REESE LIBRARY 
 
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 UNIVERSITY OF CALIFORNIA. 
 
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SCIENTIFIC MEMOIRS 
 
 BEING 
 
 EXPERIMENTAL CONTRIBUTIONS 
 
 TO 
 
 A KNOWLEDGE OF RADIANT ENERGY 
 
 BY 
 
 JOHN WILLIAM DRAPER, M.D., LL.D. 
 
 PRESIDENT OF THE FACULTY OF SCIENCE IN THE UNIVERSITY OF NEW YORK 
 AUTHOR OF U A TREATISE ON HUMAN PHYSIOLOGY*' "HISTORY OF THE 
 INTELLECTUAL DEVELOPMENT OF EUROPE 
 
 THE AMERICAN CIVIL WAR " ETC. 
 
 (5* 
 
 *-vu 
 
 NEW YORK 
 HARPER & BROTHERS, PUBLISHERS 
 
 FRAXKLIN SQUARE 
 
 1878 
 
Entered according to Act of Congress, in the year 1878, by 
 HARPER & BROTHERS, 
 
 /4Jy& . 
 
 In the Office of the Librarian of Congress, at Washington. 
 
PREFACE. 
 
 DURING the past forty years I have devoted much time to the experi- 
 mental investigation of scientific topics, and have published the results 
 in various journals, pamphlets, and the transactions of learned societies. 
 They have been largely disseminated in European languages, and many 
 of the conclusions they have presented have been admitted into the 
 accepted body of scientific knowledge. 
 
 It has therefore become desirable for me to collect these scattered 
 memoirs and essays together, and, since they are too voluminous to be 
 published in full, to offer an abridgment or condensation of those that 
 are of less interest. I propose in this book to include only such as are 
 connected with the effects of Radiations or of Radiant energy, these 
 having been distinguished by the American Academy of Science, as mani- 
 fested by its award to me of the Rumford medal for discoveries in light 
 and heat. A statement of the action of the Academy is annexed. 
 
 Besides these, I have several other memoirs on chemical, electrical, 
 and physiological topics, some of them hitherto unpublished. These, for 
 the present, I must reserve. 
 
 Among many other subjects treated of in these pages, the reader will 
 find an investigation of the temperature at which bodies become red-hot, 
 the nature of the light they emit at different degrees, the connection be- 
 tween their condition as to vibration and their heat. It is shown that 
 ignited solids yield a spectrum that is continuous, not interrupted. This 
 has become one of the fundamental facts in astronomical spectroscopy. 
 At the time of the publication of this Memoir, no one in America had 
 given attention to the spectroscope, and, except Fraunhofer, few in Eu- 
 rope. I showed that the fixed lines might be photographed, doubled 
 their number, and found other new ones at the red end of the spectrum. 
 The facts thus discovered I applied in an investigation of the nature of 
 flame and the condition of the sun's surface. I showed that" under cer- 
 tain circumstances rays antagonize each other in their chemical effect, 
 and that the diffraction spectrum has great advantages over the prismat- 
 ic, which is necessarily distorted. I attempted to ascertain the distribu- 
 tion of heat in the diffraction spectrum, and pointed out that great ad- 
 
x PKEFACE. 
 
 vantages arise if wave-lengths are used in the description of photographic 
 phenomena. I published steel engravings of that spectrum so arranged. 
 I made an investigation of phosphorescence,.and obtained phosphores- 
 cent pictures of the moon. Up to this time it had been supposed that 
 the great natural phenomenon of the decomposition of carbonic acid 
 by plants was accomplished by the violet rays of light, but by perform- 
 ing that decomposition in the spectrum itself, I showed that it is effected 
 by the yellow. Under very favorable circumstances, I examined the ex- 
 periments said to prove that light can produce magnetism, and found 
 that they had led to an incorrect conclusion. The first photographic 
 portrait from the life was made by me : the process by which it was ob- 
 tained is herein described. I also obtained the first photograph of the 
 moon. I made many experiments on and discovered the true explana- 
 tion of the crystallization of camphor towards the light. When Da- 
 guerre's process was published, I gave it a critical examination, and 
 described the analogies existing between the phenomena of the chemical 
 radiations and those of heat. For the purpose of obtaining more accu- 
 rate results in these various inquiries, I invented the chlor - hydrogen 
 photometer, and examined the modifications that chlorine undergoes in 
 its allotropic states. Since in such researches more delicate thermom- 
 eters are required than our ordinary ones, I entered on an investigation 
 of the electro-motive power of heat, and described improved forms of 
 electric thermometers. In these memoirs will be found a description of 
 the method made use of for obtaining photographs of microscopic ob- 
 jects, together with specimens of the results. In a physiological digres- 
 sion respecting interstitial movements of substances, I examined the pas- 
 sage of gases through thin films such as soap-bubbles, and the force with 
 which these movements are accomplished, applying the facts so gathered 
 to an explanation of the circulation of the sap in plants, and of the blood 
 in animals. Returning to an inquiry as to the distribution of heat and of 
 chemical force in the spectrum, I was led to conclude, in opposition to 
 the current opinion, that all the colored spaces are equally warm ; and 
 that, so far from one portion the violet being distinguished by pro- 
 ducing chemical effects, every ray can accomplish special changes. This 
 series of experiments on radiations is concluded in this volume by an 
 examination of the chemical action of burning-lenses and mirrors. 
 
 I have endeavored to reproduce these memoirs as they were original- 
 ly published. When considerations of conciseness have obliged me to 
 be contented with an abstract, it has always been so stated, and the 
 place where the original may be found has been given. Sometimes, the 
 circumstances seeming to call for it, additional matter has been intro- 
 
PREFACE. x j 
 
 duced ; but this lias always been formally indicated under the title of 
 NOTES, or included in parentheses. An instance of the former occurs on 
 page 45, of the latter on page 30. Wood-cuts and their explanation, sel- 
 dom occurring in the original publications, have been introduced. They 
 have for the most part been obtained from some popular articles pub- 
 lished by me in Harper's Magazine. 
 
 Except in a few instances, I have adhered, in these memoirs, to the chem- 
 ical nomenclature in use at the time they were written. Though often 
 very weighty reasons may be given that the designations under which 
 substances pass might be made more in accordance with their constitu- 
 tion or properties, and therefore more correct, yet such are the con- 
 fusions, the inconveniences, the difficulties attending an introduction of 
 new names, that sweeping changes of nomenclature should never be in- 
 troduced until they have become absolutely indispensable. In Memoir 
 X., which treats of the action of the leaves of plants under the influence 
 of yellow light, I have preferred to retain the term carbonic acid instead 
 of any of its more recent synonyms. 
 
 Here and there the reader will detect statements that seem to be con- 
 tradictory. On examination, however, he will find that this arises from 
 changes which the general progress of science had made necessary. As 
 an illustration of what I mean, I may refer to what is given as regards 
 wave-lengths (from Mosotti) on page 112. These numbers do not agree 
 with the more exact ones of Angstrom, page 120. It is better in such 
 cases to let the original statements stand. 
 
 The pages offered in this volume, though not very numerous, repre- 
 sent a very large amount of work, the occupation of many years. Ex- 
 perimental investigation, to borrow a phrase employed by Kepler respect- 
 ing the testing of hypotheses, is " a very great thief of time." Some- 
 times it costs many days to determine a fact that can be stated in a line. 
 The things related in these memoirs have consumed much more than 
 forty years. 
 
 Such a publication, therefore, assumes the character of an autobiog- 
 raphy, since it is essentially a daily narrative of the occupations of its 
 author. To a reader imbued with the true spirit of philosophy, even 
 the shortcomings easily detectable in it are not without a charm. From 
 the better horizon he has gained he watches his author, who, like a 
 pioneer, is doubtfully finding his way, here travelling on a track that 
 leads to nothing, then retracing his footsteps, and again, undeterred, 
 making attempts until success crowns his exertions. To explore the 
 path to truth implies many wanderings, many inquiries, many mis- 
 takes. 
 
 Perhaps, then, since this book is a sort of autobiography, its reader 
 
x {{ PREFACE. 
 
 will bear with me if I try to make it more complete by here referring to 
 other scientific or historical works in which I have been engaged. 
 
 In early life I had felt a strong desire to devote myself to the experi- 
 mental study of nature ; and, happening to see a glass containing some 
 camphor, portions of which had been caused to condense in very beauti- 
 ful crystals on the illuminated side, I was induced to read everything I 
 could obtain respecting the chemical and mechanical influences of light, 
 adhesion, and capillary attraction. Experiments I soon made in con- 
 nection with these topics are described in these memoirs. Some of 
 them I used in a Thesis for the degree of Doctor of Medicine in the 
 University of Pennsylvania. My thoughts were thus directed to physi- 
 ological studies, and I published papers on these topics in the Ameri- 
 can Journal of Medical Sciences. The favorable impression they made 
 caused me to be appointed, in 1836, Professor of Chemistry and Physi- 
 ology in Hampden Sidney College, Virginia, an appointment which en- 
 abled me to convert experimental investigation, thus far only an amuse- 
 ment, into the appropriate occupation of my life. 
 
 Several of the memoirs contained in this volume were composed at 
 that time. To them I was indebted, without any application on my part, 
 for an appointment to the Professorship of Chemistry and Physiology 
 in the University of New York. Soon afterwards I published a work 
 on the Forces that Produce the Organization of Plants. The lectures on 
 Physiology I gave at that time I improved from year to year, and at 
 length published them as a treatise on Human Physiology. It was very 
 favorably received by the medical profession. 
 
 Among new experiments and explorations on physiological subjects 
 contained in that book may be mentioned the selecting action of mem- 
 branes ; cause of the coagulation of blood ; theory of the circulation of 
 the blood ; explanation of the flow of sap ; endosmosis through thin films ; 
 measure of the force of endosmosis; respiration of fishes; action of the 
 organic muscle fibres of the lungs ; allotropism of living systems ; new 
 observations on the action of the skin ; functions of nerve vesicles and 
 their electrical analogies ; function of the sympathetic nerve ; explanation 
 of certain parts of the auditory apparatus, particularly of the cochlea 
 and semicircular canals ; the theory of vision ; the theory of muscular 
 contraction. 
 
 From the study of individual man it is but a step to the consideration 
 of him in his social relation, and this, accordingly, had been done in the 
 second part of my work on Physiology. But the subject being too ex- 
 tensive to be dealt with satisfactorily in that manner, I published the 
 
 
 
PREFACE. 
 
 materials that I had collected in a separate book, under the title of " A 
 History of the Intellectual Development of Europe." The object of 
 this was mainly to point out that the intellectual progress of nations 
 proceeds in the same course as the intellectual development of the indi- 
 vidual ; that the movement of both is not fortuitous, but under the 
 dominion of law ; that the stages of personal development are paralleled 
 by the stages of social development, and, indeed, as palaeontology has 
 proved, by the evolution of all animated nature ; that there is an ascent 
 of man through well-marked epochs from the most barbarous to the most 
 highly civilized condition. This book was translated into many lan- 
 guages : in some of them several editions of it were issued. Portions 
 of it relating to Mohammedan science were translated into Arabic. 
 
 About this time, circumstances led me to deliver before the New York 
 Historical Society a course of lectures on American topics, considered 
 from a similar point of view. These were enlarged, and published un- 
 der the title of "Thoughts on the Civil Policy of America." This, like 
 the preceding, was extensively translated and circulated. The train of 
 investigation on which I had thus entered led me to a far more serious 
 undertaking a " History of the American Civil War," which had just 
 then closed. To this, moreover, I was incited by the earnest request of 
 some who had been chief actors in the events, and who ve"ry effectively 
 aided me. I had the inestimable advantage of enjoying the friendship 
 of many whose names have now become illustrious in connection with 
 those times. The Secretary of War gave me access to the public docu- 
 ments on both sides, and to him I was indebted for guidance in the de- 
 scription of many of the most important incidents and the course of na- 
 tional policy. To generals who had commanded in great battles and 
 conducted great campaigns I owed information which they alone could 
 impart, and, in like manner, most valuable assistance was given me in 
 special cases by persons eminent in military and civil life. It is often 
 said that the history of any very great social event cannot be correctly 
 written by a contemporaneous author, and that we must wait until 
 passions have subsided and interests ceased for a narrative of the truth. 
 But this is not so. More depends on the impartiality of the writer than 
 on the deadening lapse of time. The best history will always be written 
 by one who has had the best opportunity of getting at the facts, who 
 has had the privilege of the friendship or personal acquaintance of those 
 who have been conspicuous in the events. 
 
 No one can consider the intellectual development of Europe without 
 contemplating the forces that have brought that continent to its present 
 social condition forces that have never ceased to be in active opposition. 
 Under the title of a " History of the Conflict of Religion and Science," 
 
x [ v PREFACE. 
 
 I endeavored to describe their warfare. This work has passed through 
 a great many editions in America and England. It has been translated 
 into French, Spanish, German, Russian, Italian, Polish, Servian, etc. It- 
 finds very many readers in Eastern Europe. And of some of these 
 translations several editions have been issued. 
 
 When I thus look back on the objects that have occupied my atten- 
 tion, I recognize how they have been interconnected, each preparing the 
 way for its successor. Is it not true that for every person the course of 
 life is along the line of least resistance, and that in this the movement 
 of humanity is like the movement of material bodies ? 
 
 To my American reader I need say nothing for the purpose of secur- 
 ing his kind appreciation of this work. I know that he, recognizing the 
 difficulties encountered in such a long series of experiments, will exten- 
 uate its imperfections, and regard it as a contribution from this side of 
 the Atlantic to the common fund of human knowledge, and especially 
 to one of its most important departments, at a period when the main 
 subject on which it treats had scarcely attracted scientific attention. 
 
 NEW YORK, January, 1878. 
 
CONTENTS. 
 
 MEMOIR I. 
 
 EXAMINATION OF THE RADIATIONS OF RED-HOT BODIES. THE PRO- 
 DUCTION OF LIGHT BY HEAT. 
 
 Ascertainment of the temperature at which bodies become self-luminous ; 
 it is 977 Fahr. Proof that all solids begin to shine at the same de- 
 gree. The spectrum of incandescent solids has no fixed lines. The 
 reference spectrum. Colors of light emitted as the heat increases are 
 in the order of the spectrum. Frequency of vibration increases with 
 the temperature. Intensity of the light emitted. Intensity of the heat 
 radiated Page 23 
 
 MEMOIR II. 
 
 SPECTRUM ANALYSIS OF FLAMES. PRODUCTION OF LIGHT BY CHEM- 
 ICAL ACTION. 
 
 Spectrum analysis of a candle flame. Examination of various other 
 flames. Spectrum analysis of the light of a burning solid. Flames 
 consist of a succession of shells. Spectrum of cyanogen. Combustion 
 of flames in oxygen. Effect of air in the interior of a flame. The 
 blowpipe cone. General result, the more energetic the chemical action 
 the higher the refrangibility of the resulting light; the vibrations 
 increase in frequency as the chemical action is more violent. Fraun- 
 hofer j s fixed lines 52 
 
 MEMOIR III. 
 
 ON INVISIBLE FIXED LINES IN THE SUN 5 S SPECTRUM DETECTED BY 
 
 PHOTOGRAPHY. 
 
 The photography of Fraunliofer* s fixed lines. The lines in the red due 
 to absorption by the earth's atmosphere. Nomenclature of the Fraun- 
 hofer lines. Discovery of the invisible fixed lines. Original map of 
 them. The new ultra spectral red lines a,/3, y. Rediscovered by Fou- 
 cault and Fizeau. M. BecquereTs discovery. Experiments in 1 834. . 74 
 
XVI 
 
 CONTENTS. 
 
 MEMOIR IY. 
 
 ON THE NATURE OF FLAME AND ON THE CONDITION OF THE SUN 5 S 
 
 SURFACE. 
 
 Dove's experiments on electric light. Dark lines replaced by bright ones. 
 Electric spark between metallic surfaces. The lines depend on the 
 chemical nature of the substance from which the light issues. They 
 may be used for determining the physical condition of the sun and 
 stars. Three hypotheses of the condition of the sun's surface exam- 
 ined Page 81 
 
 MEMOIR Y. 
 
 ON THE NEGATIVE OK PROTECTING RAYS OF THE SUN. 
 
 Original discovery of protecting radiations. Case of a daguerreotype 
 plate. Spectrum -photographs made in Virginia. Protecting action 
 of the less refrangible rays. Protecting action of the extreme violet. 
 Variations of the protecting action. Spectrum of darkness and spec- 
 trum of daylight. Interference of rays of different colors. Action of 
 waves of red, yellow, and violet light with ivave-lengths 2, 1^, 1. Use 
 of the diffraction spectrum 87 
 
 MEMOIR VI. 
 
 ON THE DIFFRACTION SPECTRUM. 
 
 Mode of obtaining the diffraction spectrum. The yellow is in its mid- 
 dle ; it is a centre of chemical action. It is also the hottest ray. 
 Diffraction spectra by reflection. Cold lines. Dilatation of the more 
 refrangible rays in the prismatic spectrum, compression of the less re- 
 frangible. Action of the diffraction spectrum on salts of silver. Use 
 of wave-lengths for spectrum division 97 
 
 MEMOIR VII. 
 
 STUDIES IN THE DIFFRACTION SPECTRUM. 
 
 Elementary description of the diffraction spectrum. Young's discovery 
 of interference. FresneVs discovery of transverse vibrations. Gratings 
 and the spectra they yield. Gratings on reflecting surfaces. Optical 
 action of a grating. Its spectra of different orders. Interpretation 
 of wave-lengths by the mind. Mental appreciation of multiple wave- 
 lengths. Refutation of the principle that to every color there belongs 
 an invariable wave-length. Increase in the range of perception in the 
 
CONTENTS. 
 
 eye. Extension to photographic impressions. Encroachment on the 
 first dark space. Photographs of the diffraction spectrum. Proposal 
 to use wave-lengths for spectrum divisions. Replacement of imaginary 
 imponderable principles by wave-lengths Page 115 
 
 MEMOIK VIII. 
 
 ON THE PHOSPHORESCENCE OF BODIES. 
 
 Early observations on phosphorescence. The diamond. Duration of 
 shadows. Lemery's theory. Du Fay's theory. Qualities of dia- 
 mond and fluor-spar. The volume of a phosphorescent body does not 
 change during its glow. A structural change accompanies the phos- 
 phorescence of bodies ; there is a minute disengagement of heat. Phos- 
 phorescence is not communicable. Absolute quantity of light emit- 
 ted 133 
 
 MEMOIR IX. 
 
 ON THE EFFECTS OF HEAT ON PHOSPHOEESCENCE. 
 
 Experiment of Albertus Magnus. Degree of phosphorescence at different 
 temperatures. The quantity of light a substance can retain is inversely 
 as its temperature ; the quantity it can receive is directly as the inten- 
 sity and quantity of light to which it has been exposed. Phosphores- 
 cent images of the moon. Action of ether waves. Effects of cohesion. 
 Reason that gases, liquids, and metals are non-phosphorescent. . 159 
 
 MEMOIR X. 
 
 ON THE DECOMPOSITION OF CARBONIC- ACID GAS BY PLANTS IN THE 
 PRISMATIC SPECTRUM. 
 
 The decomposition of carbonic acid by light formerly attributed to the 
 violet ray. It can be successfully accomplished in the prismatic spec- 
 trum. It takes place not in the violet but in the yellow ray. Decom- 
 position by yellow absorbent media. Analysis of the gas evolved ; it 
 always contains nitrogen. Decomposition of alkaline carbonates and 
 bicarbonates. Analysis of the gas evolved in different rays 167 
 
 MEMOIR XL 
 
 OF THE FORCE INCLUDED IN PLANTS. 
 
 Growth of a seed in darkness and in light. Action of plants and ani- 
 mals respectively on the atmosphere. Examination of Rumford's Ex- 
 periments. Dark heat rays cannot decompose carbonic acid. Germi- 
 
 B 
 
xv iii CONTENTS. 
 
 nation in colored rays. Greening of leaves takes place in the yellow 
 and adjacent rays. The essential condition of all chemical changes 
 by radiation is absorption. Plants absprb force from the sun ; it is 
 associated with their combustible parts, and is disengaged by oxida- 
 tion Page 177 
 
 MEMOIR XII. 
 
 EXPERIMENTS TO DETERMINE WHETHER LIGHT PRODUCES ANY MAG- 
 NETIC EFFECTS. 
 
 Mrs. Somerville 's experiments. Christie's experiment. Their results 
 cannot be substantiated. The violet ray has no effect. Blue glasses 
 and blue ribbons also ineffective 191 
 
 MEMOIR XIII. 
 
 AN ACCOUNT OF SOME EXPERIMENTS ON THE LIGHT OF THE SUN, 
 MADE IN THE SOUTH OF VIRGINIA. 
 
 Absorption of luminous radiations. The reference spectrum. Absorp- 
 tion of heat radiations ; the apparatus employed. Absorption of chem- 
 ical radiations. Screen of bromide of silver. Coloration of chloride 
 and bromide of silver by radiations that have passed through corre- 
 spondingly colored solutions. Early application of photography to 
 the investigation of physical problems. Crystallization of camphor to- 
 wards the light. The side of vessels towards the sky is the colder. . 197 
 
 MEMOIR XIY. 
 
 EXAMINATION OF THE PROCESS OF DAGUERREOTYPE. NOTE ON 
 LUNAR PHOTOGRAPHY. 
 
 Description of the process. Cause of the deposition of mercury and pro- 
 duction of the light parts of the picture. Polishing of the plate. The 
 operation of iodizing. Effect of keeping the iodide. The achromatic 
 lens. Reduction of focal length in the non-achromatic. The develop- 
 ment. Fixing by hyposulphite and galvanism. Necessity of heating 
 the plates. Lunar impressions. Artificial light. Note on lunar 
 photography. The first photographs of the moon 206 
 
 MEMOIR XY. 
 
 ON THE TAKING OF PORTRAITS FROM LIFE BY PHOTOGRAPHY. 
 
 History of the invention. First attempts by whitening the face. Use of 
 reflecting mirrors. Use of a blue-colored trough. Kind of camera 
 
CONTENTS. 
 
 necessary. The seat or support. The background. Appropriate 
 dresses. Ladies' 1 dresses. Arrangement of the shadow. Reflecting 
 camera Page 215 
 
 MEMOIR XYI. 
 
 ON THE CHEMICAL CONDITION OF A DAGUERREOTYPE SURFACE. 
 
 Mercury exists all over a daguerreotype surface. There is no superpo- 
 sition of the parts. The shadows have metallic mercury; the lights 
 silver amalgam. No iodine is ever evolved from the plate. Action of 
 a solution of gum and one of gelatine in tearing off the films. The 
 starch experiments. The etching of daguerreotypes 222 
 
 MEMOIR XVII. 
 
 ON SOME ANALOGIES BETWEEN THE PHENOMENA OF THE CHEMICAL 
 RAYS AND THOSE OF RADIANT HEAT. 
 
 The chemical rays are absorbed. Photographic effects are transient. 
 The chemical rays are not conducted ; they become latent. Optical 
 qualities control chemical action. The active rays are absorbed and 
 the complementary reflected. Relation of optical conditions and chem- 
 ical affinities 230 
 
 MEMOIR XVIII. 
 
 DESCRIPTION OF THE CHLOR-HYDROGEN PHOTOMETER. 
 
 Properties of a mixture of chlorine and hydrogen. It is acted upon by 
 lamplight, an electric spark at a distance, etc. The gases unite in pro- 
 portion to the amount of light. Mode of measuring out known quan- 
 tities of radiations. The maximum action is in the indigo space. 
 Construction of the instrument. The gases are evolved by electricity 
 and combined by light. Theoretical conditions of equilibrium. Pre- 
 liminary adjustment. Method of continuous observation. Method of 
 interrupted observation. Remarkable contraction and expansion . . 245 
 
 MEMOIR XIX. 
 
 ON MODIFIED CHLORINE. 
 
 Description of the experiment. The change in the chlorine is not tran- 
 sient. There are two stages in the phenomenon. Rays are absorbed 
 in producing this change. The indigo ray is absorbed. The action is 
 positive from end to end of the spectrum. The indigo ray forms hy- 
 drochloric acid and also produces the preliminary modification. 
 
xx CONTENTS. 
 
 Change in oilier elementary bodies. Verification of these results with 
 the chlor-hydrogen photometer Page 271 
 
 MEMOIR XX. 
 
 ON THE ALLOTROPISM OF CHLORINE AS CONNECTED WITH THE 
 THEORY OF SUBSTITUTIONS. 
 
 Chlorine exists in two states, active and passive. Decomposition of water 
 by it in the sunlight. Facts connected with this decomposition. The 
 relations of chlorine and hydrogen. The allotropism of chlorine. 
 Connection of these facts with the theory of substitutions 284 
 
 MEMOIR XXL 
 
 ON THE INFLUENCE OF LIGHT UPON CHLORINE, AND SOME REMARKS 
 
 ON ALCHEMY. 
 
 Modification of chlorine by the sun-rays. The modification is not tran- 
 sient. Alchemical attempts to modify metals. Exposure of silver 
 chloride to a burning-lens. The resulting silver is not acted upon by 
 nitric acid 312 
 
 MEMOIR XXII. 
 
 ON THE ACTION OF GLASS AND QUARTZ ON THE RADIATIONS THAT 
 PRODUCE PHOSPHORESCENCE. 
 
 Phosphorescence by rays from a Leyden spark through quartz. From 
 the voltaic arc. It is occasioned by the more refrangible rays. Im- 
 perviousness of glass. Professor Henry's experiments. Comparison 
 of the chemical and phosphorogenic rays 316 
 
 MEMOIR XXIII. 
 
 ON A REMARKABLE DIFFERENCE BETWEEN THE RAYS OF INCANDES- 
 CENT LIME AND THOSE EMITTED BY AN ELECTRIC SPARK. 
 
 Non-permeability of glass to spark radiations. Permeability to calcium- 
 light radiations. Different refrangibility of the spark and the cal- 
 cium-light radiations. Shadows imbedded in Canton's phosphorus. 
 Evolution of old shadows in an order of succession 320 
 
 MEMOIR XXIY. 
 
 ON THE ELECTRO-MOTIVE POWER OF HEAT. 
 
 Experimental arrangement to determine the electro-motive power. Tem- 
 peratures calculated from quantities of electricity. Increase of tension 
 
CONTENTS. xx { 
 
 with increase of temperature. Depends on increased resistance to con- 
 duction. Quantity of electricity independent of heated surface. In 
 thermo-electric piles the quantity of electricity is proportional to 
 the number of pairs. Best forms of construction of thermo-electric 
 pairs Page 324 
 
 MEMOIE XXV. 
 
 ON MICROSCOPIC PHOTOGRAPHY. 
 
 Method of making microscopic photographs by condensed sunlight. 
 Specimens of the-art 338 
 
 MEMOIE XXYI. 
 
 ON CAPILLARY ATTRACTION AND INTERSTITIAL MOTIONS. THE 
 
 CAUSE OF THE FLOW OF SAP IN PLANTS AND THE 
 CIRCULATION OF THE BLOOD IN ANIMALS. 
 
 Interstitial motions of solids. Motions of metal in coins. Movement of 
 liquids in crevices. Capillary attraction. Conditions for a continu- 
 ous flow. Capillary attraction an electrical phenomenon. Motions 
 of liquid conductors. Dutrochefs experiment of endosmosis. Pas- 
 sage of gases through liquids. Soap bubbles. Passage against heavy 
 pressure. Distribution of sap in plants. Circulation in plants due to 
 sunlight producing gum. Circulation of blood in animals explained. 
 The systemic, the pulmonary, and the portal circulation 342 
 
 MEMOIE XXVII. 
 
 ON THE EXISTENCE AND EFFECTS OF ALLOTROPISM ON THE CON- 
 STITUENTS OF LIVING BEINGS. 
 
 Allotropism of elementary substances. Frankenheim? s nomenclature. 
 Brought about by light, heat, electricity, and by the nervous principle. 
 Explanation of inflammation and congestion 375 
 
 MEMOIE XXVIII. 
 
 ON THE DISTRIBUTION OF HEAT IN THE SPECTRUM. 
 
 Early experiments seeming to prove that the maximum of heat is in the 
 less refrangible spaces. Comparison of the dispersion and diffraction 
 spectra. Effect of compression in the less refrangible regions and of 
 dilatation in the more refrangible. Measure of heat in the two halves 
 of the visible dispersion spectrum. Description of the apparatus em- 
 ployed. The different colored spaces are equally warm 383 
 
xx ii CONTENTS. 
 
 MEMOIR XXIX. 
 
 ON THE DISTRIBUTION OF CHEMICAL FORCE IN THE SPECTRUM. 
 
 The curves of the calorific, luminous, and chemical spectra. Their er- 
 rors. Inappropriateness of the term actinic rays. There is no local- 
 isation of chemical effects. Every radiation can produce some specific 
 effect. Case of the silver compounds. Bitumens and resins. Car- 
 bonic acid. Colors of flowers. Law of Grotthus Chlorine and hy- 
 drogen. Bending of stems of plants. Absorption essential to chemical 
 action. Decomposition of silver iodide. Union of chlorine and 
 hydrogen. Angstrom's law. General conclusions. Chemical force 
 exists in every portion of the spectrum. Each radiation exercises 
 chemical influences proper to itself Page 404 
 
 MEMOIR XXX. 
 
 ON BURNING GLASSES AND MIRRORS THEIR HEATING AND CHEM- 
 ICAL EFFECTS. 
 
 Can concentrated rays produce new chemical decompositions? Effects of 
 amplitude, frequency, and direction of vibration in the ether-waves. 
 Clock lenses for long exposures. Decomposition of water by chlorine. 
 Attempt to decompose it by bromine and iodine. Use of absorbing 
 troughs. Dry silver iodide not decomposed by light. Decompositions 
 under water. Decompositions in a spherical concave. Effect of ex- 
 traneous mixtures. They do not make collodion more sensitive. 
 Antagonize tion of radiations. Case of electric spark. Effects of 
 polarized light. Attempts to polarize light by an electro-magnet. 
 Mechanical cause of decompositions by light 436 
 
SCIENTIFIC MEMOIRS. 
 
 MEMOIR I. 
 
 EXAMINATION OF THE KADIATIONS OF RED-HOT BODIES. 
 THE PRODUCTION OF LIGHT BY HEAT. 
 
 From the American Journal of Science and Arts, Second Series, Vol. IV., 1847; 
 London, Edinburgh, and Dublin Philosophical Magazine, May, 1847 ; Harper's 
 New Monthly Magazine, No. 322. 
 
 CONTENTS : Ascertainment of the temperature at which bodies become 
 self-luminous ; it is 977 Fahr. Proof that all solids begin to shine 
 at the same degree. The spectrum of incandescent solids has no fixed 
 lines. The reference spectrum. Colors of light emitted as the heat 
 increases are in the order of the spectrum. Frequency of vibration 
 increases with the temperature. Intensity of the light emitted. In- 
 tensity of the heat radiated. 
 
 ALTHOUGH the phenomenon of the production of light 
 by all solid bodies, when their temperature is raised to 
 a certain degree, is one of the most familiar, no person 
 so far as I know has hitherto attempted a critical in- 
 vestigation of it. The difficulties environing the inquiry 
 are so great that even among the most eminent philoso- 
 phers a diversity of opinion has prevailed respecting 
 some of the leading facts. Thus Sir Isaac Newton fixed 
 the temperature at which bodies become self-luminous 
 as 635; Sir Humphrey Davy at 812; Mr. Wedgwood 
 at 947 ; and Mr. Daniel at 980. As respects the nature 
 of the light emitted, there are similar contradictions. In 
 some philosophical works of considerable repute it is 
 
24 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 stated that when a solid begins to shine it first emits 
 red and then white rays; in others it is asserted that a 
 mixture of blue and red light is th first that appears. 
 
 I have succeeded in escaping or overcoming many of 
 the difficulties of this problem, and have arrived at 
 satisfactory solutions of the main points; and as the 
 experiments now to be described lead to some striking 
 and perhaps unexpected analogies between light and 
 heat, they commend themselves to our attention, as hav- 
 ing a bearing on the question of the identity of those 
 principles. It is known that heretofore I have been led 
 to believe in the existence of cardinal distinctions not 
 only between these, but also other imponderable agents, 
 and I may therefore state that when this investigation 
 was first undertaken it was in the expectation that it 
 would lead to results very different from those which 
 have actually arisen. 
 
 The following are the points on which I propose to 
 treat : 
 
 1. To determine the point of incandescence of plati- 
 num, and to prove that different bodies become incan- 
 descent at the same temperature. 
 
 2. To determine the color of the rays emitted by self- 
 luminous bodies at different temperatures. This is done 
 by the only reliable method analysis by the prism. 
 
 From these experiments it will appear that as the 
 temperature rises the light increases in refrangibility; 
 and making due allowance for the physiological imper- 
 fection of the eye, the true order of the colors is red, 
 orange, yellow, green, blue, indigo, violet. 
 
 3. To determine the relation between the brilliancy 
 of the light emitted by a shining body and its tempera- 
 ture. 
 
 Here we shall find that the intensity of the light in- 
 creases far more rapidly than the temperature. For 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 25 
 
 example, platinum at 2600 emits almost forty times as 
 much light as it does at 1900. 
 
 The source of light I have employed is in all in- 
 stances a very thin strip of platinum, 1.35 inch long, 
 and .05 of an inch wide, brought to the temperature 
 under investigation by a voltaic current. Platinum was 
 selected from its indisposition to oxidize, and its power 
 of resisting a high temperature without fusion. 
 
 The strip of platinum thus to be brought to different 
 temperatures by an electric current of the proper force 
 was fastened at one end to an inflexible support, and 
 at the other was connected with a delicate lever- index, 
 which enabled me to determine its expansion, and there- 
 by its temperature. For this purpose I have used the 
 coefficient of dilatation of Dulong and Petit. The tem- 
 peratures here given are upon the hypothesis of the in- 
 variability of that coefficient at all thermometric degrees; 
 they are therefore to some extent in error. 
 
 In Fig. 1, a b represents the strip of platinum, the 
 upper end of which is soldered to 
 a stout and short copper pin, a, 
 firmly sunk in a block of wood, c, 
 which is immovably fastened to the 
 basis, d d, of the instrument. A 
 cavity, 0, half 
 an inch in di- 
 ameter is sunk 
 in the block <?, 
 and into this 
 cavity the pin a projects, so that when the cavity is filled 
 with mercury a voltaic current may be passed through 
 the pin and down the platinum. The other extremity 
 of the platinum, , is fastened to a delicate lever, b f, 
 which plays on an axis at g, the axis working in brass 
 holes supported on a block, li. Immediately beneath 
 
 Fig. 1. 
 
26 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 the platinum strip, and in metallic communication with 
 it, a straight copper wire dips down into the mercury 
 cup m; on this wire there is a metal ball, n, weighing 
 about 100 grains. The further end of the index plays 
 over a graduated ivory scale, p p, supported on a block, 
 q ; the scale can be moved a little up and down, so as to 
 bring its zero to coincide with the index at common 
 temperatures. 
 
 The action of the instrument is readily understood. 
 In the mercury cup e let there dip one of the wires, N, 
 of a Grove's battery of three or four pairs, the other 
 wire, P, being dipped into the cup m. The current 
 passes through the platinum, which immediately ex- 
 pands, the weight n lightly stretching it. The index f 
 moves promptly over the scale, indicating the amount of 
 expansion, and therefore the degree of heat. If the wire 
 N be removed out of its mercury cup e, the platinum 
 instantly becomes cold, and pulls the lever to the zero 
 point. 
 
 When the platinum is thin, so as to be quite flexible 
 at the point #, where it is fastened to the index, the 
 movements take place with such promptitude and pre- 
 cision as to leave nothing to be desired. When the 
 heat has been very high and long continued, the limit of 
 elasticity of the platinum is somewhat overpassed, and 
 it suffers a slight permanent extension. But as the 
 ivory scale p p can slide up and down a little, the index 
 is readily readjusted to the zero point. 
 
 The temperature of the platinum depends entirely on 
 the force of the current passed through it. By inter- 
 vening coils of brass wire of lengths adjusted before- 
 hand, so as to resist the current to a given extent, any 
 desired temperature may be reached. I found it con- 
 venient to intervene in the course of the current a 
 rheostat, so as to be able to bring the index with pre- 
 
MEMOIR L] THE RADIATIONS OF IGNITED BODIES. 27 
 
 cision to any degree, notwithstanding slight changes in 
 the force of the voltaic battery. 
 
 The following are the dimensions and measures of the 
 instrument I have used : Length of the platinum strip, 
 1.35 inch; length of the part actually ignited, 1.14 inch; 
 w 7 idth of ditto, -fa of an inch ; length of the index from 
 its centre of motion to the scale, 7.19 inches; distance 
 of the centre of motion of index from the insertion of 
 the platinum at the point Z>, .22 inch ; multiplying effect 
 of the index, 32.68 times; length of each division on the 
 ivory scale, .021 inch. From this it would appear by a 
 simple calculation, using the coefficient of dilatation of 
 platinum given by Dulong and Petit, that each of the 
 divisions here used is equal to 114.5 Fahrenheit degrees. 
 For the sake of perspicuity I have generally taken them 
 at 115. 
 
 The Grove's battery I have employed has platinum 
 plates thr.ee inches long and three quarters wide; the 
 zinc cylinders are two inches and a half in diameter, 
 three high, and one third thick. As used in these ex- 
 periments it could maintain a current nearly uniform for 
 an hour. I commonly employed four pairs. 
 
 By the aid of resisting wires of different lengths, or 
 the rheostat, the force of the current in the platinum 
 could be varied, and therefore its temperature. The first 
 attempt was, of course, to discover the degree at which 
 the metal began to emit light. 
 
 The platinum and the voltaic battery were placed in 
 a dark room, the temperature of which was 60 Fahr.; 
 and after I had remained therein a sufficient length of 
 time to enable my eyes to become sensible to feeble 
 impressions of light, I caused the current to pass, gradu- 
 ally increasing its force until the platinum \vas visible. 
 In several repetitions of this experiment it was uniform- 
 
28 THE KADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 ly found that the index to which the platinum was at- 
 tached stood at the eighth division when this took place. 
 The metal had therefore dilated -^ of its length ; the 
 elevation of its temperature was about 917, which, add- 
 ed to the existing height of the thermometer, 60, gave 
 for the temperature of incandescence 977 Fahr. 
 
 To the correctness of this number it might be objected 
 that, owing to the narrowness of the metallic strip, it was 
 not well calculated to make an impression on the eye 
 when the light emitted was feeble, and that we ought 
 not to take the dilatations given by the index as repre- 
 senting the uniform temperature of the whole platinum, 
 which must necessarily be colder near its points of sup- 
 port, on account of the conducting power of the metals 
 to which it was attached. 
 
 Physiological considerations might also lead to a suspi- 
 cion that the self-luminous temperature must vary as es- 
 timated by different eyes. The experiments of Bouguer, 
 hereafter to be referred to, indisputably show that some 
 persons are much more sensitive to the impressions of 
 light than others. So far as my limited investigation of 
 this matter has gone, I have not, however, found appreci- 
 able differences in the estimation of the temperature of 
 incandescence. Different individuals observing the plat- 
 inum have uniformly perceived it at the same time. 
 
 Against the number 977, it may also be objected that 
 antimony melts at a much lower temperature, and yet 
 emits light before it fuses. If this statement were true, 
 it would lead us to believe that all bodies have not the 
 same point of incandescence. But I think that the ex- 
 periments of Mr. Wedgwood on gold and earthenware 
 are decisive in this particular; and, moreover, I have 
 reason to believe that the melting-point of antimony is 
 much higher than commonly supposed. 
 
 With a view of determining directly whether different 
 
MEMOIR I.] THE KADIATIONS OF IGNITED BODIES. 29 
 
 bodies vary in their point of incandescence, I took a clean 
 gun-barrel, and having closed the touch-hole, exposed the 
 following substances in it to the action of a fire : plati- 
 num, chalk, marble, fluor-spar, brass, antimony, gas-carbon, 
 lead ; each specimen was small : the platinum was in the 
 form of a coil of stout wire. 
 
 When one of these bodies was placed in the gun-bar- 
 rel and the temperature raised, it is clear that any differ- 
 ence in their point of incandescence could be detected by 
 the eye. Thus if the ignition of platinum required a 
 higher degree than that of iron, on looking down the 
 barrel the coil of wire should be dark when the barrel 
 itself had begun to shine ; or, if the platinum was incan- 
 descent first, the wire should be seen before the barrel 
 had become visibly hot, and these results might be cor- 
 roborated by observing the inverse phenomena, when the 
 barrel was taken from the fire and suffered to cool. 
 
 In Fig. 2, a t> is the gun-barrel passing through a hole, 
 0, of suitable size in the side 
 of a stove. At the bottom of 
 the barrel, &, the substances to 
 be examined are placed. Their 
 ignition is observed by looking 
 in at the projecting end, a. 
 
 With respect to platinum, 
 brass, antimony, gas - carbon, 
 and lead, they all became in- 
 candescent at the same time as 
 the iron barrel itself. I could 
 not discover the slightest dif- 
 ference among them, either in heating or cooling ; and it 
 is worthy of remark that the lead was, of course, in the 
 liquid condition. But the chalk and marble were vis- 
 ible before the barrel was red-hot, emitting a faint white 
 light; and the fluor-spar still more strikingly so, its light 
 
30 THE RADIATIONS OF IGNITED BODIES. [MBMOIB I. 
 
 being of a beautiful blue ; and even when the barrel had 
 become bright red I could still see the spar, which had 
 decrepitated to a coarse powder, fey its faint blue rays. 
 In these cases, however, it was not incandescence, but 
 phosphorescence that was taking place. I infer, then, 
 that all solids, and probably melted metals, begin to 
 shine at the same thermometric point. 
 
 (When phosphorescent substances are to be examined, 
 they must be first exposed to a high temperature and 
 carefully guarded from access of light until they are 
 placed in the gun-barrel. A diamond which, among oth- 
 er bodies, had been thus tried, would recover its quality 
 of phosphorescing by a very short access of light after it 
 had been cooled, but if that had been carefully avoided, 
 it began to shine at the same time as other specimens 
 with which it was placed in the barrel.) 
 
 The temperature of incandescence seems to be a natu- 
 ral fixed point for the thermometer; and it is very inter- 
 esting to remark how nearly this point coincides w 7 ith 
 1000 of the Fahrenheit scale when Laplace's coefficient 
 for the dilatation of platinum is used. Upon that coef- 
 ficient the point of incandescence is 1006 Fahr. 
 
 In view of these considerations, and recollecting that 
 the number given by Daniel is 980, and that of Wedg- 
 wood 947, I believe that 977 is not very far from the 
 true temperature at which solids begin to shine. It is 
 to be understood, of course, that this is in a very dark 
 room. 
 
 I pass now to the second proposition. The rays emit- 
 ted by the incandescent platinum strip were received on 
 a flint-glass prism, placed so as to give the minimum de- 
 viation, and, after dispersion, viewed in a small telescope. 
 A movement could be given to the telescope, which was 
 read off on a graduated circle. However, instead of 
 bringing the parts of the spectrum tinder measurement 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 
 
 31 
 
 to coincide with the cross wires in the field of the instru- 
 ment, it was found more satisfactory to determine them 
 by bringing them to one or other of the edges of the 
 field a process by which the extreme rays could be bet- 
 ter ascertained, their faint light being thus more easily 
 perceived in the darkness by which it was surrounded. 
 It would scarcely be possible to see them accurately 
 while the rest of a bright spectrum was in view. 
 
 In Fig. 3, a b is the ignited platinum strip, c the prism, 
 d e the telescope, 
 moving upon the 
 centre of a gradu- 
 ated circular ta- 
 We,// 
 
 As it was ab- 
 solutely necessa- 
 ry to have fixed 
 points of refer- 
 ence, that all the 
 observations 
 might be brought to a common standard of comparison, 
 and as there are no fixed lines in the light of incandes- 
 cence such as are in the sunshine and daylight, I there- 
 fore previously determined the position of the fixed lines 
 in a spectrum formed by a ray of reflected daylight 
 which passed through a fissure -^ of an inch wide, and 
 one inch long, occupying exactly the position subsequent- 
 ly to be occupied by the incandescent platinum. In Fig. 
 4, 1 represents the result.* 
 
 (I expected to use the Fraunhofer lines for this pur- 
 pose, and was not a little surprised to find that they are 
 not to be seen in the spectrum of ignited solid bodies. 
 
 Fig. 3. 
 
 * The letters used to indicate the fixed lines are those employed at that 
 time, 1847. 
 
32 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 Thus was discovered one of the fundamental facts in 
 spectrum analysis, a fact that has become of the highest 
 importance in astronomy, as furnishing a means for de- 
 termining the physical condition of the heavenly bodies, 
 and a test for the nebular hypothesis. An ignited solid 
 will give a continuous spectrum, or one devoid of fixed 
 lines; an ignited gas will give a discontinuous spectrum, 
 one broken up by lines or bands or spaces. 
 
 About twenty years subsequently to this discovery, 
 Mr. Huggins (1864) made an examination of a nebula 
 in the constellation of Draco. It proved to be gaseous. 
 Subsequently, of sixty nebulae examined, nineteen gave 
 discontinuous or gaseous spectra, the remainder contin- 
 uous ones. 
 
 It may therefore be admitted that physical evidence 
 has through this means been obtained demonstrating the 
 existence of vast masses of matter in a gaseous condition, 
 and at a temperature of incandescence. The nebular hy- 
 pothesis of Laplace and Herschel has thus a firm basis.) 
 The strip of platinum was now placed in the position 
 
 of the slit which had 
 given the spectrum rep- 
 resented at 1, and its 
 temperature was raised 
 , o by the passage of a vol- 
 
 1 132 % taic current. Though the 
 
 jl44 metal could be distinctly 
 
 j2i3o seen by the naked eye 
 . 4. when the temperature 
 
 Spectra of daylight and of incandescent platinum had reached about 1000 
 
 Fahr., yet the loss of light 
 
 in passing the prism and telescope was so great that it 
 was necessary to carry the temperature to 1210 before 
 a satisfactory observation could be made. At this de- 
 gree the spectrum extended from the position of the fixed 
 
 E h 
 
 .1.1 
 
 5. 
 
MKMOIR I.] THE RADIATIONS OF IGNITED BODIES. 33 
 
 line B in the red almost as far as the line F in the green, 
 the colors present being red, orange, and a tint which 
 may be designated as gray. There was nothing answer- 
 ing to yellow. The rays first visible through this ap- 
 paratus may therefore be designated as red and greenish- 
 gray ; the former commencing at the line B, and the lat- 
 ter continuing to F, as at 3. 
 
 The voltaic current was now increased, and the tem- 
 perature rose to 1325. The red end of the spectrum 
 remained nearly as before, but the more refrangible ex- 
 tremity reached the position of the little fixed line d. 
 Traces of yellow were now visible, and, with a certain 
 degree of distinctness, the red, orange, yellow, green, and 
 a fringe of blue could be seen ; 4 shows the result. 
 
 The temperature was now carried to 1440. The red 
 extremity appeared to be advancing towards the line A ; 
 the blue had undergone a well-marked increase. It 
 reached considerably beyond the line G, as shown at 5. 
 
 On bringing the platinum to 2130, all the colors were 
 present, and exhibited considerable brilliancy. Their ex- 
 tent was somewhat shorter than that of the daylight 
 spectrum, as seen at 6. 
 
 Having thus by repeated experiments ascertained the 
 continued extension of the more refrangible end as the 
 temperature rose, it became necessary to obtain obser- 
 vations for degrees below 1210, the limit of visibility 
 through the telescope. I therefore carried the prism 
 nearer to the platinum, and looking with the unassisted 
 eye directly through it at the refracted image, found that 
 it could be distinctly seen at a temperature as low as 
 1095. Under these circumstances, the total length 
 could not be compared by direct measurement with the 
 other observations, and the result given at 2 is as cor- 
 rect as could be obtained. The colors were red and 
 greenish-gray. 
 
 C 
 
34 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 The gray rays emitted by platinum just beginning to 
 shine appear to be more intense than the red ; at all 
 events, the wires in the field of "the telescope are more 
 distinctly seen upon them than upon the other color. 
 The designation of gray may be given them, for they ap- 
 pear to approach that tint more closely than any other, 
 and yet it is to be remarked that they are occupying the 
 position of the yellow and green regions. 
 
