the green I believe to be an effect of 
 contrast, as where similar streamers were distant from the red 
 light they were white. 
 
 The white or green streamers appeared to eclipse the red 
 light ; they changed their size, shape, and position, while the 
 red continued comparatively unchanged. There were also dark 
 streamers, which might have been mere spaces without light, 
 and be caused by the darkness beyond, but they certainly ap- 
 peared to form a part of the phenomenon itself. These 
 streamers or long brushes could be seen beyond and clear of 
 the luminous portion of the aurora, leaving the normal light 
 of the sky between them and it, and hanging like long horse- 
 tails, or like the fringes of rain seen on the edges of a distant 
 rain-cloud, changing their shape and position just as the 
 luminous streamers are seen to do. 
 
 The most remarkable part of the phenomenon, however, 
 was the circle of sky, or what may be called the pole of the 
 aurora, to which the streamers converged. It appeared to em- 
 brace about from 7 to 10 of space. To an ordinary observer 
 it might have seemed occasionally to shift its position to some 
 extent, but, as far as I could judge during an hour's observa- 
 tions, this was not really the case ; flickerings at times covered 
 portions of it, and at other times the whole became faintly 
 luminous ; but by marking its position with reference to some 
 small stars the position of the circle seemed to me to be un- 
 altered. Most singular were the terminations of the streamers 
 as they culminated at this circle, not being undefined or gra- 
 dually evanescent, but having angular tips far brighter than 
 the portions immediately beneath, the nearest illustration to 
 which I can give is an inverted fish-tail or bat's-wing gas- 
 
448 EXPERIMENTAL INVESTIGATIONS. 
 
 burner, except that this gives a feeble light at the point, while 
 the aurora tips were whitest and brightest there, the streamers 
 now fading off, and now becoming brighter and tinged with 
 red as they got to 40 or 50 from the horizon ; the tips varied 
 constantly, but preserved the mean distance from the pole or 
 focus of the aurora. The position of this was, as far as I 
 could ascertain without star-maps or instruments for observa- 
 tion, about 15 WNW. of Vega. The convergence of the 
 beams was not in appearance conical, but dome or cupola 
 shaped ; this was, however, in all probability an optical illu- 
 sion. Whether there was really a convergence or whether the 
 beams were parallel, and the convergence an effect of per- 
 spective, can only be decided if some approximate measures 
 of the distance of the streamers be ascertained. It appears to 
 have been at a greater distance from the earth than is usually 
 attributed to aurora borealis, having been seen in different 
 parts of Europe and, I believe, in America. Doubtless the 
 comparison of these observations will give some parallax or 
 approximation to measurement of the distance. I remember 
 about seven or eight years ago seeing an aurora at Chester, 
 where the flashes appeared close to the observer, so that gleams 
 of light continuous with the streamers could be seen between 
 the houses of the town and myself, like the portions of a rain- 
 bow intervening between terrestrial objects and the observer. 
 I tried then to ascertain if there was any reflection or other 
 cause of optical illusion, but could not see it as other than a 
 real effect ; I seemed, so to speak, to be in the aurora. The 
 effect on the 25th was very different, and gave me the idea of 
 great distance. 
 
 The light was sufficient to enable me to tell the time by 
 my watch easily, but not to read newspaper print. 
 
 Between half-past six and seven o'clock it faded away, 
 and at from half-past seven to ten had become an ordinary 
 white aurora, confined to the northern portion of the heavens. 
 
ROCKING-STONES 449 
 
 ARTIFICIAL ROCKING-STONES AN EXPERIMENT. 
 
 Communicated to the Geological Sec. of the British Association, Norwich. 
 Norfolk Mws, Aug. 25, 1868. 
 
 SOME short time ago, during an excursion in Cornwall, my 
 attention was naturally directed to rocking-stones, and those 
 approximations to rocking-stones which are seen in the granite 
 where it is exposed to the action of the heat and cold, air and 
 water. I presume that I need not argue here that rocking- 
 stones are natural results, and not superposed on their pedes- 
 tals, as was once believed, by the hand of man. 
 
 Throughout the greater part of the granite rocks of the 
 west coast of Cornwall formations are to be seen approaching 
 in character to rocking-stones, or to discoid piles, like the 
 Cheesewring. 
 
 If we suppose a slab of stone of a parallelepiped form 
 lying on another, both having flat surfaces, or in other words 
 such slabs as are formed by fissures in horizontal and perpen- 
 dicular directions which are common in exposed granite rocks, 
 the attrition and disintegration produced by changes of wea- 
 ther, of temperature, &c., would necessarily act to the greatest 
 extent at the corners and next to that at the edges, because 
 those parts expose respectively the greater surfaces compared 
 with the bulk of the stone. This would tend to round off all 
 the angles, and gradually change the rhomb more or less towards 
 an oblate spheroid. This would account for the Cheesewring, 
 &c. But then, it may be asked, why should this process gra- 
 dually work on to a rocking-stone ? in other words, why should 
 the last unworn point, points, or line be in the line joining the 
 centre of gravity of the upper stone with that of the earth ? 
 Such an accident, it may be said, might happen, but the 
 chances are almost infinity to a unit against it. Not so. 
 Assume the wearing away between the slabs to reach a point 
 which is not in the line of centres of gravity, the upper stone 
 would then fall on one side, leaving the unworn point more 
 exposed to climatal and probably to electro-chemical action 
 from the water lying in the angle of the crevice, evaporation 
 