 Already we have encountered a fact of considerable 
 importance. The conclusion that as the temperature of 
 a body rises it emits rays of increasing refrangibility has 
 obviously to be taken with a certain restriction. Instead 
 of first the red, then the orange, then the yellow rays, 
 etc., in succession making their appearance, in w^hich case 
 the spectrum should regularly increase in length as the 
 temperature rises, we here find that at the very first mo- 
 ment it is visible to the eye it reaches from the fixed 
 line B nearly to F, that is to say, it is equal to about two 
 thirds of the whole length of the diffraction spectrum, 
 and almost one half of the prismatic. 
 
 It is to be remarked that while the more refrangible 
 end undergoes a great expansion, the other extremity ex- 
 hibits a corresponding though a less change. As very 
 important theoretical conclusions depend on the proper 
 interpretation of this fact, it must not be forgotten that 
 to a certain extent it may be an optical deception, arising 
 from the increased brilliancy of the light. While the 
 rays are yet feeble, the extreme terminations may be so 
 faint that the eye cannot detect them, but as the inten- 
 sity rises they become better marked, and an apparent 
 elongation of the spectrum is the consequence. 
 
 It is agreed among optical writers that to the human 
 eye the yellow is the brightest of the rays. In the pris- 
 matic spectrum the true relationship of the colors is not 
 perceived, because the less refrangible are crowded to- 
 
MEMOIR I.] THE KADIATIONS OF IGNITED BODIES. 35 
 
 gather, and the more refrangible unduly spread out. But 
 in the diffraction spectrum, where the colors are arranged 
 side by side in the order of their wave-lengths, the centre 
 is occupied by the most luminous portion of the yellow, 
 and from this point the light declines away on one side 
 in the red, and on the other in the violet, the termina- 
 tions being equidistant from the centre of the yellow 
 space. 
 
 Now if the rays coming from shining platinum were 
 passed through a piece of glass on which parallel lines 
 had been ruled with a diamond point, so as to give a 
 diffraction spectrum, even admitting the general results 
 of the foregoing experiments to be true, viz., that as the 
 temperature rises rays of a higher refrangibility are 
 emitted, it is obvious that it by no means follows that 
 the ray first visible should be the extreme red. Our 
 power of seeing that depends on its having a certain in- 
 tensity. Even when it has assumed the utmost brillian- 
 cy which it has in a solar beam, it is barely visible. We 
 ought, therefore, to expect that rays of a higher refrangi- 
 bility should be first seen, because they act more ener- 
 getically on our organ of vision ; and as the temperature 
 rises, the spectrum should undergo a partial elongation 
 in the direction of its red extremity. 
 
 I may here remark that the general result of these 
 experiments coincides exactly with that of M. Melloni 
 respecting heat at lower thermometric points. In his 
 second Memoir (Taylor's " Scientific Memoirs," vol. i., 
 p. 56) he shows that when rays from copper at 390 and 
 from incandescent platinum are compared by transmis- 
 sion through a rock-salt prism, as the temperature rises 
 the refrangibility of the calorific emanations correspond- 
 ingly increases. Those who regard light and heat as the 
 same agent will therefore see in this coincidence another 
 argument in favor of their opinion. 
 
36 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 In view of the foregoing facts, I conclude that as the 
 temperature of an incandescent body rises, it emits rays of 
 light of an increasing ref Tangibility, and that the appar- 
 ent departure from this law, discovered by an accurate 
 prismatic analysis, is due to the special action of the eye 
 in performing the function of vision. And as the lumi- 
 nous effects are undoubtedly owing to a vibratory move- 
 ment executed by the molecules of the platinum, it seems 
 from the foregoing facts to follow that the frequency of 
 those vibrations increases with the temperature. 
 
 In this observation I am led by the principle that " to 
 a particular color there ever belongs a particular wave- 
 length, and to a particular wave-length there ever be- 
 longs a particular color;" but in the analysis of the spec- 
 trum made by Sir D. Brewster by the aid of absorption 
 media, this principle is indirectly controverted, that emi- 
 nent philosopher showing that red, yellow, blue, and con- 
 sequently white light, exist in every part of the spectrum. 
 This must necessarily take place when a prism which 
 has a refracting face of considerable magnitude is used ; 
 for it is obvious that a ray falling near the edge and one 
 falling near the back, after dispersion, will depict their 
 several spectra on the screen ; the colors of the one not 
 coinciding with, but overlapping the colors of the other. 
 In such a spectrum there must undoubtedly be a general 
 commixture of the rays ; but may we not fairly inquire, 
 whether, if an elementary prism were used, the same facts 
 would hold good ; or if the anterior face of the prism 
 were covered by a screen, so as to expose a narrow fis- 
 sure parallel to the axis of the instrument, would there 
 be found in the spectrum it gave every color in every 
 part, as in Sir David Brewster's original experiment? 
 M. Melloni has shown how this very consideration com- 
 plicates the phenomena of radiant heat; and it would 
 seem a very plausible suggestion tha-t the effect here 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 37 
 
 pointed out must occur in an analogous manner for the 
 phenomena of light. 
 
 I next pass to the third branch of this investigation 
 to examine the relation between the temperatures of self- 
 luminous bodies and the intensity of the light they emit, 
 premising it with the following considerations : 
 
 The close analogy which has been traced between the 
 phenomena of light and radiant heat lends countenance 
 to the supposition that the law which regulates the es- 
 cape of heat from a body will also determine its rate of 
 emission of light. Sir Isaac Newton supposed that while 
 the temperature of a body rose in an arithmetical pro- 
 gression, the amount of heat escaping from it increased 
 in a geometrical progression. The error of this was sub- 
 sequently shown by Martin, Erxleben, and Delaroche, 
 and finally Dulong and Petit gave the true law : " When 
 a hot body cools in vacuo, surrounded by a medium the 
 temperature of which is constant, the velocity of cooling 
 for excess of temperature in arithmetical progression in- 
 creases as the terms of a geometrical progression, dimin- 
 ished by a constant quantity." The introduction of this 
 constant depends on the operation of the theory of the 
 exchanges of heat ; for a body when cooling under the 
 circumstances here supposed is simultaneously receiving 
 back a constant amount of heat from the medium of con- 
 stant temperature. 
 
 While Newton's law represents the rate of cooling of 
 bodies, and therefore the quantities of heat they emit 
 when the range of temperature is limited, and the law of 
 Dulong and Petit holds to a wider extent, there are in 
 the present inquiry certain circumstances to be taken 
 into account not contemplated by those philosophers. 
 Dulong and Petit, throughout their memoir, regard radi- 
 ant heat as a homogeneous agent, and look upon the 
 theory of exchanges, which is indeed their starting-point 
 
38 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 and guide, as a very simple affair. But the progress of 
 this department of knowledge since their time has shown 
 that precisely the same modifications found in the colors 
 of light occur also for heat; a fact conveniently desig- 
 nated by the phrase " ideal coloration of heat," and, fur- 
 ther, that the wave-length of the heat emitted depends 
 upon the temperature of the radiating source. It is one 
 thing to investigate the phenomena of the exchanges of 
 heat-rays of the same color, and another when the colors 
 are different. A complete theory of the exchanges of 
 heat must include this principle, and, of course, so too 
 must a law of cooling applicable to any temperature. 
 
 There is another fact to some extent considered by 
 Dulong and Petit, but not of such weight in their in- 
 vestigations, where the range of temperature was small, 
 as in these where it rises as high as nearly 3000 Fahr. 
 This is the difference of specific heat of the same body 
 at different temperatures. At the high temperatures 
 herein employed, there cannot be a doubt that the 
 capacity of platinum for heat is far greater than that at 
 a low point. This, therefore, must affect its rate of calo- 
 rific emission, and probably that for light also. 
 
 From these and similar considerations we should be 
 led to expect that as the temperature of an incandescent 
 solid rises, the intensity of the light emitted increases 
 very rapidly. 
 
 I pass now to the experimental proofs which substan- 
 tiate the foregoing reasoning. 
 
 The apparatus employed as the source of the light 
 and measure of the temperature was the same as in the 
 preceding experiments a strip of platinum brought to 
 a known temperature by the passage of a voltaic current 
 of the proper force, and connected with an index which 
 measured its expansion. 
 
 The principle upon which the intensities of the light 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 39 
 
 were determined was that originally described by Bou- 
 guer, and subsequently used by Masson. After many 
 experiments, I found that it is the most accurate method 
 known. 
 
 Any one who will endeavor to determine the intensi- 
 ties of light by Rumford's method of contrasting shad- 
 ows, or by that of equally illuminated surfaces, will find, 
 when every precaution has been used, that the results 
 of repeated experiments do not accord. There is, more- 
 over, the great defect that when the lights differ in color, 
 it is impossible to obtain reliable results except by re- 
 sorting to such contrivances as that described in the 
 Philosophical Magazine, August, 1844. 
 
 Bouguer's principle is far more exact ; and where the 
 lights differ in color, that difference actually tends to 
 make the result more correct. As it is not generally 
 known, I will indicate the nature of it briefly : 
 
 Let there be placed at a certain distance from a sheet 
 of white paper a candle, so arranged as to throw the 
 shadow of an opaque body, such as a rod of metal, on 
 the sheet. If a second candle be placed also in front of 
 the paper and nearer than the former, there is a certain 
 distance at which its light completely obliterates all 
 traces of the shadow. This distance is readily found, 
 for the disappearance of the shadow can be determined 
 with considerable exactness. When the lights are equal, 
 Bouguer ascertained that the relative distances were as 
 1 : 8, and therefore inferred correctly that in the case of 
 his eye the effect of a given light was imperceptible 
 when it was in presence of another sixty-four times as 
 intense. The precise number differs according to the 
 sensibility of different eyes, but for the same organ it is 
 constant. 
 
 Upon a paper screen I threw the shadow of a rod of 
 copper, which intercepted the rays of the incandescent 
 
40 
 
 THE RADIATIONS OF IGNITED BODIES. [MEMOIR 1. 
 
 platinum ; then taking an Argand lamp, surrounded by 
 a cylindrical metal sliade, through an aperture in which 
 the light passed, and the flame of which I had found by 
 previous trial would continue for an hour of almost the 
 same intensity, I approached it to the paper sheet, until 
 the shadow cast by the copper disappeared. The dis- 
 tance at which this took place was then measured, and 
 the temperature of the platinum determined. 
 
 The temperature of the platinum was now raised, the 
 shadow became more intense, and it was necessary to 
 bring the Argand lamp nearer before it was effaced. 
 When this took place, the distance of the lamp was 
 again measured, and the temperature of the platinum 
 again determined. 
 
 In Fig. 5, a b is the strip of ignited platinum. It casts 
 a shadow, A, of the metal rod e on the white screen/^; 
 is a metallic cylinder containing an Argand lamp, the 
 light of which issues through an aperture, d, and extin- 
 guishes the shadow on the screen. 
 
 Fig. 5. 
 
 In this manner I obtained several series of results, one 
 of which is given in the following table. They exhib- 
 ited a more perfect accordance among each other than I 
 had anticipated. 
 
 The intensity of the light of the platinum is of course 
 inversely proportional to the square of the distance of 
 
MEMOIH I.] THE RADIATIONS OF IGNITED BODIES. 
 
 41 
 
 Table of the Intensity of Light emitted by Platinum at Different 
 
 Temperatures. 
 
 Temperature 
 of the platinum. 
 
 Distance of Argand lamp. 
 
 Mean. 
 
 Intensity of 
 light. 
 
 Experiment I. 
 
 Experiment II. 
 
 980 
 
 
 
 
 0.00 
 
 1900 
 
 54.00 
 
 54.00 
 
 54.00 
 
 0.34 
 
 2015 
 
 39.00 
 
 41.00 
 
 40.00 
 
 0.02 
 
 2130 
 
 24.00 
 
 24.00 
 
 24.00 
 
 1.73 
 
 2245 
 
 18.00 
 
 19.00 
 
 18.50 
 
 2.92 
 
 2360 
 
 14.50 
 
 15.50 
 
 15.00 
 
 4.40 
 
 2475 
 
 11.50 
 
 12.00 
 
 11.75 
 
 7.24 
 
 2590 
 
 9.00 
 
 9.00 
 
 9.00 
 
 12.34 
 
 the Argand lamp at the moment of the extinction of the 
 shadow. 
 
 In this table the first column gives the temperatures 
 under examination in Fahrenheit degrees; the second 
 and third the distances of the Argand lamp from the 
 screen in English inches, in two different sets of experi- 
 ments; the fourth the mean of the two; and the fifth 
 the corresponding intensity of the light. 
 
 The results thus obtained proved that the increase in 
 the intensity of the light of the ignited platinum, though 
 slow at first, became very rapid as the temperature rose. 
 At 2590 the brilliancy of the light was more than 
 thirty-six times as great as it was at 1900. 
 
 Thus, therefore, the theoretical anticipation founded 
 on the analogy of light and heat was completely veri- 
 fied, the emission of light by a self-luminous solid as its 
 temperature rises being in greater proportion than would 
 correspond to mere difference of temperature. 
 
 To place this in a more striking point of view, I made 
 some corresponding experiments in relation to the heat 
 emitted. No one thus far had published results for high 
 temperatures, or had endeavored to establish through an 
 extensive scale the principle of Delaroche, that " the 
 quantity of heat which a hot body gives off in a given 
 time, by way of radiation to a cold body situated at a 
 
42 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 distance, increases, other things being equal, in a progres- 
 sion more rapid than the excess of the temperature of 
 the first above that of the second.'* 
 
 As the object thus proposed was mainly to illustrate 
 the remarkable analogy between light and heat, the ex- 
 periments now to be related were arranged so as to re- 
 semble the foregoing; that is to say, as in determining 
 the intensities of light emitted by a shining body at 
 different temperatures, I had received the rays upon a 
 screen placed at an invariable distance, and then deter- 
 mined their value by photometric methods, so in this 
 case I received the rays of heat upon a screen placed at 
 an invariable distance, and measured their intensity by 
 therrnometric methods. In this instance the screen em- 
 ployed was, in fact, the blackened surface of a thermo- 
 electric pile. It was arranged at a distance of about one 
 inch from the strip of ignited platinum, a distance suffi- 
 cient to keep it from any disturbance from the stream 
 of hot air arising from the metal ; care was also taken 
 that the multiplier itself was placed so far from the rest 
 of the apparatus that its astatic needles could not be af- 
 fected by the voltaic current igniting the platinum, or 
 the electro-magnetic action of the wires or rheostat used 
 to modify the degrees of heat. 
 
 In Fig. 6, a 1) is the ignited platinum strip, c the ther- 
 mo-electric pile, d d the multiplier. 
 
 The experiments were conducted as follows : The 
 needles of the thermo- multiplier standing at the zero 
 of their scale, the voltaic current was passed through 
 the platinum, which immediately rose to the correspond- 
 ing temperature, and radiated its heat to the face of the 
 pile. The instant this current passed, the needles of the 
 multiplier moved, and kept steadily advancing on the 
 scale. At the close of one minute the deviation of the 
 needle and the temperature of the platinum were si- 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 
 
 43 
 
 multaneously noted, 
 and then the voltaic 
 current was stopped. 
 
 Sufficient time was 
 
 f ,, 
 now given for the 
 
 needles of the multiplier to come back to zero. This 
 time varied in the different cases, according to the in- 
 tensity of the heat to which the pile had been exposed ; 
 in no instance, however, did it exceed six minutes, and 
 in most cases was much less. A little consideration will 
 show that the usual artifice employed to drive the needles 
 back to zero by warming the opposite face of the pile was 
 not admissible in these experiments. 
 
 The needles having regained their zero, the platinum 
 was brought again to a given temperature, and the ex- 
 periment conducted as before. The following table ex- 
 hibits a series of these results. 
 
 In this table the first column gives the temperatures 
 of the platinum in Fahrenheit degrees ; the second and 
 third, two series of experiments expressing the arc passed 
 over by the needle at the close of a radiation lasting one 
 minute, each number being the mean of several suc- 
 cessive trials, and the fourth the mean of the two. It 
 therefore gives the radiant effect of the incandescent 
 platinum on the thermo-multiplier for the different tem- 
 peratures. 
 
 Of course it is understood that I here take the angu- 
 
44 
 
 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 Table of the Intensity of Radiant Heat emitted by Platinum at Dif- 
 ferent Temperatures. 
 
 Temperature 
 of the platinum. 
 
 Intensity of heat emitted. 
 
 Mean. 
 
 Experiment I. 
 
 Experiment II. 
 
 980 
 
 .75 
 
 1.00 
 
 .87 
 
 1095 
 
 1.00 
 
 1.20 
 
 1.10 
 
 1210 
 
 1.40 
 
 1.60 
 
 1.50 
 
 1325 
 
 1.60 
 
 2.00 
 
 1.80 
 
 1440 
 
 2.20 
 
 2.20 
 
 2.20 
 
 1555 
 
 2.75 
 
 2.85 
 
 2.80 
 
 1670 
 
 3.65 
 
 3.75 
 
 3.70 
 
 1785 
 
 5.00 
 
 5.00 
 
 5.00 
 
 1900 
 
 6.70 
 
 6.90 
 
 6.80 
 
 2015 
 
 8.60 
 
 8.60 
 
 8.60 
 
 2130 
 
 10.00 
 
 10.00 
 
 10.00 
 
 2245 
 
 12.50 
 
 12.50 
 
 12.50 
 
 2360 
 
 15.50 
 
 15.50 
 
 15.50 
 
 lar deviations of the needle as expressing the force of 
 the thermo-electric current, or, in other words, as being 
 proportional to the temperatures. This hypothesis, it is 
 known, is admissible. 
 
 It therefore appears that if the quantity of heat radi- 
 ated by platinum at 980 be taken as unity, it will have 
 increased at 1440 to 2.5, at 1900 to 7.8, and at 2360 
 to 17.8, nearly. The rate of increase is, therefore, very 
 rapid. Further, it may be remarked, as illustrative of 
 the same fact, that the quantity of heat radiated by a 
 mass of platinum in passing from 1000 to 1300 is near- 
 ly equal to the amount it gives out in passing from com- 
 mon temperatures up to 1000. 
 
 I cannot here express myself with too much emphasis 
 on the remarkable analogy between light and heat which 
 these experiments reveal. The march of the phenomena 
 in all their leading points is the same in both cases. The 
 rapid increase of effect as the temperature rises is com- 
 mon to both. 
 
 It is not to be forgotten, however, that in the case of 
 light we necessarily measure its effects by an apparatus 
 which possesses special peculiarities. The eye is insen- 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 45 
 
 sible to rays not comprehended within certain limits of 
 refrangibility. In these experiments it is requisite to 
 raise the temperature of the platinum almost to 1000 
 before we can discover the first traces of light. Meas- 
 ures obtained under such circumstances are dependent 
 on the physiological action of the visual organ itself, and 
 hence their analogy with those obtained by the thermom- 
 eter becomes more striking, because we should scarcely 
 have anticipated that it could be so complete. 
 
 Among writers on Optics it has been a desideratum to 
 obtain an artificial light of standard brilliancy. The 
 preceding experiments furnish an easy means of supply- 
 ing that want, and give us what might be termed a " unit 
 lamp." A surface of platinum of standard dimensions, 
 raised to a standard temperature by a voltaic current, 
 will always emit a constant light. A strip of that metal, 
 one inch long and -^ of an inch wide, connected with a 
 lever by which its expansion might be measured, would 
 yield at 2000 a light suitable for most purposes. More- 
 over, it would be very easy to form from it a photome- 
 ter by screening portions of the shining surface. An in- 
 genious artist would have very little difficulty, by taking 
 advantage of the movements of the lever, in making a 
 self-acting apparatus in which the platinum should be 
 maintained at a uniform temperature, notwithstanding 
 any change taking place in the voltaic current, 
 
 UNIVERSITY or NEW YORK, Feb. 27, 1 847. 
 
 NOTE. The experiments related in the foregoing pages 
 were made by me between 1844 and 1847, They were 
 published in May in the latter year. 
 
 In the following July, M. Melloni, who was at that 
 time recognized as the chief authority on the subject of 
 
46 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 radiant heat, read before the Royal Academy of Sciences 
 at Naples a memoir entitled " Researches on the Radi- 
 ations of Incandescent Bodies ancl on the Elementary 
 Colors of the Solar Spectrum." This was translated into 
 French from the Italian, and published in the BilMo- 
 iheque Universelle of Geneva. It was also translated into 
 English, and published both in England and America. 
 
 M. Melloni commences his memoir as follows : " Among 
 the more recent scientific publications will be found a 
 memoir by the American professor J. W. Draper ' On the 
 Production of Light by Heat,' which appears to me to 
 merit the attentive consideration of those who interest 
 themselves in the progress of the natural sciences. The 
 author treats in a very ingenious manner some questions 
 allied to my own researches on light and radiant heat. 
 In reading this interesting work, several ideas have pre- 
 sented themselves to me, which I have submitted to the 
 test of experiment. I believe that an analysis of the 
 memoir of M. Draper, accompanied with a brief account 
 of what I have done, will not be without interest. 
 
 " Every one knows that heat, when it accumulates in 
 bodies, at last renders them incandescent, that is to say, 
 more or less luminous and visible in the dark. Is the 
 temperature necessary to produce this state of incandes- 
 cence always the same, or does it vary with the nature 
 of the body ? In either case, what is its degree, and what 
 is the succession of colored lights emitted by a given sub- 
 stance when brought to temperatures more and more ele- 
 vated? Finally, what is the relation that subsists at 
 different periods of incandescence between the tempera- 
 ture and the quantity of light and of heat emitted by a 
 body?" 
 
 Melloni then describes the apparatus and the processes 
 I had used to determine the thermometric degree of in- 
 candescence and its uniformity for different substances. 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 47 
 
 He dwells on the fact that melted metals, such as lead, 
 have the same point of ignition. He agrees in excepting 
 the phenomena of phosphorescence, and those in which 
 light is developed in chemical combinations. He re- 
 marks that " some philosophers of the highest eminence, 
 among them M. Biot, suppose that the first light disen- 
 gaged by incandescent bodies is blue, and they have ac- 
 counted for this on the principle of a theory now univer- 
 sally abandoned. But these cases," he adds, " ought to 
 be carefully distinguished from incandescence properly 
 speaking, which arises. directly and solely from an eleva- 
 tion of temperature in the body, and which always com- 
 mences with a red light. 
 
 " As to the exact degree of this temperature, the ob- 
 jections which might be raised against the mode em- 
 ployed by M. Draper are of very little importance. If 
 we compare the results at which he arrives with those 
 that have been obtained by Wedgwood and Daniel, the 
 difference is only 30 in excess in the first case, and 3 too 
 little in the second. The differences are much greater 
 when compared with the deductions of Davy and Sir I. 
 Newton, which gave 812 and 635, respectively. But 
 those numbers, and especially the latter, were obtained 
 by methods too imperfect to be trustworthy. Conse- 
 quently the number 977 Fahr., given by M. Draper, 
 must approach very closely the degree of heat which 
 produces the first incandescence of bodies." 
 
 Melloni then describes the method I had resorted to 
 for investigating the nature of the colors which are de- 
 veloped by an ignited body as its temperature is in- 
 creased. 
 
 He dwells on the employment of a reference spectrum, 
 which was resorted to in consequence of the spectrum of 
 a solid having no fixed lines a discovery which has 
 become of the utmost value in astronomical spectrum 
 
48 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 analysis. He states the results given in the foregoing 
 pages, and adds : " In other words, the spectrum of the 
 strip of platinum which corresponds to the red extremity 
 of the prismatic spectrum is at first very short, and con- 
 tains only the less refrangible colors ; but as the tempera- 
 ture rises, the spectrum of incandescence extends towards 
 the violet extremity, obtaining the more refrangible tints, 
 and at last acquiring all the colors and all the extent of 
 the solar spectrum, except the terminal rays at the two 
 extremities, which escape the observer evidently on ac- 
 count of their extreme feebleness. The same cause (in- 
 sensibility due to a want of luminous energy) makes the 
 first spectrum appear at the red end a little shorter than 
 the last, because the less refrangible rays of that color 
 are, as is well known, so feeble even in the solar spec- 
 trum that we are unable to perceive them, unless they 
 are isolated in a place that is totally dark. Much more, 
 therefore, ought they to remain invisible to the observer 
 when the spectrum arises from luminous agencies so little 
 energetic as are those of the first periods of incandescence. 
 
 " To a perfectly sensitive eye the variations of length 
 would evidently have taken place in the direction of the 
 more refrangible rays only, and all the spectra would 
 have commenced at the extreme limit of the red rays. 
 
 " It results from all these observations that when the 
 incandescence of a body becomes more and more vivid 
 and brilliant by the elevation of its temperature, there is 
 not only an augmentation in the intensity of the result- 
 ing light, but also in the variety of elementary colors 
 which compose it ; there is, too, an addition of rays so 
 much the more refrangible as the temperature of the in- 
 candescent body is higher. In this there is, therefore, 
 established an intimate analogy between the progressive 
 development of light and that of heat. Indeed,' 1 M. Mel- 
 loni adds, " as soon as I had convinced myself of the 
 
MEMOIR I.] THE RADIATIONS OF IGNITED BODIES. 49 
 
 immediate transmission of every variety of radiant heat 
 through rock-salt, I availed myself of that valuable prop- 
 erty to study the refraction of heat from various sources, 
 and I discovered that radiations coming from those of 
 a high temperature contain elements more refrangible 
 than those which are derived from sources that are not 
 so hot." 
 
 M. Melloni then passes to a criticism of the methods I 
 had used in investigating the law of the increase of the 
 luminous and calorific radiations, according as the tem- 
 perature of the source of heat is elevated. 
 
 He adds: "The method invented by Bouguer to deter- 
 mine the relative intensities of different luminous sources, 
 and employed by Draper to measure the quantities of 
 light emitted by a strip of platinum brought to different 
 degrees of incandescence, is the only one by which we 
 could hope for a successful result. The method of the 
 equality of shadows, well known under the name of 
 Rumford's method, would have furnished in the research- 
 es of the learned American uncertain data, on account of 
 the difficulty of establishing an exact comparison between 
 the accidental green tint introduced into the shadow en- 
 lightened by the yellow rays of the lamp and the red 
 light emitted by the ignited metal. As to the measures 
 of the radiant heat, they were determined by the aid of 
 the thermo-multiplier, that admirable instrument which 
 has revealed to science so many new properties of calo- 
 rific radiations, and which still is rendering eminent 
 services in the hands of able chemists far beyond the 
 Alps. 
 
 " The numbers obtained by M. Draper show evidently 
 that the augmentations of both light and heat, though 
 feeble at first, become very rapid at last, from which it 
 results that the radiations both of light and heat follow 
 in the progression of quantity the same analogy that we 
 
 D 
 
5Q THE KADIATIONS OF IGNITED BODIES. [MEMOIR I. 
 
 have just observed in the progression of quality. These 
 researches then conduct, as do others heretofore known 
 on light and radiant heat, to a perfect analogy between 
 the general laws which govern these two great agents of 
 nature." 
 
 The law of the radiation of heat, as illustrated by the 
 foregoing experiments with an ignited strip of platinum, 
 has been applied in recent discussions respecting the age 
 of the earth. Geological evidence has satisfactorily es- 
 tablished that the temperature of the earth was formerly 
 much higher than now, and the decline that has hap- 
 pened could only have taken place by radiation into 
 space. Considering how slow the cooling now is a 
 scarcely perceptible fraction of a degree in the course of 
 many centuries it would seem that to accomplish the 
 whole descent, if even w r e go no further back than the 
 paleozoic era, an amazing lapse of time would be re- 
 quired. And if we accept the nebular hypothesis, since 
 the original temperature must have been at least that of 
 the surface of the sun, the time must be correspondingly 
 extended. Even if numbers could be given, the imagina- 
 tion would altogether fail to appreciate them. 
 
 But we have here experimental proof that the higher 
 the temperature of a body, the more rapidly it cools. A 
 descent through a given number of degrees is more quick- 
 ly made when a body is at a high than when it is at a 
 low temperature. Anciently the cooling of the earth 
 was more rapid than it is now. Not that there was any 
 change or breach in the general law under which the op- 
 eration was taking place, for the same mathematical ex- 
 pression applies to all temperatures, no matter how high 
 or how low they may be. Mr. Croll, in his recent re- 
 searches on the distribution of heat over the globe, points 
 out the bearing of these experiments. 
 
MEMOIR L] THE RADIATIONS OF IGNITED BODIES. 
 
 51 
 
 Our estimate of the age of the earth, as deduced from 
 the cooling she has undergone, must therefore, in view 
 of these considerations, be diminished a result insisted 
 upon by many recent authors. Too much weight must, 
 however, not be given to this conclusion, since it ought 
 to be borne in mind that the cooling was not taking 
 place by radiation into space from the earth alone as a 
 solitary body. She was in presence of a high extrane- 
 ous temperature, which diminished her speed of cooling, 
 and correspondingly increased the time. 
 
 Though the problem of the age of the earth, as inves- 
 tigated through the changes of her temperature, may not 
 at present be capable of exact solution, it must be ad- 
 mitted that the time required to bring her heat to its 
 present degree must have been inconceivably long. 
 
 LIB R A R Y 
 
 UNIVKHW1TY OF 
 
 CALIF I A. 
 
 
52 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 MEMOIR II. 
 
 SPECTRUM ANALYSIS OF FLAMES. PRODUCTION OF LIGHT 
 BY CHEMICAL ACTION. 
 
 From the American Journal of Science, Second Series, Vol. V., 1848; Philosophical 
 Magazine, London and Edinburgh, Feb., 1848; Harper's New Monthly Maga- 
 zine, No. 323. 
 
 CONTENTS : Spectrum analysis of a candle flame. Examination of 
 various other flames. Spectrum analysis of the light of a burning 
 solid. Flames consist of a succession of shells. Spectrum of cyano- 
 gen. Combustion of flames in oxygen. Effect of air in the interior 
 of a flame. The blowpipe cone. General result, the more energetic 
 the chemical action the higher the refrangibility of the resulting light; 
 the vibrations increase in frequency as the chemical action is more 
 violent. Fraunhofer's fixed lines. 
 
 THE production of light and heat by the combustion 
 of various bodies is, of all chemical processes, that which 
 ministers most to the comfort and well-being of man. 
 By it the rigor of winter is abated, and night made 
 almost as available for our purposes as day. 
 
 One would suppose that, of a phenomenon on which 
 so much of our personal and social happiness depends, 
 and which must have been witnessed by every one, all 
 the particulars ought to have been long ago known. 
 Among scientific men its importance has been universal- 
 ly recognized. The earlier theories of chemistry, such as 
 those of Stahl and Lavoisier, are essentially theories of 
 combustion. 
 
 It is nevertheless remarkable how little positive 
 knowledge, until quite recently, was possessed on this 
 subject. Some chemists thought that the light emitted 
 by flames is due to electric discharges ; others, regarding 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. 53 
 
 light and heat as material bodies which can be incor- 
 porated or united with ponderable substances, supposed 
 that they are disengaged as chemical changes go on. In 
 this confusion of opinions a multitude of interesting and 
 hitherto unanswered questions present themselves. It 
 is known that different substances, when burning, emit 
 rays of different colors. Thus sulphur and carbonic 
 oxide burn blue, wax yellow, and cyanogen lilac. What 
 are the chemical conditions that determine these singular 
 differences? How is it that, by changing the conditions 
 of combustion, we can vary the nature of the light? 
 We turn aside the flame of a candle by means of a 
 blow-pipe, and a neat blue cone appears. Why does it 
 shine with a blue light? 
 
 Such inquiries might be multiplied without end ; but 
 a little consideration shows that their various answers 
 depend on the determination of a much more general 
 problem, viz., Can any connection be traced between the 
 chemical nature of a substance, or the conditions under 
 which it burns, and the nature of the light it emits ? It 
 is to the discussion of that problem that this memoir is 
 devoted. 
 
 Sir H. Davy has already furnished us with two im- 
 portant facts in relation to the nature of flame : 1st, All 
 common flames are incandescent shells, the interior of 
 which is dark ; 2d, the relative quantity of light emitted 
 depends upon the temporary disengagement of solid par- 
 ticles of carbon. 
 
 It is only by a very general examination of the light 
 arising from various solids, vapors, and gases, when 
 burning, that we can expect to obtain data for a true 
 theory of combustion. This is what I shall endeavor 
 to furnish on the present occasion. 
 
 As was foreseen by all the older chemists, the true 
 theory of combustion, whatever it may prove to be, 
 
54 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 must necessarily be one of the fundamental theories of 
 chemistry. It must include the nature of all chemical 
 changes whatsoever. The subject is therefore not alone 
 interesting in a popular sense, but of great importance 
 in its scientific connections. 
 
 I. Prismatic analyses of the flames of various vapors 
 and gases ; proving that they yield all the colors of the 
 spectrum. 
 
 I commenced this investigation of the nature of flame 
 and of combustion generally by an optical examination 
 of various bodies in the act of burning. Some authors 
 have asserted that certain flames yield monochromatic 
 light. It is necessary to verify this assertion, if true, or 
 set it aside if false. 
 
 The instrumental arrangement I resorted to for the 
 determination of the structure of a flame may be thus 
 described : The rays of the flame of which the examina- 
 tion was to be made passed through a horizontal slit, 
 one thirtieth of an inch wide and one inch long, in a 
 metallic screen placed near to the flame, and were re- 
 ceived at a distance of six or eight feet on a flint-glass 
 prism, the axis of which was parallel to the slit. After 
 passing the prism, they entered a telescope, which had a 
 divided micrometer and parallel wires in its eye-piece. 
 Through this telescope the resulting spectrum was 
 viewed. In this form of spectroscope no collimating 
 lens was used. 
 
 In Fig. 7, a is the candle, or flame which is to be ex- 
 amined, b the screen with a horizontal slit, c the prism, 
 d d the telescope. 
 
 If it be the flame of a lamp of any kind that is to be 
 examined, by using a movable stand we are able to raise 
 or lower it, and thus analyze different horizontal elements 
 in its lower, its middle, or its upper parts at pleasure. 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. 
 
 55 
 
 Fig. T. 
 
 If, instead of a horizontal, we wish to examine a vertical 
 element of the flame, the slit and the prism must, of 
 course, be set vertically. The former mode possesses 
 great advantages, as will be presently pointed out. It 
 is to be understood in all cases that the eye-piece of the 
 telescope is adjusted to give a sharp image of the slit, 
 and the prism is at its angle of minimum deviation. 
 
 By this arrangement I have examined a great number 
 of different flames, as those of oil, alcohol, solutions of 
 boracic acid arid nitrate of strontian in alcohol, phos- 
 phorus, sulphur, carbonic oxide, hydrogen, cyanogen, 
 arseniuretted hydrogen, etc. Among these, it will be 
 noticed that different colors occur. Oil gives a yellow 
 flame, alcohol a pale blue, boracic acid green, strontian 
 red, phosphorus yellowish - white, sulphur and carbonic 
 oxide blue, hydrogen pale yellow, cyanogen lilac, arseni- 
 uretted hydrogen white, etc. 
 
 Notwithstanding this diversity of color, all these 
 flames, as well as many others I have tried, yield the 
 same result: every prismatic color is found in them. 
 Even in those cases where the flame is very faint, as in 
 alcohol and hydrogen gas, not only may red, yellow, 
 green, blue, and violet light be traced, but even bright 
 Fraunhoferian lines of different colors. 
 
56 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 This observation holds good for those flames reputed 
 to be monochromatic ; for example, alcohol burned from 
 a wick imbued with common salt. It is not only a 
 yellow light which is evolved ; the other colors plainly, 
 though more faintly, appear. 
 
 All flames, no matter what their special colors may 
 be, evolve all the prismatic rays. Their special tints 
 arise from the preponderance of one class of rays over 
 another; thus in cyanogen the red predominates, and 
 in sulphur the blue. 
 
 (Later experiments have proved that the spectrum 
 thus containing all the prismatic colors, and acting as a 
 background to the bright fixed lines, is not due, except 
 indirectly, to the flame under examination, but to other 
 causes, not here taken into account, especially the acci- 
 dental presence of daylight, combustion of dust in the 
 air, etc. The statements here given are, however, in 
 accordance with observations actually made. These are 
 the eifects that will be seen in a partially illuminated 
 room such as the laboratory in which these experiments 
 were made. In a dark room the spectrum background 
 disappears.) 
 
 The production of light in the case of flames is thus 
 proved to be a very complex phenomenon. The chem- 
 ical conditions under which the burning takes place are 
 likewise very complex. The combustible vapor is sur- 
 rounded on all sides by atmospheric air; diffusion oc- 
 curs, and rapid currents are established by the high 
 temperature. Such circumstances complicate the result; 
 and it is only by observing the burning of an elementary 
 solid, in which most of these disturbances are cut off, 
 that we can hope to effect a proper resolution of the 
 problem. 
 
 II. Prismatic analysis of the light of an elementary 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. 5f 
 
 solid burning at different temperatures; proving that as 
 the temperature rises the more refrangible rays appear. 
 
 I took from the fire a piece of burning anthracite coal 
 the fuel ordinarily used in domestic economy in New 
 York and which from its compactness, the intense heat 
 it evolves, and other properties, appears to be well fitted 
 for these investigations. This coal was placed on a 
 support so as to present a plane surface to the slit in 
 the metal screen. The rays coming from it and passing 
 the slit were received on the flint-glass prism, and viewed 
 through the telescope. 
 
 When the coal was first taken from the fire, and was 
 burning very intensely, on looking through the telescope 
 all the colored rays of the spectrum were seen in their 
 proper order. I had previously passed through the slit 
 a beam of sunlight reflected from a mirror, so as to have 
 a reference spectrum with fixed lines. Now when the 
 coal was burning at its utmost vigor, the spectrum it 
 gave did not seem to differ, either as respects length or 
 the distribution of its colors, from the spectrum of sun- 
 light ; but as the combustion declined and the coal 
 burned less brightly, its spectrum became less and less, 
 the shortening taking place first at the more refrangible 
 extremity, one ray after another disappearing in due 
 succession. First the violet became extinct, then the 
 indigo, then the blue, then the green, until at last the 
 red, with an ash-gray light occupying the place of the 
 yellow, was alone visible, and presently this also went 
 out. 
 
 From numerous experiments of this kind, I conclude 
 that there is a connection between the refrangibility of the 
 light which a burning body yields and the intensity of the 
 chemical action going on, and that the refrangibility al- 
 ways increases as the chemical action increases. It might, 
 perhaps, be objected that, in the form of experiment here 
 
58 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 introduced, two totally different things are confounded, 
 and that the burning coal not only gives forth its rays 
 as a combustible body, strictly speaking, but also as an 
 incandescent mass. 
 
 To avoid this objection as far as possible, and also to 
 reach a much higher temperature than could have been 
 otherwise obtained, I threw a stream of oxygen gas on 
 that part of the anthracite which was opposite the slit ; 
 but my expectations were disappointed, for, instead of 
 the combustion being increased, the coal was actually 
 extinguished by the jet playing on it. I therefore re- 
 placed the anthracite with a flat piece of well-burned 
 charcoal, kindled at the portion opposite the slit, and 
 throwing a stream of oxygen on this part, the combus- 
 tion was greatly increased. A spectrum rivalling that 
 of the sunbeam in brilliancy was produced, all the col- 
 ors from the extreme red to the extreme violet being 
 present. 
 
 On shutting off the supply of oxygen, the combustion, 
 of course, declined, and while this was going on the vio- 
 let, the indigo, the blue, the green, etc., faded away in 
 succession. By merely turning the gas on or off, the 
 original colors could be re-established or made to decline. 
 It was very interesting to see with what regularity, as 
 the chemical action became more intense, the more re- 
 frangible colors were developed, and how, as it declined, 
 they disappeared in due succession; the final tint being 
 red and that ash-gray in the position of the yellow which 
 has been described in the preceding memoir. 
 
 In the form of experiment here made the combustion 
 is, of course, merely superficial ; and the rays come from 
 the charcoal not as an incandescent, but as a burning 
 body. 
 
 III. Of tlu constitution of flames; proving that they 
 
MKMOIR II.] SPECTRUM ANALYSIS OF FLAMES. 59 
 
 consist of a series of concentric and differently colored 
 shells. 
 
 I regard the foregoing experiments as affording the 
 means of explanation of the much more complicated 
 phenomena of flames, and proceed to inquire whether 
 the principle I have just brought forward of the co-or- 
 dinate increase of refrangibility and of chemical action 
 will hold good ; premising the experiments now to be 
 detailed with the following considerations. 
 
 All common flames, as is well known, consist of a thin 
 shell of ignited matter, the interior being dark, the com- 
 bustion taking effect on those points only which are in 
 contact with the air. From the circumstances under 
 which the air is usually supplied, this ignited shell can- 
 not be a mere mathematical superficies, but must have a 
 sensible thickness. If we imagine it to consist of a series 
 of cone-like strata, it is obvious that the phenomena of 
 combustion are different in each. The outer stratum is 
 in contact with the air, and there the combustion is most 
 perfect; but by reason of the rapid diffusion of gases 
 into one another, currents, and other such causes, the at- 
 mospheric air must necessarily pervade the burning shell 
 to a certain depth, and in the successive strata, as we ad- 
 vance inward, the activity of the burning must decline. 
 On the exterior stratum oxygen is in excess, at the in- 
 terior the combustible vapor, and between these limits 
 there must be an admixture of the two, which differs 
 at different depths. Admitting the results of the fore- 
 going experiments with anthracite coal and charcoal to 
 be true, viz., that as the combustion is more active, rays 
 of a higher degree of refrangibility are evolved, it fol- 
 lows that each point of the superficies of such a flame 
 must yield all the colors of the spectrum, the violet coming 
 from the outer strata, the yellow from the intermediate, 
 the red from those within. If we could isolate an ele- 
 
60 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 raentary horizontal section of such a flame, it would ex- 
 hibit the appearance of a rainbow ring, and when those 
 compound rays are received on ttie face of a prism, the 
 constituent colors are parted out by reason of their dif- 
 ferent refrangibility, and the eye is thus made sensible 
 of their actual existence. 
 
 When thus by the aid of a prism we analyze the light 
 coming from any portion of the superficies of a flame, we 
 in effect dissect out in a convenient manner and arrange 
 together side by side rays that have come from different 
 strata of the burning shell. These, without the prism, 
 would have pursued the same normal path, and pro- 
 duced a commixed effect as white light on the eye, but 
 with it are separated transversely, and each becomes 
 perceptible. 
 
 It is immaterial whether we impute the light emitted 
 by an ordinary flame to the liberation of solid particles 
 of carbon in an ignited condition, and becoming hotter 
 and hotter as they pass outwardly towards the surface, 
 or consider these particles to be in a state of combus- 
 tion. The experiments of an ignited wire in one case, 
 and of charcoal in presence of oxygen in the other, lead 
 to the same explanation. We are not to suppose that 
 it is simply a gas which is burning; we are examining 
 the light emitted by an incandescent solid the carbon 
 particles that for the moment are set free. 
 
 (This explanation, that the luminosity of a flame is 
 due to the temporary extrication of solid carbon, was 
 given by Sir H. Davy. It has been called in question 
 by Frankland. Experiments and criticisms have since 
 been offered by Deville, Knapp, Stein, Blockmann, and 
 others, but Davy's theory still remains substantially un- 
 affected. This is the conclusion to which Heumann has 
 come in his recently published researches on luminous 
 flames (1876).) 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. Ql 
 
 It might be supposed that in the familiar instance of 
 an oil lamp, if we put any check on the supply of the 
 air, and thereby check the intensity of the combustion, 
 we ought to produce a flame emitting rays of light the 
 refrangibility of which becomes less and less, and which, 
 from their being originally white, should pass through 
 various shades of orange, and end in a dull red. But 
 the compound nature of the burning vapor interferes 
 with that result ; for when a certain point is gained the 
 hydrogen, for the most part, alone burns, the carbon be- 
 ing set free as smoke, and such a flame cannot support 
 itself in strict accordance with the principle given. 
 
 We must, then, search for other conditions under which 
 carbon is found which are free from this difficulty. Two 
 at once present themselves : they are carbonic oxide 
 and cyanogen gas. In the former the carbon is already 
 united with half the oxygen required for maximum ox- 
 idation : its complete combustion can therefore be carried 
 on with a limited supply of atmospheric air ; in the lat- 
 ter the carbon is united with nitrogen, which during the 
 combustion is set free, and interferes with the process by 
 cutting off the more complete access of the atmosphere. 
 
 In place of the burning coal of the former experiments 
 I substituted a jet-pipe through which the various gases 
 might be made to pass, and the rays emitted by their 
 flames enter the telescope after passing through the slit 
 and prism. In this arrangement the slit should be hori- 
 zontal and not vertical. So far from it being immaterial 
 which of the two positions is selected, very great advan- 
 tages arise from the former. If the slit be vertical, the 
 prism, it is true, will separate the constituent colors from 
 one another, but it fails to show their relative positions. 
 If it be horizontal, the relative positions of the different 
 colors can be demonstrated, and it can be proved that a 
 horizontal section of a flame is in reality, as has been al- 
 
52 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 ready remarked, a colored ring, the red being the inner- 
 most color, and the violet outside; for if this be the 
 order in which the colors occur, the red ring must neces- 
 sarily have a less diameter than the green, and the green 
 less than the violet; and when the prism, set in a hori- 
 zontal position, separates those colors from each other, 
 the sides of the resulting spectrum ought not to be par- 
 allel, but inclined to one another, the breadth being least 
 in the red and increasing towards the violet end. This 
 increasing breadth proves that the constituent colored 
 shells of the flame envelop each other, the violet being 
 outermost, and therefore broadest. This valuable indi- 
 cation would be wholly lost if the slit were vertical. 
 
 This being understood, I may illustrate the facts now 
 to be brought forward by an example of the prismatic 
 analysis of a horizontal element of the flame of a spirit- 
 lamp ; it being understood that the prism is at its angle 
 of minimum deviation, and the spectrum seen through 
 the telescope. All the prismatic colors, in their proper 
 order, are visible ; the sides of the spectrum not being par- 
 allel, the inclination being quite rapid towards the red 
 extremity, the rays of which come from the interior of 
 the flame, where the diameter is less. Mere inspection is 
 sufficient to show the rapid approach of the red sides to 
 each other, and I satisfied myself that even in the more 
 refrangible regions there is the same want of parallelism, 
 by rotating the telescope on its vertical axis so that the 
 vertical wires in its eye-piece might coincide with first 
 one and then the other side of the spectrum. It will be 
 understood that I took the proper precaution not to be 
 deceived by a partial want of achromaticity in the tele- 
 scope, which might have led to a mistake. 
 
 But further, the yellow space of such a spirit -flame 
 spectrum is crossed by a bright fixed line Brewster's 
 monochromatic ray. It is a beautiful example of the 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. (53 
 
 principles just pointed out in this method of horizontal 
 analysis, being of much greater width than the rest of 
 the spectrum, and recalling to the imagination the ap- 
 pearance of Saturn's ring when nearly closed and seen 
 through a telescope of moderate power. This ray, from 
 its superior breadth, must necessarily come from that 
 pale, tawny light which invests the bright part of the 
 flame. This, which is readily seen when the flame is 
 large, envelops the middle and upper parts, but cannot 
 so easily be detected low down. It is to be attributed 
 to the carbonic acid and steam that have risen at a high 
 temperature in the burning shell, and are escaping at a 
 degree above that of incandescence into the air, and are 
 mingled with oxygen diffusing from the air into them. 
 A similar tawny cloak surrounds the upper part of the 
 flame of a candle ; it answers to the oxidizing flame of 
 the blow-pipe, and yields Brewster's monochromatic yel- 
 low light. 
 
 (A few years subsequently this yellow ray was dis- 
 covered by Swan to be due to sodium. It is now known 
 as the lines D. At the date of this memoir it was not 
 suspected that sodium is so universally diffused.) 
 
 IV. Explanation of tlie nature of colored flames, show- 
 ing, for example, why carbonic oxide burns blue, and cy- 
 anogen red. 
 
 To return now to carbonic oxide and cyanogen. Fig. 
 8, No. 1, represents the solar spectrum with its fixed 
 lines ; No. 3 represents the spectrum of carbonic oxide 
 burning in the air. It begins in the red region short of 
 the fixed line C, and terminates between the lines G and 
 H. It yields, therefore, rays of every color, and this in 
 accordance with the principles we have laid down ; but 
 when the relative quantity and force of the rays are es- 
 timated in comparison with the sunlight spectrum, the 
 
G4 
 
 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR IL 
 
 Spectra of Various Flames. 
 
 iBC ! 
 
 E F G 
 
 Hfc 
 
 IL i 
 
 fl Solar 
 1 || ||i spectrum. 
 