 G G 
 
450 EXPERIMENTAL INVESTIGATIONS, 
 
 being less rapid there than at other parts. This point would 
 then be worn away, and the stone would fall back a little ; 
 then fresh action upon new surfaces, another oscillation, and 
 so on. The effects which I have explained as taking place 
 by steps would, in fact, take place by insensible progression. 
 By assuming this process, unless there be some interfering 
 action, it becomes not improbable that the last point or line 
 worn away would be the point or line on which, from its 
 being in the line of centres of gravity, the upper stone would 
 rock. After seeing the great Logan-stone, near the Land's 
 End, I traced so many other approximations to rocking-stones 
 along that coast that I became satisfied, as far as one ought 
 to be satisfied on any subject of human enquiry, that this was 
 a correct theory. It then occurred to me, if this view be true, 
 may we not be able to hasten the operations of nature so as to 
 produce artificially (if such a term may be used) the rocking- 
 stone results ? A very little thought suggested the experiment. 
 Two parallelepipeds of iron, which had been made for keepers 
 of magnets, were taken, similar each to each, but that one was 
 twice the length of the other. The shorter was superposed on 
 the longer, and both immersed in sulphuric acid diluted with 
 three times its volume of water ; some nitric acid was added at 
 first, to hasten the corrosion. The liquid was changed from 
 time to time as it became nearly saturated, but without chang- 
 ing the position of the iron. At the end of three or four days 
 the pieces of iron were taken out, washed, and examined, when 
 the upper one was found to be a perfect analogue of a rocking- 
 stone, so delicately balanced on two points that it could be 
 made to rock by blowing on it with the mouth. (Result 
 shown.) It will be observed in this experiment that the iron 
 rocks only in one direction. Such is the case with the great 
 Logan-stone, and I believe with the greater number of rock- 
 ing-stones. It is obviously more probable that a stable equi- 
 librium should be obtained on two points than on one. I 
 have not yet got a specimen to rock or spin upon one point. 
 (Approximation to this shown by two zinc discs and explained.) 
 If the surfaces of the slabs be in such close contact that there 
 is not room for circulation of the saturated liquid, a formation 
 like those near the Cheesewring will be effected ; or if a num- 
 
DOCKING-STONES. 451 
 
 her of discs or slabs be superposed, and the lower ones more 
 exposed to the weather, and to catch the dripping and drifting 
 water from the upper, we should get a formation exactly like 
 the Cheesewring, which may be called an incipient compound 
 rocking-stone, in that each slab is worn away at the edges, 
 and the lower ones much more than the upper, so that if left 
 alone, which it won't be, and if it does not topple over too 
 soon, which it probably will, it might well end in a rocking- 
 stone. I should not. be surprised if it rocked now in a 
 great storm. 
 
 G G2 
 
NOTES AND REFERENCES 
 TO THE CORRELATION OF PHYSICAL FORCES. 
 
 5. THE reader who is curious as to the views of the ancients 
 
 regarding the objects of science, will find clues to them in the 
 second book of ARISTOTLE'S Physics, and in the first three 
 books of the Metaphysics. See also the Timseus of PLATO, 
 and RITTER'S History of Ancient Philosophy, where a sketch 
 of the Philosophy of LEUCIPPUS and DEMOCRITUS will be 
 found. 
 
 6. BACON'S Novum Organum, book ii. aph. 5 and 6. 
 
 7. HUME'S Enquiry concerning Human Understanding, S. 7, 
 
 London, 1768. 
 
 BROWN'S Enquiry into the Relations of Cause and Effect, Lon- 
 don, 1835. 
 
 The illustration I have used of a floodgate has been objected 
 to, as being one to which the term cause would scarcely be 
 applied, but after some consideration I have retained it j if 
 cause be viewed only as sequence, it must be limited to 
 sequence under given conditions or circumstances, and here, 
 given the conditions, the sequence is invariable. I see no 
 difference quoad the argument, between this illustration and 
 that of BROWN of a lighted match and gunpowder (4th edit, 
 p. 27), to which my reasoning would equally well apply. 
 
 HERSCHEL'S Discourse on the Study of Natural Philosophy, 
 pp. 88 and 149. 
 
 8. Quarterly Review, vol. Ixviii, p. 212. 
 
 WHEWELL, On the Question * Are Cause and Effect Successive 
 or Simultaneous ? ' (Cambridge Philosophical Transactions, 
 vol. vii. p. 319). 
 
454 NOTES AND REFERENCES. 
 
 PAGE 
 
 9. HERSCHEL'S Discourse, p. 93 . 
 
 AMPERE, Theorie des Phe'nomenes Electro-dynamiques, Me- 
 moirs in the Ann. de Chimie et de Physique, and works from 
 1820 to 1826, Paris. 
 
 15. LAMARCK, Sur la Matiere du Son (Journal de Physique, vol. 
 xlix. p. 397). 
 
 17. BABBAGE, On the Permanent Impression of our Words and 
 Actions on the Globe we inhabit, gth Bridgewater Treatise, 
 ch. ix. 
 