 1 j : Spirit-lamp. 
 
 i 
 
 1 
 j Carbonic oxide. 
 
 OOOOOQQ1Q 
 iDOilDQ 
 
 1 E 1 
 
 \ Cyanogen In air. 
 
 II H 1 
 
 i Cyanogen 
 |in oxyeen. 
 
 r 
 
 
 j Oil-lamp In air. 
 
 } 1 
 
 [Oil-lamp 
 in oxygen. 
 
 I i 
 
 I Hydrogen In oxygen. 
 
 nfii ^ 
 
 m b 
 
 I Nitrate of strontian. 
 
 m D D D 
 
 Brewster's yellow ray. 
 
 Blowpipe cone. 
 
 Figr. 8. 
 
 red and orange are deficient, and the more refrangible 
 colors predominate, and, indeed, it is the excess of these 
 that gives the flame its characteristic blue tint. This 
 agrees with what has been observed as to anthracite and 
 charcoal; for with carbonic oxide a limited supply of 
 oxygen can bring about the maximum chemical action, 
 and therefore liberate in abundance rays of maximum 
 refrangibility. 
 
 This condition of things is inverted in the case of cy- 
 anogen. It is the nature of its flame to be enveloped, as 
 it were, in a sheet of nitrogen arising from its own burn- 
 ing, and this necessarily impedes the access of air and 
 checks the intensity of the chemical change : a check 
 which is at once betokened by the emission of a predom- 
 inant number of rays of low refrangibility or of a red 
 color. 
 
 But there is a striking difference in the chemical con- 
 ditions under which carbonic oxide and cyanogen burn. 
 In the case of the former the whole gas is combustible, 
 in the latter the carbon alone, and we have, in reality, 
 
MEMOIR II.] SPECTKUM ANALYSIS OF FLAMES. 55 
 
 introduced an incombustible element into the flame ; for 
 as the carbon burns the incombustible nitrogen is set 
 free. It occurred to me, in selecting the gas for experi- 
 ment, that this condition should impress a physical char- 
 acteristic on the flame. I thought it was not impossible 
 that dark lines in its spectrum might be the result, be- 
 cause there must be a peculiar arrangement of the burn- 
 ing strata which together make up the shell of the flame, 
 every two atoms of carbon setting free one of nitrogen. 
 I did not know, until subsequently, that this flame had 
 already been examined by Faraday. Having therefore 
 confined some cyanogen, made from cyanide of mercury, 
 in a glass gas-holder filled with a saturated solution of 
 common salt, I burned it from the jet-pipe, and found 
 that what I had surmised was actually the fact. There 
 was a spectrum so beautiful that it is impossible to de- 
 scribe it by words, or depict it in colors. It was crossed 
 throughout its extent by black lines, separating it into 
 well-marked divisions. I could plainly count four great 
 red rays of definite refrangibility, followed by one or- 
 ange, one yellow r , and seven green rays ; while in the more 
 refrangible spaces were two extensive groups of black 
 lines, recalling somewhat from their position, but greatly 
 exceeding in extent, Fraunhofer's lines G and H in the 
 sun-rays. I shall return to the consideration of this 
 spectrum and to the relation of fixed lines presently, 
 here only making the remark that the burning of cyan- 
 ogen, both as respects the color of the light and the oc- 
 currence of fixed lines, is a direct consequence of the 
 principle I am establishing. 
 
 The unassisted eye detects two well-marked regions 
 in the cyanogen flame: a greenish-gray stratum on the 
 outside, and a lilac-colored nucleus within. Decomposed 
 by the prism, a horizontal element of this flame shows 
 that the exterior shell contains all the prismatic colors, 
 
 E 
 
(]g SPECTRUM ANALYSIS OF FLAMES. [MKMOIK JJ. 
 
 except, perhaps, the yellow ; but the green, the blue, and 
 the violet greatly predominate. The interior lilac flame 
 is the source of the bright spectrum with fixed lines just 
 described. 
 
 V. Continuation of the same principle in the case in 
 which combustion is carried on in oxygen gas instead of 
 atmospheric air. 
 
 If the principle that high refrangibility is connected 
 with intense chemical action be true, it must hold good 
 when the nature of the atmosphere in which the burning 
 is carried forward is changed. If, instead of being the 
 common air, it is oxygen gas, we ought to be able to 
 foresee the result. Carbonic oxide, when made to burn 
 in that gas, should not change its tint ; because if the air 
 can carry on the process to its maximum effect, oxygen 
 can do no more. But the result should be just the re- 
 verse with cyanogen, which, if made to burn in oxygen, 
 should be capable of emitting rays of higher refrangi- 
 bility. 
 
 Foreseeing this result, I submitted the two gases to 
 experiment, and first arranged the carbonic oxide so 
 that its spectrum might be examined in the telescope 
 as already described; then causing a clean bell -jar full 
 of oxygen to be inverted over it, the flame diminished 
 somewhat in size, emitted a slight crackling sound, but 
 retained its color unchanged. Its spectrum appeared pre- 
 cisely the same both as respects extent and the distribu- 
 tion of color, whether the burning took place in oxygen 
 gas or in atmospheric air. 
 
 If cyanogen be made to burn in oxygen, we should 
 expect that it would lose to a great extent its character- 
 istic lilac tint, and emit a whiter light. It was therefore 
 very interesting to find that the moment the flame was 
 immersed in oxygen it lost much of its pinkish color, 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. 57 
 
 and became of a dazzling brilliancy; and on examina- 
 tion through the telescope, though all the colors ha'd 
 increased in brightness, the most remarkable effect took 
 place among the extreme refrangible rays. Far out of 
 the limits of the ordinary spectrum a ray of great purity 
 and force was developed, as represented in the Fig. at 
 No. 5. Its color is violet. 
 
 I have made similar experiments on many other flames 
 besides those here mentioned. It is not necessary to re- 
 late them in detail, for they give the same results. In 
 every instance of combustion in the air, when the flame 
 is bright enough, all the colors are visible; and when 
 the combustion takes place in oxygen they are increased 
 in intensity. With hydrogen gas and alcohol the light 
 is so feeble that the eye cannot catch the terminal rays ; 
 but as soon as the combustion is made in oxygen the 
 red and the violet both appear, the latter, however, pre- 
 dominating. Several of these spectra both in air and 
 oxygen are represented in Fig. 8. In No. 9, the letters 
 m g and m b indicate a maximum of green and blue 
 light in the form of bright lines. 
 
 It does not require the use of a prism to satisfy one's 
 self of the change of tint that flames exhibit when the 
 chemical action increases. In reality it is only necessary 
 to compare by eye-sight the color of the light emitted 
 in air and in oxygen gas. In the latter case rays of a 
 higher refrangibility uniformly arise. 
 
 On the evidence furnished by the foregoing experi- 
 ments I regard a common flame as consisting of a shell 
 of ignited matter in which combustion is going on with 
 different degrees of rapidity at different depths, being 
 most rapid at the exterior, where there is a more perfect 
 contact with the atmosphere, and diminishing inward. 
 In a horizontal section, the interior space consisting of 
 unburned vapor is black; this is surrounded by a ring 
 
68 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 where the combustion is incipient, and from which red 
 light issues; then follow orange, yellow, green, blue, 
 indigo, and violet circles in succession, the production 
 of each of these tints being dependent on the rapidity 
 with w 7 hich chemical action is going forward that is, 
 on the amount of oxygen present the tints gradually 
 shading off into one another and forming, as I have said, 
 a circular rainbow. An eye placed on the exterior of 
 such a flame sees all the colors conjointly, and from their 
 general admixture arises the predominant tint. 
 
 An examination of the flame of a candle vertically 
 confirms this conclusion, for the red projects on the top 
 of the flame, and the blue towards the bottom. 
 
 From this, which may be regarded as the normal 
 flame, the flame of cyanogen differs. It must consist of 
 as many concentric shells as the prism separates it into 
 regions of definite refrangibility. Its interior part is 
 therefore divided into four red layers, followed by one 
 of orange, one of yellow, seven of green, etc. There are 
 two great inactive spaces towards the outside of the 
 flame, corresponding to the two great groups of fixed 
 lines. Perhaps through all these inactive parts the in- 
 combustible nitrogen chiefly escapes. 
 
 VI. Effects of the introduction of air into the interior 
 of a flame, producing the destruction of the red and 
 orange strata, and converting them into violet. 
 
 It now becomes a curious subject to determine what 
 takes place when an ordinary flame is disturbed by the 
 introduction of air into its interior. When a blow-pipe 
 jet is thrown through the flame of an oil-lamp, the sharp 
 blue cone which forms indicates, on the principles here 
 set forth, that the combustion is much more active. But 
 if the colors of the common flame come from different 
 depths, the red being the innermost, it is clear that the 
 
MEMOIR II.] SPECTKUM ANALYSIS OF FLAMES. 9 
 
 introduction of a jet of air by a blow-pipe should make 
 the combustion rapid where before it was slowest, and 
 the less refrangible colors ought to be destroyed. A 
 prismatic analysis should exhibit the spectrum of a 
 blow-pipe flame without any red or orange. 
 
 In this examination no slit was required, as in the 
 former experiments, for the cone itself, when at a dis- 
 tance of six or eight feet, was narrow enough for the 
 purpose. It yielded a very extraordinary spectrum. As 
 I anticipated, all the red rays were gone; not a vestige 
 of either them or of the orange could be found. But 
 the spectrum was divided into five well-marked regions, 
 separated from one another by dark spaces. There were 
 five distinct images of the blue cone : one yellow, two 
 green, one blue, and one violet. In Fig. 9 this result is 
 represented. 
 
 This experiment may be verified without a telescope. 
 On looking through a prism, set horizontally at its angle 
 of minimum deviation, at a blow-pipe cone some six or 
 eight feet distant, there will be seen a spectrum of that 
 part of the flame which does not join in the production 
 of the blue cone. It contains, of course, all the prismatic 
 colors. But projecting from this are five colored images 
 of the cone one yellow, two green, one blue, and one 
 violet. They are entirely distinct from one another, and 
 are parted by dark spaces. 
 
 Such is the effect of introducing air into the interior 
 of a flame, and destroying those 
 strata that yield the red and 
 orange colors. The effect of a 
 blow -pipe is to produce two 
 strata of blue light, one being 
 external, the other internal; also F . y 
 
 two strata of green, one again 
 external, the other internal, and the escaping products 
 
70 SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 of combustion, steam and carbonic acid, mingled with 
 atmospheric air, constitute the oxidizing flame, which 
 envelops the blue cone, and emits Brewster's mono- 
 chromatic yellow light. That the yellow light comes 
 from this flame is proved by the greater length of its 
 image. 
 
 VII. Physical cause of the production of light by chem- 
 ical action . 
 
 Do not the various facts here brought forward prove 
 that chemical combinations are attended by a rapid 
 vibratory motion of the particles of the combining 
 bodies, which vibrations become more frequent as the 
 chemical action is more intense? 
 
 The burning particles constituting the inner shell of 
 a flame are executing about four hundred billions of 
 vibrations in one second ; those in the middle about 
 six hundred billions, and those on the exterior, in con- 
 tact with the air, about eight hundred billions in the 
 same time. The quality of the emitted light, as re- 
 spects its color, depending on the frequency with which 
 these vibrations are accomplished, increases in refran- 
 gibiHty as the energy of the chemical action becomes 
 greater. 
 
 The parts of all material bodies are in a state of 
 incessant vibration ; that which we call temperature de- 
 pends on the frequency and amplitude of these vibra- 
 tions conjointly. If by any process, as by chemical 
 agencies, we increase that frequency to between four 
 and eight hundred billions of vibrations in one second, 
 ignition or combustion results. In the case of the for- 
 mer of these numbers, the temperature is 977 Fahr. 
 At this temperature the waves propagated in the ether 
 impress the organ of vision with a red light. This also 
 is the temperature of the innermost shell of a flame. If 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. ^\ 
 
 the frequency of vibration still increases, the temperature 
 correspondingly rises, and the light successively becomes 
 orange, yellow, green, blue, etc., and this condition ob- 
 tains in the successive strata of a flame, as we pass from 
 its interior to its exterior surface. 
 
 The general principle at which I thus arrive, as the 
 result of this experimental investigation, viz., that there 
 is a connection between the energy with which chemical 
 affinity is satisfied and the refrangibility of the resulting 
 light, assumes the position of a simple consequence of 
 the undulatory theory. Is it not very natural, if all 
 chemical changes are attended by vibratory motions in 
 the particles of the bodies engaged, that those vibrations 
 should increase in frequency as the action becomes more 
 violent ? But an increased frequency of vibration is the 
 same thing as an increased refraugibility. 
 
 VIII. On the physical cause of Fraunliofer* s dark lines. 
 
 Although I have extended this memoir to so great a 
 length, I have omitted many facts which have been 
 made the subject of experiment. I cannot conclude, 
 however, without offering some remarks on the artificial 
 production and cause of Fraunhofer's fixed lines. 
 
 It has been stated that I was led to expect the pro- 
 duction of these lines in the flame of cyanogen, from 
 considering the circumstances under which its combus- 
 tion takes place. Returning to this phenomenon, I 
 shall here point out a very remarkable numerical re- 
 lation existing among the fixed lines of the solar spec- 
 trum. 
 
 The following table contains Fraunhofer's determina- 
 tion of the wave-lengths of the seven great fixed lines 
 of the spectrum, which are designated by the capital 
 letters of the alphabet from B to H. I have added the 
 wave-length of A from my own experiments. 
 
SPECTRUM ANALYSIS OF FLAMES. [MEMOIR II. 
 
 Table of wave-lengths corresponding to the eight great fixed lines of 
 the solar spectrum, the Paris inch being supposed to be divided into 
 one hundred millions of equal parts. 
 
 A = 2660 
 B = 2541 
 C = 2422 
 
 E = 1945 
 F = 1794 
 G = 1587 
 
 An examination of tins table proves that 
 
 The wave-length of B is 119 parts less than A; 
 
 238 
 
 485 
 
 715 
 
 866 
 
 1073 
 
 1196 
 
 and these differences of length are obviously very nearly 
 as the whole numbers 1, 2, 4, 6, 7, 9, 10. This coincidence 
 is far too striking to be merely accidental. Moreover, 
 it must not be forgotten that the observed numbers as 
 determined by Fraunhofer are wholly independent of 
 any hypothesis. 
 
 If the relation of whole numbers were rigorously true, 
 the numbers in the foregoing table would stand as fol- 
 lows: 119, 238, 476, 714, 833, 1071, 1190. 
 
 The wave-length of the most luminous portion of the 
 spectrum, the centre of the yellow space, is 2060 parts. 
 If we take this as an optical centre, it will be found that 
 the great lines are situated symmetrically in relation to 
 it. E and D are equidistant above and below it; the 
 same observation applies to G and B, and also to H and 
 A. The only departure from this symmetry is in the 
 case of F, which is not symmetrical with C. It will be 
 understood that I am here speaking of one of those 
 spectra which are formed when a grating or ruled sur- 
 face is used. In this the colors are arranged side by 
 side, according to their wave-length, the centre of the 
 spectrum, which is its most luminous portion, is occupied 
 
MEMOIR II.] SPECTRUM ANALYSIS OF FLAMES. f 3 
 
 by the centre of the yellow space, and the light termi- 
 nates at equal distances in the violet and red. 
 
 Do not these observations lead us to conclude that 
 the cause, whatever it may be, which produces these 
 fixed lines is periodic in its action ? 
 
 What that cause in reality is we have not now facts 
 sufficient to determine. I would not affirm that the 
 disengagement of incombustible matter by a flame will 
 always give rise to dark lines. But this is very clear, 
 that in all those cases, as cyanogen, alcoholic solutions 
 of nitrate of strontia, of boracic acid, etc., in which these 
 lines are developed, incombustible matter is uniformly 
 disengaged. 
 
 UNIVERSITY OF NEW YORK, Dec. 25, 1847. 
 
74, INVISIBLE LINES IN THE SUN'S SPECTKUM. [MEMOIK III. 
 
 MEMOIR III. 
 
 ON INVISIBLE FIXED LINES IN THE SUN^S SPECTRUM DE- 
 TECTED BY PHOTOGKAPHY. 
 
 From the Philosophical Magazine, May, 1843. 
 
 CONTENTS: The photography of Fraunhofer's fixed lines. The lines in 
 the red due to absorption by the earth's atmosphere. Nomenclature of 
 the Fraunhofer lines. Discovery of the invisible fixed lines. Origi- 
 nal map of them. The new ultra spectral red lines a, /3, y. Redis- 
 covered by Foucault and Fizeau. M. E. BecquereVs discovery. Ex- 
 periments in 1834. 
 
 WHEN a beam of the sun's light, directed horizontally 
 by a heliostat, is admitted into a dark room, and, passing- 
 through a slit with parallel edges, is received on the sur- 
 face of a flint-glass prism, which refracts it at the angle 
 of minimum deviation, and, after its passage through the 
 prism, is converged to a focal image on a white screen 
 by the action of an achromatic lens, the resulting spec- 
 trum is given in great purity, and Fraunhofer's lines are 
 very distinct. If a photographic surface be set in the 
 place of the white screen, it will exhibit the representa- 
 tion of multitudes of dark lines, varying greatly in di- 
 mensions. 
 
 After several attempts last summer, I succeeded in 
 discovering these lines, and have obtained impressions 
 of them sufficiently perfect. 
 
 Before proceeding to the description of the mode to be 
 followed, and of the characters of the lines themselves, I 
 cannot avoid calling attention to the remarkable circum- 
 stance which has frequently presented itself to me of a 
 
MEMOIR III.] INVISIBLE LINES IN THE SUN'S SPECTRUM. f5 
 
 great change in the relative visibility of Fraunhofer's 
 lines when seen on different occasions. There are times 
 at which the strong lines seen in the red ray are so fee- 
 ble that the eye can barely catch them, and then again 
 they come out as dark as though marked with India-ink 
 on the paper. During these changes the other lines may 
 or may not undergo corresponding variations. The same 
 remark applies equally to the blue and yellow rays. It 
 has seemed to me that the lines in the red are more visi- 
 ble as the sun approaches the horizon, and those at the 
 more refrangible end of the spectrum are plainer in the 
 middle of the day. 
 
 (I subsequently substantiated this remark, and satis- 
 fied myself that many of the lines in the red are due to 
 absorption by the earth's atmosphere, and therefore more 
 distinct with a rising or setting sun. Those in the more 
 refrangible regions, the blue, the indigo, and the violet, 
 are due to absorption by the atmosphere of the sun.) 
 
 A sunbeam, passing horizontally from a heliostat mir- 
 ror into a dark room, was received on a metal plate with 
 a slit in its centre, the slit being formed by a pair of 
 parallel knife edges, one of which was movable by a 
 micrometer screw, the instrument being, in fact, the com- 
 mon one used for showing diffracted fringes. The screw 
 was adjusted so as to give an aperture -% inch wide, and 
 the light passing through fell upon an equiangular flint- 
 glass prism placed at a distance of eleven feet. Imme- 
 diately on the posterior face of the prism the ray was 
 received on an achromatic lens, the object-glass of a tele- 
 scope, and brought to a focus at the distance of six feet 
 six inches, at which an arrangement was adjusted for ex- 
 posing white paper screens, on which the greater fixed 
 lines might be seen, or sensitive plates substituted for 
 the screens, occupying precisely the same position. The 
 lines on the screens could, therefore, be compared with 
 
76 INVISIBLE LINES IN THE SUN'S SPECTRUM. [MEMOIR III. 
 
 those on the sensitive surfaces as to position and magni- 
 tude with considerable accuracy. 
 
 In order to identify these lines I have made use of 
 the map of the spectrum published by Professor Powell 
 in the Report of the British Association for 1839. With 
 the apparatus, as above described, they are exceedingly 
 distinct ; no difficulty arises in the identification of the 
 more prominent ones. The spectrum with which I have 
 worked occupies upon the screen a space of nearly four 
 inches and a quarter in length from the red to the violet, 
 or, more correctly speaking, from the ray marked in that 
 map A to the one marked ~k. In stating, however, that 
 no difficulty arises in identifying these lines, I ought to 
 add that I am referring to that particular map. In 
 the figure annexed to Sir John Herschel's " Treatise on 
 Light," in the "Encyclopaedia Metropolitans," the rays 
 marked Gr seem to differ from that in the report. But 
 Professor Powell's map being drawn from his personal 
 observations, with reference to these very difficulties, and 
 as it agrees with my own observations and measures, I 
 have employed it, and therefore take the letters he gives. 
 (I may add that in all the earlier of these memoirs his 
 nomenclature of the fixed lines is used. It differs a lit- 
 tle from that now employed by spectroscopists.) 
 
 It will be understood that the whole spectrum and all 
 its lines cannot be obtained at one impression. The dif- 
 ficulty is that the different regions of the spectrum act 
 with different power in producing the proper effect. 
 Thus, if on common yellow iodide of silver the attempt 
 were made to obtain all the lines at one trial, it would 
 be found that the blue region would have passed to a 
 state of high solarization, and that all its fine lines were 
 extinguished by being overdone long before any well- 
 marked action could be traced in the less refrangible ex- 
 tremity. It is necessary, therefore, to examine the differ- 
 
MEMOIR III.] INVISIBLE LINES IN THE SUN'S SPECTRUM. 
 
 ent regions in succession, exposing the 
 sensitive surface to each for a suitable 
 length of time. 
 
 In Fig. 10 I have given on the left 
 side a representation of the larger lines 
 of Fraunhofer ; the right side gives them 
 as obtained on a daguerreotype plate 
 which has been iodized to a yellow, 
 brought by the vapor of bromine to a 
 red, and then slightly exposed to the 
 vapor of chloride of iodine. The pho- 
 tograph is so adjusted as to have its H 
 lines by the side of those of Fraunhofer ,- 
 which have the same name. It will be 
 seen that there are beyond the red ray 
 three extra spectral lines, which I have 
 marked a, |3, 7. These, however, I have 
 only occasionally found, for from the 
 general diminution of effect in that re- 
 gion they do not always come out in a 
 plain and striking manner. None of 
 Fraunhofer's lines in the yellow and 
 green are given, but G and its compan- 
 ions are very strongly impressed, as also 
 the group about i. But by far the most 
 striking in the whole photograph are 
 those marked H and Ic. Then passing 
 beyond the violet and out of the visible 
 limits of the spectrum, four very strik- 
 ing groups make their appearance. To the first line of 
 each of these, in continuation of Fraunhofer's nomencla- 
 ture, I gave the designations M, N, O, P. In I there are 
 three lines, in M eight, in N three, in O four, and in P 
 five. 
 
 Besides these larger groups, the photographs were 
 
 Fig. 10. 
 
78 INVISIBLE LINES IN THE SUN'S SPECTRUM. [MEMOIR III. 
 
 crossed by hundreds of minuter lines, so that it was im- 
 possible to count them. If nearly six hundred have been 
 counted between A and H, I should think there must be 
 quite as many between H and P. In speaking of these 
 lines as though they were strong individual ones, the 
 statement is to be taken with some limitation. It is 
 quite likely that each of these bolder lines is made up 
 of a great number that are excessively narrow and close 
 together. 
 
 If the absorptive action of the sun's atmosphere be the 
 cause of this phenomenon, that action must take place 
 much more powerfully on the more refrangible and ex- 
 tra-spectral region. The lines exhibited there are bold 
 and strongly developed ; they are crowded in groups to- 
 gether. 
 
 (Scarcely was the paper from which the foregoing ex- 
 tracts are made published in the Philosophical Magazine, 
 when I learned that in France M. E. Becquerel had al- 
 ready photographed the more refrangible lines, and pub- 
 lished statements to that effect. But he had not ob- 
 served those in the less refrangible regions, designated 
 by me a, ]3, y. 
 
 In fact, the process I was using was one I had recently 
 discovered: it consisted in permitting the daylight to 
 fall along with the sun rays on the photographic surface. 
 The daylight and the sunlight antagonized each other, 
 and these hitherto undiscovered lines made their appear- 
 ance as positive photographs. The peculiarities of this 
 singular and interesting process I will describe hereafter 
 in one of these memoirs. 
 
 In 1846, MM. Foucault and Fizeau, having repeated the 
 experiment thus originally made by me, presented a com- 
 munication to the French Academy of Sciences. They 
 had observed the antagonizing action above described, 
 and had seen the ultra spectrum heat lines a, ]3, y. They 
 
MEMOIR III.] INVISIBLE LINES IN THE SUN'S SPECTRUM. ^9 
 
 had taken the precaution to deposit with the Academy 
 a sealed envelope containing an account of their discov- 
 ery, not knowing that it had been made and published 
 long previously in America. 
 
 Hereupon M. E. Becquerel communicated to the same 
 Academy a criticism on their paper. In this he remarks : 
 " M. Draper, in examining the image produced by the 
 action of the spectrum on plates of iodized silver, an- 
 nounced before those gentlemen the existence of protect- 
 ing rays antagonizing the action of the solar rays, and 
 even acting negatively on iodide of silver." He strength- 
 ened his views by adding some observations that had 
 been made by Sir J. Herschel, who did not assent to the 
 existence of this protecting action, but thought that the 
 daguerreotype impressions could be explained on New- 
 ton's theory of the colors of thin plates. 
 
 Herschel had made some investigations on the distri- 
 bution of heat in the spectrum, using paper blackened on 
 one side and moistened with alcohol on the other. He 
 obtained a series of spots or patches, commencing above 
 the yellow and extending far below the red. Some writ- 
 ers on this subject have considered that these observa- 
 tions imply a discovery of the lines a, )3, 7 ; they forget, 
 however, that Herschel did not use a slit, but the direct 
 image of the sun an image which was more than a 
 quarter of an inch in diameter, as I know from the speci- 
 mens he sent me, and which are still in my possession. 
 Under such circumstances it was physically impossible 
 that these or any other of the fixed lines should be seen. 
 
 As I had thus been unsuccessful in obtaining impres- 
 sions of the fixed lines D and E once only I thought I 
 perceived a line corresponding to Fraunhofer's F, but it 
 was exceedingly faint and on the whole doubtful I sup- 
 posed that this furnished an argument for the physical 
 independence of the luminous and actinic rays, as they 
 
gO INVISIBLE LINES IN THE SUN'S SPECTRUM. [MEMOIR III. 
 
 were subsequently called. The lines D, E, F were, how- 
 ever, afterwards photographed by my son, Henry Draper, 
 and so the argument fell to the ground.) 
 
 In 1834, when my attention was first drawn to these 
 subjects, and I began to make prismatic analyses by the 
 aid of sensitive paper, some of my earliest attempts were 
 directed to the detection of these fixed lines. At that 
 time I was employing sensitive paper, made with bro- 
 mide of silver, precisely as has been subsequently done 
 in Europe a number of the results were published in 
 the American journals during the year 1837. In the de- 
 tection of the fixed lines I failed at that time entirely ; 
 but the bromuretted paper enabled me at that early pe- 
 riod, when the attention of no other chemist was as yet 
 turned to these matters, to trace the blackening action 
 from far beyond the confines of the violet, down almost 
 to the other end of the spectrum. I distinctly made out 
 that the dark rays underwent interference, after the man- 
 ner of their luminous companions, a result originally due 
 to Arago, and printed some long papers in proof of the 
 physical independence of the chemical rays and light and 
 heat throughout the spectrum. 
 
MEMOIR IV.] CONDITION OF THE SUN'S SURFACE. 
 
 MEMOIR IV. 
 
 ON THE NATURE OF FLAME AND ON THE COND 
 THE SUN'S SURFACE. 
 
 From the American Journal of Science and Arts, Second Series, Vol. XXVI., 1858 ; 
 Philosophical Magazine, Feb., 1858. 
 
 CONTENTS: Dove's experiments on electric light. Dark lines replaced 
 by bright ones. Electric spark between metallic surfaces. The lines 
 depend on the chemical nature of the substance from which the light is- 
 sues. They may be used for determining the physical condition of the 
 sun and stars. Three hypotheses of the condition of the sun's surface 
 examined. 
 
 AMONG the more recent publications on Photo-chem- 
 istry, there is one by Professor Dove on the electric light 
 (Philosophical Magazine, Nov., 1857) which will doubt- 
 less attract the attention of those interested in that 
 branch of science. Examination by the prism, and by 
 absorbing and reflecting colored bodies, leads him to the 
 conclusion that it is necessary to consider the luminous 
 appearance as having two distinct sources: 1st, the igni- 
 tion or incandescence of the material substances bodily 
 passing in the course of the discharge ; 2d, the proper 
 electrical light itself. As respects the former, he illus- 
 trates its method of increase from low to high tempera- 
 tures by supposing a screen to be withdrawn from the 
 red end of the spectrum through the colored spaces suc- 
 cessively towards the violet ; and that of the latter from 
 the bluish brush to the bright Leyden spark, by a like 
 screen drawn from the violet towards the red. 
 
 The true electric light exhibits properties resembling 
 those observed in actual combustions, as though there 
 
 F 
 
32 CONDITION OF THE SUN'S SURFACE. [MEMOIR IV. 
 
 were an oxidation of a portion of the translated matter 
 when the spark is taken in air. The order of evolution 
 of rays in this instance happens to be the same as in the 
 second illustration of Professor Dove, that is, from the 
 violet to the red. There are certain facts connected 
 with these appearances of color which are not generally 
 known, and deserve to be pointed out. 
 
 (I then give an abstract of the preceding memoir, and, 
 after speaking of the production of dark lines in the cy- 
 anogen flame, continue as follows :) 
 
 In other cases dark lines are replaced by bright ones, 
 as in the well-known instance of the electric spark be- 
 tween metallic surfaces. The occurrence of lines, whether 
 bright or dark, is hence connected with the chemical nature 
 of the substance producing the flame. For this reason 
 these lines merit a much more critical examination than 
 has yet been given to them, for by their aid we may be 
 able to ascertain points of great interest in other depart- 
 ments of science. Thus if we are ever able to acquire 
 certain knowledge respecting the physical state of the 
 sun and other stars, it will be by an examination of the 
 light they emit. Even at present, by the aid of the few 
 facts before us, we can see our way pretty clearly to cer- 
 tain conclusions respecting the sun. For since substances 
 which are incandescent, or in an ignited state, through 
 the accumulation of heat in them, show no fixed lines, 
 their prismatic spectrum being uninterrupted from end 
 to end, it would appear to follow that the luminous con- 
 dition of our sun, whose light contains fixed lines, cannot 
 be referred to such incandescence or ignition. At vari- 
 ous times those who have studied this subject have of- 
 fered different hypotheses: one regarding the sun as a 
 solid or perhaps liquid mass in a condition of ignition ; 
 another considering the light to be electrical ; a third 
 supposing him to be the seat of a fierce combustion. 
 
MEMOIR IV.] CONDITION OF THE SUN'S SURFACE. 33 
 
 Of such hypotheses we have given reasons for declining 
 the first. Prismatic analysis, which demonstrates no re- 
 semblance between the light of the sun and that of any 
 form of electric discharges with which we are familiar, 
 enables us in like manner to reject the second ; and, upon 
 the whole, facts seem most strongly to prepossess us in 
 favor of the third ; in artificial combustions similar fixed 
 lines being observed. If such is to be regarded as the 
 physical condition of the sun, we can no longer contem- 
 plate him as an immense mass slowly and tranquilly 
 cooling in the lapse of countless centuries by radiation 
 into space, as so many considerations drawn from other 
 branches of science have hitherto led us to suppose, but 
 he must be regarded as the seat of chemical changes 
 going on upon a prodigious scale, and with inconceiv- 
 able energy. 
 
 If the law designated above, that the more energetic 
 the chemical action in combustion the more refrangible 
 the emitted light, be translated into the conceptions of the 
 undulatory theory, it not only puts us in possession of 
 a distinct idea of the manner in which the combustive 
 union of bodies is accomplished, the quickness of vibra- 
 tion increasing with the chemical energy, but it also en- 
 ables us to transfer for the use of chemistry some of the 
 most interesting numerical determinations of optics. 
 
 UNIVERSITY OF NEW YORK, Dec. 10, 1857. 
 
 NOTE. I have thus presented the four preceding 
 memoirs as early contributions to the history of spec- 
 trum analysis, and applications of spectroscopic researches 
 to solar physics or astronomical problems. I think that 
 perhaps they will not be less interesting to the scien- 
 tific reader from the circumstance of their imperfections. 
 
84 CONDITION OF THE SUN'S SURFACE. [MEMOIR IV. 
 
 He will not look upon them from the present elevated 
 point of view., but regard them as results gathered with 
 much labor by a pioneer results that have had some- 
 thing to do with the development of the subject. 
 
 The history of science shows that there have some- 
 times been occasions on which one investigator, writing 
 at an opportune moment, has carried off from his pred- 
 ecessors all the credit they were entitled to, and, per- 
 haps without any definite intention on his part, the 
 world has unjustly awarded it to him. In America, as 
 the fore^oin^ memoirs show, some attention had been 
 
 o o / 
 
 paid to the use of the spectroscope long previously to 
 the time when M. Kirchoff occupied himself with the 
 subject. 
 
 Thirteen years after the publication of the first of 
 these memoirs, M. Kirchoff (1860), in a memoir regarded 
 at that time as the origin of spectrum analysis, and en- 
 titled, " On the Relation between the Radiating and Ab- 
 sorbing Powers of Different Bodies for Light and Heat," 
 published, under the guise of mathematical deductions, 
 many of these facts as discoveries of his own. This 
 memoir appeared in German in Poggendorff s Annalen, 
 Vol. CIX., p. 275, and was translated into English in the 
 Philosophical Magazine, July, 1860. 
 
 Among these deductions are the following. I quote 
 M. Kirch off 's own language : 
 
 "If a body (a platinum wire, for example) be gradu- 
 ally heated up to a certain 'temperature, it only emits 
 rays consisting of waves longer than those of the visible 
 rays. Beyond that point waves of the length of the ex- 
 treme red begin to appear, and as the temperature rises, 
 shorter and shorter waves are added, so that, for every 
 temperature, rays of a corresponding length of wave are 
 originated, while the intensity of the rays of greater 
 wave-length is increased. 
 
MEMOIR IV.] CONDITION OF THE SUN'S SURFACE. 5 
 
 " Whence, applying the same proposition to other 
 bodies, it follows that all bodies, when their tempera- 
 ture is gradually raised, begin to emit waves of the same 
 length at the same temperature," etc. (Draper, Phil. 
 Mag., Vol. XXX., p. 345. Berl., 1847.) 
 
 "For the same temperature the magnitude (I) is a 
 continuous function of the wave-length, except for such 
 values of the latter as render (I) evanescent. The truth 
 of this assertion may be concluded from the continuity 
 of the spectrum of a red-hot platinum wire, provided it 
 be admitted that the power of absorption of such a body 
 is a continuous function of the length of the waves of 
 the incident rays." 
 
 These, together with other facts, were presented by M. 
 Kirchoff, not as experimental, but as mathematical results. 
 No allusion was made to the fact that the whole subject 
 had been extensively investigated, as shown in the pre- 
 ceding pages, many years before, the only reference to 
 such investigation being that contained, as shown above, 
 in a parenthesis, which in the original is in a foot-note, 
 and even this was omitted in an historical memoir on the 
 subject shortly afterwards published by M. Kirchoff. 
 
 As an example of the effect of this, I may quote from 
 the Cours de Physique de V&ole Poly technique, of Paris, 
 by Professor Jamin : 
 
 "M. Kirchoff has deduced the following important 
 consequences : 
 
 " Black bodies begin to emit at 977 Fahr. red radia- 
 tions, to which are added successively and continuously 
 other rays of increasing refrangibility as the temperature 
 rises. 
 
 " All substances begin to be red-hot at the same tem- 
 perature in the same enclosure. 
 
 " The spectrum of solids and liquids contains no fixed 
 lines." 
 
6 CONDITION OF THE SUN'S SURFACE. [MEMOIR IV. 
 
 Subsequently, in the Philosophical Magazine (April, 
 1863), a memoir appeared, under the title of " Contribu- 
 tions toward the History of Spectrum Analysis and of 
 the Analysis of the Solar Atmosphere," by M. Kirchoff. 
 In this all allusion to the foregoing memoirs is avoided. 
 
MEMOIR V.] INTERFERENCE OF RADIATIONS. 
 
 MEMOIR V. /t > 
 
 ON THE NEGATIVE OK PKOTECTING RAYS OF ' THENS^N. 
 
 From the Philosophical Magazine, Feb., 1847. 
 
 CONTENTS: Original discovery of protecting radiations. Case of a 
 daguerreotype plate. Spectrum-photographs made in Virginia. Pro- 
 tecting action of the less refrangible rays. Protecting action of the 
 extreme violet. Variations of the protecting action. Spectrum of 
 darkness and spectrum of daylight. Interference of rays of different 
 colors. Action of waves of red, yellow, and violet light with wave- 
 lengths 2, 1^, 1. Use of the diffraction spectrum. 
 
 IN a letter published in the Philosophical Magazine 
 Nov., 1842, I had occasion to make some incidental re- 
 marks respecting a class of rays existing in the sunlight 
 which have the quality of exerting a negative or antag- 
 onizing action upon those engaged in producing daguerre- 
 otype results. 
 
 In October last, MM. Foucault and Fizeau having 
 made a communication to the French Academy of 
 Sciences to a similar effect, and M. Edmond Becquerel, 
 in criticising their results, having referred to me as the 
 original author of the fact, I may on this occasion be 
 excused for offering a few observations on this, which 
 perhaps is destined to become one of the most important 
 phenomena in relation to the chemical action of the sun- 
 light. 
 
 That the opposite ends of the solar spectrum possess 
 opposite qualities is an idea which has been floating 
 among chemists for many years. The first distinct 
 statement in relation to it with which I am acquainted 
 occurs in a work published by Mr. B.Wilson, the second 
 
gg INTERFERENCE OF RADIATIONS. [MEMOIR V. 
 
 edition of which dates as early as 1776. It is entitled 
 "A Series of Experiments on Phosphori." He shows 
 that it is the more refrangible rays which excite the 
 phosphorescence of sulphide of lime, but the less refran- 
 gible ones extinguish it when shining. 
 
 In 1801 Bitter found that chloride of silver which 
 had been blackened in the violet rays had its color par- 
 tially restored when placed in the red. He states also 
 that phosphorus, which is oxidized with the production 
 of fumes in the invisible red, is instantly extinguished 
 in the violet. 
 
 The well-known experiments of Wollaston with guaia- 
 cum served to show the opposite relations of the red 
 and violet rays. It is remarkable that he subsequently 
 abandoned this interpretation of the phenomenon, on 
 discovering that green guaiacum changed its color by 
 the application of a hot silver spoon. 
 
 In 1839 Sir J. Herschel encountered the same action 
 in the case of some of the preparations of silver. His 
 first idea was that of a positive and negative polarity 
 of the spectrum ; but this was subsequently modified 
 for the reasons set forth in his memoir (Phil. Trans., 
 1840, 60, etc.). 
 
 In 1842 I had obtained some very fine daguerreotype 
 impressions of the solar spectrum in Virginia, and since 
 that time have never doubted the actual existence of 
 these negative or protecting rays; and on this occasion, 
 when that existence is reasserted by Lerebours, Fizeau, 
 and Foucault, I will make known certain new facts, 
 premising that I do not think the views taken by M. 
 Becquerel are correct. They are founded on what seems 
 to me to be a misapprehension of the phenomenon of 
 the daguerreotype. 
 
 A daguerreotype plate can exhibit three different 
 varieties of surface: 1st, a black aspect on those re- 
 
MEMOIK V.] INTERFERENCE OF KADIATIONS. 89 
 
 gions where it has been unaffected by light ; 2d, various 
 shades of white; 3d, a colored blackness, the tint of 
 which may be of a deep watch-spring lustre, or some- 
 times of an olive shade. Persons familiar with the proc- 
 ess will understand completely what I mean. The first 
 of these conditions is represented in the deep shadows 
 of such a photograph, the places where the light never 
 acted ; the second is exhibited in the various intensities 
 of whiteness, which constitute the figures of the picture, 
 the whiteness varying in intensity according to the in- 
 tensity of the light; the third is the solarized or overdone 
 condition, which arises from too long an exposure to the 
 rays. Like the first, this may be spoken of as a black- 
 ness, but in reality it is a dark green or blue or tawny 
 tint. It is this solarized condition of surface which M. 
 Becquerel confounds with the first, the blackness arising 
 from the unchanged state; and it is precisely on this 
 point that the whole argument turns. For the sake of 
 having distinctive words to mark out these three con- 
 ditions, I will call the first the unaffected state, the sec- 
 ond the white state, and the third the solarized state. 
 
 The observations I made in Virginia were as follows : 
 That if a solar spectrum be received on a daguerreotype 
 plate on which a weak daylight was simultaneously 
 acting, the red, orange, yellow, green, and part of the 
 blue rays arrested the action of the daylight on that 
 portion of the plate on which they fell, and maintained 
 it in the unaffected state; while the residue of the blue, 
 the indigo and violet, carried their part of the plate to 
 a completely solarized condition. This therefore seemed 
 to justify the assertion that the less refrangible rays 
 protect Daguerre's preparation from the action of a dif- 
 fused daylight. 
 
 It was also found that if the plate were exposed to 
 the daylight for a few seconds, so that had it been then 
 
9Q INTERFERENCE OF RADIATIONS. [MEMOIR V. 
 
 mercurialized it would have whitened uniformly all 
 over, on being made to receive the spectrum the less 
 refrangible rays actually carried if back to the unaffect- 
 ed condition, reversing what had been already done. 
 While the more refrangible rays were forcing it on to 
 the solarized state, these were returning it into the con- 
 dition of shadow : they therefore not only protect, but 
 seem even to exert a negative or antagonistic action. 
 
 Sir J. Herschel has critically examined one of these 
 specimens, and has suggested an explanation of their 
 appearance on the Newtonian principle of the tints ex- 
 hibited by thin films (PM. Mag., Feb., 1843). But I 
 found that it was immaterial whether the exposure to 
 the spectrum was for thirty seconds or one hour the 
 result was the same. The final action had been pro- 
 duced, the less refrangible rays had carried their region 
 to the unaffected state, while the more refrangible had 
 solarized theirs. Now if the phenomenon were due, as 
 M. Becquerel supposes, to an unequal action of the same 
 kind in the different rays, it is obvious that the final 
 result ought to depend on the time of exposure; the 
 red ray, aided by the daylight, should carry its portion 
 through the various shades of white, and solarize it at 
 last. But this in the longest exposure never takes place; 
 that part of the plate remains as though a ray of light 
 had never fallen upon it. 
 
 Such are the facts I observed, and they seem to have 
 been reproduced by MM. Foucault and Fizeau ; but 
 there are also others of a much more singular nature. 
 In these Virginia specimens the same protecting action 
 reappears beyond the violet. 
 
 The only impressions in which I have ever seen this 
 protecting action beyond the violet are those made in 
 Virginia in 1842 ; they were made in the month of July. 
 Struck with this peculiarity, on my return to New York 
 
MEMOIR V.] INTERFERENCE OF RADIATIONS. 91 
 
 the following August I made many attempts to obtain 
 similar specimens, but in no instance could the extra- 
 violet protecting action be traced, though the analogous 
 action of the red, orange, yellow, green, and blue was 
 perfectly given. Supposing, therefore, that the differ- 
 ence must be due either to impurities in the iodine or 
 to differences in the method of conducting the experi- 
 ment, I tried it again and again in every possible way. 
 To my surprise I soon found that the negative effect was 
 gradually disappearing ; and on Sept. 29 it could no 
 longer be traced, except at the highest part correspond- 
 ing to the yellow and green rays. In December it had 
 become still more imperfect, but on the 19th of the fol- 
 lowing March the red and orange rays had recovered 
 their original protective power. It seemed, therefore, 
 that in the early part of the year a protective action 
 had made its appearance in the red ray, and about July 
 extended over all the less refrangible regions, and as the 
 year went on it had retreated upwards. 
 
 Are there, then, periodic changes in the nature of the 
 sun's light ? The absorptive action of the earth's atmos- 
 phere is out of the question : if that were the cause, the 
 character of these spectrum impressions should vary 
 with the hour of the day. Or is it not more probable 
 that these singular phenomena rather depend on inci- 
 dental changes in the experiment, such as external tem- 
 perature, variations of moisture, the color of the sky, etc.? 
 
 Under proper circumstances there is no difficulty in 
 exhibiting the power which the less refrangible rays 
 exert in arresting the action of the daylight : under such 
 circumstances a daguerreotype impression of the sun's 
 spectrum yields all three of the varieties of surface be- 
 fore alluded to. The plate in the less refrangible and 
 extreme violet region is unaffected ; a narrow space of 
 white separates these unaffected portions from the in- 
 
92 INTERFERENCE OF RADIATIONS. [MEMOIR V. 
 
 digo and violet spaces, which are in a highly solarized 
 condition. 
 
 But a totally different result is obtained when the 
 daylight is not allowed to fall on the plate, either before 
 or during its exposure to the spectrum. Under these 
 circumstances the rays which would otherwise protect 
 now act on the plate and slowly whiten it. A daguerre- 
 otype spectrum formed in darkness and without pre- 
 vious exposure to the light exhibits a white stain over 
 all the less refrangible regions, and bears a marked con- 
 trast to one formed under the simultaneous action of a 
 weak daylight. For brevity I will call the former the 
 spectrum of darkness, and the latter the spectrum of day- 
 light. The following are some additional observations : 
 
 In the spectrum of darkness there is in the white stain 
 a point of maximum action. This corresponds with the 
 maximum of protection in the spectrum of daylight. 
 
 The white stain of the spectrum of darkness is appar- 
 ently narrower than the protected space in the spectrum 
 of daylight. 
 
 Rays of luminous or of non-luminous heat projected 
 on the dai'kness or daylight spectra during their forma- 
 tion appear to exert no kind of special influence on the 
 result. 
 
 The white fringe which borders the solarized portion 
 is not due to anything analogous to conduction. These 
 chemical changes, unlike thermal changes, cannot be con- 
 ducted. 
 
 By interposing between the prism and the daguerreo- 
 type plate a small convex lens of short focus, so as to 
 intercept in succession each of the colored rays, I threw 
 all over the plate, while the spectrum was in the act of 
 being impressed upon it, red, orange, yellow, and other 
 lights in succession ; the object being to ascertain how 
 far the impressed spectrum would change when these 
 
MEMOIR V.] INTERFERENCE OF RADIATIONS. 93 
 
 monochromatic rays were used along with daylight, 
 Herschel having previously shown in similar experi- 
 ments that new phenomena arise during the conjoint ac- 
 tion of rays (PM. Trans., 1840, 64). The following 
 are some of the observations I made; their date is Sept. 
 24, 1842. 
 
 The red ray when projected increases the length of 
 the solarized portion, and also of its white extremities. 
 
 The yellow ray shortens the solarized portion. 
 
 The green ray exerts a greater action of the same kind. 
 
 The indigo ray gives a most remarkable result. It to- 
 tally inverts the action of the less refrangible rays, and 
 they solarize the plate, acting in the same way that the 
 more refrangible rays commonly do, causing it to exhibit 
 a watch-spring lustre. 
 
 I further found that when different rays are brought 
 to act upon each other, the result does not alone depend 
 upon their intrinsic differences, but also on their relative 
 intensities. Thus the green and lower half of the blue 
 rays, when of a certain intensity, protect the plate from 
 the action of the daylight ; but if of a less intensity, they 
 aid the daylight. 
 
 The red and orange rays, when of a certain intensity, 
 increase the action of daylight on the plate ; but if of a 
 less intensity, they restrain it. 
 
 These facts seem to be connected with the circum- 
 stance that there is often to be traced on daguerreotype 
 plates a remarkable difference between the central and 
 lateral parts of a spectrum. Thus if a line be drawn 
 through the centre of such a spectrum and a parallel to 
 it on one of the edges, the action at any point on the cen- 
 tral line is the reverse of that at the corresponding point 
 on the edge. A similar remark, as respects impressions 
 on paper, has been previously made by Herschel. 
 