 20. MAYER, Annalen der Pharmacie Liebig und Wohler, May 1852. 
 
 RANKINE, Measure of Moving Force. (The Engineer, October 
 26, 1866). 
 
 22. JOULE, On the Mechanical Equivalent of Heat (Phil. Trans. 
 1850, p. 61). 
 
 ERMAN, Influence of Friction upon Thermo-electricity (Reports 
 of the British Association, 1845). 
 
 24. BECQUEREL, De'gagement de PElectricite' par Frottement, Traite 
 
 de TElectricite, torn. ii. p. 113, et seq. 
 
 25. SULLIVAN, Currents produced by the Vibration of Metals (Archiv. 
 
 de TElectricite, t. 10, p. 480). LEROUX, Vibrations Arrested 
 Produce Heat (Cosmos, March 30, 1860). 
 
 Mr. B. STEWART considers that the radiant forces are not pro- 
 duced directly by visible energy, by which I presume he means 
 motion. The difference is perhaps one of terms. Arrested mo- 
 tion produces heat in the body stopped, and this heat radiates. 
 It may be doubtful if this radiant heat should be said to be a 
 direct or intermediate result. (See B. STEWART, Conservation 
 of Energy, p. 105). 
 
 26. WHEATSTONE, On the Prismatic Decomposition of Electrical 
 
 Light (Notices of Communications to the British Association, 
 P- IJ > l8 35-) 
 
 27. BACON, De Forma Calidi, Nov. Org. book ii. aph. 20. 
 
 RUMFORD, An Enquiry concerning the Source of Heat which is 
 excited by Friction (Phil. Trans., p. 80, 1798). 
 
 DAVY, On the Conversion of Ice into Water by Friction (West of 
 England Contributions, p. 16). 
 
 Of Heat or Calorific Repulsion (Elements of Chemical Philo- 
 sophy, p. 69). 
 
NOTES AND REFERENCES. 455 
 
 AGE 
 
 29. BADEN POWELL, On the Repulsive Power of Heat (Phil. Trans. 
 
 1834, p. 485). 
 FRESNEL, Annales de Chimie, torn. xxix. pp. 57 and 107. 
 
 30. MOSER, On Invisible Light (Taylor's Scientific Memoirs, vol. iii. 
 
 pp. 461 and 465). 
 BLACK, On Latent Heat (Elements of Chemistry, p. 144, et 
 
 passim, 1803). 
 
 32. The experiments of HENRY and DONNY have shown that the 
 cohesion of liquids, as far as their antagonism to rupture goes, 
 is much greater than has been generally believed. These ex- 
 periments, however, make no difference in the view I have put 
 forth, as, whatever be the character of the attraction, there is a 
 molecular attraction to be overcome in changing bodies from the 
 solid to the liquid state, which must require and exhaust force. 
 
 DONNY, Stir la Cohesion des Liquides (Memoires de 1' Academic 
 Royale de Bruxelles, 1843). 
 
 HENRY, Proceedings of the American Philosophical Society, 
 April 1844 (Silliman's Journal, vol. xlviii. p. 215). 
 
 35. THILORIER, Solidification de TAcide Carbonique (Ann. de Ch. et 
 de Phys., torn. Ix. p. 432). 
 
 3 7. I. WEDGWOOD, Thermometer for Measuring the Higher Degrees 
 of Heat (Phil. Trans. 1782, p. 305; and 1786, p. 390). 
 
 TYNDALL, On the Physical Properties of Ice (Phil. Trans. 1858, 
 p. 211). 
 
 DESPRETZ, Recherche s sur le Maximum de Densite de 1'Eau 
 pure et des Dissolutions aqueuses (Ann. de Ch. et de Ph., 
 torn. Ixx. p. 45, and torn. Ixxiii. p. 296). 
 
 38. BIOT (Comptes rendus de F Academic des Sciences, Paris 1850, 
 
 p. 281). The experiments on circular polarisation by water 
 were, I believe, by Dr. Leeson. 
 
 39. I. THOMPSON, Trans. R. S. Edin. vol. xvi. p. 575. 
 W. THOMPSON, PhD. Mag., August 1850, p. 123. 
 
 BUNSEN, Pogg. Ann., vol. Ixxxi. p. 562 ; Ann. de Ch. et de 
 Phys., vol. xxxv. p. 383. Effects of Pressure on the Freezing 
 Point. 
 
 JOULE, Phil. Trans. 1859, p. 104. 
 
 40. DULONG and PETIT, and REGNAULT. See their Memoirs ab- 
 
 stracted and referred to in Ginelin's Handbook of Chemistry, 
 
456 NOTES AND REFERENCES. 
 
 PAGE 
 
 translated by Watts for the Cavendish Society, vol. i. p. 242, 
 et seq. 
 
 41. WOOD, Phil. Mag. 1851, 1852. 
 
 42. SENARMONT, Conduction of Heat by Crystals (Gmelin's Hand- 
 
 book, vol. i. p. 222). 
 
 43. KNOBLAUCH; Ann de Ch. et de Ph., vol. xxxvi. p. 124. 
 TYNDALL, Transmission of Heat through Organic Structures 
 
 (Phil. Trans., vol. cxliii. p. 217). 
 
 44. GROVE, Electricity produced by Approximating Metals (Expe- 
 
 rimental Investigations), p. 301. 
 