 Such are the chief facts I have observed in relation to 
 
94 INTERFERENCE OF RADIATIONS. [MEMOIR V. 
 
 the daguerreotype spectrum. It would seem at first 
 sight that their diversity is so great that we can have 
 but little hope of reducing them to* a common system of 
 results originating in the same cause. I have, however, 
 been long led to believe that the explanation is to be 
 met with in the great and fertile principle of interference. 
 From this point of view I regard the action of rays of 
 every kind as being essentially positive, and that action 
 mainly consists in impressing a vibratory movement on 
 the atoms of the decomposing substance. It is to my 
 mind a fact of no common significance, that in those Vir- 
 ginia specimens the places of maximum protection in the 
 less and more refrangible regions fall where the lengths 
 of the luminous waves have the extraordinary relation 
 of 2 : 1. Then, when we also see that, before a perfect 
 neutralization of action between two rays ensues, those 
 rays must be adjusted in intensity to each other, does it 
 not show that interference of some kind is going on ? 
 Again, the yellow ray is in numerous instances the ray 
 which most completely antagonizes those at the red and 
 violet extremes of the spectrum : to use the language of 
 Herschel, "This ray may be considered as marking a 
 sort of chemical centre, a point of equilibrium, or rather 
 a change of action in the spectrum." I cannot avoid see- 
 ing that these phenomena are connected with the remark- 
 able fact that the waves of red, yellow, and violet light 
 are of lengths which correspond to 2, 1^, 1. 
 
 If, then, a powerful yellow ray can hold in check a fee- 
 ble violet one, and prevent it from decomposing iodide 
 of silver merely because their relation of length is in 
 the ratio of 1-J : 1, it should follow on the same princi- 
 ples that a red ray acting conjointly with a violet should 
 give rise to an increased effect, because the lengths have 
 now become 2 : 1. And that this is in reality the case I 
 found by direct experiment; for on projecting the red 
 
MEMOIR V.] INTERFERENCE OF RADIATIONS. 95 
 
 upon the violet, so that the colors should half overlap 
 each other, I found that at the point of concourse the 
 plate instantly solarized, and assumed a splendid green 
 metallic color. 
 
 I have now explained the acceptation in which I re- 
 ceive the negative ray as a synonym (in this instance of 
 iodide of silver) for the yellow ray, and alluded to the 
 mechanism which seems to be the cause of protecting ac- 
 tion generally. Perhaps on a review of his own experi- 
 ments M. Becquerel may find reason to believe that there 
 are in reality antagonizing actions in different parts of 
 the spectrum ; actions not limited to the daguerreotype, 
 but occurring in all kinds of cases. They have been met 
 with by every one who has examined the spectrum with 
 sensitive papers, and, in a different series of phenomena, 
 M. Becquerel has himself furnished a conclusive illustra- 
 tion. He shows that when sulphide of lime and other 
 phosphorescent bodies in a shining state are exposed to 
 the spectrum, the more refrangible rays increase the glow, 
 but the less extinguish it. 
 
 It is proper to observe that some of the phenomena 
 recorded in this communication which seem to be in op- 
 position to the principle set forth are not so in reality. 
 All reasonings founded on the decomposition of light by 
 the prism, and the action of the prismatic spectrum on 
 changeable surfaces, are liable to error. The only meth- 
 od free from these difficulties is to employ the diffraction 
 spectrum formed by a ruled surface or a grating, a meth- 
 od which was proposed eight years ago by Herschel 
 with a view of getting rid of the disturbing agencies 
 arising from the ideal coloration of glass, and which I 
 first carried into effect in 1844 with so much success 
 that the resulting daguerreotype impressions contained 
 Fraunhofer's lines, even with microscopic minuteness. 
 With this spectrum we avoid a far more serious diffi- 
 
96 INTERFERENCE OF RADIATIONS. [MEMOIR V. 
 
 culty than that of the ideal coloration of glass, a difficulty 
 arising from the magnitude of the ^refracting faces of the 
 prism. It is this which makes a prismatic spectrum 
 blacken paper, made sensitive with the bromide of silver, 
 from the red to the violet end; whereas the diffraction 
 spectrum shows that the true action is confined to the 
 more refrangible side, and stops short of the centre of 
 the yellow space. 
 
 UNIVERSITY OF NEW YORK, Dec. 24, 1846. 
 
MEMOIR VI.] THE DIFFRACTION SPECTRUM. 97 
 
 MEMOIR VI. "</ , ' v '.y . 
 
 N^ X/V, ~*Jr 
 
 ON THE DIFFRACTION SPECTRUM. (j^ J 
 
 From the Philosophical Magazine, March, 1857. ^</ 
 
 CONTENTS: Mode of obtaining the diffraction spectrum. The yellow is 
 in its middle ; it is a centre of chemical action. It is also the hottest 
 ray. Diffraction spectra by reflection. Cold lines. Dilatation of the 
 more refrangible rays in the prismatic spectrum, compression of the less 
 refrangible. Action of the diffraction spectrum on salts of silver. Use 
 of wave-lengths for spectrum division. 
 
 M. EISENLOHR having published in PoggendorfFs An- 
 nalen for June and August, 1856, some researches on the 
 diffraction spectrum, from which it appeared that he was 
 not aware of the results I had obtained and published in 
 1844, 1 thought it well to call attention to them, and this 
 I did in a letter to the editors of the Philosophical Mag- 
 azine, which they published in their journal for March, 
 1857, and from which the following extracts are made: 
 
 At first I obtained the diffraction spectrum very much 
 in the same manner that M. Eisenlohr has done, by pass- 
 ing a beam of light directed through a vertical slit by a 
 heliostat, and through a piece of ruled glass at twelve 
 feet distance. It was then received on an achromatic 
 lens of about four-feet focus, which gave a sharply de- 
 fined spectrum on a ground-glass or photographic sur- 
 face. Subsequently I found that it was much better to 
 silver the ruled surface with tin amalgam, in the manner 
 of a looking-glass, and thus employ a reflected spectrum. 
 This is far more brilliant than the transmitted one, and 
 the silvering acts so perfectly that the most minute fixed 
 lines may be seen. 
 
 G 
 
98 THE DIFFRACTION SPECTRUM. [MiiMoiR VI. 
 
 In a work published in 1844 "On the Forces which 
 Produce the Organization of Plants," the results I ob- 
 tained were illustrated by steel engravings, one of them 
 colored, with a view of comparing the prismatic and dif- 
 fraction spectra, and of showing the photographic ac- 
 tion on iodide, chloride, and bromide of silver. These 
 results are substantially the same as those of M. Eisen- 
 lohr. They give the wave-lengths at which action begins 
 and ceases on each of those substances. Some of the de- 
 tails may be found in a former number of the Philosoph- 
 ical Magazine (June, 1845), and in that article it is par- 
 ticularly recommended to use wave-lengths for designat- 
 ing rays, instead of titles of color, as red, yellow, etc. 
 
 The chemical action of the diffraction spectrum is, 
 however, only given in part in M. Eisenlohr's publica- 
 tion. He speaks of what occurs in the more refrangible 
 regions, and takes no notice of the action in the centre of 
 the spectrum, and in its less refrangible space. 
 
 I will state in detail what I here mean. It is well 
 known that in such a spectrum the yellow occupies the 
 middle space, and that the light grades off in one direc- 
 tion to the red, and in the other to the violet. These 
 terminate at equal distances from the yellow, so that 
 the wave-lengths for the extreme violet, the centre of 
 the yellow, and the extreme red, are as 1, 1^, 2. 
 
 Now from the extreme violet to the yellow the spec- 
 trum blackens silver preparations, and this is what is 
 commonly understood by its chemical action by those 
 who have written on photographic subjects. But from 
 the yellow to the extreme red the spectrum is also active, 
 though in a different way. This half is in antagonism 
 with the other half. It can suspend or arrest the black- 
 ening which would be caused by contemporaneously act- 
 ing diffused light ; nay, even more, it can undo what such 
 light may have done some time before. Some remarks 
 
MEMOIR VI.] THE DIFFRACTION SPECTRUM. 99 
 
 on this topic may be seen in the Philosophical Magazine, 
 February, 1847 (and in Memoir V. of the present work). 
 
 The centre of the yellow space is therefore the point 
 of a change in photographic action. From the commence- 
 ment of the red to that point the action is negative ; from 
 that point to the extreme violet it is positive. The phase 
 of action changes in the centre of the yellow. 
 
 M. Eisenlohr has made an allusion to the heating pow- 
 er of the spectrum, and to that point my attention has 
 also been directed. This is the result at which I event- 
 ually arrived that the centre of the yellow is the hottest 
 space, and that the heat declines equally to the two ends 
 of the spectrum. I attempted to form a diffraction spec- 
 trum without the use of any dioptric media, endeavoring 
 to get rid of all the disturbances which arise through 
 the absorptive action of glasses by using as the grating 
 a polished surface of steel, on which parallel lines had 
 been drawn with a diamond, and employing a concave mir- 
 ror instead of an achromatic lens; and though my results 
 were imperfect and incomplete, I saw enough to convince 
 me that this is the right course to be taken in investi- 
 gating the problem. It is absolutely necessary to employ 
 a spectrum which has been formed by reflection alone. 
 
 I will also add that, in the experiments here referred 
 to, a method was employed for determining the tempera- 
 tures of narrow spaces, which may be recommended to 
 those who are disposed to resume such inquiries. It is 
 to use a blackened platinum wire, f of an inch long, and 
 3*0 of an inch in diameter, one end of which is fastened 
 to the end of a bar of bismuth, and the other end to the 
 end of a bar of antimony, each of these bars being f of 
 an inch square on the end and 4 inches long, their dis- 
 tant extremities communicating with a galvanometer. 
 By carrying the platinum wire transversely along the 
 spectrum, I expected not only to determine the distribu- 
 
100 THE DIFFRACTION SPECTRUM. [MEMOIR VI. 
 
 tion of heat, but also to ascertain whether the heat spec- 
 trum has fixed lines, like the luminous and chemical ones. 
 
 This was at a time when I first began to suspect that 
 the essential difference between light and heat is this, 
 viz., that while the vibrations of light are transverse, 
 those of heat are normal, and its waves in that respect 
 analogous to the waves of sound ; but so feeble is the 
 intensity of these spectra that I could do no more than 
 satisfy myself that in the diffraction spectrum the centre 
 of the yellow is really the hottest space, as well as the 
 most luminous. I believe that there is a cold line in the 
 spectrum answering in position to the dark line H, but 
 I could not absolutely demonstrate it. 
 
 Nevertheless, so certain does it appear that the distri- 
 bution of heat corresponds to the distribution of light, 
 that I have not hesitated since that time to look upon 
 the centre of the yellow space as the point of maximum 
 heat, from which there is a decline to each end of the 
 spectrum. And I accordingly made this the basis of a 
 theory of vision, published in my Treatise on " Human 
 Physiology." Such a view has the advantage of being 
 sustained by many facts of comparative anatomy. It 
 obliges us, it is true, to return to the opinions entertained 
 of the functions of the black pigment more than a cen- 
 tury ago; but then it gives a very elegant explanation 
 of the uses of the different parts of the retina, examined 
 in its radial section after the manner of M. H. Muller, 
 and of those of the choroid coat structures which are 
 without meaning on the ordinary theory of vision. 
 
 UNIVERSITY OF NEW YORK, Jan. 26, 1857. 
 
 NOTE. To the foregoing paragraphs I may add 
 the following as contributions to the theory of vision. 
 
MEMOIR VL] HYPOTHESES OF VISION. 101 
 
 I may also refer to my Treatise on "Human Physiology" 
 (1856). 
 
 There are many rays emitted by the sun and other 
 shining bodies to which our eyes are entirely blind. 
 
 Two different reasons may be alleged for our inability 
 to perceive such rays : first, they may not be able to reach 
 the retina, the media of the eye not transmitting them ; 
 second, the retina may be so constituted as to be unable 
 to receive their impressions. 
 
 It has long been known that rays which come from 
 sheet-iron heated by a lamp cannot pass either through 
 the cornea or through the crystalline lens. Even of 
 those that are furnished by an Argand flame, used as a 
 luminous source of heat, less than one fifth pass through 
 the cornea alone, and scarcely one fiftieth when the crys- 
 talline lens is interposed. Cima showed that of the heat- 
 rays emitted by a flame, less than one tenth pass through 
 the cornea, lens, and vitreous humor conjointly. Janssen, 
 using a flame, compared the heat transparency of the 
 separate media of the eye with that of water included 
 between glass plates, showing that there is a perfect 
 accordance between them if taken of equal thickness. 
 From this it is to be concluded that invisible rays to a 
 certain extent reach the retina. Franz, by carefully con- 
 ducted experiments with a thermo-electric pile, came to 
 the conclusion that a quantity of obscure rays detectable 
 by the thermometer can reach the retina, which there- 
 fore must be so constituted as not to be able to per- 
 ceive them. 
 
 This settles the question so far as the less refrangible 
 or ultra-red rays are concerned. We have then to de- 
 termine how it is with those at the opposite or more re- 
 frangible end of the spectrum. Do these pass through 
 the media of the eye, or are they arrested and never 
 reach the retina ? 
 
102 HYPOTHESES OF VISION. [MEMOIR VI. 
 
 I made a series of experiments on these rays, and 
 found that they passed through the different media of 
 the eye, examined separately, and* what is more to the 
 point, through them all collectively, with but little loss. 
 There was no difficulty in obtaining a dark stain on pa- 
 per made sensitive with chloride of silver, and placed at 
 the back of the eye of an ox, from which the sclerotic 
 and pigment had been suitably removed. In a gen- 
 eral manner the media of the eye act like water on the 
 transmissibility of these rays. 
 
 Admitting from these experiments that invisible as 
 well as visible rays reach the retina, we may next con- 
 sider the nature of the impression made upon it, and are 
 thus brought directly to an investigation of the act of 
 vision. 
 
 There are three hypotheses to be considered : 
 
 1. That rays falling on the retina or black pigment 
 impart to those structures a rise of temperature. This 
 may be termed the calorific hypothesis. 
 
 2. That rays falling on the retina occasion a chemical 
 change or metamorphosis in its structure, implying the 
 occurrence of waste in it, and therefore the necessity of 
 repair. This may be termed the chemical hypothesis. 
 
 3. That rays falling on the retina throw its parts into 
 a vibratory movement, not necessarily attended by any 
 metamorphosis of tissue, as waves of sound occasion con- 
 sentaneous pulsations in the auditory apparatus of the 
 ear. This may be termed the mechanical hypothesis. 
 
 FIRST, of the calorific hypothesis of vision. Compara- 
 tive anatomy offers certain facts which lend plausibility 
 to this hypothesis. Some of the most remarkable of 
 these relate to the construction of the eye in lower ani- 
 mals. The ocelli, which consist of dark-colored or black 
 spots, or black cup-shaped membranes containing w r ithiri 
 them the rudiment of an optic nerve, are the beginning 
 
MEMOIR VI.] HYPOTHESES OF VISION. 1Q3 
 
 of an organ of vision. There being no optical apparatus 
 for the production of images, the luminous impression 
 must be felt as heat. For this the dark pigment is well 
 designed. It is an old physical experiment to lay upon 
 the snow on a sunshiny winter day pieces of differently 
 colored cloth. They will melt their way to a greater 
 depth in proportion as their tint is deeper; the black, 
 becoming the warmest, sinks deepest; the white, reflect- 
 ing most of the heat, scarcely melts the snow at all. 
 Now an animal destitute of any visual organ can only 
 be affected by the impressions of light in a very doubt- 
 ful manner; but if there be upon its exterior a black 
 spot, not only is there a much higher sensitiveness be- 
 cause of the increased absorptive power for heat, but 
 the sphere of its consciousness is greatly extended, from 
 the possibility of acquiring a knowledge of directions in 
 space a knowledge that becomes more and more exact 
 with the increasing number and symmetrical arrange- 
 ment of these ocelli. 
 
 If we apply these principles to a more perfect form of 
 eye, as that of man, we are led to a new interpretation 
 of the function of some of its parts. The black pigment 
 becomes the receiving surface for images of external 
 things, and rays falling upon it, in their diversity of col- 
 or, brightness, and shade, in the act of becoming extin- 
 guished, engender heat. As with the tip of the finger 
 passing over an object we can discover, even in the dark, 
 spaces that are warm and those that are cool, so the rods 
 and cones of Jacob's membrane, acting as tactile organs, 
 convey to the brain a knowledge of the momentary dis- 
 tribution of heat on the dark concave of the eye. The 
 pigment has therefore a far more important office to dis- 
 charge than that of merely extinguishing stray light and 
 darkening the inside of the globe. 
 
 But this calorific hypothesis is not without great dif- 
 
1Q4 HYPOTHESES OF VISION. [MEMOIR VI. 
 
 ficulties. Heat suffers conduction. If this black pig- 
 ment officiated as a transformer of light rays into heat 
 by producing extinction, there must unavoidably be a 
 lateral spread from the boundaries of warm to cooler 
 spaces, the edges of images must be nebulous and with- 
 out sharpness of contour. Moreover, there is reason to 
 believe that the visual apparatus cannot take cognizance 
 of heat merely as such. Calorific rays reach the black 
 pigment and raise its temperature without the retina 
 being affected. 
 
 Such considerations seem, therefore, to exclude the cal- 
 orific hypothesis, and prepare us for an examination of 
 the chemical. 
 
 SECOND, of the chemical hypothesis of vision. Numer- 
 ous discoveries made of late years in relation to the 
 chemical action of light put us in possession of many 
 facts having a bearing on this hypothesis. A majority 
 of compound substances, both inorganic and organic, suf- 
 fer chemical modifications when exposed to the access of 
 light, and, what is very significant, these changes are oc- 
 casioned by definite classes of rays. One substance finds 
 its maximum of action in the violet region, another in 
 the yellow, another in the red. The effect in every in- 
 stance grades off towards the less and more refrangible 
 spaces respectively. 
 
 In these actions of decomposition there is nothing like 
 lateral spreading, nothing answering to conduction. No 
 better proof of this is necessary than the exquisite sharp- 
 ness of photographic pictures a sharpness only limited 
 by the optical imperfections of the lens with which they 
 are made. The molecules on which the light falls are 
 the only ones that experience change ; there is no propa- 
 gation of effect from part to part an important particu- 
 lar, because it is what we observe in the case of sight. 
 
 The retina, the nervous expansion of the eye, is so con- 
 
MEMOIR VI.] HYPOTHESES OF VISION. 
 
 sti tuted that a maximum effect upon it is occasioned by 
 the yellow ray, the action declining on one side to the 
 red, and on the other towards the violet, and ceasing 
 at the extremes of those rays. For this reason, when a 
 solar spectrum is examined by the eye, the yellow is the 
 most brilliant space, there being a decline in intensity 
 from it to the two extremes. 
 
 In my experiments on the decomposition of carbonic 
 acid by plants in the sunlight, to be described hereafter 
 in these memoirs, the maximum of action was found to 
 be in the yellow, with a gradation of effect towards the 
 red and violet ends of the spectrum respectively. From 
 this it would appear that a relation exists between light 
 and compounds having a carbon nucleus, answering to 
 that observed in the case of the retina of the eye. Such 
 a relation is very well illustrated in the case of other 
 chemical elements, as silver, a metal which is the basis 
 of all ordinary photographic preparations. The ray of 
 maximum action is in the indigo space. Objects viewed 
 by a retina having a silver sensitive nucleus would pre- 
 sent an appearance altogether unlike that they would 
 offer to a carbon nucleus. The order of brilliancy in 
 the lights would be no longer the same. The red and 
 yellow parts of objects would be black, that is to say, 
 invisible, and other rays beyond the violet would come 
 into view. 
 
 Among experiments I have made on this subject, 
 there is one of much physiological interest. The ele- 
 ment phosphorus finds its maximum impression in the 
 more refrangible portion of the spectrum, in that respect 
 resembling silver. Upon a portion of translucent phos- 
 phorus, enclosed out of contact of air in a flattened glass 
 tube, into which it had been drawn while melted, and 
 then suffered to solidify, a solar spectrum was cast. The 
 effect of light upon this kind of phosphorus is to turn it 
 
105 HYPOTHESES OF VISION. [MEMOIR VI. 
 
 eventually to a deep mahogany red, and chemically to 
 throw it from an active into an inactive state. As amor- 
 phous phosphorus, otherwise prepared, it ceases to shine 
 in the dark. In the experiments now alluded to, it ap- 
 peared that this reddening takes place in the indigo and 
 violet spaces, so that the fixed lines known by spectro- 
 scopists as those about H were beautifully depicted. 
 Now some physiologists have supposed that nerve ves- 
 icle tissue owes its property to the presence of uuox- 
 idized phosphorus, but if the principles we are contem- 
 plating be correct, and this were the case, the most brill- 
 iant ray in the spectrum should be the indigo, and not 
 the yellow. Therefore, if vision be performed by chem- 
 ical change in the substance of the retina, it is carbon 
 and not phosphorus that is concerned. 
 
 If we admit that during the act of vision the retina, 
 as a structure with a carbon nucleus, undergoes meta- 
 morphosis, the principles of photo-chemistry would lead 
 us to expect that the yellow must be the brightest ray, 
 and a harmony is thus established between this and 
 other functional changes in the body. We also perceive 
 the significance of certain structures of the eye which 
 otherwise would appear to be without meaning. The 
 rapid retrograde metamorphosis which must be taking 
 place in the retina involves the provision of some means 
 for moving away the wasted products and of supplying 
 nutrition with the utmost quickness. And this is the 
 office discharged by the choroid. 
 
 But such removals and supplies require time. Time, 
 therefore, enters as an element in the visual operation. 
 Sight commences instantaneously, but the image of an 
 object may be seen long after the reality has disap- 
 peared. This instantaneous commencement of a retinal 
 impression may be very strikingly illustrated. The 
 spark of a Ley den-jar, though it does not last, as is af- 
 
MEMOIR VI.] HYPOTHESES OF VISION. 1Q7 
 
 firmed, the millionth of a second, can without any diffi- 
 culty be photographed even on so sluggish a compound 
 as silver iodide. On the far more sensitive retina the 
 chemical impression must be practically contemporane- 
 ous with the impinging of the light. 
 
 If after the eyelids have been closed for some time, 
 we suddenly and steadfastly gaze at a bright object, 
 and then quickly close the lids again, a phantom image 
 is perceived existing in the indefinite darkness before 
 us. By degrees the image becomes less and less distinct; 
 in a minute or two it has disappeared. 
 
 The chemical hypothesis renders a very clear explana- 
 tion of this effect an explanation that commends itself 
 to our attention as casting light in many cases on the 
 curious phenomena of apparitions: phenomena that have 
 been not without influence on the history of mankind. 
 
 The duration and gradual extinction of the retinal 
 phantoms correspond to the destruction and renovation 
 taking place in the retina itself. The blood supply is 
 very ample, as are likewise the channels for the removal 
 of waste, but the operations require time to be accom- 
 plished. As in machines contrived by man, so in natural 
 organs, the practical working does not always come up 
 to the theoretical standard. Theoretically, as the retina 
 suffers change under the incident light, the removal of 
 waste and nutrition should go on in an equal manner 
 both as to time and quantity. A marvellous approach 
 to the ideal perfection is attained, for though the action 
 of light must necessarily be cumulative, that is, increas- 
 ing with the continuance of exposure, objects do not 
 become brighter and brighter as we look at them, but 
 they attain their predestined distinctness at once. The 
 action of the light, the removal of the waste it is occa- 
 sioning, and the supply for renovation are all contem- 
 poraneously going on with an equal step, or so nearly 
 
103 HYPOTHESES OF VISION. [MEMOIR VI. 
 
 so that such may be considered to be the practical 
 effect. 
 
 THIKD, of the mechanical hypothesis of vision. There 
 is a growing belief among those who are cultivating 
 photo-chemistry that the mode of operation of a ray of 
 light in accomplishing chemical changes is by establish- 
 ing vibratory movements among the molecules of the 
 substance affected. As has been affirmed, perhaps fanci- 
 fully, of certain singers, that they could cause a glass 
 goblet to fly to pieces by a proper intonation of their 
 voice, through the attempt of the glass by resonance to 
 execute incompatible vibrations, so it is thought that an 
 incident ray may break asunder a group of molecules by 
 establishing among them discordant agitations. Chem- 
 ical decompositions by radiations become thus connected 
 theoretically with vibratory movements. 
 
 But these are vibrations not necessarily attended by 
 any destruction of tissue. Waves of sound occasion such 
 pulsations in the apparatus of the ear without producing 
 any chemical change in the auditory nerve. 
 
 If we consider the retina as an elastic shell, of which 
 the parts are put into a purely mechanical movement 
 by the pulsations of light, w r e abandon without explana- 
 tion some of the most interesting portions of the struct- 
 ure of the eye. Of what use is that wonderful net- work 
 of vessels constituting the choroid ? It is a principle in 
 physiology that the supply of blood to a part is propor- 
 tional to its functional activity. The elaborate vascular 
 mechanism in juxtaposition with the retina will bear no 
 other interpretation than that that tissue is the seat of 
 incessant chemical changes. 
 
 Moreover, physical science in its present state is not 
 sufficiently advanced to furnish the means of clearly 
 comprehending such purely mechanical motions exe- 
 cuted by the ultimate particles of things. We may 
 
MEMOIR VI.] HYPOTHESES OF VISION. 1Q9 
 
 conceive of the comparatively slow swaying of groups 
 of molecules under the influence of normal pulsations in 
 the air, but not of the dance of atoms disturbed by trans- 
 verse vibrations in the ether. If, therefore, there were 
 no arguments of an anatomical kind to be presented 
 against the admission of this hypothesis, we should be 
 compelled to turn aside from it because of the inade- 
 quacy of our knowledge in tracing its conditions to their 
 applications. 
 
 This, therefore, is the conclusion at which we finally 
 arrive that vision depends on chemical changes, especial- 
 ly of oxidation, in the retina, and that they approach in 
 their nature those that we speak of as photographic. 
 There is no difficulty in understanding how such changes 
 may give rise to an influence transmitted along the optic 
 nerve to the brain, when we reflect that the oxidation 
 of a few particles of zinc may accomplish specific me- 
 chanical results through many miles of intervening tele- 
 graphic wire, producing mechanical motions as in the 
 telegraph of Morse, or chemical changes as in that of 
 Bain. 
 
 We have remarked that a critical study of the func- 
 tion of vision cannot fail to lead to interesting results 
 respecting the nervous system generally. Guided by 
 that remark, we may perhaps profitably consider further 
 the vestiges of visual impressions, and the physical con- 
 ditions under which they disturb us or spontaneously 
 obtrude themselves on our attention. 
 
 The perception of external objects depends on the 
 rays of light entering the eye, and converging so as to 
 produce images, which make an impression on the retina, 
 and through the optic nerve are delivered to the brain. 
 The direction of these influences, so far as the observer 
 is concerned, is from without to within, from the object 
 to the brain. 
 
HO INVERSE VISION. [MEMOIR VI. 
 
 But the inverse of this is possible. Impressions ex- 
 isting in the brain may take, as it were, an outward 
 direction, and be projected or localized among external 
 forms ; or if the eyes be closed, as in sleep, or the ob- 
 server be in darkness, they will fill up the empty space 
 before him with scenery of their own. 
 
 Inverse vision depends primarily on the condition 
 that former impressions, enclosed in the optic thalami, or 
 registering ganglia at the base of the brain, assume such 
 a degree of relative intensity that they can arrest the 
 attention of the mind. The moment that an equality 
 is established between the intensity of these vestiges 
 and sensations contemporaneously received from the 
 outer world, or that the latter are wholly extinguished, 
 as in sleep, inverse sight occurs, presenting, as the occa- 
 sions may vary, apparitions, visions, dreams. 
 
 From the moral effect that arises, we are very liable 
 to connect these with the supernatural. In truth, how- 
 ever, they are the natural results of the action of the 
 nervous mechanism, which of necessity produces them 
 whenever it is placed, either by normal or morbid or 
 artificial causes, in the proper condition. It confounds 
 the subjective and the objective together. It can act 
 either directly, as in ordinary vision, or inversely, as in 
 cerebral sight, and in this respect resembles those instru- 
 ments which equally yield a musical note whether the 
 air is blown through them or drawn in. 
 
 The hours of sleep continually present us, in a state 
 of perfect health, illusions that address themselves to 
 the eye rather than to any other organ of sense, and 
 these commonly combine into moving and acting scenes, 
 a dream being truly a drama of the night. In certain 
 states of health appearances of a like nature intrude 
 themselves before us even in the open day, but these, 
 being corrected by the realities by which they are sur- 
 
MEMOIR VI.] INVERSE VISION. 
 
 rounded, impress us very differently. The want of uni- 
 son between such images and the things among which 
 they have intruded themselves, the anachronism of their 
 advent, or other obvious incongruities, restrain the mind 
 from delivering itself up to that absolute belief in their 
 reality which so completely possesses us in our dreams. 
 Yet, nevertheless, such is the constitution of man, the 
 bravest and the wisest encounter these fictions of their 
 own organization with awe. 
 
 The visions of an Arab merchant have ended in tinct- 
 uring the daily life of half the people of Asia and Africa 
 for a thousand years. A spectre that came into the 
 camp at Sardis the night before the battle of Philippi 
 unnerved the heart of Brutus, and thereby put an end 
 to the political system that had made the Roman repub- 
 lic the arbiter of the world. A phantom that appeared 
 to Constantine strengthened his hand to that most diffi- 
 cult of all the tasks of a statesman, the destruction of an 
 ancient faith. 
 
 Hallucinations are of two kinds those seen when the 
 eyes are open and those perceived when they are closed. 
 To the former the designation of apparitions, to the lat- 
 ter that of visions, may be given. 
 
 In a physiological sense, simple apparitions may be 
 considered as arising from disturbances or diseases of 
 the retina; visions, from the traces of impressions en- 
 closed at a former time in the corpora quadrigemina and 
 optic thalami. 
 
 From flying specks floating before us, the first rudi- 
 ments of apparitions, it is but a step to the intercalation 
 of simple or even grotesque images among the real ob- 
 jects at which we are looking; and indeed this is the 
 manner in which they always offer themselves, as resting 
 or moving among the actually existing things. 
 
THE DIFFRACTION SPECTRUM. [MEMOIR VI. 
 
 The method by which the first photograph of the dif- 
 fraction spectrum was obtained is described as follows 
 in the Philosophical Magazine, Jline, 1845. 
 
 The prismatic spectrum, even when every precaution 
 has been used to obtain it in a state of purity, its fixed 
 lines being visible, is liable to lead us into many errors. 
 As respects its luminous or photic properties, we cannot 
 determine the distribution or intensity of the light be- 
 cause the violet extremity is unduly dilated. As respects 
 the chemical effects, the same difficulty occurs, for these 
 are necessarily controlled and disturbed by the law of 
 distribution. All chemical actions occurring in the more 
 refrangible regions, by being spread over a great space 
 appear to be more feeble than what they actually are. 
 
 In a perfect spectrum the most luminous portion of the 
 yellow should be in the centre ; and from this the inten- 
 sity of the light should gradually decline, fading away 
 on one side in the red, and on the other in the violet. 
 At equal distances from the middle yellow point on 
 either hand the intensity of the light should be equal. 
 These beautiful results are due to Mosotti, who also 
 states that the length of the extreme red wave is to that 
 of the extreme violet in the simple ratio of 2 : 1. 
 
 The prismatic spectrum does not exhibit these facts. 
 The yellow is not in the centre ; the blue, indigo, violet 
 are abnormally spread out, the spectrum having its own 
 law of distribution. But the diffraction spectrum en- 
 ables us to observe them. 
 
 By the aid of a heliostat, I arranged horizontally in a 
 dark room a narrow ribbon of light, coming through a slit 
 3*0 of an inch wide, set vertically. At the distance of 
 twelve feet it fell perpendicularly on a piece of flat glass, 
 the surface of which was ruled with equidistant parallel 
 lines by a diamond ; and having been silvered with tin 
 foil, after the manner of a mirror, it served the purpose 
 
MEMOIR VI.] 
 
 THE DIFFRACTION SPECTRUM. 
 
 113 
 
 of a grating. The reflected beam went out through the 
 slit at which it entered, and on either side of it to the 
 right and left the well-known double series of diffraction 
 spectra made their appearance. I selected, for the obvi- 
 ous reason that it was not overlapped by its successor, 
 the first of the series, and intercepting it by an achro- 
 matic object-glass, placed in the focus a frame capable 
 of holding a ground -glass or sensitive surface. This 
 frame was adjusted until the fixed lines were distinctly 
 depicted upon it. 
 
 For a further description of the reflected diffraction 
 spectrum I may refer to any of the elementary works on 
 optics. It is sufficient for my purpose here to recall that 
 the angular deviations of any two colors from the primi- 
 tive incident ray are to one another as the lengths of 
 their respective undulations. 
 
 On the ground-glass we see the fixed lines, and the 
 length of waves corresponding to those lines has been 
 carefully determined by Fraunhofer. The following 
 table is extracted from Herschel's treatise on Light, the 
 Paris inch being divided into one hundred millions of 
 equal parts: 
 
 Length of wave corresponding to the fixed line B, 2541 parts. 
 
 C, 2422 
 
 D, 2175 
 
 E, 1945 
 
 F, 1794 
 
 G, 1587 
 H, 1464 
 
 When, therefore, we have any chemical, luminous, ther- 
 mic, phosphorogenic, or any other effect under discussion, 
 in relation to the spectrum, we have only to determine 
 its place among the fixed lines, remembering in the dif- 
 fraction spectrum the simple law that connects the devi- 
 ations and wave-lengths. In the mode of operation here 
 described absolute exactitude is not reached because our 
 
 H 
 
THE DIFFRACTION SPECTRUM. [MEMOIR VI. 
 
 measures are obtained from a flat surface, and not upon 
 a circular arc. 
 
 To the diffraction spectrum thns formed I exposed for 
 half an hour a daguerreotype plate, rendered sensitive 
 by iodine and then by bromine. It resulted that the 
 bromide of silver is decomposed at a maximum by a 
 wave which is 0.00001538 of a Paris inch in length. The 
 action does not extend equally, as we might have sup- 
 posed, towards the more and less refrangible regions. 
 
 I exposed a silver plate which had been prepared by 
 iodine, bromine, arid chloride of iodine successively. The 
 point of maximum fell as before at 0.00001538. The 
 time of exposure one hour. The decomposition com- 
 menced by a wave in the green space, the length of 
 which was 0.00002007, and was terminated by one in 
 the violet, the length of which was 0.00001257. The 
 point of maximum action, therefore, inclined to the vio- 
 let, and was not midway between the extremities of the 
 photograph. The absolute length of the stain depends, 
 however, on the time of exposure. 
 
 I need not multiply these results. It is sufficient to 
 add that in several trials I obtained in these delicate ex- 
 periments photographs of the diffraction spectrum on 
 different surfaces in great perfection. The fixed lines 
 which are crowded close together were beautifully dis- 
 tinct. 
 
 I would suggest, therefore, that when we wish to in- 
 dicate spectrum regions with precision, we should use 
 wave-lengths. By doing this we shall connect the vari- 
 ous actinic phenomena with a great many of the numer- 
 ical results of optics, and have fixed points of comparison. 
 
 UNIVERSITY OF NEW YORK, June, 1845. 
 
MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 
 
 MEMOIR VII. 
 
 STUDIES IN THE DIFFRACTION SPECTRUM. 
 
 A popular exposition condensed from Harper's New Monthly Magazine, Vol. LV. 
 
 CONTENTS : Elementary description of the diffraction spectrum. 
 Young's discovery of interference. FresneTs discovery of transverse 
 vibrations. Gratings and the spectra they yield. Gratings on reflect- 
 ing surfaces. Optical action of a grating. Its spectra of different 
 orders. Interpretation of wave-lengths by the mind. Mental appreci- 
 ation of multiple wave-lengths. Refutation of the principle that to 
 every color there belongs an invariable wave-length. Increase in the 
 range of perception in the eye. Extension to photographic impressions. 
 Encroachment on the first dark space. Photographs of the diffrac- 
 tion spectrum. Proposal to use wave-lengths for spectrum divisions. 
 Replacement of imaginary imponderable principles by wave-lengths. 
 
 WHAT is a diffraction spectrum ? Every person who 
 has read a book on light is familiar with the prismatic 
 spectrum, in the study of which Newton displayed his 
 transcendent philosophical powers. The diffraction I 
 have had occasion to refer to several times, and since it 
 is less known, will now describe it. Some very curious 
 phenomena connected with it I have personally exam- 
 ined. It carries us to a true interpretation of the rela- 
 tions of heat, light, and actinism ; it offers some impor- 
 tant suggestions respecting the mode of action of that 
 most wonderful of all organs, the brain, and therefore 
 commends itself to our most earnest attention. 
 
 If we look at a candle flame placed ten or a dozen feet 
 distant, the eyelids being so nearly closed that the eye- 
 lashes intercept the incoming rays, we see on either side 
 of the true image of the flame a succession of colored 
 ones rainbow streaks or fringes, as it were. Examining 
 
STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 
 
 these particularly, we find that each of them is blue on 
 the side nearest to the true image, and red on the more 
 distant. Our investigation will be simplified if we con- 
 sider the action of a single eyelash. We can then reason 
 from that to the conjoint action of all. 
 
 It is necessary, however, in the first place, to recall 
 some facts connected with the wave theory of light. 
 The foundations of this theory were laid by Huyghens, 
 the great Dutch philosopher, contemporary with Newton, 
 but its construction advanced very slowly, being opposed 
 by the great authority of Newton, who favored the cor- 
 puscular or emission theory, and regarded light as con- 
 sisting of particles emitted with excessive velocity from 
 shining bodies. Although there were facts, such as those 
 connected with double refraction, easily accounted for 
 by the system of undulations, but inexplicable on the 
 emission theory, these were put aside, in the expectation 
 that they would in the course of time be successfully 
 dealt with. It was not until the publication of a 
 course of lectures on natural and experimental philos- 
 ophy by Dr. Young in 1802, in which he announced the 
 great discovery of the interference of light, that the un- 
 dulatory theory could no longer be overlooked. This 
 discovery was, however, still ridiculed by the Edinburgh 
 Review, and Young's explanations so bitterly attacked 
 that he was constrained to publish a pamphlet in reply. 
 Of this it is said that only a single copy was sold. 
 
 In 1819, a memoir by Fresnel was crowned by the 
 French Academy of Sciences. He discovered that the 
 vibratory movements in the ether constituting light are 
 transverse to the course of the ray. His views are em- 
 bodied in what is now known as the theory of trans- 
 verse vibrations. 
 
 The conflict between the rival theories was eventually 
 settled by the experiments of Fizeau and Foucault. On 
 
MEMOIR VII. ] STUDIES IN THE DIFFRACTION SP 
 
 > V x 
 
 Newton's principles the particles of light shcmSl motfe 
 faster through water than through air ; on the theory of A 
 Huyghens, waves of light must move slower in wafer 
 than in air. The experiments of the French physicist^ 
 proved that the latter is the case. This may, 
 be considered as the successful establishment of the un- 
 dulatory theory. It has, moreover, given that strik- 
 ing proof of its truth which may be considered as the 
 criterion of any theory the ability to foretell results. 
 This it did in the case of the discovery of conical re- 
 fraction. 
 
 Light, therefore, consists in the transference of energy 
 or force, not in the transference of matter. 
 
 The grating I employed in the experiments hereinafter 
 related was made for me by Mr. Saxton, at the United 
 States Mint in Philadelphia, more than thirty years ago. 
 Though from the work it did for me I cannot but speak 
 of it with admiration it enabled me to make the first 
 photograph that was ever executed of the diffraction 
 spectrum yet it was far from being equal to the mag- 
 nificent ones of Mr. Rutherfurd, This grating was five 
 eighths of an inch long and one third of an inch in 
 breadth. Mr. Rutherfurd's gratings have in some speci- 
 mens 17,240 lines to the inch. I had found previously 
 to 1843 that it is more advantageous practically to use a 
 reflecting than a transparent grating, arid accordingly I 
 silvered mine with mercury-tin amalgam, such as is used 
 in ordinary looking-glasses. Mr. Rutherfurd's reflecting 
 gratings are coated with pure silver, by an operation 
 more recently discovered. 
 
 I will now relate the use of these gratings, and de- 
 scribe some of the important discoveries made by them. 
 
 Let a beam of light, S A', Fig. 11, pass through a nar- 
 row slit, S, and fall perpendicularly on the ruled grating, 
 
STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 
 
 G, the lines of which are parallel to the sides of the slit. 
 Concentric with the middle line of the grating let there 
 be placed a circular zone or screen, Q', Q", Q'", Q"", of 
 white paper, through which there is an opening at A, to 
 admit the intromitted beam. 
 
 A beam of parallel rays passing along S G will give a 
 bright image of the slit S when it impinges on the screen 
 at A'. This is the image by transmission. It would 
 also give another similar image at A, were it not for the 
 opening arranged there. This is the image by reflection. 
 Also from G, as from a central axis, there falls upon the 
 cylindrical paper zone, covering its surface all over, an 
 infinite number of radiations. 
 
 These effects are seen with much more precision if 
 
MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 
 
 there be placed behind the grating a convex lens, or, still 
 better, if the lens be the objective of a telescope. 
 
 Now the eye can only be impressed by special radia- 
 tions consisting of waves of a determinate length. Its 
 vision is limited to those that impart to it a sensation of 
 red on one hand, and of violet on the other. To all oth- 
 ers it is blind. Then, though the whole paper zone is 
 receiving radiations of every kind, the eye selects out 
 only those that it can perceive, and, as a result, sees in 
 the four quadrants, Q', Q", Q'", Q"", those only for which 
 it is fitted. 
 
 It follows, therefore, that at A' there is a white image 
 of the slit S, and to the right and left of this there are 
 equal spaces, p, p', completely dark. Beyond, and sym- 
 metrically on each side, there is a series of spectra, v r, 
 v r /, v" r" , etc., of which the violet ends are nearest A', 
 and the red ends most distant. These spectra are des- 
 ignated respectively as being of the 1st, 2d, 3d, etc., order. 
 On each side the 1st spectrum is separated from the 2d 
 by an obscure space, r v', which is shorter than the first 
 dark spaces, p,p' , and the red end of the 2d spectrum is 
 overlapped by the violet of the 3d. In like manner the 
 3d is overlapped by the 4th, etc. If the intromitted ray 
 be of sunlight, and a convex lens or small telescope be 
 used, the dark Fraunhofer lines are seen in these spectra. 
 
 Such are the results seen in the quadrants Q"', Q x/// , 
 from the light transmitted through the grating. In the 
 quadrants Q', Q", exactly the same train of phenomena 
 will be discovered dark spaces and spectra, the latter 
 having their violet ends nearest to A, and the overlap- 
 ping of successive ones taking place in the manner above 
 described. 
 
 Since the results are thus symmetrical in all the four 
 quadrants, it is sufficient to select one of them for de- 
 tailed examination. Let it be the quadrant Q"". 
 
120 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 
 
 Selecting one of the fixed lines, that in the yellow 
 space, the sodium line D, for example, in the successive 
 spectra, it will be found that the distance which inter- 
 venes between it and the middle of the white image A' 
 is in the second double, in the third triple, etc., the dis- 
 tance it is in the first. These angular distances are des- 
 ignated as the deviations of the ray under examination. 
 Fraunhofer proved that 
 
 (1) The deviation of the same ray, e. g., D, depends on 
 the sum of the width of a groove in the grating and of a 
 transparent interval, being in the inverse ratio of that 
 sum. 
 
 (2) The deviation of any one of the colors of the spec- 
 trum of the first order, multiplied by the sum of a trans- 
 parent interval and a groove, gives the length of a wave 
 of light of that color. 
 
 (3) The deviations of the same color in the successive 
 spectra increase as the whole numbers 1, 2, 3, 4, etc. 
 
 (4) The deviations of two colors in the same spectrum 
 are to each other as the length of their undulations. 
 Hence in all the violet is nearest to A', and the red the 
 most distant. 
 
 The undulatory theory gives a rigorous explanation 
 of all these facts. The lengths of the waves of light 
 have hence been most critically and accurately deter- 
 mined. 
 
 We may now examine more closely the spectrum that 
 is nearest to A' the spectrum of the first order. Be- 
 ing completely separated from the others, it presents the 
 special facts most distinctly. At the point where the 
 light first becomes visible, the violet or inner end of this 
 spectrum, the wave-length of the incident radiation is, as 
 Angstrom has proved, 3933, and the wave-length of the 
 last visible radiation at the outer or red end is 7604, 
 ten millionths of a millimeter. If we accept the velocity 
 
MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 
 
 of light as determined by the experiments of Foucault, 
 the number of vibrations made by the ether in the for- 
 mer of these radiations is 754 millions of millions in one 
 second, and the number in the latter case is 392 millions 
 of millions in one second. 
 
 Or, to quote measures which are perhaps more familiar, 
 and numbers as given by Herschel, though not so exact 
 as those of Angstrom, the number of undulations con- 
 tained in one English inch at the extreme violet end is 
 59,750, and the number of vibrations executed in one 
 second is 727,000,000,000,000. The number of undu- 
 lations in one English inch at the extreme red end is 
 37,640, the number of vibrations executed there in one 
 second being 458,000,000,000,000. The velocity of light 
 used in these computations is 192,000 miles per second, 
 that used in the preceding paragraph, 186,000. 
 
 Knowing the rate at which light moves in a second, 
 and the wave-length of any particular color, it is easy to 
 compute the number of vibrations made by the ether 
 in one second for the production of that color. This 
 is obtained by dividing the distance that light passes 
 over in one second by the wave-length of the color in 
 question. 
 
 The numbers we thus obtain give us an idea of the 
 scale of space and time upon which Nature carries for- 
 ward her works among the particles of matter. They 
 also indicate to us the amazing activity of those portions 
 of the brain which execute motions in accordance with 
 those scales. 
 
 The distribution of the colored spaces in the diffrac- 
 tion spectrum is not the same as in the prismatic. In 
 the former the yellow space, which is the most luminous 
 radiation, is in the middle of the spectrum, and is not 
 crowded down or compressed towards the red end, as in 
 
122 STUDIES IN THE D1FFKACTION SPECTRUM. [MEMOIR VII. 
 
 the latter. So the maximum intensity or illuminating 
 power is, as Mosotti first observed, in the centre, the in- 
 tensity of the light declining symmetrically on each side 
 to the end. 
 
 The Italians have a clear perception, a quick apprecia- 
 tion of the symmetrical and beautiful. When Mosotti 
 first stated this peculiarity of the diffraction spectrum, at 
 a meeting of one of the Italian scientific societies, the an- 
 nouncement was received by the audience with loud ac- 
 clamations of joy. 
 
 I may now describe some of nay own studies of these 
 beautiful spectra. 
 
 Recalling, then, the principle that the wave-length of 
 an incident radiation is proportional to its deviation, let 
 us select upon the paper zone previously described the 
 point where a ray is falling having a wave-length 7866. 
 It is, of course, twice as far from A' as was the violet 
 end of the first spectrum, for the selected deviation is 
 double. If we inquire what interpretation the mind 
 will give of a radiation having such a wave-length, an 
 inspection of the zone shows that not only is it visible, 
 but that it is regarded as being of a violet color. 
 
 This is an important fact. We find that a radiation 
 consisting of waves of a given length which is visible 
 will also be visible when the constituent waves are twice 
 that length. And in like manner it might be shown 
 that the same will hold good when they are three, four, 
 five, etc., times that length. Moreover, in all these cases 
 the color impression imparted to the mind will be the 
 same. 
 
 Again, let us select upon the paper zone another point 
 where the wave-length is 15,208. It will have double 
 the deviation of the red end of the first spectrum. Now, 
 agreeably to the foregoing remarks, this point should be 
 visible to the eye, and, for anything that has thus far 
 
MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 123 
 
 been said, it should be interpreted by the mind as red 
 light, its wave-length being twice that of the red of the 
 first spectrum. But it is obvious that here a new con- 
 sideration must enter into account. If this radiation 
 has double the wave-length of the first red, it has triple 
 the wave-length of the first yellow-green. On the prin- 
 ciple just laid down, the mind may interpret it as red 
 light or as yellow-green. Which will it do ? 
 
 Examination of the paper zone, or, better still, through 
 a telescope, shows that the mind adopts both these in- 
 terpretations, and the same principle applying to other 
 wave-lengths, this constitutes what we have spoken of 
 as the overlapping of the second spectrum by the third, 
 etc. At the point here specially considered, both red 
 and yellow-green light are seen. 
 
 From what has here been presented, it follows that 
 the principle considered as established in optics, that 
 to every color there belongs a determinate wave-length, 
 must be modified, since the same color impression will 
 be given to the mind by waves that have twice, thrice, 
 etc., that determinate wave-length. But should the 
 wave-lengths under consideration answer to multiples 
 of that of some other color, the mind will interpret them 
 as being of that color too. 
 