 GASSIOT, Phil. Mag., October 1844. 
 
 ROGET, On the Improbability of the Contact exciting Force ; 
 
 Treatise on Galvanism (Library of Useful Knowledge, 
 
 S. 113). 
 FARADAY, Phil. Trans. 1840, p. 126. 
 
 45. MELLONI, Sur la Polarisation de la Chaleur : Recherches sur 
 
 plusieurs Phenomenes Calorifiques (Annales de Chimie et de 
 Ph., torn. xlv. pp. 5-68; torn. xli. pp. 375-410 ; torn, xlviii. 
 pp. 198, 218). 
 
 FORBES, On the Refraction and Polarisation of Heat (Transac- 
 tions of the Royal Society of Edinburgh, vol. xiii.pp. 131, 168). 
 
 47. KIRCHOFF, Trans. Berlin Acad. 1861. 
 
 BALFOUR STEWART on the Theory of Exchanges (Report Bri- 
 tish Association, 1861). 
 
 48. T. WEDGWOOD, On the Production of Light and Heat by dif- 
 
 ferent bodies (Phil. Trans., vol. Ixxxii. p. 272). 
 
 49. TYNDALL, Proc. R. S., vol. iv. p. 487 ; Phil. Trans., vol. clvi. 
 
 p. i. 
 
 50. GROVE, On the Decomposition of Water into its Constituent 
 
 Gases by Heat (Experimental Investigations, p. 306). 
 
 ROBINSON, On the Effect of Heat in lessening the Affinities of 
 the Elements of Water (Transactions of the Royal Irish Aca- 
 demy, vol. xxi. p. 2). 
 
 52. GROVE, Water Decomposed by Chlorine and Heat (Phil. Trans. 
 1847, p. 20). 
 
 55. CARNOT, Reflexions sur la Puissance motrice du Feu, Paris, 
 1824. 
 
NOTES AND REFERENCES. 457 
 
 'AGE 
 
 60. SEGUIN, Influence des Chemins de Fer, p. 378 et seq. 
 
 6 1. JOULE and THOMSON, On the Thermal Effects of Fluids in 
 
 Motion (Phil. Trans. 1854, p. 338). 
 
 64. ROGERS, Consumption of Coal for Man-power (Cosmos, vol. ii. 
 
 p. 56). 
 
 67. MEYER and WATERSTON have suggested that solar heat may 
 
 arise from the mechanical action of meteoric stones falling into 
 the sun, and Mr. THOMPSON has written an elaborate paper 
 on the subject (Trans. Brit. Assoc. 1853). If a number of 
 gravitating bodies exist in the neighbourhood of the sun, and 
 form, as is conjectured, the zodiacal light, it is difficult to 
 conceive how comets, as they approach this region, steer 
 clear of such bodies, and are not even deflected from their 
 orbits. 
 
 For Mr. THOMPSON'S various and valuable papers, see Phil. Mag. 
 passim. B. STEWART, Conservation of Energy, p. 153. 
 
 68. POISSON, Comptes rendus, Paris, January 30, 1837. 
 
 69. WILLIAMS, MATTIEU, On the Fuel of the Sun. 
 
 71. DUFAYE, SYMMER, WATSON, and FRANKLIN, Theories of Elec- 
 
 tric Fluid and Electric Fluids (Priestley's History of Electri- 
 city, pp. 429-441). 
 
 GROTTHUS, Sur la Decomposition de 1'Eau et des Corps qu'elle 
 tient en dissolution a aide de PElectricite Galvanique (Ann. de 
 Chimie, torn. Iviii. p. 54). 
 
 72. FARADAY, On the Question whether Electrolytes Conduct with- 
 
 out Decomposition (Proceedings of the Weekly Meetings of 
 the Royal Institution, 1855). 
 
 GROVE, Comptes rendus, Paris, 1839. 
 
 FARADAY, On Induction as an Action of Contiguous Particles 
 (Phil. Trans. 1838, p. 30). 
 
 MATTEUCCI, Plates of Mica Polarised by Electricity (De la Rive's 
 Electricity, p. 140). 
 
 73. GROVE, Experimental Investigations, 415. 
 
 KARSTEN on Electrical Figures (Archiv. de 1'Elec., vols. ii. iii. 
 and iv.) 
 
 74. GROVE, Etching Electrical Figures and Transferring them to Col- 
 
 lodion (Experimental Investigations, p. 402). 
 
458 NOTES AND REFERENCES. 
 
 PAGE 
 
 75. FUSINIERI, Du Transport des Matieres ponderables qui s'opere 
 
 dans les Decharges Electriques (Archives de 1'Electricite ; 
 Supplement a la Bibliotheque Universelle de Geneve, torn. iii. 
 P- 597)- 
 
 76. GROVE, On the Voltaic Arc (Report of Lecture at the Royal 
 
 Institution, Lit. Gaz. and Athenaeum, Feb. 7, 1845 ; Experi- 
 mental Investigations, p. 328). 
 
 77. GROVE, On the Electro-chemical Polarity of Gases (Experimental 
 
 Investigations, p. 356). 
 