 Moreover, these observations lead us to extend the 
 range of perception of the eye. The prism would lead 
 us to infer that it can only be affected by waves the 
 length of which is between 3933 and 7604. Compari- 
 sons have hence been drawn between the organ of vision 
 and the organ of hearing, to the disparagement of the 
 former. The ear, it is said, can embrace a range of sev- 
 eral octaves, but the eye is influenced by less than one. 
 The grating, however, leads us to reject the restriction, 
 and to place the eye more nearly on an equality with 
 the ear. 
 
124 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 
 
 It is also to be borne in mind that by using very con- 
 densed sunlight, or by resorting to fluorescent or other 
 optical contrivances, as several experimenters have done, 
 the range of vision may be carried beyond the proper 
 violet limit. 
 
 The principles here indicated must not be restricted 
 to the luminous radiations; they apply to all others too. 
 Thus if a photographic sensitive surface be made to re- 
 ceive the first spectrum, it will be impressed by certain 
 of its radiations, chiefly by those above the line Gr. If 
 it be exposed in the second, third, etc., spectra, it will 
 again be impressed by the corresponding undulations, 
 having two, three, etc., times the former length. From 
 this it may, therefore, be inferred that a chemical decom- 
 position of a given substance, brought about by undula- 
 tions of a certain length, will also be accomplished by 
 radiations that are octaves of the first. 
 
 It has been stated that a dark space,j9, intervenes be- 
 tween the violet end of the first spectrum and the bright 
 streak A'. This dark space is at present an attractive 
 and wonderful field of optical investigation. 
 
 A' 
 
 Fig. 12. 
 
 In Fig. 12, let A' represent the white streak in the 
 position of A' in Fig. 11 ; then from A' to v is the first 
 dark space,^) ; from v to r, the spectrum of the first order, 
 its violet end, v, nearest to A', its red end, r, more dis- 
 tant; from r to v' the second dark space; and from v' 
 to r' the spectrum of the second order. The third spec- 
 trum overlaps this second, and the fourth the third, etc. ; 
 
MEMOIR VII. ] STUDIES IN THE DIFFRACTION SPECTRUM. 125 
 
 but these it is not necessary to consider. The two dark 
 spaces, and especially the first, are the objects mainly to 
 be examined. 
 
 Previously to 1844 I had attempted to obtain diffrac- 
 tion photographs with the grating that Mr. Saxton gave 
 me, and had met with great success. In that year I 
 published engravings of them, the originals having been 
 made on silver daguerreotype tablets, in use at that time. 
 By these I carried spectrum impressions as far as the 
 wave-length 3800, and therefore encroached considerably 
 on the dark space p, towards A'. But collodion, since 
 introduced, is a much more sensitive preparation. It 
 has enabled Henry Draper, who has produced superb 
 photographs of the more refrangible regions, to carry 
 the impressions as far as 3032. 
 
 According to M. Mascart, waves are emitted by incan- 
 descent cadmium having a length not exceeding 2200. 
 These stand still further in the dark space p. 
 
 In these excursions into the dark space the experi- 
 ments of Professor Stokes on the long spectrum of elec- 
 tric light become not only interesting, but very impor- 
 tant ; for as we gradually approach A', the wave-length 
 of the incident radiation is continually diminishing, and 
 at A 7 it becomes zero. That point is the supreme limit, 
 beyond which no radiant manifestation of any kind is 
 possible. 
 
 The goal towards which experimental investigation is 
 tending is therefore obvious. We are gradually groping 
 the way across the dark space, and expect one day to 
 reach the bright streak that lies at its terminus. At 
 every step of advance the ether waves are becoming 
 shorter and shorter, and the vibrations more and more 
 rapid. When the journey is accomplished, a region will 
 have been gained in which the waves are infinitely short, 
 and the vibrations infinitely rapid. 
 
126 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIK VII. 
 
 Several years before the announcement of the discov- 
 ery of photography by Daguerre and Talbot (1839) I 
 had made use of that process for the purpose of ascer- 
 taining whether the so-called chemical rays exhibited 
 interference, and in 1837 published the results in the 
 Journal of the Franklin Institute, Philadelphia (July, 
 1837, p. 45). In this, as will be seen by consulting that 
 publication, I was successful. 
 
 Encouraged by this result, I some years subsequently 
 attempted to photograph the diffraction spectrum itself. 
 
 The following is an extract from the publication I 
 made of this experiment in 1844: "Through a narrow 
 fissure or slit, #, Fig. 13, I direct a beam of light hori- 
 zontally, and at a distance of twelve feet receive it on a 
 grating, b c, the lines of which are parallel to the slit. 
 Having found that there are advantages in using a re- 
 flecting grating, I silvered this with tin amalgam, which 
 copies the ruling perfectly. There is no difficulty in 
 placing b c so that the ray coming from a falls perpen- 
 dicularly on it, for all that is required is to move the 
 grating into such a position that the light, after reflection 
 from it, goes back through the fissure a. At a distance 
 from b c of 
 six inches I 
 
 place an ach- L _ m _ J 
 romatic ob- ::: *^ c 
 
 ject- glass, dj 
 in such a po- 
 sition that it 
 
 shall receive perpendicularly the reflected rays of the 
 spectrum of the first order. The lens is brought as near 
 to the grating as possible without its edge intercepting 
 the ray coming from a. In the focus of this lens, at <?/, 
 a ground glass is placed. This portion of the apparatus 
 is, however, nothing more than the sliding part of a com- 
 
MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 
 
 mon photographic camera, which contains the ground 
 glass and shields for sensitive preparations." 
 
 In the publication above referred to I gave engravings 
 of the results thus obtained; the fixed lines were marked 
 by their wave-lengths. The photographs were very clear 
 and beautiful; they bore magnifying six or eight times 
 without injury to their sharpness. 
 
 I may here be permitted to add that it was on the 
 publication of these researches in 1844 that I first made 
 the suggestion to describe spectrum effects by wave- 
 lengths, or what, perhaps, is still better, by ether vibra- 
 tions a method now generally adopted. I may give 
 the following extracts: 
 
 " In the earlier discussions of the chemical effects of 
 light, the different regions of the spectrum were marked 
 out by the designations of the different colored rays, and 
 effects were described as taking place in the red or yel- 
 low or violet regions respectively. An improved plan 
 was proposed by Herschel, and followed by him in his 
 various writings. It consists essentially in dividing the 
 space w r hich exists between the red and yellow ray as 
 insulated by cobalt blue glass into 13.30 parts, taking 
 the centre of the yellow ray as the zero point, and 
 continuing the divisions equally into the more and less 
 refrangible regions. 
 
 u Over these methods the use of the fixed lines pos- 
 sesses very great advantages, inasmuch as we make refer- 
 ence to actually visible points existing in the spectrum. 
 
 "It has been stated that the deviations of the different 
 fixed lines in the diffraction spectrum are proportional 
 to the lengths of the undulations which they respective- 
 ly represent. By designating the different points of the 
 spectrum by their wave-lengths, the subdivision may be 
 carried to any degree of minuteness, the measures of one 
 author will compare with those of another, and the dif- 
 
128 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 
 
 ferent phenomena of chemical changes occurring through 
 the agency of light become at once allied to a multitude 
 of other optical results. If it we're necessary, by a very 
 simple arithmetical process we could determine the num- 
 ber of vibrations executed by a ray bringing about a 
 given decomposition in billionths of a second. The 
 fixed lines used in this way enable us at once to divide 
 the diffraction spectrum into any number of parts, and, 
 by comparing wave-lengths and the velocity of light, to 
 indicate effects either in space or in time." 
 
 The diffraction spectrum, as we have seen, differs 
 strikingly from the prismatic in the arrangement of its 
 colored spaces. In the latter, the less refrangible parts 
 are compressed more and more in proportion as their 
 refrangibility is less. Now there is reason to believe 
 that in the former the colored spaces are equally warm, 
 though so feeble is the calorific effect that all attempts 
 at the direct measurement of the heat have proved 
 unsatisfactory. I first made such attempts with very 
 delicate thermo-electric apparatus, but could not obtain 
 sufficiently striking results. Admitting, however, that 
 every ray, irrespective of its color, in the act of extinc- 
 tion will generate the same amount of heat, it necessarily 
 follows that in the prismatic spectrum the heat should 
 appear to increase steadily from the more to the less 
 refrangible end, because in it the compression of the 
 colored spaces is becoming greater and greater, and this 
 is what is actually observed. 
 
 These considerations respecting the distribution of 
 heat in the spectrum lead naturally to the examination 
 of a much more comprehensive problem indeed, one of 
 the most important problems that science presents viz., 
 the constitution of the sunbeam. 
 
 Until the time of Newton it was universally admitted 
 
MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 129 
 
 that light is a pure undecomposable elementary principle. 
 He showed that this conclusion must be modified. No 
 one, except Leibnitz, in those days, and no one for a long 
 time subsequently, discerned the ominous import of the 
 discoveries of this Prince of geometers. Of his detec- 
 tion of the origin of Kepler's laws, and its necessary 
 consequence of the mode in which the government of 
 the universe is conducted, I have nothing here to say. 
 Let us see how it was with his discoveries concerning 
 light. 
 
 His interpretation of the experiment he made in the 
 "dark chamber" was this, that light is not an undecom- 
 posable element, as was at that time supposed, but that 
 in reality it consists of not fewer than seven different 
 constituents, recognizable by their color. These, if mixed 
 in any manner together, whether by grinding tinted pig- 
 ments or revolving parti - colored sectors, or converging 
 the spectrum through a convex, lens, would, by their 
 union, produce white light. His felicitous experiment 
 with the two reversed prisms silenced the carping critics 
 of that day, who had declared that the colored tints 
 with which he was working had no such origin as he 
 affirmed in difference of refrangibility but were anal- 
 ogous to the iridescent play of light on a pigeon's breast, 
 or the more gorgeous lustre of a peacock's tail. It cost 
 only a short struggle, and the theory of the composite 
 nature of light made good its ground. 
 
 When, therefore, Herschel, in his examination of the 
 sun's surface through colored glasses, came to the conclu- 
 sion that the heat emitted by the sun is essentially and 
 intrinsically distinct from the light, and that these ele- 
 ments may be parted from each other by refraction, he 
 did no more than develop the principle that had been 
 announced by Newton ; and when, at a later period, 
 Melloni extended these researches, and it was universal- 
 
 I 
 
130 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 
 
 ly admitted that there are heat rays which, like light 
 rays, have various refrangibilities, this conclusion was 
 quite accordant with Newton's results. Heat was con- 
 sidered as existing in the solar beam independent and 
 irrespective of light. In fact, the one might be easily 
 separated from the other. 
 
 When, again, the Swedish chemist Scheele, investigat- 
 ing the chemical action of light, showed that there are 
 rays invisible to the eye, and of greater refrangibility 
 than the violet, which can produce the decomposition 
 of certain compounds of silver, these were considered to 
 be an additional element, and passed under the designa- 
 tion of chemical rays, deoxidizing rays, etc. Treated of 
 in the works of physics of those times as imponderable 
 bodies, there seemed to be no necessary limit to their 
 number. More than half a hundred ponderable sub- 
 stances w r ere known. Why, then, should there not be 
 as many of these imponderable ones? This was the 
 view universally entertained at the time I began the 
 experimental study of radiations. For such as are con- 
 cerned in producing chemical changes I suggested a 
 special designation, which, however, did not find accept- 
 ance : the inappropriate and unmeaning appellation, ac- 
 tinic rays, was preferred. 
 
 Meantime, however, the undulatory theory of light 
 had been steadily making its way. It was exhibiting 
 all the aspect of a great physical truth, in not only 
 rendering an explanation of known facts, but also in 
 predicting the occurrence of other facts previously un- 
 known. Persons who were in the front of the scientific 
 movement in this direction had thus their attention 
 forcibly drawn to a contemplation of the whole subject 
 from this new point of view. They very soon perceived 
 that from it bonds of interconnection between facts 
 hitherto supposed to be isolated might be discerned; 
 
MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 
 
 things that were fragmentary and confused spontaneous- 
 ly fell into an orderly arrangement. 
 
 While the theory of optics was making this great 
 advance, another important science, physiology, was pre- 
 senting a similar development. It was casting off the 
 Vital Force of the older medical authors, and acknowl- 
 edging the dominion of chemical and physical forces. 
 It had become plain that the interpretation of many 
 phenomena, as hitherto received, must be changed. 
 
 We may apparently have heat without light, and 
 light without heat. In the darkest room we cannot 
 perceive vessels filled with boiling water, yet the warmth 
 we experience on approaching them assures us that they 
 are emitting radiations. Is not this heat without light ? 
 If we stand in the rays of the full moon, we cannot de- 
 tect any increase of temperature. Is this not light with- 
 out heat ? It is true that in this latter instance we are 
 mistaken as to the fact; but overlooking that for the 
 heat to be detected in the moonbeams requires the most 
 sensitive apparatus do not such observations assure us 
 that heat and light are independent of each other, phys- 
 ical principles having an existence separate from each 
 other ? 
 
 Such were some of the arguments on which was sus- 
 tained the hypothesis of the intrinsic difference of light 
 and heat. In this, no account was taken of the optical 
 functions of the eye. Qualities were incorrectly attrib- 
 uted to radiations which, in truth, were due to peculiari- 
 ties in the organ of vision. 
 
 The great service which the diffraction spectrum has 
 rendered to science is the abolishment of all these im- 
 aginary independent existences heat, light, actinism, 
 etc. and the substitution for them of the simpler con- 
 ception of vibratory motions in the ether. The only 
 difference existing among the radiations that issue from 
 
132 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 
 
 a grating, in the manner we have been describing, is in 
 their wave-lengths, or what comes to the same thing, in 
 their times of vibration. The diversity of effects pro- 
 duced depends on the quality of the surface on which 
 they fall. If on a dark surface, and the more so in pro- 
 portion to its blackness, they engender heat ; if on the 
 retina, they are interpreted by the mind as light ; if on 
 photographic preparations, they produce decompositions, 
 designated actinic effects. 
 
 Heat, light, actinism, are, then, not natural principles 
 existing independently of each other, but effects arising 
 in bodies from the reception of motions in the ether, 
 motions which differ from each other ,in their rapidity. 
 Of those that the eye can take cognizance of, the most 
 rapid impart to the mind the sensation of violet light, 
 the slowest the sensation of red, and intermediate ones 
 the intermediate optical tints. Colors, like light itself, 
 are nothing existing exteriorly. They are merely mental 
 interpretations of modes of motion in the ether, and in 
 this they represent musical sounds, which exist only as 
 interpretations by the mind of waves in the air. 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 133 
 
 ' 
 
 MEMOIR 
 
 ON THE PHOSPHORESCENCE OF 
 
 From the Philosophical Magazine, Feb., 1851 ; Harper's New Monthly Ma. 
 
 No. 325. 
 
 CONTENTS: Early observations on phosphorescence. The diamond. 
 Duration of shadows. Lemery's theory. Du Fay's theory. Quali- 
 ties of diamond and fluor-spar. The volume of a phosphorescent body 
 does not change during its glow. A structural change accompanies the 
 phosphorescence of bodies ; there is a minute disengagement of heat. 
 Phosphorescence is not communicable. Absolute quantity of light 
 emitted. 
 
 THERE are some surfaces on which if a shadow falls, it 
 can be brought into view a long time subsequently. 
 
 A belief in the existence of the carbuncle, a stone 
 supposed to have the property of shining in the dark, 
 appears to have been current from the very infancy 
 of chemistry. It gave rise to many legends among the 
 alchemists, and early travellers relate marvellous stories 
 respecting self-shining mountains and gems. Thus it 
 was said that the King of Pegu wore a carbuncle so 
 brilliant that if any of his subjects looked upon him in 
 the dark, his countenance seemed as though it was irra- 
 diated by the sun, and that in a certain part of North 
 America there was a mountain which illuminated the 
 country for many miles, and served by its rays to guide 
 the Indians at night. The story seems to indicate that 
 the locality of this wonder was somewhere in the west- 
 ern part of Pennsylvania. Mr. Boyle relates that a gov- 
 ernor of one of the American colonies imparted this fact 
 to him at a time when he was charged with the superin- 
 
134 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 tendence of those important settlements, and that an ex- 
 pedition had been despatched to ascertain the facts cor- 
 rectly. It saw the shining wonder from afar, but the 
 light diminished as the place was approached, and be- 
 coming at length invisible, the locality could not be 
 determined with certainty. 
 
 These legends had for some time been passing into 
 discredit, when Vincenzio Cascariola, a cobbler of Bolog- 
 na in Italy, who had abandoned the mending of shoes 
 for the purpose of rinding the philosopher's stone, discov- 
 ered his celebrated phosphorus, the Bolognian stone, or, 
 as it was then designated, sun-stone (lapis Solaris). He 
 had seduced himself into the expectation that a heavy 
 mineral he had met with barium sulphate contained 
 silver, and in an attempt to melt out that precious metal 
 was astonished to see that the burned substance shone 
 like an ignited coal in the dark. This was in the year 
 1602. 
 
 Some time afterwards a Saxon of the name of Baldwin 
 conceived the idea of obtaining the soul of the world by 
 distilling in a retort chalk which had been dissolved in 
 aqua fortis. In this extraordinary pursuit accident led 
 him to observe that the substance he was working with 
 possessed the quality of shining in the dark after it had 
 been exposed to the light of the sun. The alchemist 
 Kunckel, who relates the incident, tells us with gravity 
 how he stole a piece of this substance on the occasion of 
 a visit he made to Baldwin one night when that adept 
 was trying to make his phosphorus shine by the light 
 absorbed from a candle, and also from its image reflect- 
 ed by a concave mirror. In consequence of this theft, 
 Kunckel succeeded in discovering what the substance 
 was, and made known the method of its preparation. 
 
 The special condition under which these preparations 
 shine in the dark was very, quickly detected. Isidore, 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 of Seville, speaking of the "lightning-stone," says, "Si 
 sub divo positus fuerit fulgorem rapit sidereum." That 
 condition is previous exposure to light. 
 
 The discovery of the elementary substance now known 
 as phosphorus drew the attention of the cultivators of 
 natural science to this singular property, and under the 
 names of sun-stones, light magnets, noctilucas, etc., vari- 
 ous shining bodies were introduced. But the first truly 
 scientific examination of the subject was made by Boyle, 
 on the occasion of observing that a certain diamond be- 
 longing to Mr. Clayton, and subsequently purchased by 
 Charles II., emitted light in the dark. Though he does 
 not seem to have been aware of it, the fact itself was not 
 new, for the alchemist Albertus Magnus says in the thir- 
 teenth century that he had seen a diamond which glowed 
 when it was put into warm water. A diamond rubbed 
 upon gold becomes beautifully luminous; as Bernouilli 
 remarks, it shines like a burning coal excited by the bel- 
 lows. 
 
 A diamond rubbed upon gold emitting light ! the 
 imaginary or intrinsic value of the substances employed 
 adds to the glory of the phenomenon. A light, too, that 
 cannot be extinguished by water, and yet so ethereal and 
 pure that it can set nothing on fire. Here certainly were 
 facts of interest enough to excite the philosophers of the 
 last century. 
 
 The chief points ascertained by Boyle respecting the 
 diamond were that it shone by friction with various 
 bodies, and at the same time displayed electrical devel- 
 opment ; that it also glowed when warmed by a candle, 
 the fire, a hot iron, or even when placed on the skin. 
 Under the latter circumstances it exhibited no electricity, 
 being unable to attract a hair held near to it. He also 
 found that it would shine under water, various acid or 
 alkaline liquids, or when covered with saliva, and that 
 
136 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 the glow was increased when the gern was put into hot 
 water. 
 
 These results led to the conclusion that though under 
 certain circumstances the light was accompanied by elec- 
 trical development, as when friction had been used, there 
 was no necessary connection between the two properties. 
 The gem would shine without the least trace of attract- 
 ive power. 
 
 (Among substances endowed with this property, one 
 of the best was discovered about a century ago by Can- 
 ton. Still known as Canton's phosphorus, it is easily 
 made by burning oyster-shells in an open fire until they 
 have become white ; then, having pulverized them with 
 about a quarter of their weight of flowers of sulphur, 
 they are once more brought to a dull red heat in a cru- 
 cible. This completes the preparation. A convenient 
 mode of using the substance is to provide a piece of tin 
 plate two or three inches square, brush over one side of 
 it with gum or glue water, then dust upon it from a fine 
 sieve some of the powdered phosphorus. In this manner 
 a uniform white surface is procured, well adapted for 
 experiments. 
 
 If on such a surface a key or other opaque object be 
 laid, and it then be exposed for a moment to daylight, 
 on carrying it into a dark room and removing the key, 
 a spectral shadow will be seen, depicted in black, and its 
 contour marked out by the brilliantly glowing phos- 
 phorus surrounding it. After continuing to shine for 
 some minutes, the light gradually fades, and finally be- 
 comes extinct. If, this having been accomplished, the 
 phosphorized plate be put away in a box or drawer 
 where not a ray of light can reach it, and kept therein 
 for days or even weeks, on exposing it in a dark room, 
 on a plate of warm metal, the phantom shadow will 
 emerge, perhaps even more strongly than at first. 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 A wonderful experiment, truly. Shadows, then, are 
 not such fleeting, such fugitive things as poets say. 
 They may bury themselves in stony substances, and be 
 made to come forth at our pleasure. 
 
 The persistence of such surface phantoms may be strik- 
 ingly illustrated by a simple experiment in which light 
 is not concerned. If on a cold polished metal, as a new 
 razor, an- object such as a small coin be laid, and the 
 metal be then breathed upon, and when the moisture 
 has had time to disappear, the coin be thrown off, though 
 now the most critical inspection of the polished surface 
 can discover no trace of any form, if we breathe once 
 more upon it a spectral image of the coin comes plainly 
 into view. And this may be done again and again. 
 Nay, more, if the razor be put carefully aside where noth- 
 ing can deteriorate its surface, and be so kept for many 
 months, on breathing again upon it the shadowy form 
 emerges.) 
 
 Early in the last century two hypotheses were intro- 
 duced for the explanation of the "various cases of phos- 
 phorescence : 
 
 1. That phosphorescent bodies act like sponges to 
 light, absorbing it, and retaining it by so feeble a power 
 that very trivial causes suffice for its extrication. This 
 was the view of Lemery, and was published in 1709. 
 
 2. That phosphorescence arises from an actual combus- 
 tion taking place in the sulphureous parts of the glow- 
 ing body. It is to be remembered that sulphur figured 
 largely in the chemistry of those days. This was the 
 hypothesis of Du Fay. 
 
 To this celebrated electrician we owe a very able in- 
 vestigation of the phosphorescence of various bodies, and 
 especially of the diamond. He recognized the fact, over- 
 looked by Boyle, that the gem must first be exposed to 
 
133 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 the light ; and then, when taken into a dark place, it 
 shines for a time, the light gradually fading away. But 
 the glow can be re-established by raising the tempera- 
 ture, and an exposure of a single second to the sun is 
 quite enough to commence the process. 
 
 To recognize feeble degrees of luminosity, it is neces- 
 sary for the observer to remain in the dark until the 
 pupil of the eye is quite dilated, and the impression of 
 light to which the retina has been exposed is worn off. 
 Du Fay gives a singular but very serviceable practical 
 process. He recommends the experimenter to keep one 
 eye bound up or closed for the purpose of observing in 
 the dark, and to use the other in conducting his process- 
 es in the light. He remarks the curious fact that the 
 eye which has been shut will not have the delicacy of 
 its indications affected by that which has been exposed 
 to the light. 
 
 In this manner Du Fay found that of four hundred 
 yellow diamonds all were phosphorescent; but some 
 that were white or rose-colored or blue or green were 
 not. Nor was there any external indication by which 
 it could be told whether any given one of these kinds 
 would shine. He discovered, too, that the glow took 
 place under various - colored media, as stained - glass, 
 water, milk, but not under ink. He also made attempts 
 to compel the gem to preserve its light by enveloping it 
 in opaque media, such as ink, black wax, etc., under the 
 idea that the light could not get out, and concluded that 
 he had partially succeeded, because in some instances the 
 diamonds would shine after being so shut up for six or 
 twelve hours. He verified Boyle's fact on the effects of 
 hot water and heating generally, and carried his tem- 
 peratures to far higher degrees, even above a white heat, 
 finding that the stone had lost none of its qualities, for 
 it would take light again when it was cold on a momen- 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 tary exposure to the sun. He also investigated, how far 
 the glow was connected with electrical relations, and 
 showed its perfect independence. He also greatly in- 
 creased the list of phosphori, asserting that, so far from 
 the quality being a peculiarity of the Bolognian stone, 
 Baldwin's compound, the diamond, all solid substances 
 except the metals, are phosphorescent when rightly treat- 
 ed, and even these he believed would eventually be 
 found to have the same property. 
 
 There is one point upon which Du Fay dwells that 
 deserves more than a passing remark the connection 
 between phosphorescence and temperature. He proved 
 that phosphori cannot absorb light so well when they 
 are warm as when they are cold, and that a rise of tem- 
 perature always makes them disengage their light. 
 
 It is obvious that these early investigators labored 
 under great difficulties arising from the imperfect chem- 
 ical science of those times. They confounded together 
 things that were essentially different, such as the shining 
 of urine-phosphorus with the glow of the diamond, and 
 this again with the electrical light arising when friction 
 has been employed. Then, again, their erroneous views 
 of the composition of bodies were constantly leading 
 them astray. Thus Du Fay, finding that the Bolognian 
 stone (barium sulphate) emitted a sulphury smell, and 
 thinking that it shone because of the burning of the 
 
 o o 
 
 sulphur, transferred the same explanation to the case of 
 yellow diamonds, and asserted that they also glowed 
 through the combustion of the sulphur that discolored 
 them. 
 
 I do not intend now to give a review of the subse- 
 quent discoveries and hypotheses brought forward by 
 the numerous experimenters of the last century, or by 
 Heinrich, the Becquerels, Biot, Poggendorff, Pearsall, and 
 many others in this. I may, however, recall attention to 
 
140 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 a very elaborate memoir by Osann (Poggendorff s Anna- 
 len, 1834, vol. xxxiii., p. 405), in which he discusses the 
 various theories of combustion, absorption, excitation, 
 etc., and gives many new facts. 
 
 (Respecting the phosphorescence of diamonds, I have 
 recently had an opportunity of making a curious obser- 
 vation. A lady, a relative of mine, has a pair of ear- 
 rings in which are set two large and beautiful gems, 
 both of which phosphoresce after exposure to an electric- 
 al spark; she has also another pair in which both the 
 diamonds in like manner phosphoresce. Judging from 
 these four instances, one might regard this property as 
 very common. Curiously enough, the necklace belong- 
 ing to this set, containing thirty-eight stones of very fine 
 water, has only one that will phosphoresce. This neck- 
 lace would, therefore, lead us to reverse the conclusion 
 to which the ear-rings had led us, and to infer that phos- 
 phorescing diamonds are comparatively rare.) 
 
 All solid substances, except the metals, possess the 
 phosphorescent quality. We may, however, by making 
 a judicious selection of the bodies which are to serve as 
 our means of experiment, disembarrass the inquiry of 
 many of its complications. If we employ the Bolognian 
 stone (barium sulphate) or Canton's phosphorus (cal- 
 cium sulphide), or, indeed, any other substance liable 
 to undergo chemical changes in the air, we introduce 
 unnecessary phenomena, and cannot distinctly prove 
 whether the shining is due to a direct combustion of 
 the parts or to other causes. 
 
 Among selections that might be made, diamond and 
 fluor-spar possess qualities rendering them very eligible 
 for these purposes unchangeability in the air and 
 under water. Even between these there is a choice, for 
 fluor-spar possesses all the good qualities of diamond. 
 It might be said, considering the chemical relationships 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 of diamond, that when it glows it undergoes a kind of 
 surface combustion, which is the cause of the light ; but 
 though direct experiments prove that this is not the 
 case, it 'is much better to resort to fluor-spar, which is 
 free from such an objection. It is absolutely incombus- 
 tible. Besides, it can be obtained perfectly transparent 
 or nearly opaque ; it occurs of many tints of color ; can 
 be easily cut and polished to any figure, and obtained in 
 pieces of any required size. Its phosphorescent powers 
 are very high ; indeed, it yields, when properly treated, 
 to no other substance, not even to Canton's phosphorus, 
 in that respect, and greatly exceeds the potash sulphate, 
 a substance which, however, possesses many eligible 
 qualities. It will therefore be understood that in select- 
 ing fluor-spar and its varieties as the subjects for experi- 
 ment, it has been done with a view of bringing the re- 
 sults to their simplest conditions. In such inquiries 
 Canton's phosphorus and bodies chemically changeable 
 are wholly inadmissible. 
 
 The specimens of fluor-spar employed by me were de- 
 rived from many different sources, American and Euro- 
 pean. The color of the light they emitted was in some 
 cases blue, in some green, in some yellow. Among them 
 was an American variety of chlorophane of a pale flesh- 
 colored aspect, translucent on the edges, and excelling 
 all the others in the splendor of its light. It equalled 
 the best Canton's phosphorus in power, yielding a superb 
 emerald green light when it received the rays of the sun 
 or of an electric spark, or had its temperature raised. 
 The warmth of the hand in a dark place made it shine. 
 Considering the facility with which we can regulate the 
 intensity of an electric spark, measuring out the quanti- 
 ties of light used in a given experiment, it is clear that 
 there are great advantages in resorting to it in prefer- 
 ence to the variable rays of the sun. Our choice of a 
 
142 THE PHOSPHORESCENCE OF BODIES. [MoioiR VIII. 
 
 substance should be controlled by these conditions, and 
 fluor-spar completely fulfils all the indications. 
 
 It will be seen, however, that I have not restricted 
 myself to the use of this body, but whenever other sub- 
 stances could be compared with it, have resorted to them 
 also. The general principles here set forth as applicable 
 to fluor-spar may be likewise extended to them. 
 
 To what cause are we to attribute phosphorescence? 
 What are the changes taking place in the glowing 
 body? 
 
 We have already seen that a century ago two different 
 answers had been given to these questions. Lemery 
 supposed that all bodies act towards light as they do to 
 heat, absorbing it and then giving it out: Du Fay that 
 all phosphorescences are cases of combustion. 
 
 Before we can reach a decision there are evidently 
 many preliminary points to be settled. If .chemical 
 changes between the glowing body and the air are dis- 
 posed of, and the action is recognized to be of a purely 
 physical or molecular kind, it is necessary to determine 
 (1) whether there is any expansion or contraction of the 
 shining body during its glow ; (2) whether there is any 
 structural change; (3) whether there is any evolution 
 of heat along with the light ; or (4) any development of 
 electricity. These inquiries will now be taken up in 
 succession. 
 
 1. Is there any change of volume in a phosphorescent 
 ~body during its glow ? 
 
 I attempted to ascertain this by causing various bodies 
 to shine brightly when enclosed in glass vessels filled 
 with water, so that if there were any expansion the 
 water might be pressed out into a slender tube, and the 
 
MEMOIR VIIL] THE PHOSPHORESCENCE OF BODIES. 143 
 
 amount of dilatation thereby determined. The arrange- 
 ment was as follows : 
 
 A glass tube about two inches long and three quar- 
 ters of an inch in diameter was closed at its upper end 
 by means of a plate of polished quartz, cemented air- 
 tight. Immediately beneath the quartz the phosphores- 
 cent body was supported. Through a cork which closed 
 the other end of the tube there passed a piece of ther- 
 mometer tube bent on one side, and to it was affixed a 
 scale. The arrangement was supported on a suitable 
 stand, so that the quartz was uppermost, and at a little 
 distance above it the spark from a Leyden-jar could be 
 passed between a pair of stout iron wires maintained at 
 an invariable distance, and thus produced phosphores- 
 cence in the body. It may be remarked that these ef- 
 fects of an electric spark do not take place well through 
 glass, and hence a plate of quartz, which readily trans- 
 mits them, must be used. 
 
 In Fig. 14, a a is the glass tube, b b the plate of pol- 
 ished quartz, c the phosphorescent body, 
 d d the cork closing the lower end of the 
 tube, e e the bent tube,/ its scale, g g the 
 iron wires connected with a Leyden-jar, 
 and giving a spark. The index drop at 
 li refers not to this, but to a subsequent 
 experiment. 
 
 The large tube containing the phosphor- 
 escent body must be filled quite 
 full of water, free from air, as also 
 f must be the thermometer tube to 
 
 Fig. i4. a given mark on its scale. If an 
 
 electric spark be now passed between the wires to make 
 the phosphorus shine, it is clear that if there be any ex- 
 pansion or contraction of its volume, there will be a corre- 
 sponding movement in the water of the thermometer tube. 
 
144 TIIE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 On making the trial, and using in succession a crystal 
 of violet-colored fluor-spar, a piece of flesh-colored chloro- 
 phane, and a mass of Canton's phosphorus, the result in 
 all cases was negative ; for, though these different sub- 
 stances glowed very brilliantly as soon as the spark 
 passed, there was not the smallest movement perceptible 
 in the index liquid of the thermometer tube. 
 
 With a view of estimating the delicacy of the means 
 thus used for determining any change in the volume of 
 the spar, the solid content of a piece of chlorophane was 
 determined by weighing in water; also the value of each 
 division of the scale was ascertained. The value of each 
 such division was equal to -j^Vo of the volume of the 
 spar, and a movement equal to one tenth of that value 
 could have been detected. 
 
 It may therefore be concluded that a phosphorescent 
 body, when at its maximum of glow, has not changed its 
 volume perceptibly. 
 
 The conclusion thus arrived at is strengthened by an- 
 other mode of experiment. If change of volume be con- 
 nected with this evolution of light, it might reasonably 
 be expected that a sudden, severe, but equable compres- 
 sion, exerted on a piece of spar, the light of which is just 
 fading out, would compel it to regain a portion of its 
 brilliancy. A piece of chlorophane in that condition was 
 placed in water contained in the apparatus known as 
 Oersted's instrument for measuring the compressibility 
 of water, and which is described in most of the treatises 
 on physics ; but though, by suitably turning the screw, 
 pressures varying from one to four atmospheres were 
 suddenly put on the spar and as suddenly removed, no 
 change whatever was seen in the glowing mass, the light 
 of which continued steadily to die away. 
 
 In Fig. 15, Oersted's instrument for proving the com- 
 pressibility of water, a a is the glass cylinder filled with 
 
MKMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 water, b the pressure screw, c the phosphores- 
 cent spar or substance. 
 
 These experiments have a bearing on Le- 
 mery's theory. A mass of iron suddenly com- 
 pressed grows hot; so, too, does atmospheric 
 air. It would, therefore, not be unreasonable 
 to expect that if a phosphorus acted like a 
 sponge to light, and were thus pressed upon, 
 it would yield up its light. But conceptions 
 derived from the old theories of specific heat r Fig . 15 1 
 are perhaps scarcely applicable here. 
 
 When unequal pressure is applied, the result is differ- 
 ent. A piece of chlorophane pressed by a forceps glows 
 brightly; if crushed, the fragments sparkle like little 
 fire-works as they fly through the air. If the spar be 
 previously powdered, a shining is still produced, and 
 when the pulverization is conducted in an agate mortar 
 in the dark, bright eddies of light follow the track of 
 the pestle. In these cases, however, the separation of 
 the laminae of the crystal and the heat produced by 
 friction probably determine the result. Canton's phos- 
 phorus did not shine when compressed or submitted to 
 friction. 
 
 2. Does any structural change accompany the phos- 
 phorescence of bodies? 
 
 The foregoing experiments appearing to prove that 
 if there be any expansion of a phosphorescing body, it 
 is to a very small amount, I next endeavored to de- 
 termine whether there is any molecular change, or new 
 structural arrangement, which can be detected by polar- 
 ized light. 
 
 A flat piece of fluor-spar, polished on both sides, was 
 placed in a polariscope, and a pair of blunt iron wires 
 connected with a Leyden-jar were adjusted near the 
 
 K 
 
146 THE PHOSPHOKESCENCE OF BODIES. [MEMOIR VIII. 
 
 front of it, so that when the spark passed, a brilliant 
 glow arose in the spar, which was at once viewed 
 through the analyzer of the instrument. But though 
 the experiments were made both by daylight and lamp- 
 light, no kind of effect could be detected. Had any 
 molecular change occurred, it could not have escaped 
 notice. 
 
 In Fig. 16, a I) is the polariscope, c the flat piece of 
 
 fluor-spar, b the analyzer, d d 
 the wires giving the electric 
 spark. 
 
 These experiments were first 
 made by using as the analyzer 
 a doubly refracting achromatic 
 prism ; they were, however, re- 
 peated with a Nicol, in which 
 the eye is not disturbed by a 
 bright image as in the other 
 
 case. Having fixed the plate of polished fluor in the 
 polariscope, it was readily perceived that it possessed 
 naturally a structural arrangement, for there were cloudy 
 spaces or lines in it which contrasted with the faint 
 white light passing in the adjacent parts. It was also 
 seen that this structural arrangement could be deranged 
 in a transient manner, either by pressure or an unequal 
 warming, as is well known of other bodies ; but when a 
 powerful electric discharge was passed near the spar, 
 and a brilliant phosphorescence took place, no impres- 
 sion could be detected. Even when the iron wires rest- 
 ed on the spar, and the explosion passed over its surface, 
 nothing was perceptible except along the line between 
 the ends of the wires, where the surface was roughened 
 or abraded by the force of the discharge. 
 
 But though these experiments with polarized light 
 give a negative result, or, at all events, prove that a 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 phosphorus when shining has its molecular condition so 
 little disturbed that the change cannot be detected in 
 this way, there can be no doubt that if the means of 
 testing were more delicate, such a change would be dis- 
 covered, for many years ago Mr. Pearsall found that 
 specimens of fluor, not possessing phosphorescence natu- 
 rally, might have that quality communicated to them by 
 repeated exposure to many powerful electric discharges, 
 which also gave rise to a change in their natural color. 
 Now there can be no doubt that such an alteration of 
 tint implies an alteration of structure. 
 
 Besides the test by polarized light, there is another 
 which may be resorted to for the detection of structural 
 changes when they are merely superficial; it is the 
 mode in which various vapors will condense. I de- 
 scribed several such cases in the Philosophical Magazine 
 for September, 1840, some time previously to the pub- 
 lications on the subject that were made by M. Moser. 
 They were brought forward at that time as an illustra- 
 tion of the manner in which mercurial vapors condense 
 on a daguerreotype plate arid develop images which it 
 has received. Proceeding on this principle, a large plate 
 of fluor-spar, the surface of which was finely polished, 
 was made to phosphoresce brightly along a given line 
 determined by the ends of two iron wires, which served 
 as a discharger for a Leyden spark, and were placed 
 near to the polished surface. The spar was forthwith 
 suspended in the mercurial box of a daguerreotype ap- 
 paratus and kept there an hour. The mercury condensed 
 upon it faintly in the manner it would have done on a 
 daguerreotype plate, especially on and in the^vicinity 
 of those parts that were more immediately exposed to 
 the spark. This, therefore, seems to prove that there is 
 in these cases a molecular modification of the shining 
 surface. 
 
148 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 3. When a phosphorescent body glows, does it likewise 
 emit Tieat? 
 
 A very thin bulb, half an inch in diameter, was blown 
 on a piece of thermometer tube, and after being washed 
 over with gum-water, finely powdered chlorophane was 
 dusted on until it was neatly coated all over. A drop 
 of water was then introduced into the tube to serve as 
 an index. Although the instrument was very sensitive 
 to heat, when the chlorophane was made to shine and 
 emit a gorgeous emerald green light by the passage of 
 a powerful electric spark near it, no movement whatever 
 of the index ensued. From this it would appear that 
 the quantity of heat developed by phosphorescence must 
 be very small. 
 
 In Fig. 17, a a is the glass bulb covered with a coat- 
 ing of powdered chlorophane, /> 
 a drop of water serving as an 
 index. 
 
 A modification of this exper- 
 iment, which appeared to offer 
 
 several advantages, was tried. The instrument repre- 
 sented in Fig. 14 was emptied of its water, and a single 
 drop, h, put into the index tube. It was supposed that 
 when the rays of the electric spark passed through the 
 quartz and made the phosphorus shine, the air contained 
 in the tube, warmed thereby, would expand, and a move- 
 ment in the index liquid of the thermometer tube take 
 place. But in several trials, in which different bodies 
 chlorophane, Canton's phosphorus, etc. were employed, 
 the results were uniformly negative; for though tlaese 
 different substances glowed splendidly as soon as the 
 spark passed, there was not the slightest rise of tem- 
 perature perceptible. 
 
 A further attempt was made as follows : The disk of 
 quartz being removed and replaced by a cork, through 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 which a pair of iron wires to serve as a discharger passed 
 air-tight, and descended to within a short distance of the 
 phosphorus, sufficient time was allowed in various repeti- 
 tions for the index liquid to come to rest. It was hoped 
 that this form of experiment would have advantages over 
 the preceding, because the discharging wires could be 
 brought nearer to the phosphorus, and the effect take 
 place without the intervention of the quartz. When 
 the spark was made to pass, there was a great move- 
 ment in the index tube, as in the instrument known as 
 Kinnersley's electrometer, but the liquid immediately re- 
 turned to within a short distance of its first place ; then 
 a slow dilatation occurred, as though the air was gradu- 
 ally warming. Thus in one experiment the liquid stood 
 at 24, after the explosion it returned to 26, and then 
 there was a gradual dilatation to 32. 
 
 To eliminate the various disturbing causes in this ex- 
 periment, it was repeated many times, the spar being al- 
 ternately introduced into the glass tube, and alternate- 
 ly removed. It was found that whenever the spar was 
 present the gradual dilatation alluded to took place; 
 but when the spar was not in the tube, instead of a 
 dilatation, there was a gradual contraction until the in- 
 dex liquid recovered its original position. 
 
 From this it appears that with the evolution of light 
 there is a feeble extrication of heat. 
 
 The quantities of heat thus liberated are so small, and 
 the causes of error are so numerous, that I endeavored 
 by other methods to obtain more trustworthy results. 
 Thus I attempted to determine the surface temperature 
 of a flat piece of chlorophane while phosphorescing by 
 means of the thermo-electric multiplier. The thermo- 
 pile was placed in a vertical position, and the spar 
 having been attached to a piece of wood, which served 
 as a handle, intense phosphorescence was communicated 
 
150 
 
 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 by a Leyden spark, and the flat and shining surface 
 instantly put on the upper face^ of the pile. But there 
 was no movement of the astatic needles. 
 
 Then, taking the stone by its handle, it was touched 
 with the tip of the finger for one second, and quickly 
 placed on the pile. A prompt movement of the needles, 
 amounting to four degrees, ensued. These experiments 
 were repeatedly tried, and the results were uniformly 
 the same. 
 
 In Fig. 18, a a is the thermo-electric pile, b b the 
 
 plate of chlorophane, c the 
 handle. 
 
 On considering these re- 
 sults, it appears that as the 
 temperature of the air near 
 the multiplier in one of 
 the experiments was 53, 
 and the estimated temper- 
 ature of the skin 94, the 
 amount of heat which the 
 stone received from the 
 touch of the finger must 
 have been very small. I 
 made a comparative trial 
 by touching the bulb of a 
 thermometer for the same space of time, in the same 
 way, and found that there was a rise of about 1|. But 
 the conductibility of quicksilver is much greater than 
 that of chlorophane. 
 
 It is to be inferred, therefore, that the quantity of 
 heat set free during phosphorescence is very small, 
 and that the surface of the chlorophane does not 
 change its temperature by one fourth of a degree ; 
 for had it done so, the multiplier would have instant- 
 ly detected it. 
 
 Fig. 18. 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 4. Is phosphorescence accompanied with a development 
 of electricity ? 
 
 It has been stated already that the experimenters of 
 the last century paid a good deal of attention to this 
 point. Du Fay established the fact that though in many 
 cases of phosphorescence there is a development of elec- 
 tricity, there are many others in which the light seems 
 to be wholly unattended by any disturbance of that 
 kind. 
 
 I have repeated some of these experiments, and with 
 the same result, proper care being taken to avoid fric- 
 tion and other obvious causes of electrical excitement. 
 Thus a flat piece of chlorophane, phosphorescing power- 
 fully, was put on the cap of a very delicate gold-leaf 
 electroscope, but no disturbance whatever was percep- 
 tible. 
 
 A large crystal of fluor-spar was made to phosphoresce 
 brilliantly along a line about half an inch in length by 
 passing the spark of a Leyden-jar between two blunt 
 iron wires, the ends of which were that distance apart, 
 and resting on the face of the crystal. Over this line of 
 blue light, which was pretty sharply marked, and which 
 lasted for several minutes, a fine hair was held. This 
 would have been readily attracted and repelled by the 
 feeblest excitation of sealing-wax, but in this case it 
 wholly failed to yield any indication whatsoever. 
 
 In connection with the foregoing experiments, I may 
 mention some miscellaneous facts. Some attempts were 
 made to determine whether phosphorescent bodies in 
 the field of a powerful electro-magnet would exhibit any 
 change of property. Six Grove's pairs were caused to 
 magnetize a good electro-magnet ; the power they could 
 give to it would enable the keeper to support about 
 half a ton. Between its polar pieces chlorophane, Can- 
 
152 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 ton's phosphorus, etc., which were made to glow by ex- 
 posure to a Ley den spark, were placed. But it made no 
 difference in the light whether *the magnetism was on 
 or not. 
 
 It was also found that the electric spark from a con- 
 tact-breaker would communicate phosphorescence to all 
 the various bodies in use in these experiments, and that 
 up to a certain point the intensity of the light increased 
 with the number of sparks received. 
 
 Phosphorescence is not communicable from on 3 body 
 to another. Having provided two polished plates of 
 fluor-spar, one of them was made to glow by an electric 
 spark, and the other was immediately put upon it. No 
 communication of phosphorescence took place; the sec- 
 ond piece remained perfectly dark. 
 
 Some authors state that fluor-spar does not become 
 phosphorescent by exposure to the sun ; but this re- 
 mark does not apply to all varieties of it. Thus some 
 chlorophane, which had been ignited in a glass tube till 
 it had ceased to shine, was pulverized and again ignit- 
 ed in a platinum crucible. It emitted an emerald light. 
 A slip of wood was now put on it to screen a part of 
 its surface, and it was exposed to the sun for a few 
 minutes. On ignition, it shone again finely, with a green 
 light, the shadow of the wood being beautifully depict- 
 ed. The same having been repeated a great many 
 times, it appeared that the phosphorescence at last be- 
 gan to decrease, perhaps by frequent ignition causing a 
 change. 
 
 A screen of yellow glass intervening between the sun 
 and some powdered chlorophane prevented phosphores- 
 cence, but it took place through a plate of polished fluor- 
 spar. When the light of an electric spark was used in- 
 stead of the sunshine in this experiment, the fluor-spar 
 prevented phosphorescence. 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 GENERAL CONCLUSIONS. 
 
 The results to which the foregoing experiments bring 
 us are therefore 
 
 1st. That the methods employed in these experiments 
 are not sufficiently delicate to detect any increase in the 
 dimensions of a phosphorus while it is in a glowing state. 
 
 2d. No structural change can be discovered by resort- 
 ing to polarized light ; but there is reason to believe 
 from the change of color which certain bodies exhibit 
 when the quality of shining is communicated to them, 
 and from the manner in which vapors condense on their 
 surfaces, that such has actually taken place. 
 
 3d. That phosphorescence is attended with a minute 
 rise of temperature. 
 
 4th. That it is not necessarily connected with any elec- 
 trical disturbance. 
 
 On comparing these conclusions, it is obvious that if 
 the third be correct there must necessarily be a change 
 of volume, and that the reason the dilatation is not dis- 
 covered by direct experiment is owing to the insufficiency 
 of the means employed. 
 
 The general definition given of phosphorescence is that 
 it is the extrication of light without heat (Grnelin). 
 But these results show that that definition is essentially 
 incorrect ; for if the experiment be made with due care, 
 a rise of temperature can be detected, though its absolute 
 amount may be very small. 
 
 'Determination of the absolute quantity of light emitted 
 ~by plwspliori. 
 
 And now we may inquire how it is with the light it- 
 self? do we not deceive ourselves respecting it? We 
 ought to recollect that it is barely perceptible in the 
 open day, and that these experiments require to be made 
 
154 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 in the dark. We should also recollect the great sensi- 
 tiveness of the eye, and how feeble a luminous impres- 
 sion it can detect. Impressed with these facts, I have 
 endeavored to compare the absolute quantity of light 
 given by the most brilliant phosphori with some well- 
 known standards. The result of these experiments puts 
 a new view on the whole subject. 
 
 The first attempts I made for this purpose were con- 
 ducted on the principle of comparing the stains formed 
 on a daguerreotype plate by the phosphorus under trial, 
 and by an oil-lamp, receiving the rays from each on a 
 concave metallic mirror eight inches in diameter and 
 fifteen in focus, arranged as a reflecting camera obscura. 
 There were set, side by side, a small oil-lamp, a piece of 
 white paper illuminated by the lamp, and a fragment of 
 chlorophane, arranging things in such a manner that the 
 chlorophane might be illuminated by rays coming from 
 a contact-breaker worked by two Grove's pairs. The 
 contact-breaker was kept in action fifteen minutes, and 
 then, to prove the sensitiveness of the plate, the lamp 
 was moved for one minute to a new position, and the 
 experiment closed. 
 
 On developing, it was found that the impressions of 
 the lamp had solarized, both that of fifteen minutes and 
 that of one, proving that such a light in one minute is 
 amply sufficient to change the plate to its maximum. 
 Also the electric spark of the contact-breaker was solar- 
 ized, and the image of the piece of white paper beauti- 
 fully given of a clear white ; but the phosphorescing 
 spar had made no impression, except from one portion 
 where it had reflected the rays of the spark. 
 
 Suspecting that the spark from the contact-breaker 
 might not have been powerful enough, I repeated the 
 experiment, using sparks from a Leyden-jar. The oil- 
 lamp was exposed in front of the mirror one minute, and 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 155 
 
 then removed ; then ten strong sparks were passed over 
 the spar, each of which made it emit an emerald light ; 
 but during the moment of the passage of each spark a 
 screen was interposed, that no direct or reflected light, 
 and, indeed, none but that of the phosphorus, could reach 
 the mirror and sensitive plate. 
 
 On mercurializing, it was found as before that the 
 lamp was beautifully depicted but the spar was invisible. 
 
 In Fig. 19, a is the oil-lamp, b the white paper, c the 
 chlorophane, cut and polished, d the contact - breaker 
 with its wires,// the concave mirror. Its concavity 
 faces the above-named objects, and reflects their images 
 inverted and reversed on a sensitive plate, e. The mir- 
 ror and sensitive plate are enclosed in a darkened box 
 not shown in the figure. 
 
 Fig. 19. 
 
 Estimated, therefore, by the chemical effects they can 
 produce, the light from chlorophane is incomparably less 
 intense than that from a common lamp. For there can 
 be no doubt that each of the ten Leyden sparks gave a 
 light which made the spar phosphoresce brilliantly for six 
 seconds, and the whole phosphorescence was equal in du- 
 ration to that produced by the light of the lamp; yet the 
 latter had changed the plate to a maximum, while the 
 former had not made the smallest perceptible impression. 
 
 As the foregoing attempt to obtain photographic ef- 
 
15(5 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 fects had failed, I varied the experiment as follows : In a 
 Bohemian glass tube a quantity of chlorophane in coarse 
 fragments, sufficient to occupy about three inches in 
 length of the tube, was placed. The reflecting camera 
 with its sensitive silver plate was set in a proper po- 
 sition. When everything was arranged, a spirit-lamp 
 was applied to the chlorophane, which soon emitted a 
 superb emerald light, and continued to do so for about 
 two minutes. As the light began to decline, the spar 
 splintered by decrepitation. The process went on in a 
 very satisfactory way. An oil-lamp was then placed in 
 front of the camera for five seconds. On developing, the 
 image of the lamp-flame came out, but no trace whatever 
 of the chlorophane could be detected. Thus it appears 
 that the splendid green light emitted when the spar is 
 heated is at least twenty-four times less intense than the 
 light emitted by a small oil flame. It should be remem- 
 bered that this is a measure of absolute intensity, and 
 not of illuminating power. 
 
 But as it is known that green light is not very ef- 
 ficient in changing a sensitive surface, I tried to deter- 
 mine the intensity of the light emitted by chlorophane 
 by the optical method of Bouguer, described in the first 
 of these Memoirs, p. 39. 
 
 The spar being heated by a current of hot air arising 
 from the flame of a spirit-lamp, the light of which was 
 carefully screened by a chimney and other contrivances 
 of sheet-iron, a comparison was made with a very small 
 oil-lamp, the flame of w 7 hich was about six tenths of an 
 inch high and the wick one sixth of an inch thick. It 
 was covered with a glass shade. 
 
 The spar, when it began to glow, cast a reddish shad- 
 ow on the paper, which shadow was extinguished when 
 at its maximum by the lamp at about 25 inches, the 
 spar being at 5 inches. 
 
MEMOIR VIII.] THE PHOSPHORESCENCE OF BODIES. 
 
 The distances of the chlorophane and lamp from the 
 paper were, therefore, as 1:5. The illuminating effect is 
 as the squares of those numbers, and therefore 1 : 25. 
 But for extinction it requires that one light should be 
 sixty times as intense as the other ; it follows, therefore, 
 that at those distances the illuminating power of the 
 lamp is fifteen hundred times as intense as the illumi- 
 nating effect of the spar. 
 
 But the quantity of spar used in this experiment ex- 
 posed a surface much greater than that of the flame ; it 
 w r as estimated to be at least twice as great. This, there- 
 fore, would bring us to the conclusion that the intrinsic 
 brilliancy of the chlorophaue is not ^-^ part that of 
 the lamp. 
 
 This experiment was several times repeated. Thus it 
 was found that the lamp extinguished the shadow from 
 the spar when the relative distances were 1 : 4. The 
 lamp at 4 was therefore sixty times as luminous as the 
 spar at 1 ; that is, their illuminating power was as 1 : 960. 
 But it was estimated that the surface of the spar was 3| 
 times that of the flame of the lamp, so this would make 
 the intrinsic brilliancy -^Vs? a resu ^ f the same order 
 as the preceding. 
 
 From this we conclude that the intrinsic brilliancy of 
 phosphori is very small ; a fine specimen of chlorophane, 
 at its maximum of brightness, yielding a light three thou- 
 sand times less intense than the flame of a very small oil- 
 lamp. 
 
 It was stated above that these photometric experi- 
 ments put a new view on the \vhole subject; in fact, 
 they explain all the difficulties of the foregoing inquiries. 
 How could w r e expect to be able to measure the heat of 
 phosphorescence? The radiant heat of the little oil- 
 lamp here employed would require at such distances a 
 very delicate thermometer to measure it. Is it likely, 
 
158 THE PHOSPHORESCENCE OF BODIES. [MEMOIR VIII. 
 
 then, that we could detect that of a source three thou- 
 sand times less intense ? 
 
 I conclude, therefore, that all phosphoric bodies emit 
 radiant heat as well as light ; but that its quantity is so 
 small that we have no means delicate enough to measure 
 it, though the eye is so sensitive that it can detect the 
 light, the absolute intensity of which has, however, hith- 
 erto been greatly overrated. I believe that the quanti- 
 ties of both are of the same order, and if this be true we 
 should scarcely expect to discover any dilatation of the 
 glowing body unless means much more refined than 
 those here resorted to are employed. 
 
MEMOIR IX.] EFFECTS OF HEAT ON PHOSPHORESCENCE. 15 9 
 
 MEMOIR IX. 
 
 ON THE EFFECTS OF HEAT ON PHOSPH 
 
 From the Philosophical Magazine, Feb., 1851. 
 
 ^. 
 
 CONTENTS: Experiment of Albertus Magnus. Degree of phosphores- 
 cence at different temperatures. The quantity of light a substance can 
 retain is inversely as its temperature ; the quantity it can receive is di- 
 rectly as the intensity and quantity of light to which it has been ex- 
 posed. Phosphorescent images of the moon. Action of ether waves. 
 Effects of cohesion. Reason that gases, liquids, and metals are non- 
 phosphorescent. 
 
 IT has been already observed that the effect of heat 
 in promoting the disengagement of light is an old dis- 
 covery. Albertus Magnus remarked it in the case of 
 a diamond plunged into hot water. 
 
 It is customary in later works which treat systemat- 
 ically on phosphorescence to group the different facts 
 under two heads 1st, phosphorescence produced by in- 
 solation ; 2d, by heat. An example of this is offered in 
 the standard work on chemistry by Gmelin. 
 
 A division of this kind brings the whole subject into 
 confusion. It assigns different causes for things that are 
 essentially allied. It leads to the inference that as un- 
 der certain circumstances the sunlight or an electric 
 spark can make bodies glow, so under other circumstan- 
 ces heat will produce the same effect, and this wholly 
 independent of incandescence. 
 
 But what are the facts? If a yellow diamond placed 
 upon ice be submitted to the sun, and then brought into 
 a dark room the temperature of which is 60, for a time 
 there is a glow, but presently the light dies out. If the 
 
160 EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX. 
 
 diamond be now put into water at 100, it shines again, 
 and again its light dies away. If next it be removed 
 from that water and suffered to cool, and then be re- 
 immersed, it will not shine again ; but if the water be 
 heated to 200, and the diamond be dropped into it, 
 again it glows, and again its light dies away. 
 
 There is, therefore, a correspondence between the light 
 disengaged and the temperature. We are not to con- 
 clude from the foregoing illustration that when the dia- 
 mond has its temperature raised from 100 to 200 the 
 light is due to the heat. On the contrary, the light is 
 unquestionably due to the primitive exposure to the 
 sun ; just as in Lemery's illustration of the sponge, if we 
 exert a little pressure a portion of the water flows out ; 
 if a stronger pressure, still more ; and for each degree of 
 pressure there will be a corresponding quantity of water 
 expelled. 
 
 The connection between phosphorescence and temper- 
 ature may be instructively illustrated as follows : 
 
 Suppose that three yellow diamonds, a, b, c, have been 
 simultaneously exposed to the sun, a being kept at 32, 
 b at 60, c at 100, and that they are then simultaneously 
 removed to a bath of water at 100 in a dark room; it 
 will be found that a emits a bright li^ht, b shines more 
 
 ~ O > 
 
 feebly, and c scarcely at all. This is what ought to be 
 expected from the principle laid down above ; for if at a 
 particular temperature a certain quantity of light is set 
 free, it is clear that a has the advantage of Z>, so that it 
 will disengage all the light to be set free between 32 
 and 60. 
 
 From such experiments and considerations it is to be 
 inferred that there is an intimate connection between 
 temperature and phosphorescence which may be conven- 
 iently expressed in the following terms. The quantity of 
 light a substance can retain is inversely as its temperature. 
 
MEMOIR IX.] EFFECTS OF HEAT ON PHOSPHORESCENCE. 
 
 This principle furnishes the explanation of a multitude 
 of facts. Thus Du Fay discovered that the Bolognian 
 stone shines brighter when exposed to the sky than to 
 the sun. In the latter case the temperature rises, and 
 the quantity of light retained is less. Under violet 
 and other glasses, stained with such colors as impede 
 the warming effect, phosphorescence is even more vivid 
 than when no glass has intervened. On the same prin- 
 ciple we have an explanation of Du Fay's apparently 
 successful attempt to prevent the escape of light from 
 diamonds by putting them in ink or covering them with 
 black wax. When removed from the ink and brought 
 into the air, they became somewhat warmer perhaps 
 the touch of the finger aided the effect and a corre- 
 sponding quantity of light was set free. 
 
 But though temperature is a controlling, it is not the 
 only condition involved. If it were, phosphorescence 
 after insolation should occur only after a rise of tempera- 
 ture. The fundamental fact of the whole inquiry proves 
 that a glowing body can retain more light in presence 
 of a lucid surface than it can in the dark. 
 
 Is not this fact analogous to what we meet with in 
 the exchanges of heat? A substance can retain more 
 heat in presence of a hot body than a cold one. The 
 brilliancy and quantity of light to which a phosphorus 
 is exposed goes very far to determine the intensity of 
 the subsequent glow. Thus I found that a piece of 
 chlorophane exposed to one spark of a contact-breaker 
 shone feebly, but if it had received one hundred sparks, 
 its light was very vivid ; and it has long been known 
 that in delicate phosphori a certain degree of luminosity 
 can be communicated by the moonbeams, a more intense 
 one by lamp-light, and one still more brilliant by the 
 sunshine or a Leyden spark. This, therefore, leads to 
 the conclusion that the quantity of light a phosphorus 
 
 L 
 
162 EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX. 
 
 can receive is directly as the intensity and quantity of 
 light to which it has been exposed. 
 
 (With respect to the light of the moon, I have suc- 
 ceeded in obtaining an image of that satellite on Canton's 
 phosphorus, by the aid of a concave metallic mirror.) 
 
 The various facts herein cited indicate that when a 
 ray of light falls on a surface, it throws the particles 
 thereof into vibration. An examination of the action of 
 the differently colored rays dispersed by a prism shows 
 that in general the greater the frequency of vibration 
 of the impinging ray, the more brilliant is the phosphor- 
 escence. But in such a prismatic examination we have 
 constantly to bear in mind the disturbing agencies which 
 are present, and especially the antagonizing effects of 
 heat ; that this determines the amount of light that a 
 phosphorus can receive, and also the rate of its subse- 
 quent extrication. In Memoir V., p. 87, I have shown 
 how the photographic action of light betrays the general 
 principle of an interference of vibratory movements, and 
 the production of antagonizing results in different parts 
 of the solar spectrum. An argument is there brought 
 forward to the effect that as the violet end produces 
 phosphorescence and the red extinguishes it, this is a 
 proof of opposition of action. In explaining this fact, 
 M. E. Becquerel supposes the darkening power of the 
 red rays to be due to the more rapid disengagement of 
 the phosphorescence by reason of the heat produced by 
 those rays, arid the apparent antagonization is not at- 
 tributable to the supposition of vibratory movements of 
 light rays of different frequency, but to the relations of 
 caloric and light. The force of this explanation, how- 
 ever, disappears when it is understood that light and 
 heat, the chemical and phosphorogenic rays, are, accord- 
 ing to the principles of the able experimenter, all mani- 
 festations of the same agent. It avails us nothing to say 
 
MEMOIR IX.] EFFECTS OF HEAT ON PHOSPHORESCENCE. 
 
 that a want of phosphorescence at the less refrangible end 
 of the spectrum is due to the heat-giving powers of those 
 rays, when that very heat-giving power is under the hy- 
 pothesis dependent on their comparative rapidity of vi- 
 bration. 
 
 We may, therefore, in the explanation of phosphor- 
 escence, abandon expressions derived from the material 
 theory of light, and assume that whenever a radiation 
 falls upon any surface, it throws the particles thereof 
 into a state of vibration, just as in the experiment of 
 Fracaster, in which a stretched string is made to vibrate 
 in sympathy with a distant sound, and yield harmonics, 
 and form nodes. Such a view includes at once the facts 
 of the radiation of heat and the theory of calorific ex- 
 changes ; it also offers an explanation of the connection 
 of the atomic weights of bodies and their specific heats. 
 It suggests that all cases of decomposition of compound 
 molecules under the influence of a radiation are owing 
 to a want of consentaneousness in the vibrations of the 
 impinging ray and those of the molecular group, which, 
 unable to maintain itself, is broken down under the pe- 
 riodic impulses it is receiving into other groups, which 
 can vibrate along with the ray. 
 
 If a hot body, #, be placed in presence of a cold body, 
 , the theory of the exchanges of heat teaches that the 
 temperature of the latter will steadily rise until equi- 
 librium between the two takes place. The molecules of 
 a communicate their vibratory movement to the ether, 
 and this in its turn imparts an analogous movement to 
 the molecules of b. For, as the ethereal medium is of 
 vastly less density than the vibrating molecules, each of 
 these oscillations will produce in it a determinate wave, 
 which is propagated through it according to the ordi- 
 nary laws of undulations, in such a way that the ether 
 would be in repose after the wave had passed, were it 
 
EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX. 
 
 not for the recurrence of the continuing vibration of the 
 molecules. At each vibration the molecules of a lose a 
 part of their vis viva, lay the quantity they have com- 
 municated to the ethereal wave, the intensity or ampli- 
 tude of the wave becoming less and less as this abstrac- 
 tion of force is going on. But the ether being of uniform 
 density and elasticity throughout, each of its particles 
 communicates the whole vis viva it has received to the 
 next adjacent, and would instantly come to rest were it 
 not again disturbed by the vibrations of the material 
 molecules. These elementary considerations show how 
 it is that a wave of sound passes through the air, or of 
 light through the ether, and the particles of those media 
 come instantly to rest; but a hot body or a vibrating 
 string persists in its motions, which only undergo a grad- 
 ual decline. If the vibrating molecule were in a medium 
 of the same density, it would impart to it all its motion 
 at once, and in the same way that a heavy molecule grad- 
 ually communicates its motion to the ether, so in its turn 
 does the ether to other systems of molecules. 
 
 Upon these principles we may explain the phenomena 
 of phosphorescence. From a shining body undulations 
 are propagated in the ether, and these, impinging on a 
 phosphorescent surface, throw its molecules into a vi- 
 bratory movement. These in their turn impress on the 
 ether undulations ; but by reason of the difference of its 
 density compared with that of the molecules, they do 
 not lose their motion at once; it continues for a time, 
 gradually declining away and ceasing when the vis viva 
 of the molecules is exhausted. 
 
 When a phosphorescent surface is exposed to the lu- 
 minous source, it necessarily undergoes a rise of temper- 
 ature, and the cohesion of its parts is diminished ; but 
 after its removal from that source, as the temperature 
 declines and radiation goes on, the cohesion increases, 
 and a restraint is put on those motions. 
 
MEMOIR IX.] EFFECTS OF HEAT ON PHOSPHORESCENCE. 
 
 Now let the phosphorus have its temperature raised, 
 and the cohesion of its molecules be thereby weakened, 
 and the restraint on their motions abated. At once they 
 resume their oscillations, and continue them to an extent 
 that belongs to the temperature used. When this has 
 passed away, a still higher temperature will release them 
 once more, and the glowing will again be resumed. 
 
 What would be the result if we could cause the sur- 
 face of a mass of water on which circular waves are rising 
 and falling to be instantaneously congealed? It might 
 be kept in that condition for a thousand years, and then, 
 if instantaneously thawed, the waves would resume their 
 ancient motion from the point at which it was arrested, 
 and it would now go on to its completion. 
 
 So with these phosphori. Exposed to light of a suit- 
 able intensity, their parts begin to vibrate; but the free- 
 dom of those motions is interfered with by their cohe- 
 sion. Amplitude of vibration must always be affected 
 by cohesion, and if the ray be removed and the tem- 
 perature be permitted to decline, the restraint becomes 
 greater and greater, and they pass into a condition some- 
 what like that which has just been illustrated. It mat- 
 ters not how long a time may intervene, rise of tempera- 
 ture will enable them to resume their motions. 
 
 These principles give an explanation of all the facts 
 we observe. We see how it is that as we advance from 
 one temperature to another the phosphorus will resume 
 its glow, and that there is, as it were, for every degree 
 a certain amount of vibratory movement that can be ac- 
 complished, or, to use a different phrase, a certain amount 
 of light that can be set free. It also necessarily follows 
 that different solids will display these motions with dif- 
 ferent degrees of facility, and hence shine for a longer or 
 shorter time, and with lights of different intensities. 
 
 But in liquids and gases, which want that particular 
 
EFFECTS OF HEAT ON PHOSPHORESCENCE. [MEMOIR IX. 
 
 condition of cohesion characteristic of the solid state, and 
 the parts of which move freely among each other, phos- 
 phorescence cannot take place, for it depends on the in- 
 fluence that cohesion has had in restraining the vibra- 
 tory movements. 
 
 Further, the condition of opacity does not permit phos- 
 phorescence to be established. The exciting ray cannot 
 find access to disturb the interior layers of the mass, and 
 even if it did, and phosphorescence ensued, how could 
 we expect to be able to discover it through the impervi- 
 ous veil of the superficial layers ? The light of the most 
 brilliant phosphorus cannot be seen through the thin- 
 nest gold-leaf. Its intensity is vastly too small. These 
 are the reasons that no one has ever yet succeeded in 
 detecting phosphorescence in metals and black bodies. 
 
 It will be gathered from this explanation that I am 
 led to believe that all the facts of phosphorescence can 
 be fully explained on the principles of the communica- 
 tion of vibratory motion through the ether; that as upon 
 that theory an incandescent body maintained at incan- 
 descence would eventually compel a cold body in its 
 presence to come up to its own temperature by making 
 its particles execute movements like those of its own, so 
 the sunshine or the flash of an electric spark compels a 
 vibratory movement in the bodies on which its rays fall ; 
 that these vibrations are interfered with by cohesion in 
 the case of solids, but that they are instantly established 
 and almost as instantly cease in the case of liquids and 
 gases ; that reducing the cohesion of a solid by raising 
 its temperature permits a resumption of the movement ; 
 and that the condition of opacity, whether metallic or 
 otherwise, is a bar to the whole phenomenon. 
 
MEMOIR X.] THE DECOMPOSITION OF CARBONIC-ACID GAS. 
 
 MEMOIR X. 
 
 ON THE DECOMPOSITION OF C AKBONIC ACID GAS BY 
 PLANTS IN" THE PRISMATIC SPECTRUM. 
 
 From the Proceedings of the American Philosophical Society, May, 1 843 ; Phil- 
 osophical Magazine, Sept., 1843; American Journal of Science and Arts, Vol. 
 XXVI., 1843; Philosophical Magazine, Sept., 1844. 
 
 CONTENTS: The decomposition of carbonic acid by light formerly at- 
 tributed to the violet ray. It can be successfully accomplished in the 
 prismatic spectrum. It takes place not in the violet but in the yellow 
 ray. Decomposition by yellow absorbent media. Analysis of the gas 
 evolved ; it always contains nitrogen. Decomposition of alkaline car- 
 bonates and bicarbonates. Analysis of the gas evolved in different rays. 
 
 FOR many years it has been known that the green 
 parts of plants, under the influence of sunlight, possess 
 the power of decomposing carbonic acid, and setting 
 free its oxygen. It is remarkable that this, which is a 
 fundamental fact in vegetable physiology, should not 
 have been investigated in an accurate manner. The 
 statements met with in the books are often far from 
 being correct. It is sometimes said that pure oxygen 
 gas is evolved, that the decomposition is brought about 
 by the so-called " chemical rays;" these and a multitude 
 of such errors pass current. So far as my reading goes, 
 no one has yet attempted an examination of the phe- 
 nomena by the aid of the prism, the only way in winch 
 it can be correctly discussed. 
 
 In a paper by Dr. Daubeny, inserted in the Philosoph- 
 ical Transactions for 1836, two facts, which I shall verify 
 in this communication, are fully established. These are, 
 1st, the occurrence of nitrogen gas in mixture with the 
 
168 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X. 
 
 oxygen, an observation originally due to Saussure, or 
 some earlier writer; and, 2d, that the act of decompo- 
 sition is due to the LIGHT of tlie sun. This latter ef- 
 fect, obtained by employing colored glasses or absorb- 
 ent media, has not been generally received. Doubt will 
 always hang about results obtained in that way, and 
 nothing but an examination by the prism can be satis- 
 factory. 
 
 In its connection with organic chemistry and physi- 
 ology the experiment of the decomposition of carbonic 
 acid by leaves assumes extraordinary interest. When 
 we remember that this decomposition is the starting- 
 point for organization out of dead matter, that com- 
 mencing with this action of the leaf the series of or- 
 ganized atoms goes forward in increasing complexity, 
 and blood and flesh and cerebral matter are at its ter- 
 minus, it is clear that unusual importance belongs to 
 precise views of this the commencing change. The rays 
 of the sun are the authors of all organization. 
 
 There is but one way by which the question can be 
 finally settled it is by conducting the experiment in 
 the prismatic spectrum itself. When we consider the 
 feebleness of effect which takes place by reason of the 
 dispersion of the incident beam through the action of 
 the prism, arid the great loss of light through reflection 
 from its surface, it might appear to be a difficult opera- 
 tion to eifect a determination in that way. Encouraged, 
 however, by the purity of the skies in America, I made 
 the trial, and met with complete success. 
 
 When the leaves of plants are placed in water from 
 which all air has been expelled by boiling, and exposed 
 to the sun's rays, no gas whatever is evolved from them. 
 When they are placed in common spring or pump wa- 
 ter, bubbles quickly form, which, when collected and 
 analyzed, prove to be a mixture of oxygen and nitro- 
 
MKMOIR X.] THE DECOMPOSITION OF CARBONIC- ACID GAS. 1(3 9 
 
 gen gases; from a given quantity of water a definite 
 quantity of air is produced. When they are exposed 
 in water which has been boiled, and then impregnated 
 with carbonic acid, the decomposition goes on with ra- 
 pidity, and large quantities of gas are evolved. 
 
 The obvious inference seeming to arise from these 
 facts is that all the oxygen collected is derived from 
 the direct decomposition of carbonic acid. 
 
 Having by long boiling and subsequent cooling ob- 
 tained water free from dissolved air, I saturated it with 
 carbonic-acid gas. Some grass leaves, the surfaces of 
 which were carefully freed from adhering bubbles or 
 films of air by having been kept in carbonated water 
 for three or four days, were provided. Seven glass 
 tubes, each half an inch in diameter and six inches 
 long, were filled with carbonated water, and into the 
 upper part of each the same number of blades of grass 
 were placed, care being taken to have all as near as could 
 be alike. The tubes were placed side by side in a small 
 pneumatic trough. It is to be particularly remarked that 
 the leaves were of a pure green aspect as seen in the wa- 
 ter; no glistening air-film such as is always on freshly 
 gathered leaves nor any air -bubbles were attached to 
 them. Great care was taken to secure this perfect free- 
 dom from air at the outset of the experiments. 
 
 The little trough was now placed in such a position 
 that a solar spectrum, kept motionless by a heliostat, 
 and dispersed in a horizontal direction by a flint-glass 
 prism, fell upon the tubes. By bringing the trough 
 nearer to the prism or moving it farther off", the different 
 colored spaces could be made to fall at pleasure on the 
 inverted tubes. The beam of light was about three 
 fourths of an inch in width. 
 
 In a few minutes after the beginning of the experi- 
 ment, the tubes on which the orange, yellow, and green 
 
170 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X. 
 
 rays fell commenced giving off minute gas-bubbles, and 
 in about an hour and a half a quantity was collected 
 sufficient for accurate measurement. 
 
 In Fig. 20, a a represents the trough, and R, O, Y, etc., 
 the tubes containing the leaves and carbonated water 
 placed so as to receive the spectrum, I b. 
 
 ROY 
 
 Fig. 20. 
 
 The gas thus collected in each tube having been trans- 
 ferred to another vessel and its quantity determined, the 
 little trough with all its tubes was freely exposed to the 
 sunshine. All the tubes now commenced actively evolv- 
 ing gas, which, when collected and measured, served to 
 show the capacity of each tube for carrying on the proc- 
 ess. If the leaves in one were more sluggish or exposed 
 a smaller surface than the others, the quantity of gas 
 evolved in that tube was correspondingly less. And 
 though I could never get the tubes to act precisely 
 alike, after a little practice I brought them sufficiently 
 near for my purpose. In no instance was this testing 
 process of the power of each tube for evolving gas omit- 
 ted after the experiment in the spectrum was over. 
 
 From the following table it appears that the rays which 
 cause the decomposition of carbonic-acid gas are the or- 
 ange, the yellow, the green ; the extreme red, the blue, the 
 indigo, and the violet exerting no perceptible effect. We 
 
MEMOIR X.] THE DECOMPOSITION OF CARBONIC-ACID GAS. 
 
 Table of the Decomposition of Carbonic Acid by Light of Different 
 
 Colors. 
 
 Experiment I. Experiment II. 
 
 Name of ray. 
 
 Volume of gas. 
 
 Name of ray. | Volume of gas. 
 
 Extreme red 
 Red and orange. . . 
 Yellow and green. 
 Green and blue. . . 
 Blue 
 
 .33 
 
 20.00 
 36.00 
 .10 
 .00 
 .00 
 .00 
 
 Extreme red and red. 
 Red and orange 
 Yellow and green . 
 
 .00 
 24.75 
 43.75 
 4.10 
 1.00 
 .00 
 .00 
 
 Green and blue 
 
 Blue 
 
 Indigo .... 
 
 Indigo . . 
 
 Violet 
 
 Violet 
 
 
 
 should therefore expect that in a beam, passing through 
 absorbent media of such a nature that the extreme red, 
 the blue, the indigo, and violet are absorbed, this de- 
 composition should nevertheless go on. A solution of 
 bichromate of potash nearly fulfils these conditions. It 
 transmits the luminous rays in question, except a trace 
 of those which correspond to the more refrangible yel- 
 low and less refrangible green. 
 
 o o 
 
 A remarkable proof of the correctness of the foregoing 
 prismatic analysis comes out when leaves are made to 
 act on carbonated water in light which has passed 
 through a solution of bichromate of potash. I took a 
 wooden box of about a cubic foot in dimensions, and 
 having removed its bottom, adjusted to it a trough made 
 of pieces of plate-glass. The box being set on one side, 
 its lid served as a door, and the trough being filled with 
 a solution of bichromate of potash, the sun's beams came 
 through it, and in the interior of the box leaves and car- 
 bonated water could be exposed to the rays that had 
 escaped absorption. The thickness of the liquid stra- 
 tum was about half an inch. I had several such boxes 
 made, so that I might compare the simultaneous effect 
 of light that had undergone absorption by different me- 
 dia. They formed, as it were, little closets, in which 
 bodies could be exposed to parti-colored light blue, 
 yellow, red, etc. 
 
172 THE DECOMPOSITION OF CAEBONIC-ACID GAS. [MEMOIR X. 
 
 Fig. 21 represents one of these closets; a a is its side 
 
 of stained - glass, or the 
 trough containing the ab- 
 sorbing solution, bichro- 
 mate of potash, etc. ; b b, 
 the door. 
 
 Whenever an experi- 
 ment was commenced in 
 these closets, a similar one 
 Ir ~n^ r was simultaneously com- 
 
 menced in the unobstruct- 
 ed sunshine. It is needless to state that in all these 
 care was taken to have the different arrangements for 
 decomposition as nearly alike as possible. 
 
 On comparing the amount of gas evolved in unab- 
 sorbed light and in light that had undergone absorption 
 by bichromate of potash, in three out of five trials the 
 gas collected under the latter circumstances exceeded in 
 volume that collected under the former; this was prob- 
 ably due to a higher temperature existing in the box. 
 
 On comparing the volume of gas collected under bi- 
 chromate of potash and under litmus -water, the latter 
 was not equal to half the former. 
 
 I compared the gas evolved in unobstructed light, 
 under bichromate of potash, and under ammonia sul- 
 phate of copper; the results were as follows: 
 
 Unobstructed light 4.75 
 
 Bichromate of potash 4.25 
 
 Ammonia sulphate of copper 75 
 
 It therefore appears that light which has passed 
 through bichromate of potash, by which the chemical 
 rays have been absorbed, can accomplish this decompo- 
 sition ; but light which has passed through ammonia 
 sulphate of copper, which transmits the chemical rays, 
 fails to produce that effect. 
 
MEMOIR X.] THE DECOMPOSITION OF CARBONIC- ACID GAS. ^73 
 
 For these reasons I conclude that the decomposition 
 of carbonic acid by the leaves of plants is brought about 
 by the rays of light, and that the calorific and so-called 
 chemical rays do not participate in the phenomenon. 
 The rays of light are therefore as much entitled to the 
 appellation of chemical rays as those which have here- 
 tofore passed under that name. 
 
 Next I examined the constitution of the gaseous mixt- 
 ure given off during these decompositions. It proved to 
 be not pure oxygen, but a variable mixture of oxygen, 
 nitrogen, and carbonic acid. Omitting the carbonic acid, 
 which diffused from the solution in variable quantities, 
 the following table represents the composition of the 
 gaseous mixture: 
 
 Analyses of Gas evolved from Carbonated Water. 
 
 Exp. 
 
 Name of plant. 
 
 Oxygen. 
 
 Nitrogen. 
 
 1 
 2 
 3 
 
 4 
 5 
 
 Pinus taeda. 
 ii 
 Poa annua. 
 
 16.16 
 27.16 
 22.33 
 90.00 
 77.90 
 
 8.34 
 13.84 
 21.67 
 10.00 
 22.10 
 
 This table contains a few out of a great number of 
 experiments, all of which might have been quoted as ex- 
 amples of the conclusions which I wish to deduce. 1st. 
 They all coincide in this, that the oxygen is never 
 evolved without the simultaneous appearance of nitro- 
 gen ; 2d. That when certain leaves are employed, as those 
 of the Pinus tseda, there seems to be a very simple rela- 
 tion between the volumes of oxygen and nitrogen. In 
 the first and second of these experiments, the volume of 
 oxygen is to that of nitrogen as two to one; in the third, 
 as one to one. In certain cases this apparent simplicity 
 of proportion is departed from ; but from its frequent oc- 
 currence in many analyses I have made it seems to de- 
 
174 THE DECOMPOSITION OF CARBONIC-ACID GAS. [MEMOIR X. 
 
 mand attentive consideration. Moreover, in other plants, 
 as in experiments 4 and 5, the amount of oxygen is rela- 
 tively greater, and between it and the nitrogen there 
 does not appear any exact proportion. 
 
 In order to ascertain whether decompositions taking 
 place under absorbent media, as bichromate of potash, 
 yield the same results as indicated in the foregoing- 
 table, I made several analyses of gas collected under 
 those circumstances. The presence of the absorbent me- 
 dium did not seem to exert any influence whatever, the 
 general results coming out as though it had not been 
 employed. 
 
 It was also found that the alkaline carbonates and 
 bicarbonates could be decomposed by leaves in yellow 
 light. The alkaline bicarbonates, as is well known, un- 
 dergo decomposition by a slight elevation of temperature. 
 When boiled in water they gradually give off their sec- 
 ond atom of carbonic acid, and slowly pass into the con- 
 dition of neutral carbonate. In the experiments I made 
 with them the boiling was not continued long enough to 
 affect to any extent the constitution of the salt, and in 
 each case any portion of carbonic acid extricated during 
 cooling was removed by the air-pump. A few leaves 
 placed in this solution showed no effect if kept in the 
 dark, but if brought into the sunshine there was a copi- 
 ous evolution of gas-bubbles, which, on detonation with 
 hydrogen, proved to be rich in oxygen gas. One experi- 
 ment gave 88 of oxygen, 12 of nitrogen. 
 
 In a subsequent communication to the Philosophical 
 Magazine (Sept., 1844), I gave additional analyses of 
 the gas emitted in yellow light as follows : 
 
 Five tubes, each three eighths of an inch in diameter 
 and six inches long, were inverted in a small trough of 
 water containing carbonic acid, with which the tubes 
 were also filled. Some blades of grass nearly of the 
 
MEMOIR X.] THE DECOMPOSITION OF CARBONIC-ACID GAS. }f5 
 
 same size and volume were placed in each tube. This 
 grass had been kept for two days in the dark in a bottle 
 filled with carbonated water. During this time the film 
 of air which envelops all new leaves was removed, the 
 grass became perfectly free from all adhering gaseous 
 matter, and when in the carbonated water exhibited a 
 dark-green aspect. 
 
 I have previously found that leaves thus soaked emit 
 under the influence of light a larger amount of nitrogen 
 than usual ; this comes from the incipient decay of some 
 of their nitrogenized constituents. When under these 
 circumstances they are placed in the sunshine, this nitro- 
 gen comes off along with the gas liberated from the car- 
 bonic acid. 
 
 In the experiment I am now relating, a tube arranged 
 like one of the foregoing five evolved in the open sun- 
 shine a certain volume of gas which was composed of 
 
 Oxygen 41 ~\ 
 
 Nitrogen 59 '- 100 
 
 Carbonic acid 00 ) 
 
 The five tubes were placed in the spectrum in the fol- 
 lowing colors, and emitted the quantities of gas repre- 
 sented in the following table : 
 
 1 
 
 Extreme red and red. 
 
 0.0 
 
 2 
 
 Orange and yellow 
 
 19.8 
 
 3 
 
 Yellow and green 
 
 27.4 
 
 4 
 
 Blue " " 
 
 0.5 
 
 5 
 
 Indigo and violet 
 
 0.0 
 
 The gas in tube 2, which had been in the orange and 
 yellow ray, was then washed with a solution of caustic 
 potassa. After this it still measured 19.8. It contained, 
 therefore, no perceptible quantity of carbonic acid. It 
 was next examined for oxygen, and with the following 
 result : 
 
176 THE DECOMPOSITION OF CARBONIC- ACID GAS. [MEMOIR X. 
 
 Constitution of Gas emitted in Orange and Yellow Light. 
 
 Oxygen 8.0 \ r Oxygen 40.4 
 
 Nitrogen 11.8 > or < Nitrogen 59.6 
 
 Carbonic acid.. 0.0 ) ( Carbonic acid. . 00.0 
 
 19.8 100.0 
 
 The gas evolved by the yellow and green rays was 
 next analyzed. Like the former, it underwent no dimi- 
 nution by washing with caustic potash. After this 
 treatment it therefore measured 27.4, and on being ex- 
 amined for oxygen yielded as follows : 
 
 Constitution of Gas emitted in Yellow and Green Light. 
 
 Oxygen 12.5^ (Oxygen 45.6 
 
 Nitrogen 14.9 > or < Nitrogen 54.4 
 
 Carbonic acid. . 00. o) ( Carbonic acid .. 00.0 
 
 ~VLA 100.0 
 
 In explanation of the large and variable amount of 
 nitrogen occurring in these analyses, it will scarcely be 
 necessary to remind the vegetable physiologist that it 
 arises from the mode of conducting the experiment. In 
 order to be absolutely certain that no atmospheric air 
 infilmed the leaves, they were soaked in water, and then 
 when brought into the sunlight the nitrogen which had 
 accumulated in their tissues from incipient decay dif- 
 fused out with the first portions of oxygen. As, there- 
 fore, more and more gas was evolved, the relative amount 
 of the nitrogen diminished. Thus the reason that the 
 third tube appeared to be richer in oxygen than the sec- 
 ond was owing to its containing more gas. Any person, 
 however, who is familiar with the physiological action 
 of leaves will understand these things without any fur- 
 ther explanation. 
 
 UNIVERSITY OF NEW YORK, June 15, 1844. 
 
MEMOIR XL] THE FORCE INCLUDED IN PLANTS. 177 
 
 MEMOIR XI. 
 
 OF THE FORCE INCLUDED IN PLANTS. 
 
 Collected and condensed from Memoirs in the Journal of the Franklin Institute and 
 Harper's Monthly Magazine. 
 
 CONTENTS : Growth of a seed in darkness and in light. Action of 
 plants and animals respectively on the atmosphere. Examination of 
 Rumford's experiments. Dark heat rays cannot decompose carbonic 
 acid. Germination in colored rays. Greening of leaves takes place in 
 the yellow and adjacent rays. The essential condition of all chemical 
 changes by radiation is absorption. Plants absorb force from the sun; 
 it is associated with their combustible parts, and is disengaged by oxi- 
 dation. 
 
 I HAVE given in Memoir II. a description of the struct- 
 ure of an ordinary flame. Its light is derived from par- 
 ticles of solid carbon issuing from combustible matters 
 with which the wick or the gas jet is fed ; these solid 
 particles, passing from a low temperature to a white 
 heat, and undergoing eventually complete oxidation, es- 
 cape into the atmosphere as carbonic-acid gas. 
 
 We now encounter a question of imposing interest: 
 Whence has the force which thus manifests itself as heat 
 and light been derived ? Force cannot be created ; it 
 cannot spring forth spontaneously out of nothing. 
 
 It may be said, without much error, of such flame-giv- 
 ing compounds as we are here considering, that they are 
 for the most part compounds of carbon with hydrogen. 
 To regard them as such will very much simplify the 
 facts we have now to present. 
 
 Under the form of oils and fats these combustible 
 substances are derived directly or indirectly from the 
 vegetable world ; directly, as, for instance, in the case of 
 
 M 
 
IfS THE FORCE INCLUDED IN PLANTS. [MEMOIR XL 
 
 olive-oil ; indirectly, as in the case of animal oils and fats. 
 These have been collected by^ the animals from which 
 we obtain them out of their vegetable food. Even fats 
 derived from the carnivora have been procured from the 
 herbivora, and came originally from plants. 
 
 This brings us therefore to a consideration of the 
 chemical facts connected with the life of plants. 
 
 If a seed be planted in moist earth, the air having ac- 
 cess and the temperature that of a pleasant spring day, 
 germination in the course of a few hours will take place. 
 Should the process be conducted in total darkness, as in 
 a closet, the young plant shoots upward, pale, or at most 
 of a faint tawny tint. We can easily verify this state- 
 ment by placing a few turnip seeds in a flower-pot con- 
 taining earth, put into a closet or drawer from which 
 light has been carefully excluded. 
 
 A sickly-looking plant thus springs from a seed in the 
 dark. It is etiolated, as botanists say. If we examine 
 it carefully, making allowance for the water it contains, 
 we shall find that no matter how tall it may be, its 
 weight has not increased beyond the original weight of 
 the seed from which it came. It has been developing at 
 the expense of the seed, the substance of which has been 
 suffering exhaustion for its supply of nourishment. We 
 cannot continue this development in the dark indefinite- 
 ly, for the seed-supply is soon exhausted, and then the 
 shoot dies. 
 
 But if instead of exposing the seed which is the sub- 
 ject of our experiment to darkness, we cause the germi- 
 nation to take place in the open clay, a very different 
 train of consequences ensues. There is no longer that 
 immoderate extension of a sickly etiolated stem upward, 
 but the parts emerging into the light turn green. Very 
 soon, to use a significant expression, they are weaned 
 from the seed ; they no longer use the material collected 
 
MEMOIR XI.] THE FORCE INCLUDED IN PLANTS. 179 
 
 for them in the preceding year by the parent plant, and 
 stored up for their use, but their leaves, expanding, turn 
 green, and expose themselves to receive the rays of the 
 sun. If they be now examined as in the previous in- 
 stance, making allowance for the water they contain, it 
 will be found that from day to day their weight is in- 
 creasing; they are living independently of the seed. 
 They are obtaining carbon and hydrogen, the former 
 from carbonic acid, and the latter from water and am- 
 monia compounds existing in the air or furnished from 
 the ground. 
 
 If a seedling, germinated in darkness and permitted to 
 grow to a certain extent, be then exposed to light, pro- 
 vided its dark -life has not continued too long, its etio- 
 lated aspect will soon disappear ; it turns green, and as- 
 sumes all the characters of a healthy plant. This is in 
 effect the natural process. For we bury seeds a little 
 under the surface, covering them lightly with earth, the 
 opacity of which secures the necessary darkness ; but the 
 mould being moist, the air having a ready access, and 
 the temperature of the season suitable, all the conditions 
 needful for germination water, air, warmth, darkness 
 are present. The plumule, or shoot, makes its way out 
 of the obscurity into the light; its reliance for nutrition 
 on the seed ends; its independent life begins. It obtains 
 carbon, hydrogen, oxygen, nitrogen from the air, and sa- 
 line substances and water from the soil. 
 
 The facts which we thus bring into relief, as necessary 
 for the further exposition of the subject, are these : In 
 the first stage of the life of a plant, its dark -life, there 
 is, excluding water, a diminution in the weight; in the 
 second, or light-life, there is an increase, due very largely 
 to the appropriation of carbon from the air. The atmos- 
 pheric carbonic acid has been decomposed, its oxygen set 
 free and, for the most part, permitted to escape, its other 
 
13Q THE FORCE INCLUDED IN PLANTS. [MEMOIR XI. 
 
 constituent, carbon, now ministering to the growth of the 
 plant. 
 
 A stone trough standing in a garden received the 
 waste water from a pump. There had accumulated on 
 its sides a green slimy growth (conferva). From this 
 growth, on the west side of the trough, which was receiv- 
 ing the morning rays of the sun, bubbles of gas were con- 
 tinually forming ; and these, as they attained a sufficient 
 size, rose through the water and escaped into the air. 
 This effect on the west side diminished as the sun passed 
 towards the meridian, but at mid-day the north side of the 
 trough was in full activity. As evening came on, that in 
 its turn gave forth fewer bubbles, and was succeeded in 
 activity by the east side. At first it was thought that 
 these bubbles were nothing more than the gas which is 
 dissolved in all water, and analogous in composition to 
 atmospheric air, but closer examination showed that it 
 was oxygen, nearly pure. During the night no gas what- 
 ever was disengaged. 
 
 Priestley, Ingenhousz, Rumford, and other experiment- 
 ers of the last century investigated these facts carefully. 
 The conclusions to which they came may be thus sum- 
 marized : All ordinary natural waters contain carbonic 
 acid in solution ; leaves or other green parts of plants 
 placed in such water and kept in darkness exert no action 
 upon it, but in the sunshine they decompose the carbonic 
 acid, appropriating its carbon and setting its oxygen free, 
 as gas. Soon, however, the supply in the sample of water 
 is exhausted, and the action even in the sunlight ceases. 
 It is again resumed if more carbonic acid be artificially 
 dissolved in the water; and since the air expired from 
 the lungs in the act of breathing contains much of that 
 gas, it is sufficient, by the aid of a tube, or in any other 
 suitable manner, to conduct such expired air into the 
 water for the disengagement of oxygen to go on. 
 
MEMOIR XL] THE FORCE INCLUDED IN PLANTS. 
 
 The experiments of these earlier chemists had thus 
 established the important fact that from carbonic acid, 
 which is extensively diffused through the atmosphere 
 and in water, and even in the soil, through the influence 
 of sunlight, oxygen is obtained. The sunlight, then, is 
 the force which carries into effect the decomposition. 
 
 There is thus a perpetual drain on the supply of car- 
 bonic acid, a perpetual tendency to its diminution, and 
 hence, for the order of nature to continue, there must be 
 an incessant supply. The source of that supply was very 
 strikingly indicated by some of Priestley's experiments. 
 " Having rendered a quantity of air thoroughly noxious 
 by mice breathing and dying in it, he divided it into 
 two receivers inverted in water, introducing a few green 
 leaves into one, and keeping the other receiver unaltered. 
 The former was placed in light, the latter in darkness. 
 After a certain time he found that the air in the former 
 had become respirable, for a mouse lived very well in it ; 
 but that in the latter was still noxious, for a mouse died 
 the moment it was put into it." 
 
 To Priestley chiefly, though he was aided by other in- 
 vestigators, we must refer the honor of one of the great- 
 est discoveries of the last century. It was this, that the 
 two great kingdoms of nature, the animal and the vege- 
 table, stand at once in antagonism and alliance. What 
 is done by the one is undone by the other. Each is ab- 
 solutely essential to the existence of the other. There is 
 a never-ending cycle through which material atoms run. 
 Now they are in the atmosphere, then they are parts 
 of plants, then they are transferred to animals, and by 
 them they are conducted back to the atmosphere, to run 
 through the same cycle of changes again. The sunlight 
 supplies the force that carries them through these revo- 
 lutions. 
 
 Previously to 1834 I had turned my attention to this 
 
182 THE FORCE INCLUDED IN PLANTS. [MEMOIR XL 
 
 interesting subject. It had been asserted by Rumford 
 that many other substances besides the leaves of plants 
 would evolve oxygen gas. He specified raw silk and 
 cotton fibres. My investigation commenced by an exam- 
 ination of this assertion. I soon found that two totally 
 distinct things had been confounded. Ordinary water 
 contains, as has been said, carbonic acid in solution, but 
 it also necessarily contains the ingredients of atmospheric 
 air. To this, for the sake of distinctness, the perhaps in- 
 correct designation of water-gas may be given. Since 
 oxygen is very much more soluble than nitrogen, this 
 dissolved gas differs in composition from atmospheric 
 air. It is relatively richer in oxygen. 
 
 I very soon found, on exposing raw silk, spun glass, 
 and other such fibres, immersed in water, to the sun, that 
 Rumford's assertion was correct gas -bubbles were set 
 free; but his inference was incorrect the gas did not 
 come from decomposed carbonic acid ; it was merely the 
 water-gas of the water. I was thus able to separate the 
 true from the false portion of the experiment. And 
 though I thus dispose of the subject in a few words, it is 
 perhaps due to the labor that was expended to say that 
 it cost several weeks of uninterrupted work and many 
 scores of analyses before I felt absolutely certain that 
 this was the indisputable interpretation of Rumford's 
 experiments. 
 
 Now as a guide to a correct exposition of the experi- 
 ments that I have to relate, it must be borne in mind that 
 the assumption of a green color by a germinating plant 
 and the decomposition of carbonic acid by it are identical 
 events. Or, perhaps, to speak more correctly, the latter 
 is the cause, the former the effect. 
 