 80. FREMY and E. BECQUEREL, Oxygen Changed to Ozone by the 
 Electric Spark (Ann. de Ch. et de Phys. 1852). This subject 
 and the nature of Ozone was first investigated by Dr. Schon- 
 bein. See also a paper by Mr. Brodie, On the Conditions of 
 certain Elements at the Moment of Chemical Change (Phil. 
 Trans. 1850). 
 
 82. Molecular Changes in Electrised Metals (NAIRNE, Phil. Trans. 
 
 1780, p. 334, and 1783, p. 223 ; GROVE, Experimental Inves- 
 tigations, p. 305 ; PELTIER, Archives de 1'Electricite, vol. v. 
 p. 182 j FUSINIERI, id. p. 516). 
 
 WERTHEIM, Change in Elasticity of Metals by Electrisation 
 (Ann. de Ch. et de Phys., vol. xii. p. 623 ; Arch. Elec., vol. iv. 
 p. 490). 
 
 DUFOUR, Alteration in Tenacity of Metals by Electrisation 
 (Bibl, Univ. de Geneve, Fev. 1855, p. 156). 
 
 83. MATTEUCCI, Conduction of Electricity by Crystals (Comptes ren- 
 
 dus de 1'Acad., Paris, March 5, 1855, p. 541). 
 
 84. E. BECQUEREL, Transmission of Electricity by Heated Gases 
 
 (Ann de Ch. et de Phys., vol. xxxix. p. 355). 
 GROVE, Experimental Investigations, p. 356, note. 
 
 BECQUEREL, Divergence of Gold-leaves in Vacuo (Traite 
 d'Electricite, vol. v., part. ii. p. 55). 
 
 85. NEWTON, Thirty-first Query to the Optics. 
 
 GROVE, Particles of Metals and Metallic Oxides detached in 
 Liquids by Electricity (Experimental Investigations, p. 303). 
 
 86. MATTEUCCI, Relations of Electricity and Nervous Force (Phil. 
 
 Trans. 1845, p. 285, 1846, p. 497 j Phenomenes Physiques 
 des Corps Vivants, p. 305 ; Lezioni di Fisica, p. 360). 
 
NOTES AND REFERENCES. 459 
 
 PAGE 
 
 GALVANI VOLTA MARIANIM et NOBILI on Physiological Effects 
 of Electricity (Ann. de Ch. et de Phys., vols. 23, 25, 29, 38, 
 40, 43, 44, 56). 
 
 88. BECQUEREL, Chemical Changes by Friction (Traite de 1'Elec., 
 vol. v., part i, p. 1 6). 
 
 91. DE LA RIVE, Heat of the Voltaic Pile (Bibl. Univ., vol. xiii. 
 
 p. 389). 
 
 DAVY, On the Properties of Electrified Bodies in their relations 
 to Conducting Powers and Temperature, (Phil. Trans. 1821, 
 p. 428). 
 
 92. GROVE, On the Effect of surrounding Media on Voltaic Ignition 
 
 (Experimental Investigations, p. 336). 
 
 OERSTED, Experiences sur 1'Effet du Conflit electrique sur FAi- 
 guille aimantee (Ann. de Ch. et de Phys., torn. xiv. p. 417). 
 
 94. COLERIDGE, Table Talk, vol. i. p. 65. 
 
 LENZ and JACOBI, Pogg. Ann., vol. xvlii. p. 403 ; Bulletin de 
 1'Acad. St. Petersburg, 1839 ; Harris, Magnetism, part 2, 
 p. 63. 
 
 DAVY, Decomposition of the fixed Alkalies (Phil. Trans. 1808, 
 
 P . i). 
 
 BECQUEREL, Des Composes electro-chimiques (Traite de FElec- 
 tricite, vol. iii. c. 13). 
 
 CROSSE, Transactions of the British Association, vol. v., p. 47; 
 Proceedings of the Electrical Society, p. 320. 
 
 95. MALUS, Polarisation of Light by Reflexion (Me'moires d'Arcueil, 
 
 torn. ii. p. 143). 
 
 ARAGO, Circular Polarisation by Solids (Memoires de PInstitut, 
 1811). 
 
 96. BIOT, Circular Polarisation by Liquids (Memoires de PInstitut, 
 
 1817). 
 
 NIEPCE and DAGUSRRE, Historique et Description des Proce / de / s 
 du Daguerreotype ; Paris, 1839. 
 
 TALBOT, Photogenic Drawing and Calotype (Phil. Mag., March 
 1839, and August 1841). 
 
 98. HERSCHEL, Chemical Action of the Solar Spectrum on various 
 Substances (Phil. Trans. 1840, p. i, and 1842, p. 181). 
 
 HUNT, Researches on Light, London, 1844. 
 
460 NOTES AND REFERENCES. 
 
 PAGE 
 
 100. B. STEWART, Conservation of Energy, pp. 129, 144. 
 
 101. GROVE, Other Forces produced by Light, and Influence of 
 
 Light on the Polarised Electrode (Experimental Investigations, 
 p. 406). 
 
 SOMERVILLE (Mrs.), On the Magnetising Power of the more Re- 
 frangible Solar Rays (Phil. Trans. 1826, p. 132). 
 
 MORICHINI'S experiments are given in Mrs. Somerville's paper, 
 
 1 02. HERSCHEL, On the Absorption of Light in Coloured Media 
 
 viewed in connection with the Undulatory Theory (Phil. Mag. 
 December 1833). 
 