 At that time very incorrect views of the nature of the 
 sun -rays were entertained. It was believed that they 
 contained three distinct principles: (1) heat, (2) light, (3) 
 
MEMOIR XI.] THE FORCE INCLUDED IN PLANTS. 
 
 183 
 
 chemical or deoxidizing radiations. In common with all 
 other chemists I accepted this view, and proposed to my- 
 self to determine to which of these principles the decom- 
 position of carbonic acid and the greening of plant leaves 
 are due. 
 
 And first, to ascertain if it were the heat radiation, I 
 converged, by the aid of a large metallic mirror, the dark 
 or invisible radiations emitted by an iron stove on some 
 leaves placed in water. The gas which was set free was 
 nothing more than the water-gas; there was no decom- 
 position of carbonic acid. 
 
 In Fig. 22, a is the concave mirror, b an inverted flask 
 containing the leaves and 
 water; it dips into a glass, 
 c, also containing water, 
 and receives the reflected 
 radiations of the stove, dd. 
 
 Then I tried a similar 
 experiment, using the radi- 
 ations emitted by a bright- 
 ly burning wood fire. The 
 result was the same as the 
 preceding, and I even push- 
 ed the experiment so far 
 that the water became very 
 hot, a portion of its carbonic acid effervesced from it, and 
 the leaves lost their bright green color. Still no decom- 
 position of the carbonic acid could be detected. 
 
 At this time it was generally received that the essen- 
 tial characteristic of the more refrangible the violet 
 rays is that they produce deoxidation. In accordance 
 with this opinion a name deoxidizing had been given 
 them. Now since the decomposition of carbonic acid is 
 an effect of deoxidation, I was not surprised at the issue 
 of the foregoing experiments, and expected to find that 
 
184 THE FORCE INCLUDED IN PLANTS. [MEMOIR XL 
 
 though the less refrangible radiations, those of heat, 
 were inoperative, the more refrangible, the chemical or 
 deoxidizing, would decompose carbonic acid readily. 
 
 But some collateral experiments had thrown a diffi- 
 culty in the way of this conclusion. I had caused seeds 
 to germinate in three little closets (Fig. 21, page 172), into 
 which, by means of panes of colored glass, or through 
 troughs filled with colored liquids red, yellow, and vio- 
 let light respectively could be admitted I remarked 
 with very great surprise that the seeds in the red-light 
 closet and those in the violet one were just as much etio- 
 lated as they would have been had they grown in dark- 
 ness; those in the yellow closet promptly assumed a 
 green color, and developed themselves as well as if grow- 
 ing under natural circumstances. 
 
 But the light that comes through stained glass and 
 colored solutions is far from being homogeneous ; it con- 
 tains rays of many refrangibilities. I therefore deter- 
 mined to attempt the greening of plants and the decom- 
 position of carbonic acid by their leaves phenomena 
 which, as has been said, are equivalent in the solar 
 spectrum itself. 
 
 I arranged things so as to have a horizontal solar spec- 
 trum of several inches in length kept motionless by a he- 
 liostat. I had previously caused to germinate in a wooden 
 box filled with earth, and of corresponding length, a crop 
 of seeds. They were etiolated, or blanched, for the ger- 
 mination had taken place in the dark. These young 
 plants I placed so as to receive the spectrum. Very soon 
 those that were in the yellow space turned green, but 
 those in the extreme red and extreme violet underwent 
 no change, though the exposure might be kept up the 
 whole day. 
 
 In Fig. 23, a a is the box containing the germinating 
 seeds, and placed so as to receive the colored spaces R, 
 
MEMOIR XI.] THE FORCE INCLUDED IN PLANTS. 
 R O Y G B I V 
 
 Fig. tt*. 
 
 O, Y, G, B, I, V red, orange, yellow, green, blue, indigo, 
 violet of the spectrum. 
 
 Many repetitions of this experiment satisfied me that 
 it is the yellow and adjacent regions of the spectrum 
 which occasion the greening of plants; the heat rays 
 and the chemical rays have nothing whatever to do 
 with it. 
 
 Next I attempted the decomposition of carbonic acid 
 in the spectrum, and succeeded. I read before the Amer- 
 ican Philosophical Society in Philadelphia, at its centen- 
 nial celebration in 1843, an account of this experiment 
 as published in Memoir X. 
 
 In addition to the special interest of these experiments 
 on plant life, they had a very important bearing on the 
 general principles of actino - chemistry. They proved 
 that it is altogether incorrect to suppose that chemical 
 changes are brought about by the more refrangible rays 
 only. They showed that every ray has its proper chem- 
 ical function ; for instance, the violet in the decomposi- 
 tion of compounds of silver, the yellow in the case of car- 
 bonic acid. And hence I proposed to abandon the con- 
 ception of a tripartite division of the spectrum into heat, 
 light, and chemical radiations, and to designate radia- 
 tions by their wave-lengths, or, better still, by their ntim- 
 
186 THE FORCE INCLUDED IN PLANTS. [MEMOIR XI. 
 
 ber of vibrations a method now universally adopted in 
 spectrum analysis. 
 
 By other experiments a narrative of which would be 
 too long for the present occasion I established this 
 result: that for any ray to produce a chemical effect, it 
 must be absorbed. For instance, when a ray has passed 
 through a mixture of chlorine and hydrogen gases, and 
 by causing them to unite has produced hydrochloric acid, 
 it can no longer produce the same effect if made to pass 
 through a second portion of the same mixture ; its act- 
 ing part has been detained or absorbed by the first. So, 
 too, the radiations which have fallen on a daguerreotype 
 plate, and impressed their image upon it, have lost the 
 quality of producing a similar effect on a second plate 
 that may be placed to receive them. Their active por- 
 tion has been taken up or absorbed by the first. The es- 
 sential preliminary of all chemical changes ly radiations 
 is absorption. 
 
 But it must not be supposed that the rays thus ab- 
 sorbed are annihilated or lost. They are simply held in 
 reserve, ready to be surrendered again, undiminished and 
 unimpaired, if the conditions under which they were ab- 
 sorbed are reversed. They may appear under some other 
 form as heat, electricity, motion but their absolute 
 energy remains unchanged. This is a necessary conse- 
 quence of the theory of the Conservation and Correla- 
 tion of Force. 
 
 From this point of view how interesting is that great 
 discovery made by Angstrom, that an ignited gas emits 
 the same rays it absorbs a discovery that explained 
 the Fraunhofer lines of the solar spectrum, and consti- 
 tuted an epoch in the history of spectrum analysis. 
 
 I have now presented the facts that are requisite for 
 answering the question proposed on one of the foregoing 
 
MEMOIR XL] THE FORCE INCLUDED IN PLANTS. 
 
 pages : " Whence has the force which manifests itself as 
 heat and light in a flame been derived? Force cannot 
 be created ; it cannot spring forth spontaneously out of 
 nothing." 
 
 The answer is, it came from THE SUN. 
 
 Under the influence of his rays the growing plant de- 
 composed carbonic acid obtained from the atmosphere, 
 appropriating its carbon and setting its oxygen free. To 
 accomplish this decomposition, this appropriation, it was 
 necessary that a portion of the energy contained in those 
 rays should be absorbed. Associated with this, the car- 
 bon could now form part of the plant, and, indeed, con- 
 stituted the solid basis of which it was composed. 
 
 But the force thus associated with the carbon atoms 
 was not annihilated ; it was only concealed : through 
 countless ages it might remain in this latent state, ready 
 at any moment to come forth. All that is requisite is 
 to oxidize the carbon, to turn it into carbonic acid, and 
 the associated energy, under the form of heat and light, 
 is set free. 
 
 When we read by gas or by the rays of a petroleum 
 lamp, the light we use was derived from the sun perhaps 
 millions of years ago. The plants of those ancient days, 
 acting, as plants do now, under the influence of sunshine, 
 separated carbon from the carbonic acid of the atmos- 
 phere by associating it with the radiant energy they had 
 absorbed, and this remained for an indefinite time en- 
 closed, as it were, in the now combustible material, ready 
 to be disengaged as soon as the reverse action, oxidation, 
 takes place, returning then to commingle as heat with 
 the active forces of the world. 
 
 Much of what has here been said applies to hydrogen 
 as well as to carbon. Hydrogen is derived, under similar 
 conditions, from the decomposition of water or ammonia. 
 When its oxidation recurs, it delivers up, under the form 
 
THE FORCE INCLUDED IN PLANTS. [MEMOIR XI. 
 
 of heat, the energy it had absorbed. As, however, I am 
 here speaking of the source of light in flames, in which 
 carbon takes the leading and hydrogen only an indirect 
 or subordinate part, it is not necessary to trace in further 
 detail the action of the latter element. 
 
 A very interesting illustration of the principles here 
 under consideration occurs in the case of the decomposi- 
 tion of water by an electric current. The constituents 
 of the water, hydrogen and oxygen, are set free in the 
 gaseous form. But for them to assume that form they 
 must be furnished with caloric of elasticity. The current 
 supplies them with this, and, indeed, the decomposition 
 can only go on at the rate which is regulated by that 
 supply. The heat they have thus assumed remains in- 
 sensible in them, imparting to them their elastic or gas- 
 eous condition, until they are caused to reunite and re- 
 form water, when it is at once given up. The part that 
 is played by that portion of the electric current which is 
 thus transformed into heat and furnishing their caloric 
 of elasticity to the evolving gases is absolutely essential 
 to the decomposition has been hitherto too much over- 
 looked by chemists. 
 
 Nature thus offers us in the instance we have been 
 considering in this Memoir a striking illustration of the 
 transmigration of matter and of force. Plants obtain 
 
 o 
 
 carbon from the atmosphere ; it constitutes the basis of 
 their combustible portions. Sooner or later it suffers ox- 
 idation, turns back into the condition of carbonic acid, 
 and is diffused again into the atmosphere. There is a 
 never-ending series of cycles through which it runs : now 
 it is in the air, now a part of a plant, now back again 
 in the air. And the same is true as regards the energy 
 with which it was associated. Derived from the sun- 
 beam, it lay hidden in the plant, awaiting re-oxidation ; 
 then it was delivered, escaping under the form of heat or 
 
MEMOIR XL] THE FORCE INCLUDED IN PLANTS. 
 
 light, and remingling with the universal cosmic force 
 from which it had been of old derived by the sun, or 
 from which, perhaps more correctly speaking, the sun 
 himself was derived, for he is the issue of nebular con- 
 densation. 
 
 I cannot close this Memoir without making reference 
 to a point of surpassing interest. What goes on in the 
 case of a flame, goes on in the case of an animal. Either 
 from other animals or from plants, combustible material 
 is obtained and used as food. Directly or indirectly it 
 undergoes oxidation in the system, brought about by 
 the air introduced through the process of respiration. 
 Speaking in a general manner, though there are many 
 intermediate products, the issue of this chemical action 
 is the evolution of carbonic acid, ammonia, water, which 
 pass into a common receptacle, the atmosphere. Thence 
 their ingredients are taken by plants, and, under the 
 agency of the sunlight, combustible material food is 
 re-formed. The same particle is, therefore, now in the 
 air, now in the plant, now in the animal, now back again 
 in the air. It suffers a perpetual transmigration. 
 
 But in the case of an animal the oxidation may not be 
 so sudden, so complete, as it is in the case of a flame. It 
 may, and indeed generally does, go on stage by stage, 
 step by step, partial oxidations occurring. It is thus 
 that, from one original hydrocarbon, a long catalogue of 
 fatty and oily substances may arise ; the inevitable issue, 
 however, is total oxidation. As the partial degradations 
 go on, in corresponding degrees the latent energy or force 
 is set free. It may assume any correlated form, as mus- 
 cular motion ; or, as heat, it may give warmth to the 
 body; in certain fishes, as the gyinnotus, it may turn 
 into an electrical or nerve current; in certain insects, 
 such as the fire-fly, into light. 
 
 As a cataract is only a form which any river may as- 
 
190 THE FORCE INCLUDED IN PLANTS. [MEMOIR XL 
 
 sume if it comes to a precipitous descent a form which, 
 though it may be outwardly unchanging, is interiorly 
 never for two successive moments the same, for it is per- 
 petually fed from above and is wasting away below so 
 the flame of a lamp is only a form, the aspect of which is 
 determined by its environment. The changes it is under- 
 going issue in the liberation, the escape, of force, chiefly 
 under the aspect of light and heat. Its life is very tran- 
 sitory. It dies out as soon as the oil that fed it is ex- 
 hausted. We blow upon it, and it passes into nonentity. 
 And so, too, with an animal, the appearance of identity 
 it presents is altogether deceptive. At no two succes- 
 sive moments are its parts the same. In a very short 
 time all the old have been removed, and new ones have 
 taken their places. The force that it derived from its 
 food has been manifested in various ways, such as mus- 
 cular motion or heat. But the material particles have 
 not been destroyed ; they have merely gone back into 
 the atmosphere, and will be used by nature for the fab- 
 rication of other plant and animal forms over and over 
 again. And so, too, the energy they have displayed it 
 has not ceased to exist ; the heat, for instance, that once 
 vivified them has merely mingled with that of the outer 
 world, and is ready to discharge its special functions 
 again and again. In the world there is thus an unceas- 
 ing transmigration of matter, an unceasing transmigra- 
 tion of force. 
 
MEMOIR XII.] THE MAGNETIC EFFECTS OF LIGHT. 
 
 MEMOIR XII. 
 
 EXPERIMENTS TO DETERMINE WHETHER LIGHT PRODUCES 
 ANY MAGNETIC EFFECTS. 
 
 From the Journal of the Franklin Institute, Feb., 1835. 
 
 CONTENTS: Mrs. Somerville 1 s experiments. Christie" 1 's experiment. 
 Their results cannot be substantiated. The violet ray has no effect. 
 Blue glasses and blue ribbons also ineffective. 
 
 " THE more refrangible rays of light are said to pos- 
 sess the property of rendering iron and steel magnetic. 
 The existence of this property was first asserted by Dr. 
 Morichini of Rome. Other observers subsequently failed 
 in obtaining the same results, but in the year 1825 the 
 fact appeared to be decisively established by the learned 
 and accomplished Mrs. Somerville in an Essay published 
 in the Transactions of the Royal Society. In her experi- 
 ments sewing-needles were rendered magnetic by expos- 
 ure for two hours to the violet ray, and the magnetic 
 virtue was communicated in still shorter time when the 
 violet rays were concentrated by a lens. The indigo 
 rays were found to possess a magnetizing power almost 
 to the same extent as the violet; and it was observed, 
 though in a less degree, in the blue and green rays. It 
 is wanting in the yellow, orange, and red. Needles were 
 likewise rendered magnetic by the sun's rays transmitted 
 through green and blue glass. These results have been 
 verified by M. Zantedeschi, of Pavia (Bill. Univ., May, 
 1829), but their accuracy has been doubted by Riess 
 and Moser, who consider that the means employed by 
 Mrs. Somerville for ascertaining the magnetic state of 
 
192 THE MAGNETIC EFFECTS OF LIGHT. [MEMOIR XII. 
 
 the needles were not sufficiently exact. They found the 
 oscillation of the needles to be wholly unaffected by ex- 
 posure to the prismatic colors (JZrewster^s Journal, Vol. 
 II., p. 225, N. S.). This must still be regarded, there- 
 fore, as one of the disputed points in science" (Turner's 
 Chemistry). 
 
 It has been supposed that these contradictory results 
 arose entirely from local circumstances. A hazy atmos- 
 phere, such as is met with in the northern and middle 
 countries of Europe, might perhaps influence in some 
 manner this peculiar property of light when the clearer 
 sky of Italy permitted the experiment to succeed. Some, 
 indeed, have thought that the observers who were said 
 to have verified the alleged results were deceived in not 
 having previously ascertained the magnetic state of the 
 needles they used. 
 
 During the past summer (1834) I have attempted to 
 satisfy myself whether the more refrangible rays really 
 exert any magnetic influence; and happening to reside 
 in the south of Virginia, on the same parallel of latitude 
 as Tunis and the more northerly African kingdoms, I 
 thought the situation too favorable to suffer such an op- 
 portunity to pass without endeavoring to gain some in- 
 formation on this contested point. 
 
 In 1824 Mr. Christie found that a needle six inches 
 long, contained in a brass compass -box with a glass 
 cover, suspended by a hair and made to vibrate alter- 
 nately shaded and exposed to the sun, came to rest much 
 sooner in the latter than in the former case. That this 
 was not occasioned by an increase of temperature was 
 proved by the needle vibrating more rapidly when its 
 temperature was raised by other means. 
 
 On repeating this experiment, I very quickly found 
 that it depended in a great measure on the mode of 
 suspension of the needle, and its position with respect to 
 
MEMOIR XII.] THE MAGNETIC EFFECTS OF LIGHT. 193 
 
 the incident light, what results would be obtained. If 
 the needle was suspended on a point, or by a thread 
 without torsion, the time and the number of vibrations 
 were the same whether the needle was exposed to the 
 sunbeam or not. But if the needle was suspended by a 
 hair or other organic substance having torsion, the sun- 
 beam would occasion a twist in the hair on its first 
 exposure to light ; and if the direction of that twist hap- 
 pened to coincide with the direction of the needle's 
 motion, the momentum of the needle was increased and 
 the vibrations continued longer. A needle which vi- 
 brated forty-four times in one minute would occasional- 
 ly, owing to this cause, vibrate forty-six when suspended 
 by a hair; but if by a silk fibre, its vibrations were al- 
 ways forty-four, the first arc of vibration being in every 
 instance 40. 
 
 Thinking to obtain more decisive effects, I concen- 
 trated the sunbeam with a lens on the south pole of the 
 suspended needle, and found that the needle was thrown 
 into a rapid, tremulous motion. But here the hot air 
 ascending from the needle acts upon it as upon the sail 
 of a windmill ; and the same effect ought to take place 
 to a certain extent on simple exposure of the half of a 
 vibrating needle to direct light. But I found that a 
 needle suspended in the vacuum of an air-pump by a 
 thread without torsion is in no way affected by exposure 
 to solar light. 
 
 It is said in the account of Christie's experiment that 
 the needle was contained in a brass compass -box. It 
 might have been that electrical currents were excited in 
 that box which were the cause of the derangement in 
 question. I therefore vibrated a needle under similar 
 circumstances, with the result above stated. I should 
 mention that this was done in a solid cylinder or ring 
 of brass without any seam or soldered junction ; but as 
 
 N 
 
194 
 
 THE MAGNETIC EFFECTS OF LIGHT. [MEMOIR XII. 
 
 Fig. 24. 
 
 compass-boxes are generally made of sheet-brass with a 
 soldered seam in the side, it was possible that the fine 
 line of solder acted with the brass as a thermo-electric 
 couple, capable of excitation by the warmth of the sun- 
 beam. I therefore made a compound cylinder, half of 
 
 copper, the other half of zinc, c z, 
 Fig. 24, the edges of which, at a 
 and b respectively, are soldered to- 
 gether, one junction, , being pol- 
 ished, the other, #, blackened. The 
 needle was suspended in an ex- 
 hausted receiver by a silk fibre, </, 
 ^ and a ray of light, rf, coming from 
 an aperture half an inch wide in 
 the shutter fell upon the junction. 
 The needle used in this experiment 
 was of watch-spring. Its first vi- 
 bration was performed in an arc of 40, and when the 
 compound cylinder was taken away it made thirty-two 
 vibrations in sixty seconds in vacuo. On placing it con- 
 centrically with the compound cylinder, and suffering 
 the ray to impinge on the polished junction, the moment 
 that the arc of vibration had become 40 the number of 
 oscillations in one minute was observed ; six experiments 
 gave severally thirty -two. On turning the blackened 
 junction to the light the result was still thirty-two, and 
 on substituting the solid brass cylinder three consecutive 
 trials gave thirty -two. The thermometer stood in the 
 sunshine at 103. 
 
 By some this magnetic action of light has been at- 
 tributed to the violet or more refrangible rays only. A 
 needle of watch-spring about four inches long, which in 
 an exhausted receiver suspended by a filament of silk 
 exhibited no polarity, had one half exposed to the violet 
 ray dispersed by a flint-glass prism. This ray was sepa- 
 
MEMOIR XII.] THE MAGNETIC EFFECTS OF LIGHT. 
 
 rated from others of the spectrum by passing through a 
 slit in a metallic screen, and half the needle was screened 
 from its action by a piece of paper. After two hours' 
 exposure to the sun it was suspended again in the ex- 
 hausted receiver, but still showed no polarity; it was 
 then exposed to the other rays successively, with the 
 same result. The needle was now slightly touched, and, 
 slowly vibrating, arranged itself in the magnetic merid- 
 ian. The first vibration was performed in a semicircular 
 arc, the number of vibrations in one hundred seconds 
 was twenty -seven. But after four hours' exposure to 
 the violet ray, as before, no evidence of any change 
 either increasing or diminishing the number of oscilla- 
 tions could be obtained. A beam of violet light passing 
 through a disk of stained glass was concentrated on one 
 end of a sewing-needle by means of a lens without pro- 
 ducing any change in the number of vibrations made in 
 one minute. This needle on some occasions would, how- 
 ever, give different results : when its first vibration was 
 performed in a semicircle, the number varied from forty- 
 one to forty-three in sixty seconds. On vibrating it in 
 vacuo, its results uniformly gave the latter number very 
 nearly. 
 
 The position of the needle to the incident ray is not 
 of any consequence, whether it receives it obliquely, in 
 the direction of the light, or across it. If soft iron be 
 substituted for steel, the results are still negative, even 
 if the needle be arranged in the magnetic meridian, the 
 line of dip, or any other position. I therefore came to 
 the conclusion that the violet ray exerts no influence on 
 the magnetic needle, and that all the other rays are 
 equally inert. 
 
 But as Mrs. Somerville found that a needle placed 
 under a piece of glass or blue ribbon, having half its 
 length protected by paper, became in a short time mag- 
 
196 THE MAGNETIC EFFECTS OF LIGHT. [MEMOIR XII. 
 
 netic, I tried the same experiment, but in every instance 
 failed to make the needle magnetic. When suspended 
 by a silk fibre in vacuo needles showed no disposition 
 to arrange themselves in any particular direction, and 
 when they came to rest were found cutting the magnetic 
 meridian at every angle, though the temperature of the 
 sunbeam to which they had been exposed on one occa- 
 sion was 124. Great care was taken to ascertain the 
 previous non- magnetic state of the needles, and they 
 were suspended by a fibre without torsion. 
 
MEMOIR Xlll.] EXPERIMENTS MADE IN VIRGIN 
 
 7 /> 
 
 c ^/> 
 
 MEMOIR XIII. 
 
 AN ACCOUNT OF SOME EXPERIMENTS ON THE L 
 THE SUN, MADE IN THE SOUTH OF VIRGINIA. 
 
 Abstract from the Journal of the Franklin Institute of Philadelphia for June, July, 
 August, and September, 1837 ; Philosophical Magazine, Feb., 1840. 
 
 CONTENTS: Absorption of luminous radiations. The reference spec- 
 trum. Absorption of heat radiations / the apparatus employed. Ab- 
 sorption of chemical radiations. Screen of bromide of silver. Color- 
 ation of chloride and bromide of silver by radiations that have passed 
 through correspondingly colored solutions. Early application of pho- 
 tography to the investigation of physical problems. Crystallization of 
 camphor towards the light. The side of vessels towards the sky is the 
 colder. 
 
 IF a beam of the sun's light be passed through a so- 
 lution of chromate of potassa, it can no longer blacken a 
 piece of sensitive paper paper covered over with chlo- 
 ride or bromide of silver. If the light which has thus 
 passed through a stratum of this liquid be converged by 
 means of a lens, the chloride of silver will remain for a 
 long time without change in the focus. 
 
 I made many such experiments with a view of deter- 
 mining the effect of absorbent media on the luminous, 
 calorific, and chemical rays. Such, at that time, was the 
 accepted subdivision of the solar radiations. A ray of 
 the sun was caused to pass through a trough with paral- 
 lel sides containing the absorbent solutions, and for the 
 sake of exactness a reference spectrum was employed, 
 such as is now used in spectroscopic experiments. In 
 this manner the particular luminous rays absorbed by 
 any given solution could be determined, the absorbed 
 
198 
 
 EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII. 
 
 Fig. 25. 
 
 heat -rays could be ascertained by an air- thermometer, 
 and the acting chemical rays by papers made sensitive 
 by chloride of silver. 
 
 Into a darkened chamber, the shutter of which is rep- 
 resented in section at A A, Fig. 25, a beam of the sun's 
 
 light is made to pass hori- 
 zontally by means of a mir- 
 ror of silvered glass, B. The 
 mirror I use is one belong- 
 ing to a solar microscope, 
 and by turning the milled 
 screws, e e, it can be brought 
 into any position required 
 to throw a beam horizontally into the room. A brass 
 tube,/, two inches in diameter, can be screwed into the 
 position figured. There is also a lens, a, Fig. 31, which 
 may be introduced ; its focus is nine inches, its diameter 
 about two inches, and the diameter of the sun's image it 
 gives nearly -^ of an inch. 
 
 A piece of sheet-lead about a quarter of an inch thick 
 is to be cut into the form of a horse-shoe of such size 
 that a circle one inch in diameter may be inscribed in it. 
 Upon this lead two pieces of glass are cemented, so as to 
 form a trough for containing liquids. In Fig. 26, a a is 
 the wooden basis or foot of the trough ; 
 c c c, the leaden horse -shoe; b b, the 
 glass plates. 
 
 A thin metallic plate three or four 
 inches square, having a longitudinal slit 
 in it about an inch long 
 and ^ f an i nc h wide, is 
 to be provided. In Fig. 27, a a is the slit. 
 The lens, $, Fig. 31, having been removed, 
 a beam of light is thrown horizontally into 
 the room, as at Fig. 25 ; the slit, Fig. 27, 
 
 Fig. 26. 
 
 Fig. 27. 
 
MEMOIR XIII.] EXPERIMENTS MADE IN VIRGINIA. 
 
 199 
 
 being placed as at a, Fig. 25, a narrow streak of light 
 passes through. The trough, Fig. 26, is then placed 
 behind in such a position that half the light coming 
 through the slit may pass through the liquid in the 
 trough, and the other half pass by its side unintercepted. 
 Behind the trough is placed a flint-glass prism, as in Fig. 
 25, and further still a white pasteboard screen,/; a is 
 the slit plate, b the trough, d the prism. 
 
 The action of this arrangement is as follows: The 
 beam of light cast by the mirror into the room is inter- 
 cepted, except the small portion that passes the slit. Of 
 this a part passes through the trough and a part on one 
 side of it. Two beams of light, therefore, fall on the 
 prism, one of which has passed through the trough 
 and suffered absorption, the other has not There are, 
 therefore, on the pasteboard screen two spectra side 
 by side. 
 
 Suppose, for example, that the trough is filled with 
 distilled water ; the two spectra on the screen are alike, 
 as in Fig. 28. If it be filled with a solution of chromate 
 of potassa, in that coming from the light that has passed 
 through the trough the blue, indigo, and violet rays are 
 missing, as in Fig. 29. If the trough be filled with am- 
 monia-sulphate of copper, the red, the orange, the yellow 
 have disappeared or been absorbed, as in Fig. 30. The 
 
 Fig. 28. 
 
 Fig. 29. 
 
 Fig. 30. 
 
 reference or undisturbed spectrum enables us to deter- 
 mine what rays have been absorbed with precision. 
 For the investigation of the absorption of heat the 
 
200 
 
 EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII. 
 
 Fig ' 31 ' 
 
 following apparatus was used. The mirror being placed 
 
 on the shutter, as in Fig. 31, a 
 plano-convex lens, $, is screwed 
 into the tube so as to bring the 
 rays to a focus on one of the 
 bulbs of a delicate differential 
 thermometer ; this gives the heat 
 of the sunbeam as concentrated 
 by the lens. To find the effect 
 of any liquid medium in absorbing heat-rays, the trough 
 filled with the substance under trial is placed as at <?, 
 Fig. 31. The cone of rays, converging from the lens, a, 
 on the blackened bulb, &, forms an image upon it, and 
 the differential thermometer yields a corresponding in- 
 dication. In trying different solutions the same trough 
 is always to be used, so that the solutions may always 
 be of the same thickness. It is also requisite to cover 
 the thermometer with a very thin shade of clear glass, 
 e 0, to prevent disturbance from air-currents. 
 
 I may select as examples the following : A solution of 
 sulphate of copper and ammonia, absorbing the red and 
 yellow light, transmitted twenty rays out of every hun- 
 dred that fell on it. 
 
 A thin stratum of pitch enclosed between two plates 
 of crown-glass, and transmitting a homogeneous red 
 light, but absorbing all the other colors of the spec- 
 trum, allowed only 19 per cent, of the heat to pass 
 through it. 
 
 In examinations of the transmissive powers of vapors 
 and gases, a cubical bottle was used instead of the 
 trough. The vapor of iodine was thus found to absorb 
 two thirds of the heat falling on it ; but the same bottle 
 filled with nitrous acid, and which, therefore, \vas in a 
 stratum of the same thickness, permitted much more 
 heat to pass. 
 
MEMOIR XIII.] EXPERIMENTS MADE IN VIRGINIA. 201 
 
 For the investigation of chemical radiations, there was 
 placed upon the screen,/, Fig. 25, a paper covered with 
 bromide of silver. It darkened in those parts on which 
 the more refrangible rays fell. 
 
 Or, having removed the differential thermometer and 
 its shade, Fig. 31, the cone of light converging from the 
 lens passed through a solution of sulphate of copper and 
 ammonia in the trough, and a piece of paper painted 
 with chloride of silver was placed so as to receive the 
 focus. Though but little heat was transmitted through 
 the solution, a dark spot was at once produced charac- 
 teristic of the blackening of the silver salts by the sun- 
 rays. Though, therefore, this double salt transmits the 
 rays of heat with difficulty, the rays of chemical action 
 pass with facility. If in the trough there be placed a 
 strong solution of bichromate of potassa, a far greater 
 quantity of light will pass, and much more heat ; but a 
 paper painted with chloride of silver being placed in the 
 focus, no chemical change whatever goes on, the chloride 
 retaining its usual whiteness. 
 
 NOTE. The foregoing experiments were made be- 
 tween 1834 and 1837. The reader of this volume must 
 not regard them from the present elevated view of Radi- 
 ations. At that time very little respecting these radi- 
 ations was known. They present to me personally this 
 point of interest that they were the beginning of a 
 series of researches to which I devoted many subsequent 
 years. 
 
 The following is a list of solutions possessing an ab- 
 sorptive action on the more refrangible or chemical radi- 
 ations : 
 
202 EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII. 
 
 Bichromate of potassa, 
 
 Chromate of potassa, 
 
 Yellow hydrosulphide of nmmonia, 
 
 Hydrosulphide of lime, 
 
 Chloride of iron, 
 
 Chloride of gold, 
 
 Chloride of platinum. 
 
 It is to be remarked that all these solutions are yel- 
 low. I also found that a great many vegetable colored 
 infusions, especially those which had a yellow tint, would 
 in like manner absorb the chemical rays. 
 
 When pieces of paper covered with chloride or bro- 
 mide of silver were exposed to a beam which had passed 
 through a solution of red sulpho- cyanide of iron, the 
 paper became of a brick-red color ; if to a beam which 
 had passed through a solution of ammonia- sulphate of 
 copper, it became of a blue-brown ; and on exposing a 
 piece for five days to light acted on by bichromate of 
 potassa, it became perceptibly of a faint yellowish green. 
 
 A beam which has passed through bichromate of po- 
 tassa does not cause the union of a mixture of chlorine 
 and hydrogen. I kept such a mixture for several hours 
 in it, and could not perceive any change. 
 
 Ritter asserted that the opposite extremities of the 
 spectrum possess opposite powers of chemical action : he 
 states that phosphorus will emit fumes in the red ray, 
 but if the violet be thrown on it, it ceases to smoke. 
 This experiment I repeated often, and under favorable 
 circumstances, but could not make it succeed. 
 
 I could succeed, however, in showing very beautifully 
 the interference of that class of chemical rays which 
 ' blacken chloride and bromide of silver, but failed for 
 want of proper apparatus in trying to produce their 
 polarization. An electric current circulating in a wire 
 does not seem to have any influence on these chemical 
 rays. I found that the same neat magnified image of 
 the wire was obtained on chloride paper when it was 
 
MEMOIR XIII.] EXPERIMENTS MADE IN VIRGINIA. 
 
 203 
 
 placed in a beam diverging from a lens, whether the 
 current was made to pass or was stopped. 
 
 NOTE. These were instances of the application of 
 photography to the solution of physical problems long 
 before the announcement of the discoveries of Daguerre 
 or Talbot. 
 
 The following are some mechanical results of solar 
 light: 
 
 (a.) Having made a large air-pump jar clean and dry, 
 a few pieces of camphor were placed on the plate of the 
 pump, and the jar exhausted. The pump and its re- 
 ceiver were then set in the sunshine, and very soon all 
 that side towards the sun was covered with crystals ; but 
 few or none were on the farther side. 
 
 (&.) A torricellian vacuum having been made in a 
 tube half an inch or more in diameter, and upwards of 
 thirty inches long, a fragment of camphor was passed up 
 through the mercury into it. The tube might be kept 
 for any length of time in the dark without anything 
 happening, but on bringing it into the beams of the sun 
 crystallization in a few minutes took place on that side 
 of the tube next the luminary. 
 
 (tf.) On the inside of an air-pump 
 jar a circle of tinfoil an inch in diam- 
 eter was pasted ; and having oper- 
 ated as in experiment (a), that side 
 was exposed to the sun. Crystals 
 soon formed, but the tinfoil protected 
 the glass in its vicinity, and none 
 were found within a certain space 
 round the metallic circle. Fig. 32. Fig. 32. 
 
204 EXPERIMENTS MADE IN VIRGINIA. [MEMOIR XIII. 
 
 (<?.) Crystallization is not necessarily connected with 
 these results; the vapor of mercury in a torricellian 
 void is condensed towards the light; so also the dew 
 which settles on the inside of a jar containing water 
 is always on the side nearest the window. The rays of 
 the sun have also the power of decomposing chloride of 
 gold : the metalline spangles are deposited on the side 
 of the glass nearest to the light. 
 
 Artificial light gives none of these results. 
 
 (<?.) Having removed the piece of tinfoil used in ex- 
 periment (V), it was placed on a stand in front of the re- 
 ceiver; it hindered crystallization taking place on the 
 parts on which its shadow was cast, and also for a cer- 
 tain space in the vicinity. 
 
 (jf.) A piece of tinfoil was placed before a jar that had 
 been already coated with crystals. It removed all those 
 crystals that were within its shadow. 
 
 (</.) Instead of using a piece of tinfoil as in experiment 
 (<?), the receiver was made hot, and a piece of resin 
 rubbed upon it, so as to leave a transparent circle of 
 that substance on it. On exposure to the light it was 
 found that the resin could not protect the glass. 
 
 (^.) Along the inside of a vessel about to be exposed 
 to the sun a glass rod was rubbed. Rows of crystals 
 were deposited on the lines described by the end of the 
 rod. But the vessel must be very dry for this experi- 
 ment to succeed. This curious fact was first 
 observed in the case of an exhausted vessel 
 having a small siphon gauge shut up in it, 
 the extremity of which rested against the 
 glass. By accident the gauge was moved 
 half round the glass, and shortly afterwards a 
 line of crystals was observed coinciding with 
 the line of motion. The appearance was such as is rep- 
 resented in Fig. 33. 
 
MEMOIR XIII.] EXPERIMENTS MADE IN VIRGINIA. 205 
 
 Now can we explain these singular results, excepting 
 the last, on any other known principle than this : that 
 the side of the jar nearest the sun radiates freely the 
 heat it receives back again, while radiation is interfered 
 with at the other side ? that, in point of fact, the anterior 
 side is the colder, and the other side the hotter? 
 
206 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV. 
 
 MEMOIR XIV. 
 
 EXAMINATION OF THE PROCESS OF DAGUERREOTYPE. 
 
 NOTE ON LUNAR PHOTOGRAPHY. 
 
 From the Philosophical Magazine, Sept., 1840; Smithsonian Contributions to 
 Knowledge, No. 180. 
 
 CONTENTS : Description of the process. Cause of the deposition of mer- 
 cury and production of the light parts of the picture. Polishing of 
 the plate. The operation of iodizing. Effect of keeping the iodide. 
 The achromatic lens. Reduction of focal length in the non- achro- 
 matic. The development. Fixing by hyposulphite and galvanism. 
 Necessity of heating the plates. Lunar impressions. Artificial light. 
 Note on lunar photography The first photographs of the moon. 
 
 MORE than one hundred instances are recorded in Ber- 
 zelius's Chemistry in which the agency of light brings 
 about changes in bodies; these are of all kinds: for- 
 mations of new compounds, re-arrangements of elements 
 already in union, changes of crystallographic character, 
 decompositions, and mechanical modifications. 
 
 The process of the daguerreotype is to expose a sur- 
 face of pure silver to the action of the vapor of iodine, so 
 as to give rise to a peculiar iodide of silver, which under 
 certain circumstances is exceedingly sensitive to light. 
 The different operations of polishing, washing with nitric 
 acid, exposure to heat, etc., are only to secure a pure silver 
 surface ; the operation of hyposulphite of soda, and the 
 process, which I shall presently describe, of galvaniza- 
 tion, are to free the plate from its sensitive coating, and 
 in nowise affect the depth of the shadows, as some of 
 the French chemists at first supposed. 
 
 There is but one part of the daguerreotype which 
 
MEMOIR XIV.] THE PKOCESS OF DAGUERREOTYPE. 207 
 
 does not yield to theory : on one point alone there is ob- 
 scurity. Why does the vapor of mercury condense in a 
 white form on those portions of the film of iodide which 
 have been exposed to the influence of light ? condense 
 to an amount almost proportional to the quantity of 
 incident light? 
 
 Even on this point there are facts which appear to 
 have a bearing. 
 
 (a.) It has long been known that if a piece of soap- 
 stone or agalmatolite be made use of as a pencil to write 
 with on glass, though the letters that may have been 
 formed are invisible, and though the surface of the glass 
 may subsequently have been well cleaned, yet they will 
 come into view as soon as the glass is breathed on. 
 
 (&.) I have often noticed that if a piece of very clear 
 and cool glass, or, what is better, a cold polished metallic 
 reflector, has a little object such as a piece of metal laid 
 upon it, and the surface be breathed over once, the object 
 being then carefully removed, as often as you breathe 
 again on the surface a spectral image of it may be seen, 
 and this singular phenomenon may be exhibited for 
 many days after the first trial was made. 
 
 (<?.) Again, in the common experiment of engraving on 
 glass by hydrofluoric acid, if the vapor has been very 
 weak, no traces will be perceived on the glass after the 
 wax has been removed ; but on breathing over it, the 
 moisture condenses in such a way as to bring the object 
 into view. 
 
 (d.) In a former Memoir (XIH.) I described a phe- 
 nomenon relating to the crystallization of camphor on 
 surfaces of dry glass, on which invisible traces have 
 been made by the pressure of a glass rod ; this also ap- 
 pears to belong to the same class of effects. 
 
 Berzelius (jBvz&?,VoL II., p. 186) has attempted to ex- 
 plain (a) and (<?) on this principle, that the changed and 
 
208 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV. 
 
 unchanged surfaces radiate heat unequally. There may 
 be strong doubts as to the correctness of this, but is not 
 the daguerreotype due to the same cause, whatever it 
 may be ? 
 
 We must separate carefully the chemical changes 
 which iodide of silver undergoes in the sunbeam from 
 the mechanical changes which happen to the sensitive 
 film : iodide of silver turns black in the solar ray the 
 whole success of the daguerreotype artist depends on 
 his checking the process before that change shall have 
 supervened. 
 
 The coating of iodine is not immediately necessary to 
 the production of images by the mercurial vapor. The 
 condition seems to be traceable to the metallic surface. 
 If you take a daguerreotype, clean off the mercury, polish 
 the plate thoroughly with rottenstone, wash it with nitric 
 acid, and bring it to a brilliant surface, yet if it has not 
 been exposed to heat, the original picture will reappear 
 on exposure to the mercurial vapor. Is not this a result 
 of the same kind as those just referred to ? 
 
 As a polishing material for the daguerreotype plate, 
 common rottenstoue and oil answer very well. The 
 plate having been planished by the workman, is to be 
 rubbed down to a good surface, and as high a polish 
 given to it as possible; it is to be heated and washed 
 with nitric acid, as indicated in the French account, and 
 finished by being rubbed with whiting (creta prcepa- 
 rata) in the state of a very fine powder, going over it for 
 the last time with a piece of clean dry cotton ; this 
 gives an intensely black lustre, that cannot be obtained 
 by rottenstone alone, arid thoroughly removes any film 
 which nitric acid may have left. 
 
 To coat with iodine, I make use of a box about two 
 inches deep, in the bottom of which that substance in 
 coarse flakes is deposited ; no cloth intervenes, but the 
 
MEMOIR XIV.] THE PROCESS OF DAGUERREOTYPE. 209 
 
 silvered plate, with a temporary handle attached to it, is 
 brought within half an inch of the crystals, and it be- 
 comes perfectly coated in the course of from one to three 
 minutes ; no metallic strips are necessary to insure this 
 effect ; if the edges and corners are thoroughly clean, the 
 golden hue will appear uniformly. 
 
 M. Daguerre recommends that the plate, after being 
 iodized, shall be placed in the camera without loss of 
 time. The longest interval, he says, ought not to exceed 
 an hour. " Beyond this space the action of the iodine 
 and silver no longer possesses the requisite photogenic 
 properties." 
 
 There may be something peculiar in the preparation 
 of the plate as I have described it, but it is certain that 
 this observation must be received with some limitation. 
 A plate which has been iodized does not appear so 
 quickly to lose its sensitiveness. On the other hand, by 
 keeping it in the dark for twelve or twenty-four hours, 
 its sensitiveness is often remarkably increased. Other 
 advantages also accrue. Those who have made many of 
 these photogenic experiments will have had frequent oc- 
 casion to remark that the film of iodine is not equally 
 sensitive all over, that there are spots or cloudy places 
 which do not evolve any impression, and often the whole 
 is in that condition that the bright parts alone come out, 
 while the parts that are in shadow do not evolve corre- 
 spondingly, nor can they be well developed, except at the 
 risk of solarizing the picture. Now a plate that has 
 been kept for several hours is by no means so liable to 
 these effects: I do not pretend to give any reason for 
 this, but merely mention it as a fact, of considerable im- 
 portance to the travelling daguerreotyper ; he will find 
 that the iodine does not lose its sensitiveness in many 
 days. 
 
 In a paper read before the Royal Society, Herschel 
 
 O 
 
210 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV, 
 
 states that there is an absolute necessity of a perfect 
 achromaticity in the object-glass of a photographic cam- 
 era. M. Daguerre appears to have been under the same 
 impression, and recommends in his published account 
 such an object-glass. 
 
 All the rays of light, with perhaps the exception of the 
 yellow, leave an impression on the iodide of silver. The 
 less refrangible rays, however, act much more slow T ly 
 than those at the opposite end of the spectrum. In the 
 common kinds of glass, the most energetic action takes 
 place in the indigo, or on the boundary of the blue. 
 Now the retina receives an impression with equal facility 
 from each of the different rays, the yellow light acting as 
 quickly upon it as the red or the blue. Vision is there- 
 fore performed independently of time, the eye catching 
 all the colors of the spectrum with equal facility and 
 with equal speed. But it is not so with these photo- 
 genic preparations. In the action of light upon them, 
 time enters as an element ; the blue ray may have effect- 
 ed its full change, while the red is yet only beginning 
 slowly to act; and the red may have completed its 
 change before the yellow has made any sensible impres- 
 sion. On these principles, it is plain that an achromatic 
 object-glass is by no means essential for the production 
 of fine photographs ; for if the plate be withdrawn at a 
 certain period, when the rays that have a maximum 
 energy have just completed their action, those that are 
 more dispersed but of slower effect will not have had 
 time to leave any stain. We work, in fact, with a tem- 
 porary monochromatic light. 
 
 Upon these principles I constructed the camera I am 
 in the habit of using, with a double convex non-achro- 
 matic lens. Some of the finest proofs were procured 
 with a common spectacle lens, of fourteen inches focus, 
 arranged at the end of a cigar-box as a camera ; a lens 
 
MEMOIR XIV.] THE DAGUERREOTYPE. 211 
 
 of this diameter answers very well for plates four inches 
 by three, reproducing the objects with the most admi- 
 rable finish, copper-plate engravings being represented in 
 the minutest particulars, and the marks of the tool be- 
 coming quite distinct under a magnifier. 
 
 In this instance, it is true, owing to the magnitude of 
 the focal length compared with the aperture, but little 
 difficulty ensues from chromatic aberration ; but when 
 with the same focal length the aperture is increased to 
 three or four inches, the dispersion becomes very sen- 
 sible, and yet good proofs can be procured by working 
 in the method here indicated, the chief difficulty then 
 arising from spherical aberration. 
 
 It has already been stated that the ray of maximum 
 action for the daguerreotype, when colorless French 
 plate-glass is used, lies probably within the indigo space: 
 it therefore follows that the length of the camera should 
 be diminished, after arranging it to the luminous focus. 
 The importance of this has been pointed out in a paper 
 by Mr. Towson ; I was, however, in the habit of using 
 that adjustment before reading the suggestions contain- 
 ed in his communication. The amount of shortening to 
 be given to the camera, where the lens is fifteen inches 
 focus, does not commonly exceed three tenths of an inch. 
 If the luminous focus be used, the proof comes out indis- 
 tinctly. 
 
 In the subsequent process of mercurializing, it is of lit- 
 tle importance what the angular position may be. Sev- 
 eral experimenters were for a time under the idea that 
 an angle of 45 or 48 is a necessary inclination, in order 
 that the plate should take the vapor; this arose from a 
 misinterpretation of the printed account. Plates mercu- 
 rialize equally well in a horizontal as in any other posi- 
 tion ; perhaps a slight inclination may be of advantage, 
 in allowing the vapor to flow with uniformity over the 
 
212 THE DAGUERREOTYPE. [MEMOIR XIV. 
 
 iodized surface, but the chief use of an angle of 45 is to 
 allow the operator to inspect the process through the 
 glass of the mercury-box. 
 
 Sometimes it is advantageous to heat the mercury a 
 second time, when the proof is not distinctly evolved at 
 first. Indeed, it occasionally happens that a proof which 
 did not evolve at all at first will come out quite fairly 
 on raising the temperature of the mercury again. 
 
 M. Daguerre recommends two methods of removing 
 the sensitive coating from the plate by washes of hypo- 
 sulphite of soda, and by a solution of common salt. The 
 former answers perfectly, the second only indifferently 
 well. There is, however, another process, which is very 
 simple, and has an advantage over the former of these in 
 cheapness. It adds not a little to the magic of the whole 
 operation, in the eyes of those who are unaccustomed to 
 chemical results. The plate, having been dipped into 
 cold water, is placed into a solution of common salt, of 
 moderate strength ; it remains without being acted upon 
 at all; but if it be now touched on one corner with a 
 piece of zinc, which has been scraped bright, the yellow 
 coat of iodide moves off like a wave and disappears. It 
 is a very pretty process. The zinc and silver forming 
 together a voltaic couple, with the salt water intervening, 
 oxidation of the zinc takes place, and the silver surface 
 commences to evolve hydrogen gas; while this is in a nas- 
 cent condition it decomposes the film of iodide of silver, 
 giving rise to the production of hydriodic acid, which is 
 very soluble in water, and hence instantly removed. 
 
 This process, therefore, differs from that with hyposul- 
 phite. The latter acts by dissolving the iodide of silver, 
 the former by decomposing it. It is necessary not to 
 leave the zinc in contact too long, or it deposits stains, 
 and in large plates the contact should be made at the 
 four corners successively, to avoid this accident. 
 
MEMOIR XIV.] PICTURES OF THE MOON. 213 
 
 After the proof is washed, all the defects in the prep- 
 aration of the plate become apparent. If a film of mer- 
 cury has existed on it, due to its not having been burned 
 sufficiently long, there will be found a want of distinct- 
 ness in the shadows ; or if the plate has not been burn- 
 ed at all, perhaps the former impressions will reappear. 
 This accident frequently happened in my earlier trials, 
 when care had not been taken to give a due exposure 
 each time to the spirit-flame. Spectral appearances of 
 former objects, on different parts of it, emerged an in- 
 terior with Paul Pry coming out, when the camera had 
 been pointed at a church. 
 