 TYNDALL, Proc. R. I., vol. iv. p. 491. 
 
 SEEBECK, Heat of Coloured Rays (Brewster's Optics, p. 90). 
 
 103. KNOBLAUCH (Ann. de Ch., vol. xxxvi. p. 124, and Pogg. Ann., 
 
 there referred to). 
 
 104. HERSCHEL, Epipolised Light (Phil. Trans., vol. cxxxv. pp. 143, 
 
 H7)- 
 
 STOKES, Change in Refrangibility of Light (Phil. Trans., vols. 
 cxlii. cxliii.) 
 
 1 08. For the first enunciations of the Corpuscular and Undulatory 
 Theories, see NEWTON'S Optics, HOOKE'S Micographia, and 
 HUYGHEN'S Tractatus de Lumine. See also BREWSTER'S 
 Optics, p. 138. 
 
 109. YOUNG, Lectures edited by Kelland, p. 359 et seq. ; Phil. 
 
 Trans. 1800, p. 126 ; HERSCHEL, Encyc. Metrop., art. Light, 
 pp. 450 and 738 j NEWTON'S Optics, p. 322 ; WHEWELL'sHist. 
 Indue. Sc., vol. ii. p. 449 ; FOUCAULT, Comptes rendus, Paris, 
 1850, p. 65 ; HARRISON, Phil. Mag., November 1856; Camb. 
 Phil. Trans. 
 
 in. SONDHAUSS, Refraction of Sound (Ann. de Ch. et de Phys., 
 vol. xxxv. p. 505) j DOVE, Polarisation of Sound (Cosmos, 
 May 13, 1859). 
 
 112. GROVE, Exp. Investns., 336. 
 
 TYNDALL, Phil. Trans. 1861, 1862, 1864. 
 
 117. PASTEUR, Rotation of Plane of Polarised Light by Solutions 
 of Hemihedral Crystals (Ann. de Ch. et de Phys., vol. xxiv. 
 p. 442. 
 
 119. COMTE, Cours de Philosophic Positive, vol. ii. p. 409. 
 
NOTES AND REFERENCES. 461 
 
 PAGE 
 
 119 to 123. WOLLASTON, Phil. Trans. 1822, p. 89; WHEWELL, Phil, 
 of the Induct. Sc., vol. i. p. 419 ; WILSON, Trans, of the Roy. 
 Soc. of Ed in., vol. xvi. p. 79 ; Sir W. HERSCHEL, Phil. Trans. 
 J 7935 P- 201 an d 1 80 1, p. 300; MORGAN, Phil. Trans., 
 vol. Ixxv. p. 272 ; DAVY, Phil. Trans. 1822, p. 64; Elements 
 of Chemical Philosophy, p. 97 ; GASSIOT, Phil. Trans. 1859, 
 
 P- 157- 
 
 Diminishing Periods of Comets (Herschel's Outlines of Astro- 
 nomy, p. 357). NEWTON, 3oth Quere to the Optics. 
 124. EMPEDOCLES. 
 -126. STRUVE, Etudes d'Astronomie Stellaire, 1847. 
 
 127. FARADAY, Evolution of Electricity from Magnetism (Phil. 
 Trans. 1832, p. 125). 
 
 129. FARADAY, Magnetic Condition of all Matter (Phil. Trans. 1846, 
 
 p. 21 ; Phil. Mag. 1846, p. 249). 
 
 BECQUEREL, Ann. de Ch. et de Ph., torn, xxxvi. p. 337 ; 
 Comptes rendus, Paris, 1846, p. 147; and 1850, p. 201. 
 
 130. FARADAY, On the Magnetisation of Light (Phil. Trans. 1846, 
 
 P . i). 
 
 131. WARTMANN, Rotation of the Plane of Polarisation of Heat by 
 
 Magnetism (Journal de ITnstitut, No. 644). 
 
 PROVOSTAYE and DESSAINES, Ann. de Ch. et de Phys., October 
 
 1849. 
 HUNT, Influence of Magnetism on Molecular Arrangement 
 
 (Phil. Mag. 1846, vol. xxviii. p. i ; Memoirs of the Geological 
 
 Society, vol. i. p. 433). 
 
 WARTMANN, Phil. Mag. 1847, v l' xxx - P- 2 ^3- 
 GROVE, Experiment on Molecular Motion of a Magnetic Sub- 
 stance, Exp. Investns., p. 354. 
 
 132. On the direct Production of Heat by Magnetism, Exp. 
 
 Investns., p. 355. 
 
 VAN BREDA, Comptes rendus, October 27, 1845. 
 JOULE, Phil. Mag. 1843. 
 
 136. The Experiments on the Effects of Magnetism on the Matter 
 Magnetised are collected by Mr. DE LA RIVE in his recently- 
 published Treatise on Electricity, vol. i. 
 
 138. DAVY, Electricity denned as Chemical Affinity acting on Masses 
 (Phil. Trans. 1826, p. 389). 
 
462 NOTES AND REFERENCES. 
 
 PAGE 
 
 VOLTA, Electricity excited by the mre Contact of Conducting 
 Substances (Phil. Trans. 1800, p. 403). 
 
 139. GROVE, Gold-Leaf Experiment, Exp. Investns., p. 232. 
 
 GROVE, Voltaic Action of Sulphur, Phosphorus, and Hydro- 
 carbons, Exp. Investns., p. 285. 
 
 140. GROVE, New Voltaic Combination, Exp. Investns., p. 231. 
 GROVE, Electricity of Blowpipe Flame, Exp. Investns., p. 392. 
 
 141. DALTON, New System of Chemistry, London, 1810. 
 
 142. I have here and elsewhere used whole numbers, as sufficiently 
 
 approximate for the argument, but without intending to 
 express any opinion as to the so-called law of PROUT. 
 
 FARADAY, Definite Electrolysis (Phil. Trans. 1834, p. 77). 
 144. WOOD, Heat Disengaged in Chemical Combinations (Phil. Mag. 
 
 i8 S2 ). 
 
 146. ANDREWS, Phil. Trans. 1844, p. 21. 
 
 HESS, Poggendorff's Annalen, Bd. Hi. p. 107. 
 
 147. FAVRE, Ann. de Ch. et de Phys., vols. xxxix. xl. ; Comptes 
 
 rendus, Paris, vol. xlv. p. 56, and vol. xlvi. p. 337. 
 
 153. CATALYSIS by Platinum (DOBEREIMER, Ann. de Ch. etde Phys., 
 torn. xxiv. p. 93 ; DULONG and THENARD, Ann. de Ch. et de 
 Phys., torn, xxiii. p. 440). 
 
 GROVE, Gas Voltaic Battery, Exp. Investns., pp. 249, 253. 
 
 155. MOSOTTI, Forces which Regulate the Internal Constitution of 
 
 Bodies (Taylor's Scientific Memoirs, vol. i. p. 448). 
 
 PLUCKER, Repulsion of the Optic Axes of Crystals by the 
 Poles of a Magnet (Taylor's Scientific Memoirs, vol. v. 
 P- 353)- 
 
 Magnetic Action of Cyanite (Lit. Gaz. 1849, p. 431). 
 
 156. MATTEUCCI, Correlation of Electric Current and Nervous 
 
 Force (Phil. Trans. 1850, p. 287). 
 
 157. CARPENTER, On the Mutual Relations of the Vital and Physical 
 
 Forces (Phil. Trans. 1850, p. 751). 
 
 158. On Effort. See BROWN, Cause and Effect ; HERSCHEL'S Dis- 
 
 course ; and Quarterly Review, June 1841. 
 
 HELMHOLTZ, M tiller's Archives, 1845 \ MATTEUCCI, Comptes 
 rendus, Paris, 1856 ; BECLARD, Archives de Medecine, 1861. 
 
NOTES AND REFERENCES. 463 
 
 PAGE 
 
 172. DULONG and PETIT, Relation between Specific Heat and Chemi- 
 cal Equivalents (Ann. de Ch. et de Phys., torn. x. p. 395). 
 NEUMANN, PoggendorfFs Annalen., Bd. xxiii. p. i. 
 AVOGADRO, Ann. de Ch. et de Phys., torn, Iv. p. 80. ' 
 
 NOTES AND REFERENCES 
 
 TO" 
 CONTINUITY. 
 
 1 86. HERSCHEL, Sir J., Astronomical Observations at the Cape of 
 
 Good Hope, 1847. 
 
 WROTTESLEY, Lord, On Double Stars, Phil. Trans. 1851 ; Proc. 
 R. S. 1849. 
 
 ROSSE, Earl of, Observations on the Nebulas (Phil. Trans. 1850, 
 p. 499). 
 
 187. OLMSTED, Silliman's Journal, July 1834, p. 138. 
 
 The first suggestion of a perspective vanishing-point for meteors 
 seems to be due to Professor Thomson, of Nashville. 
 
 HERSCHEL, ALEXANDER, Reports of the Meteor Committee of 
 the British Association. 
 
 SORBY, Idem, and Proc. R. S. June 16, 1864. 
 
 BRAYLEY, Idem, 1865, p. 140, and Proc. R. S. March 23, 
 1865. 
 
 1 88. LEVERRIER, Intra-Mercurial Planets (Comptes rendus, Paris, 
 
 1861, p. 1109). 
 
 DAUBREE, Comptes rendus, Paris, 1866 (Bulletin de la Societe 
 Geologique de France, Mars 1866). 
 
 190. PLUCKER, Variation of Spectrum Lines with Temperature 
 
 (Phil. Trans. 1865, p. 6). 
 
 191. HUGGINS and MILLER, Spectra of Fixed Stars (Phil. Trans. 
 
 1864, p. 413). 
 
 Spectrum of Temporary Star (Proc. R. S., No. 84, 1866). 
 
464 NOTES AND REFERENCES. 
 
 PAGE 
 
 HUGGINS, Spectrum of Comet, 1866 (Proc. R. S., No. 80, 
 1866). 
 
 193. CHACORNAC, On the Moon (Comptes rendus, Paris, June 1866, 
 
 p. 1406, &c.). 
 
 194. GROVE, Monthly Notices Ast, Soc. January 14, 1853, Exp. 
 
 Investns., p. 431. 
 
 196. RUMFORD, Heat of Friction (Phil. Trans. 1798, p. 80). 
 DAVY, Idem, West of England Contributions, p. 18. 
 
 197. SABINE, Magnetism and Solar Spots (Proc. R. S. 1865, p. 491). 
 AIRY, On Solar Magnetism (Phil. Trans. 1863, pp. 313-646). 
 CHAMBERS, Idem, Phil. Trans. 1863, pp. 514-516. 
 
 198. MAYER, Friction of Tidal Wave (See his Papers collected and 
 
 translated by Youmans, New York, 1865). 
 
 DELAUNAY, Acceleration of Moon's Motion (Comptes rendus, 
 Paris, Dec. 1865, Jan. 1866). 
 
 AIRY, Idem, Notices R. Ast. Soc., April 13, 1866. 
 CARRINGTON, Observations on Spots on the Sun, 1863. 
 DE LA RUE, STEWART, and LOEWY, Idem, 1865. 
 
 FAYE, On the Dynamic Theory of Solar Heat (Comptes rendus, 
 Paris, Oct. 1862, p. 564). 
 
 Constitution of Sun. Motion of Sun-spots, &c. (Comptes 
 rendus, Paris, Jan. 1866, &c.). 
 
 199. STRUVE, Etudes d'Astronomie Stellaire, 1847. 
 
 The passage in the text is so brief as to be obscure. (See the 
 idea elaborated, Corr. Phys. Forces, p. 187). 
 
 200. See Corr. Phys. Forces, p. 52. 
 
 BERTHELOT, Formate of Potash (Institut. 1864, p. 332). 
 
 201. TYNDALL, On Radiant Heat (Phil. Mag., Nov. 1864; Phil. 
 
 Trans. 1866). 
 GRAHAM, Dialysis of Air (Phil. Trans. 1866, p. 399). 
 
 202. WILDE, Increase of Magneto-electric Force (Proc. R. S., April 
 
 1866, p. 107). 
 
 HOLTZ, New Electrical Machine (Pogg. Annalen, 1865, pt i, 
 P- 157). 
 
 203. CARPENTER, Food and Force ; Physiology, Treatise on. 
 
NOTES AND REFERENCES. 465 
 
 PACK 
 
 BENCE JONES, Idem, Proc. R. I. March 23, 1866! 
 PLAYFAIR, Idem, Proc. R. I. April 28, 1865. 
 E. SMITH, Idem, Phil. Trans. 1861, p. 747. 
 FRANKLAND, Idem, Proc. R. I. 1866. 
 
 204. TRAUEE, Virchow's Archiv., vol. xxiii. p. 196, &c. 
 
 FICK and WISLICENUS, Idem, Phil. Mag., June 1866, Supple- 
 ment. 
 
 205. LAVOISIER, GEuvres, vol. ii. p. 640. 
 ANSTED, Intellectual Observer, Aug. 1864. 
 
 206. RAMSAY, Addresses to the Geological Society, 1863 and 1864. 
 
 207. HERSCHEL, Sir J., Geological Effects of Variation in Earth's 
 
 Orbit (Trans. Geol. Soc., 2nd series, vol. iii. p. 295; Outlines 
 of Astronomy, 1864, pp. 233-235). 
 
 CROLL, Idem, Phil. Mag., Aug. 1864, and April 1866. 
 
 See farther, Quarterly Journal of Sciences, 1869, p. 117, and 
 WALLACE, Nature, July 17, 1870. 
 
 211. PASTEUR and POUCHET, On Spontaneous Generation (Comptes 
 rendus, Paris, 1863 to 1865 inclusive). 
 
 CHILD, Proc. R. S. 1865, p. 178. 
 
 215. CARPENTER, On Foraminifera (Phil. Trans. 1856, p. 227 ; 1860, 
 
 p. 584). 
 
 216. H. BATES, Butterflies of South America (Trans. Linn. Soc., 
 
 xxv. xxiii. p. 495). 
 
 217. WALLACE, Butterflies of Malay, &c. (Trans. Linn. Soc., vol. 
 
 xxv. p. i). 
 
 WALSH, Proc. Entom. Soc., Philadelphia, 1864, p. 403. 
 
 FRITZ MULLER, Fiir Darwin, Leipsig, 1864 (Annals and 
 Magazine of Natural History, 1865). 
 
 LUBBOCK, Diving Hymenoptera (Trans. Linn. Soc., vol. xxiv. 
 P- 135)- 
 
 218. LOGAN, Eozoon, Communication to the British Association at 
 
 Bath, 1864. 
 
 A. DE CANDOLLE, Variability in Oaks, &c. (Bibl. Univ. de 
 Geneve, Nov. 1862). 
 
 HOOKER, On Oaks (Trans. Linn. Soc., vol. xxiii. p. 381). 
 H H 
 
466 NOTES AND REFERENCES. 
 
 PAGE 
 
 219. On Arctic Flora (Trans. Linn. Soc., vol. xxiii. p. 251). 
 
 220. The parts between brackets were not in the original disco arse. 
 
 221. For instances of retrocession or degradation, see Quarterly 
 
 Review, Oct. 1869. 
 225. DARWIN, Origin of Species through Natural Selection, 1866, in 
 
 which see also Dr. McDonnell's results. 
 HUXLEY, Address to the Geological Society, Feb. 21, 1862. 
 LYELL, Antiquity of Man, 1862. 
 
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