 There is no difficulty in procuring impressions of 
 the moon by the daguerreotype beyond that arising 
 from her motion. By the aid of a lens and a heliostat, 
 I caused the moonbeams to converge on a plate, the lens 
 being three inches in diameter. In half an hour a very 
 strong impression was obtained. With another arrange- 
 ment of lenses I obtained a stain nearly an inch in diam- 
 eter, and of the general figure of the moon, in which the 
 places of the dark spots might be indistinctly traced. 
 
 An iodized plate, being exposed for fifteen seconds 
 only close to the flame of a gas-light, was very distinctly 
 stained ; in one minute there was a very strong impres- 
 sion. 
 
 On receiving the image of a gas-light, eight feet dis- 
 tant, in the camera, for half an hour, a good representa- 
 tion was obtained. 
 
 The flame of a gas-lamp was arranged within a magic- 
 lantern, and a portion of the image of a grotesque on 
 one of the slides received on a plate ; a very good repre- 
 sentation was procured. 
 
 With Drummond's light, and the rays from a lime-pea 
 in the oxy-hydrogen blowpipe, the same results were ob- 
 tained. 
 
214 LUNAR PHOTOGRAPHY. [MEMOIR XIV. 
 
 NOTE ON LUNAR PHOTOGRAPHY. To the foregoing 
 paragraph respecting photographs of the moon, I may 
 here add an extract from a publication by my son, 
 Dr. Henry Draper, in the Smithsonian Contributions to 
 Knowledge, No. 180, p. 33, entitled, " On the Construction 
 of a Silvered Glass Telescope, and its Use in Celestial 
 Photography :" 
 
 "The first photographic representations of the moon 
 ever made were taken by my father, Professor John W. 
 Draper, and a notice of them published in his quarto 
 work 4 On the Forces that Organize Plants,' and also in 
 the September number (1840) of the London, Edin- 
 burgJi, and Dublin Philosophical Magazine. He pre- 
 sented the specimens to the New York Lyceum of Nat- 
 ural History. The Secretary of that association has 
 sent me the following extract from their minutes : 
 
 "< March 23d, 1840. Dr. Draper announced that he 
 had succeeded in getting a representation of the moon's 
 surface by the daguerreotype. . . . The time occupied was 
 twenty minutes, and the size of the figure about one inch 
 in diameter. Daguerre had attempted the same thing, 
 but did not succeed. This is the first time that any- 
 thing like a distinct representation of the moon's surface 
 has been obtained. ROBERT H. BROWNNE, Secretary. 1 ' : 
 
MEMOIR XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY. 215 
 
 MEMOIR XV. 
 
 ON THE TAKING OF PORTRAITS FROM LIFE BY PHO- 
 TOGRAPHY. 
 
 From the Philosophical Magazine, September, 1840. 
 
 CONTENTS: History of the invention. First attempts by whitening the 
 face. Use of reflecting mirrors. Use of a blue-colored trough. Kind 
 of camera necessary. The seat or support. The background. Ap- 
 propriate dresses. Ladies' 1 dresses. Arrangement of the shadow. 
 Reflecting camera. 
 
 HISTORICAL NOTE. This Memoir contains the first 
 published description of the process for taking daguer- 
 reotype portraits. Of late, since the introduction of col- 
 lodion, this art has been much cultivated and improved. 
 It now forms an important branch of industrial occupa- 
 tion. That it was possible by photogenic processes, such 
 as the daguerreotype, to obtain likenesses from the life, 
 was first announced by the author of this volume in a 
 note to the editors of the Philosophical Magazine, dated 
 March 31, 1840, as may be seen in that journal for June, 
 1840, p. 535. The first portraits to which allusion is made 
 in the following Memoir were produced in 1839, almost 
 immediately after Daguerre's discovery was known in 
 America. 
 
 In the Edinburgh Review for January, 1843, there is 
 an important article on Photography. In that the in- 
 vention of the art of taking photographic portraits is 
 attributed to its true source the author of this book. 
 It says : " He was the first, we believe, who, under the 
 brilliant summer sun of New York, took portraits by 
 the daguerreotype. This branch of photography seems 
 
216 TAKING OF PORTRAITS BY PHOTOGRAPHY. [MEMOIR XV. 
 
 not to have been regarded as a possible application of 
 Daguerre's invention, and no not;ce is taken of it in the 
 reports made to the legislative bodies of France. We 
 have been told that Daguerre had not at that period 
 taken any portraits; and when we consider the period 
 of time twenty or twenty -five minutes which was 
 then deemed necessary to get a daguerreotype landscape, 
 we do not wonder at the observation of a French author, 
 who describes the taking of portraits as ' Toujours un 
 terrain un pen fabuleux pour le Daguerreotype? " 
 
 Very soon after M. Daguerre's remarkable process for 
 photogenic drawing was known in America, I made at- 
 tempts to accomplish its application to the taking of 
 portraits from the life. M. Arago had already stated in 
 his address to the Chamber of Deputies that M. Da- 
 guerre expected by a slight advance to meet with suc- 
 cess, but as yet no account had reached us of that object 
 being attained. 
 
 In the first experiments I made for obtaining portraits 
 from the life, the face of the sitter was dusted with a 
 white powder, under an idea that otherwise no impres- 
 sion could be obtained. A very few trials showed the 
 error of this ; for even when the sun was only dimly 
 shining there was no difficulty in delineating the feat- 
 ures. 
 
 When the sun, the sitter, and the camera are situated 
 in the same vertical plane, if a double convex non-achro- 
 matic lens, four inches in diameter and fourteen in focus, 
 be employed, perfect miniatures can be obtained in the 
 open air, in a period varying with the character of the 
 light, from twenty to ninety seconds. The dress is admi- 
 rably given, even if it should be black ; the slight differ- 
 ences of illumination are sufficient to characterize it, as 
 well as to show each button, button-hole, and every fold. 
 
MEMOIR XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY. 217 
 
 Partly owing to the intensity of such light, which can- 
 not be endured without a distortion of the features, but 
 chiefly owing to the circumstance that the rays descend 
 at too great an angle, such pictures have the disadvan- 
 tage of not exhibiting the eyes with distinctness, the 
 shadow from the eyebrows and forehead encroaching on 
 them. 
 
 To procure fine proofs, the best position is to have the 
 line joining the head of the sitter and the camera so ar- 
 ranged as to make an angle with the incident rays of less 
 than ten degrees, so that all the space beneath the eye- 
 brows shall be illuminated, and a slight shadow cast from 
 the nose. This involves obviously the use of reflecting 
 mirrors to direct the ray. A single mirror would answer, 
 and would economize time, but in practice it is often con- 
 venient to employ two; one placed, with a suitable mech- 
 anism, to direct the rays in vertical lines; and the second 
 above it, to direct them in an invariable course towards 
 the sitter. 
 
 On a bright day, and with a sensitive plate, portraits 
 can be obtained in the course of five or seven minutes, in 
 the diffused daylight. The advantages, however, which 
 might be supposed to accrue from the features being 
 more composed, and of a more natural aspect, are more 
 than counterbalanced by the difficulty of retaining them 
 so long in one constant mode of expression. 
 
 But in the reflected sunshine the eye cannot support 
 the effulgence of the rays. It is therefore absolutely 
 necessary to pass them through some blue medium, which 
 shall abstract from them their heat, and take away their 
 offensive brilliancy. I have used for this purpose blue 
 glass, and also ammonia-sulphate of copper, contained in 
 a large trough of plate glass, the interstice being about 
 an inch thick, and the fluid diluted to such a point as to 
 permit the eye to bear the light, and yet to intercept 
 
218 TAKING OF PORTRAITS BY PHOTOGRAPHY. [MEMOIR XV. 
 
 no more than was necessary. It is not requisite, when 
 colored glass is employed, to make use of a large sur- 
 face ; for if the camera operation be carried on until the 
 proof almost solarizes, no traces can be seen in the por- 
 trait of its edges and boundaries; but if the process is 
 stopped at an earlier interval, there will commonly be 
 found a stain, corresponding to the figure of the glass. 
 
 The camera I have used, though much better ones 
 might be constructed, has for its objective two double 
 convex lenses, the united focus of which for parallel rays 
 is only eight inches ; they are four inches in diameter in 
 the clear, and are mounted in a tube, in front of which 
 the aperture is narrowed down to 3 inches, after the 
 manner of Daguerre's. 
 
 The chair in which the sitter is placed has a staff at 
 its back, terminating in an iron ring, which supports the 
 head, so arranged as to have motion in directions to suit 
 any stature and any attitude. By simply resting the 
 back or side of the head against this ring, it may be kept 
 sufficiently still to allow the minutest marks on the face 
 to be copied. The hands should never rest upon the 
 chest, for the motion of respiration disturbs them so 
 much as to make them of a thick and clumsy appear- 
 ance, destroying also the representation of the veins on 
 the back, which, if they are held motionless, are copied 
 with surprising beauty. 
 
 It has already been stated that certain pictorial advan- 
 tages attend an arrangement in which the light is thrown 
 upon the face at a small angle. This also allows us to 
 get rid entirely of the shadow on the background, or to 
 compose it more gracefully in the picture; for this it is 
 well that the chair should be brought forward from the 
 background from three to six feet. 
 
 Those who undertake daguerreotype portraitures will 
 of course arrange the backgrounds of their pictures ac- 
 
MEMOIR XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY. 219 
 
 cording to their own tastes. When one that is quite 
 uniform is desired, a blanket, or a cloth of a drab color, 
 properly suspended, will be found to answer very well. 
 Attention must be paid to the tint : white, reflecting too 
 much light, would solarize upon the proof before the face 
 had had time to come out, and owing to its reflecting all 
 the different rays, a blur or irradiation would appear on 
 all edges, due to chromatic aberration. It will be readily 
 understood, that if it be desired to introduce a vase, an 
 urn, or other ornament, it must not be arranged against 
 the background, but brought forward until it appears 
 perfectly distinct on the ground-glass of the camera. 
 
 Different parts of the dress, for the same reason, re- 
 quire intervals differing considerably, to be fairly copied 
 the white parts of a costume passing on to solarization 
 before the yellow or black parts have made any decisive 
 representation. We have therefore to make use of tem- 
 porary expedients. A person dressed in a black coat 
 and open waistcoat of the same color must put on a tem- 
 porary front of a drab or flesh color, or by the time that 
 his face and the fine shadows of his woollen clothing are 
 evolved, his shirt will be solarized, and be blue or even 
 black, with a white halo around it. Where, however, 
 the white parts of the dress do not expose much surface, 
 or expose it obliquely, these precautions are not essential; 
 the white shirt collar will scarcely solarize until the face 
 is passing into the same condition. 
 
 Precautions of the same kind are necessary in ladies' 
 dresses, which should not be selected of tints contrasting 
 strongly. 
 
 It will now be readily understood that the whole art 
 of taking daguerreotype miniatures, consists in directing 
 an almost horizontal beam of light, through a blue-col- 
 ored medium, upon the face of the sitter, who is re- 
 tained in an unconstrained posture, by an appropriate 
 
220 TAKING OF PORTRAITS BY PHOTOGRAPHY. [MEMOIR XV. 
 
 but simple mechanism, at such a distance from the back- 
 ground, or so arranged with resppct to the camera, that 
 his shadow shall not be copied as a part of his body ; 
 the aperture of the camera should be three and a half or 
 four inches at least, indeed the larger the better, if the 
 objective be aplanatic. 
 
 If two mirrors be made use of, the time actually occu- 
 pied by the camera operation varies from forty seconds to 
 two minutes, according to the intensity of the light. If 
 only one mirror is employed, the time is about one fourth 
 shorter. In the direct sunshine, and out in the open air, 
 the time varies upwards from half a minute. 
 
 Looking-glasses which are used to direct the solar 
 rays after a short time undergo a serious deterioration, 
 the silvering assuming a dull granular aspect, and losing 
 its black brilliancy. Hence the time, in copying, becomes 
 gradually prolonged. 
 
 The arrangement of the camera above indicated gives 
 reversed pictures, the right and left sides changing places. 
 Mr. Woolcott, an ingenious mechanician of this city, has 
 taken out a patent for the use of an elliptical mirror for 
 portraiture; it is about seven inches in aperture, and 
 allows him to work conveniently with plates two inches 
 square. The concave mirror possesses this capital advan- 
 tage over the convex lens, that the proof is given in its 
 right position, that is to say, not reversed; but it has the 
 serious inconveniences of limiting the size of the plate, 
 and representing parts that are at all distant from the 
 centre in a very confused manner. With the lens, plates 
 might be worked a foot square, or even larger. 
 
 Miniatures procured in the manner here laid down are 
 in most cases striking likenesses, though not in all. They 
 give, of course, all the individual peculiarities a mole, a 
 freckle, a wart. Owing to the circumstance that yellow 
 and yellowish browns are long before they impress the 
 
MEMOIR XV.] TAKING OF PORTRAITS BY PHOTOGRAPHY. 221 
 
 substance of the daguerreotype, persons whose faces are 
 freckled all over give rise to the most ludicrous results, a 
 white portrait mottled with just as many black dots as 
 the sitter had yellow ones. The eye appears beautifully : 
 the iris with sharpness, and the white dot of light upon 
 it with such strength and so much of reality and life as 
 to surprise those who have never before seen it. Many 
 are persuaded that the pencil of the painter has been 
 secretly employed to give this finishing touch. 
 
 UNIVERSITY OF NEW YORK, September, 1840. 
 
222 CONDITION or A DAGUERREOTYPE SURFACE. [MBMOIK xvi. 
 
 MEMOIR XVI. 
 
 ON THE CHEMICAL CONDITION OF A DAGUEEREOTYPE 
 SURFACE. 
 
 From the Philosophical Magazine, September, 1841. 
 
 CONTENTS: Mercury exists all over a daguerreotype surface. There is 
 no superposition of the parts. The shadows have metallic mercury ; 
 the lights silver amalgam. No iodine is ever evolved from the plate. 
 Action of a solution of gum and one of gelatine in tearing off the 
 films. The starch experiment. The etching of daguerreotypes. 
 
 As many of the results to be given in the next Mem- 
 oir depend on the use of daguerreotype tablets, I shall 
 in this examine the chemical and physical condition of 
 those surfaces and offer proof of the following facts : 
 
 1st. That metallic mercury exists all over the surface 
 of an ordinary daguerreotype in the shadows as well 
 as in the lights in the shadows as metallic mercury, 
 in the lights as silver amalgam. 
 
 2d. That in an iodized daguerreotype as taken from 
 the mercury-bath there is no order of superposition of 
 the parts, that is to say, the iodide is neither upon nor 
 beneath the mercury, but both are, as it were, in the same 
 plane. 
 
 3d. That when a ray of light falls upon the surface of 
 this preparation, through all the intervening steps, and 
 up to the point of maximum action, no iodine is evolved 
 from the plate, but that in the common daguerreotype 
 the light communicates a tendency to the atoms of the 
 iodide to yield up to the mercurial vapor their silver, 
 while the iodine retires and combines with the tinaf- 
 
MKMOIR XVI.] CONDITION OF A DAGUERREOTYPE SURFACE. 223 
 
 fected silver beneath. It follows that when such a plate 
 is withdrawn from the mercurial vapor, there is all over 
 it a uniform film of iodide of silver of the very same 
 thickness as at first, and that this has happened through 
 a direct corrosion of the silver by the iodine, while it 
 was undergoing the mercurial operation. 
 
 I pass at once to the proofs of these several propo- 
 sitions, and, 1st, That metallic mercury exists all over the 
 surface of an ordinary daguerreotype, in the shadows as 
 well as in the lights in the shadows it is as metallic mer- 
 cury, in the lights as silver amalgam. 
 
 I took a plated copper three inches by four in surface, 
 and having prepared it with care, I exposed half of it to 
 the diffused light of the day, screening the other half; it 
 was then mercurialized at 170 Fahr., the iodide removed 
 by hyposulphite of soda, and washed. And now, a plate 
 on which a gold-leaf was spread was placed over it, but 
 separated, as shown in Fig. 34, in 
 the points a, b, c, by three slips of i i *i i *rr 
 
 glass. By means of a spirit-lamp 
 the photographic plate d e was heated, and the gilded 
 plate g k kept cool by occasionally wetting it. On part- 
 ing the plates, it was perceived that faint but distinct 
 traces of whitening were visible all over the gold, as well 
 on that part which was over the whitened half of the 
 photograph as over that which was unchanged. 
 
 But as it might happen that the mercury diffused 
 itself laterally past the imperfect obstacle Z>, I made the 
 following decisive trials : 
 
 I iodized three silver plates, A, B, C, each three inches 
 by four in surface, conducting the processes for each in the 
 same way ; and having exposed each for two minutes to a 
 faint daylight, I laid them aside in the dark, to be pres- 
 ently used as test-plates in lieu of the gilded plate (g K). 
 
224 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIR XVI. 
 
 Then I took three other plates, D, E, F, of the same 
 size, and conducting the preparatory processes for each 
 as before, I iodized D in the dark, and mercurialized it 
 forthwith at 170 Fahr., taking the utmost care that not 
 a ray of light should be suffered to impinge upon it. 
 
 E was iodized and exposed for two minutes to dif- 
 fused daylight, and then mercurialized at 170 Fahr. 
 
 F was iodized and exposed to the sun until it began 
 to turn brown, an effect occurring almost at once. It 
 was then mercurialized at 170 Fahr. 
 
 All these plates then had their sensitive coating re- 
 moved by hyposulphite, and were thoroughly washed in 
 distilled water and dried. 
 
 I had, therefore, three plates, representing accurately 
 the conditions proposed to be investigated. D was in 
 the condition of the most perfect shadows; E in that of 
 the highest lights, and F solarized. In appearance, D 
 was black, E was white, and F bluish-gray. 
 
 Upon D, E, F, I placed A, B, C, respectively, separat- 
 ing each pair of plates one sixteenth of an inch, or there- 
 abouts, by slips of glass. Then I laid them on the level 
 surface of the sand-bath, the test-plates being kept cool 
 by sponging occasionally with water. Temperature of 
 the sand, 200 Fahr. ; duration of the experiment, fifteen 
 minutes. 
 
 On examination, A, B, C, were all found powerfully 
 mercurialized, nor did there seem to be any difference 
 between them. 
 
 I consider, therefore, that the shadows, the demi-tints, 
 the lights, and the solarized portions of a daguerreotype, 
 are covered with mercury ; for at a temperature of 200 
 Fahr. they all evolve it alike, a sufficiency of vapor rising 
 from the parts that have not been exposed to the light 
 to bring a plate that has been so exposed to its max- 
 imum of whiteness. 
 

 MEMOIR XVI.] CONDITION OF A DAGUERREOTYPE 
 
 In Memoir XIV. I described a remarkable 
 I had noticed in these investigations, that if an object 
 such as a wafer be laid upon a piece of cold glass or j 
 metal, arid you breathe once on it, and as soon as tije V\ 
 moisture has disappeared remove the object and breathe -. 
 again on the glass, a spectral image of the wafer will 
 make its appearance. The impression thus communi- 
 cated to the surface, under certain conditions, remains 
 there a long time. During the cold weather last winter 
 I produced such an image on the mirror of my heliostat ; 
 it could be revived by breathing on the metal many 
 weeks afterwards, nor did it finally disappear until the 
 end of several months. 
 
 I do not at present know what is the explanation of 
 this result, but the analogy between it and the arrange- 
 ment of mercurial globules covering the surface of a da- 
 guerreotype is too striking to be overlooked. It proves 
 that surfaces may assume such a condition as to affect 
 the deposition of vapors upon them, so as to produce 
 the reproduction of appearances of external forms. I 
 gave, therefore, particular attention to this point, but 
 eventually found that silver exists in an ordinary da- 
 guerreotype, in connection with the mercury all over the 
 plate, in a less proportion in the shadows, and in a 
 greater proportion in the lights. This result was, how- 
 ever, only obtained after the following fact was discov- 
 ered that the mucilage of gum-arabic, when slowly 
 dried in a thin layer on the surface of a daguerreotype, 
 splits up in shivers, bringing along with it the white 
 portions of the picture, and leaving the plate clean. 
 
 Having therefore prepared three plates, D, E, F, ex- 
 actly as before, I poured on them a solution of gum, 
 drained them so as to leave only a small quantity, and 
 let them dry slowly over the sand-bath. The gum sepa- 
 rated readily, and lay in chips on the surface of each 
 
 P 
 
226 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIK XVI. 
 
 plate; it was easily removed to three sheets of paper, by 
 tapping with the finger on the back of the plate. Each 
 was then treated alike as follows : 
 
 The gummy matter was incinerated on a platinum 
 leaf, and the remaining ashes transferred to a test-tube 
 half an inch in diameter. One drop of nitric acid and 
 one drop of water were added ; it was boiled over a 
 small flame, and diluted with a little water. Dilute hy- 
 drochloric acid was now added, and the chloride of silver 
 immediately fell. In repeating this, it is necessary to 
 attend to the state of dilution of the acid, for if too 
 strong it wholly dissolves the minute quantity of chlo- 
 ride of silver generated. 
 
 As, from the minuteness of that quantity, it was 
 impossible to obtain a direct quantitative analysis, I 
 adopted the foregoing method, and added the dilute 
 acid to all three tubes at the same time. In D there 
 was a faint opalescence, in E and F a cloud ; but I 
 could not always determine whether the deposit of E 
 or F was most copious, sometimes the one and some- 
 times the other appearing to have a slight advantage. 
 
 I conclude, therefore, that while the whole surface of 
 the plate is coated with mercury, it exists as silver amal- 
 gam chiefly in the lights, and as uncombined mercury 
 chiefly in the shadows, and in a mixed proportion in the 
 demi-tiuts; and that when a plate is solarized, both free 
 mercury and amalgam are present. 
 
 Such is the state of surface in a daguerreotype, recently 
 formed. In the course of time, however, a great portion 
 of the mercury that is in the shadows, and also free in 
 the lights, evaporates aw r ay. When the picture has thus 
 changed, the shadows are metallic silver, and the lights 
 silver amalgam. 
 
 2d. That in an iodized daguerreotype, as taken from 
 the mercury -bat] i, there is no order of superposition of the 
 
MEMOIR XVI.] CONDITION OF A DAGUERREOTYPE SURFACE. 927 
 
 parts, that is to say, the iodide is neither upon nor beneath 
 the mercury, but both are, as it were, in the same plane. 
 
 Soon after I had ascertained the action of gum-arabic, 
 some of it was applied to the surface of a plate on which 
 an impression had just been formed in the mercury-bath. 
 This was without removing the coat of iodine. On dry- 
 ing it, the gum chipped up, as was expected, bringing 
 away with it all the lights of the picture, and leaving 
 a uniform coat of yellow iodide of silver beneath. It 
 seerns, therefore, that the film of iodide coheres more 
 strongly to the metal plate than the amalgam ; and, far- 
 ther, from this result we should judge that the amalgam 
 is on the surface of the iodide. 
 
 But this is not true; for on three different occasions 
 I found that when Russian isinglass was employed in- 
 stead of gum for the purposes presently to be related, 
 the isinglass, from its stronger cohesive power, chipped 
 off in the act of drying, tearing up the yellow film from 
 end to end of the plate, and leaving the amalgam con- 
 stituting the lights undisturbed. It is here to be under- 
 stood that this action takes place without the smallest 
 disturbance of the lights and demi-tints, the plate remain- 
 ing in all the beauty and brilliancy and perfection that 
 it would have had if it had been carefully washed in 
 hyposulphite of soda. 
 
 This is a result, however, which cannot be produced 
 with uniformity. Most commonly the lights are torn 
 up with the iodide. Had it occurred but once, I should 
 still have cited it with decision, for from the very char- 
 acter of it, it is impossible to be mistaken or to commit 
 an error of judgment. It proves that the film of iodide 
 may be mechanically torn off from the metallic surface 
 as perfectly as it can be dissolved off by chemical agents 
 a singular fact. 
 
 This result, therefore, proving that we can tear off 
 
228 CONDITION OF A DAGUERREOTYPE SURFACE. [MEMOIR XVI. 
 
 the film of iodide and leave the amalgam, can only be co- 
 ordinated with that by gum-watgr, in which the amalgam 
 is removed and the iodide left, by supposing that there 
 is not anything like a direct superposition in the case, 
 and that the particles of amalgam and iodide lie, as it 
 were, side by side. 
 
 3d. That when a ray of light falls upon tlie surface 
 of this, etc. 
 
 There is no difficulty in proving this directly, and the 
 indirect evidence is copious. If we lay a piece of paper 
 imbued with starch on an iodized plate, and expose it 
 to the sun, although the plate presently assumes a dark 
 olive-green color, the starch remains uncolored. 
 
 This dark substance is probably a subiodide of silver; 
 the iodine therefore which has been disengaged from it, 
 not having been set free, must have necessarily united 
 with the adjacent metallic silver this, for very obvious 
 reasons, there is no difficulty in admitting. 
 
 Now, therefore, when a photogenic impression existing 
 on the surface of a plate in an invisible state is brought 
 out by the action of mercury vapor, we easily understand 
 how this is effected. No iodine is ever evolved. But 
 each atom of iodide of silver that has been acted on by 
 the light yields to the attraction of the mercury its atom 
 of silver, and the iodine thus set free unites with the me- 
 tallic silver particles around it, reproducing the same 
 yellow iodide by a direct corrosion of the plate: the 
 proofs that we have of this are two in number: 
 
 1st. Dry some mucilage of gum-arabic on a daguerre- 
 otype just brought from the mercury-bath ; when it has 
 split up, we perceive that the white amalgam of silver 
 is removed, and a uniform coat of yellow iodide of sil- 
 ver, of the very same thickness as at first, as is proved 
 by its color, is left. 
 
 2d. Dry upon the same plate a solution of Russian 
 
MEMOIR XVI.] CONDITION OF A DAGUERREOTYPE SURFACE. 229 
 
 isinglass, and when it has split up, it will be seen that 
 it uniformly rends off with it the yellow iodide, leaving 
 the metallic plate with an exquisite polish ; and wher- 
 ever the light has touched, there it is corroded. 
 
 These two facts, taken together, prove that in mercu- 
 rializing a plate no iodine is evolved, but that a new film 
 of iodide of the same thickness is formed, at the expense 
 of the metallic surface. 
 
 From these facts we readily gather that on the pres- 
 ence of the metallic silver the sensitiveness of this prep- 
 aration mainly depends, for to the tendency which the 
 light has impressed on the elements of the iodide to sep- 
 arate is added the strong attraction of metallic silver for 
 nascent iodine. 
 
 This corrosion or biting in of the silver plates, by the 
 conjoint action of the mercury and iodine, gives rise to 
 etchings that have an inexpressible charm. Could any 
 plan be hit upon of forcing the iodine to continue its ac- 
 tion, the problem of producing engraved daguerreotypes 
 would be solved. By another process, which will be 
 described hereafter, I have succeeded in producing deep 
 etchings from daguerreotypes. 
 
 UNIVERSITY OF NEW YORK, September, 1811. 
 
230 CHEMICAL KAYS AND RADIANT HEAT. [MEMOIR XVII. 
 
 MEMOIR XVII. 
 
 ON SOME ANALOGIES BETWEEN THE PHENOMENA OF THE 
 CHEMICAL KAYS AND THOSE OF KADIANT HEAT. 
 
 From the Philosophical Magazine, September, 1841. 
 
 CONTENTS: The chemical rays are absorbed. Photographic effects are 
 transient. The chemical rays are not conducted ; they become latent. 
 Optical qualities control chemical action. The active rays are ab- 
 sorbed and the complementary reflected. Relation of optical conditions 
 and chemical affinities. 
 
 IT is the object of this Memoir to establish some strik- 
 ing analogies existing between the phenomena of the 
 chemical rays and those of radiant heat. 
 
 Without saying anything respecting the laws of re- 
 flection, refraction, polarization, and interference, to which 
 the chemical rays are subject, the study of which I com- 
 menced more than five years ago on paper rendered sen- 
 sitive by bromide of silver, further than that a general 
 similitude holds in all these cases between the rays of 
 heat and the chemical rays, I shall at present confine my 
 observations to establishing the following propositions : 
 
 1st. That the chemical action produced by the rays of 
 light depends upon the absorption of those rays by sen- 
 sitive bodies; just as an increase of temperature is pro- 
 duced by the absorption of those of heat. 
 
 2d. That as a body warmed by the rays of the sun 
 gradually loses its heat by radiation, or conduction, or 
 contact with other bodies, so likewise, by some unknown 
 process, photographic effects produced on sensitive sur- 
 faces are only transient, and gradually disappear. 
 
MEMOIR XVII. ] CHEMICAL RAYS AND RADIANT HEAT. 231 
 
 3d. That, as when rays of heat fall on a mass of cold 
 ice its temperature rises degree by degree until it reaches 
 32 Fahr. and there stops until a certain molecular 
 change (liquefaction) is accomplished, and after that 
 rises again, so also the chemical rays impress certain 
 changes proportional to their quantity, up to a certain 
 point, and there a pause ensues a very large amount of 
 light being now rendered latent or absorbed, without 
 any indication thereof being given by the sensitive prep- 
 aration (as the heat of fluidity is latent to the thermom- 
 eter) ; a molecular change then setting in, the incre- 
 ments of the quantity of light are again indicated by 
 changes in the sensitive preparation. 
 
 4th. That it depends on the CHEMICAL nature of the 
 ponderable material what rays shall be absorbed. 
 
 5th. That while the specific rays thus absorbed de- 
 pend upon the chemical nature of the body, the absolute 
 amount is regulated by its OPTICAL qualities, such as de- 
 pend on the condition of its surfaces and interior ar- 
 rangement. 
 
 o 
 
 6th. It will be proved from this that the SENSITIVE- 
 NESS of any given substance depends on its chemical 
 nature and optical qualities conjointly, and that it is 
 possible to exalt or diminish the sensitiveness of any 
 chemical compound by changing the character of its op- 
 tical relations. 
 
 7th. That, as when radiant heat falls on the surface of 
 an opaque body, the number of rays reflected is the com- 
 plement of those that are absorbed, so in the case of 
 a sensitive preparation, the number of rays reflected 
 from the surface is the complement of those that are 
 absorbed. 
 
 I now commence with the proofs of the propositions 
 of this Memoir. 
 
 1st. That the chemical action produced by the rays 
 
232 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 
 
 of light depends upon the absorption of those rays by 
 sensitive bodies, etc. 
 
 I iodized a plate to a golden yellow color, and exposed 
 it to the diffused light of day, setting it in such a po- 
 sition that it reflected specularly the light falling upon 
 it from the window to the objective of the camera-obscu- 
 ra, which formed an image upon a second sensitive plate. 
 The rays falling upon the sensitive plate of course ex- 
 erted their usual influence upon the iodide, which, after 
 the lapse of a short time, began to turn brown. As 
 soon as this effect was observed, I closed the aperture of 
 the camera, and, taking out its plate, mercurialized it; 
 but it was found that the rays reflected from the sensi- 
 tive plate, although they had been converged by a lens 
 four inches in diameter, and formed a very bright image, 
 had lost the quality of changing the iodide of silver. 
 
 We see, therefore, that a ray of light which has im- 
 pinged on the surface of yellow iodide of silver has lost 
 the power of causing any further change on a second 
 similar plate on which it may fall. 
 
 In the practice of photography this observation is of 
 much importance, especially when lenses having large 
 apertures are used ; the rays converging upon the sensi- 
 tive plate are reflected by it in all directions, and the 
 camera is full of light ; its sides reflect back again in all 
 directions on the surface of the plate these rays, which, 
 if they were effective, must stain the plate in the shad- 
 ows. But if the plate has been iodized to the proper 
 tint, this light is wholly without action, and hence the 
 proof comes out neat and clean. 
 
 Upon an iodized plate I received a solar spectrum 
 formed by a flint-glass prism, the ray being kept motion- 
 less by reflection from a heliostat, and the plate so ar- 
 ranged as to receive the refracted rays perpendicularly. 
 After five minutes it was mercurialized, and the resulting 
 
MEMOIR XVII.] CHEMICAL KAYS AND RADIANT HEAT. 233 
 
 proof exhibited the place of the more refrangible colors 
 in the most brilliant hues. The less refrangible colors 
 had also left their impress of a whitish aspect, but the 
 region of the yellow was unaltered. All the different 
 rays, therefore, except the yellow, have the power of 
 changing this particular preparation. Now when sev- 
 eral pieces of cloth of different colors are placed in the 
 sunbeam, they absorb heat in proportion as their color 
 is deeper. A black cloth, which does not reflect any of 
 those calorific rays, becomes presently hot ; and in the 
 same way Daguerre's sensitive preparation absorbs all 
 the rays having any chemical action on it, and reflects 
 the yellow only, which does not affect it. In this par- 
 ticular lies the secret of its sensitiveness, compared with 
 the common preparations of the chloride and bromide 
 of silver. 
 
 2d. That as a body warmed by the rays of the sun, etc. 
 
 After a beam of light has made its impression on the 
 iodide, if the plate be laid aside in the dark before mer- 
 curializing, that impression decays away with more or 
 less rapidity ; first the faint lights disappear, then those 
 that are stronger. 
 
 Having brought 
 three plates to the 
 same condition of 
 iodization, and re- 
 ceived the image 
 of a gas -flame in 
 the camera on each 
 for three minutes, I 
 mercurialized one, A, forthwith ; the second, B, I kept 
 an hour, the third, 6 7 , forty-eight hours. The relative ap- 
 pearance of these three images is represented in Fig. 35. 
 
 Those who are in the habit of taking daguerreotypes 
 know how much they suffer when the process of mercu- 
 
234 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 
 
 rialization is deferred. To show this effect in the ex- 
 treme, I took four plates, and having prepared all alike, 
 I exposed half of the surface of each to a bright sky for 
 eight seconds. 
 
 No. 1 mercurialized immediately, came out black solarized. 
 
 "2 " in five hours, " white. 
 
 "3 " ** twenty-two hours, " same effect. 
 
 " 4 " one hundred and forty-four hours, no effect. 
 
 This last plate, on being submitted twice more to the 
 vapor of mercury, gave an indistinct mark. On exposing 
 a corner of it to the sun it blackened instantly, these re- 
 sults showing that the peculiar condition brought on by 
 the action of the light gradually disappears, the com- 
 pound all the time retaining its sensitiveness. 
 
 Similar results are mentioned by Daguerre in the case 
 of the changes produced on surfaces of resinous bodies, 
 and I have noticed them in a variety of other cases. 
 Now to whatever cause these phenomena are due, whether 
 to anything analogous to radiation, conduction, etc., it is 
 most active during the first moment after the light has 
 exerted its agency, but it must also take effect even at 
 the very time of exposure; and it is for these reasons 
 that it comes to pass that when light of a double inten- 
 sity is thrown upon a metallic plate the time required 
 to produce a given effect is less than one half. 
 
 I could conceive the intensity of a ray so adjusted that 
 in falling upon a given sensitive preparation the loss 
 from this cause, this casting off of the active agent, should 
 exactly balance the primitive effect, arid hence no observ- 
 able change result. Hereafter we shall find that one 
 cause of the non-sensitiveness of a number of bodies is 
 to be traced directly to the circumstance that they yield 
 up these rays as fast as they receive them. 
 
 It needs no other observation than a critical examina- 
 tion of the sharp lines of a daguerreotype proof with a 
 
MEMOIR XVIL] CHEMICAL RAYS AND KADIANT HP]AT. 235 
 
 magnifying glass to show that the influence of the chem- 
 ical rays is not propagated laterally on the yellow iodide 
 of silver. Of the manifestations which these rays may 
 exhibit, after they have lost their radiant form and be- 
 come absorbed, we know but little. If they conform to 
 the analogous laws for heat, and if the absorbing action 
 of bodies for this agent is inversely as their conducting 
 power, we perceive at once why a photographic effect 
 produced on yellow iodide of silver retains the utmost 
 sharpness without any lateral spreading; the absorbing 
 power is almost perfect, the conducting should therefore 
 be zero. 
 
 3d. That, as ivheti rays of heat fall on a mass of cold 
 ice, etc. 
 
 Although in the sun the iodide of silver blackens at 
 once, this is only the result of a series of preliminary 
 operations. 
 
 When we look at a daguerreotype, we are struck with 
 the remarkable gradation of tint, and we naturally infer 
 that the amount of whitening induced by mercurializa- 
 tion is in direct proportion to the amount of incident 
 light; otherwise it would hardly seem that the grada- 
 tion of tones could be so perfect. 
 
 But in truth it is not so. When the ravs be^in to 
 
 / o 
 
 act on it, the iodide commences changing, and is capable 
 of being whitened by mercury. Step by step this proc- 
 ess goes on, an increased whiteness resulting from the 
 prolonged action or increased brilliancy of the light, until 
 a certain point is gained, and now the iodide of silver 
 apparently undergoes no further visible change ; but an- 
 other point being gained, it begins to assume, when mer- 
 curialized, a pale blue tint, becoming deeper and deeper, 
 until it at last assumes the brilliant blue of a watch- 
 spring. This incipient blueness goes under the technical 
 name of solarization. 
 
236 CHEMICAL RAYS AND KADI ANT HEAT. [MEMOIR XVII. 
 
 The successful practice of the art of daguerreo typing, 
 therefore, depends oil limiting the action of the sun-ray 
 to the first moments of change in the iodide; for if the 
 exposure be continued too long, the high lights become 
 stationary, while the shadows increase unduly in white- 
 ness, and all this happens long before solarization sets in. 
 Let us examine this important phenomenon more mi- 
 nutely. Having carefully cleaned and iodized a silver 
 plate, three inches by four in size, it is to be kept in the 
 dark an hour or two. 
 
 By a suitable set of tin-foil screens, rectangular por- 
 tions of its surface, half an inch by one eighth, are to be 
 exposed at a constant distance to the rays of an Argand 
 gas-burner (the one I have used is a common twelve- 
 holed burner), the first portion being exposed fifteen sec- 
 onds, the second thirty seconds, the third forty -five sec- 
 onds, the fourth sixty seconds, etc. 
 
 We have thus a series of 
 spaces upon the plate, &, b, 
 c, d, Fig. 36, each of which 
 has been affected by known 
 quantities of light ; b being 
 affected twice as much as $, 
 having received a double 
 quantity of light; c thrice 
 as much as $, having re- 
 ceived a triple quantity, 
 etc. 
 
 The plate is now exposed 
 to the vapor of mercury at 
 170 Fahr. for ten minutes; 
 the spaces all come out 
 in their proper order, and 
 nothing remains but to remove the iodide. 
 
 An examination of one of these plates thus prepared 
 
MEMOIR XVII.] CHEMICAL RAYS AND RADIANT HEAT. 237 
 
 shows* that, commencing with the first space, $, we dis- 
 cover a gradual increase of whitening effect until we 
 reach the seventh ; that a perfect whiteness is there at- 
 tained ; that, passing on to the sixteenth, no increase of 
 whitening is to be perceived, although the quantities of 
 light that have been incident and absorbed have been con- 
 tinually increasing; but as soon as the light thus latent 
 has reached a certain quantity, visible decomposition sets 
 in, indicated by a blueness, and the sensitive surface once 
 more renders evident the increments of incident li^ht. 
 
 O 
 
 Or, by presenting a plate covered with a screen to a 
 sky that is clear or uniformly obscured, and with a regu- 
 lar motion withdrawing the screen deliberately from one 
 end to the other, and then 
 suddenly screening the 
 whole, it is plain that those 
 parts first uncovered will 
 have received the greatest 
 quantity of light, and the 
 others less and less. On 
 mercurializing, it will be 
 seen that a stain will be 
 evolved on the plate, as is 
 represented in Fig. 37; 
 from a to b the changes 
 have been successive; from 
 f) to c no variation in the 
 amount of whitening is per- 
 ceptible ; at d solarization 
 
 is commencing, which becomes deeper and deeper to the 
 end, , of the stain. 
 
 * It is impossible to represent these changes in a drawing which is simply black 
 and white ; it will be understood that the characteristic distinction of the spaces 
 from the sixteenth to the twentieth, for example, depends on their assuming a Hue 
 tint, which continually deepens in intensity. 
 
238 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 
 
 The plate from which the drawing of Fig. 37 is taken 
 gives from a to b ten parts, from to c seventeen parts, 
 from d to e twelve parts ; we perceive, therefore, how 
 large an amount of light is absorbed, and its effects ren- 
 dered latent, between the maximum of whiteness being 
 gained and solarization setting in. 
 
 4th. That it depends on the CHEMICAL nature of the 
 ponderable material what rays shall be absorbed. 
 
 I had prepared a number of observations in proof of 
 this, very much of the same kind as those which have 
 some time ago been published in the Phil. Trans, by 
 Herschel. These refer chiefly to the variable lengths of 
 the stains impressed by the prismatic solar spectrum on 
 different chemical bodies, and the points of maximum ac- 
 tion noticed in them. For the present I content myself 
 with referring to that Memoir for proofs substantiating 
 this proposition. 
 
 5th. That while the specific rays thus absorbed de- 
 pend upon the chemical nature of the body, the absolute 
 amount is regulated by its OPTICAL QUALITIES, such as 
 depend on the condition of its surfaces and interior ar- 
 rangement. 
 
 I took a polished silver plate, and having exposed it to 
 the vapor of iodine, found that it passed through the fol- 
 lowing changes of color : 1st, lemon yellow; 2d, golden 
 yellow; 3d, reddish yellow; 4th, blue; 5th, lavender; 6th, 
 metallic; 7th, yellow ; 8th, reddish ; 9th, green, etc., the 
 differences of color being produced by the differences of 
 thickness in the film of iodide, and not by any difference 
 of chemical composition. 
 
 It is a common remark, originally made by Daguerre, 
 that of these different tints that marked 2 is the most 
 sensitive, and photogenic draughtsmen generally suppose 
 that the others are less efficient from the circumstance 
 of the film of iodide being too thick. Some suppose, 
 
MEMOIR XVII.] CHEMICAL KAYS AND RADIANT HEAT. 239 
 
 indeed, that the first yellow alone is sensitive to light. 
 We shall see in a few moments that this is very far 
 from being the case. 
 
 Having brought nine different plates to the different 
 colors just indicated, I received in the camera on each 
 the image of a uniform gas-flame, treating all as nearly 
 alike as the case permitted. I readily found that in 
 No. 1 there was a well-marked action, No. 2 still stronger, 
 but that the rays had less and less influence down to 
 No. 6, in which they appeared to be almost without 
 action ; but in No. 7 they had recovered their original 
 power, being as energetic as in No. 2, and from that 
 declining again. This is shown in Fig. 38. 
 
 Blue. Lavender. 2d Yellow. 
 
 No. 4. No. 5. No. T. 
 
 Fig. 38. 
 
 Hence we see that the sensitiveness of the iodide of 
 silver is by no means constant; that it observes period- 
 ical changes depending on the optical qualities of the 
 film and not on its chemical composition; and that by 
 bringing the iodide into those circumstances that it re- 
 flects the blue rays we greatly reduce its sensitiveness, 
 and still more so when we adjust its thickness so as to 
 give it a gray metallic aspect. But the moment we go 
 beyond this, and restore by an increased thickness its 
 original color, we restore also its sensitiveness. Here, 
 then, in this remarkable result we again perceive a cor- 
 roboration of our first proposition. 
 
040 CHEMICAL RAYS AND RADIANT HEAT. [MEMOIR XVII. 
 
 I may, however, observe in passing, that although I am 
 describing these actions as if thjere were an actual ab- 
 sorption of the rays, and that films on metallic plates ex- 
 hibit colors, not through any mechanism like interference, 
 but simply because they have the power of absorbing 
 this or that ray, there is no difficulty in translating these 
 observations into the language of that hypothesis. When 
 the diffracted fringes given by a hair or wire in a cone 
 of diverging light are received on these plates, corre- 
 sponding marks are obtained, a dark stripe occupying 
 the place of a yellow fringe, and a white that of a blue. 
 I found, more than four years ago, that this held in the 
 case of bromide of silver paper, and have since verified it 
 in a more exact way with this French preparation. Sim- 
 ilar phenomena of interference may be exhibited with 
 the chloride of silver. 
 
 We have it, therefore, in our power to exalt or de- 
 press the sensitiveness of any compound by changing its 
 optical character. Until now, it has been supposed that 
 the amount of change taking place in different bodies, 
 by the action of the rays of light, depends wholly on 
 their chemical constitution, and hence comparisons have 
 been instituted as to the relative sensitiveness of the 
 chlorides, bromides, oxides, and iodides of silver, etc. But 
 it seems the liability to change depends also on other 
 principles which, being liable to variation, the sensitive- 
 ness of a given body varies with them. Thus this very 
 iodide of silver, when in a thin yellow film, is decomposed 
 by the feeblest rays of a taper, and even moonlight acts 
 with energy ; yet simply by altering the thickness of its 
 film it becomes sluggish, blackening even in the sunlight 
 tardily, and recovering its sensitiveness again on recov- 
 ering its yellow hue. 
 
 We have now no difficulty in understanding how, in 
 the preparation of ordinary sensitive paper, great varia- 
 
MEMOIR XVII.] CHEMICAL BAYS AND RADIANT HEAT. 241 
 
 tions ensue, by modifying the process slightly, and how 
 even on a sheet which is apparently washed uniformly 
 over, large blotches appear, which are either inordinately 
 sensitive or not sensitive at all. If, without altering 
 the chemical composition of a film on metallic silver, or 
 even its mode of aggregation, such striking changes re- 
 sult by difference of thickness, how much more may we 
 expect that the great changes in molecular condition, 
 which apparently trivial causes must bring about on sen- 
 sitive paper, should elevate or depress its capability of 
 being acted on by light. If I mistake not, it is upon these 
 principles that an explanation is to be given of the suc- 
 cessful modes of preparation which Talbot and Hunt have 
 described, and the action of the mordants of Herschel. 
 
 I therefore infer 
 
 6th. That the SENSITIVENESS of any given preparation 
 depends on its chemical nature and its optical qualities 
 conjointly, and that it is possible to exalt or diminish the 
 sensitiveness of a given compound by changing its optical 
 relations. 
 
 Vth. That, as when radiant heat falls on the surface of 
 an opaque body, the number of rays reflected is the comple- 
 ment of those that are absorbed, so in the case of a sensitive 
 preparation, the number of chemical rays reflected from 
 the surface is the complement of those that are absorbed. 
 
 This important proposition I prove 
 in the following way: I take a plate, 
 A Gr, Fig. 39, three inches by four, and 
 by partially screening its surface while 
 in the act of iodizing with a piece of 
 flat glass, I produce upon it five trans- 
 verse bands, b, c, d, e, f ; the fifth,/, 
 which has been longest exposed, is of 
 a pale lavender color, the fourth 
 bright blue, the third a red, the sec- tfig.39. 
 
 Q 
 
242 CHEMICAL KAYS AND RADIANT HEAT. [MEMOIR XVII. 
 
 ond a golden yellow, and the first uniodized metal ; the 
 object of this arrangement being to expose at the same 
 time and on the same plate a series of films of different 
 colors and of different thicknesses, and to examine the 
 action of the rays impinging on them and the rays re- 
 flected by them. 
 
 Having prepared a second plate, B, and iodized it uni- 
 formly to a yellow, I deposit it in the camera, and now, 
 placing the first plate, A G, so that the rays coming on it 
 from the sky through the window shall be specularly 
 reflected to the object-glass of the camera, and the image 
 of A G form upon B, I allow the exposure to continue 
 until the yellow of A G is beginning to turn brown; 
 then I shut the camera and mercurialize both plates. 
 
 In accordance with what has been said, it will be 
 readily understood that of the bands on A G, the first 
 one, which is the bare metal, does not whiten in the mer- 
 cury vapor; the second, which is yellow, mercurializes 
 powerfully ; the third, which is red, is less affected ; the 
 fourth, which is blue, still less; and the fifth, which is 
 lavender, hardly perceptibly. 
 
 But the changes on B, which have been brought about 
 by the rays reflected from A G, are precisely the con- 
 verse ; the band which is the image of b is mercurialized 
 powerfully ; that of c is untouched and absolutely black, 
 d faintly stained, e whitened, and f mercurialized but lit- 
 tle less than b. 
 
 It follows from this that a white stripe on B corre- 
 sponds to a black one on A G, and the converse ; and for 
 the depth of tint of the intermediate stripes those of the 
 one are perfectly complementary to the corresponding 
 ones of the other. 
 
 By the aid of these results we are now able to give 
 an account of the variability of sensitiveness in photo- 
 genic preparations ; the yellow iodide of silver is exces- 
 
MEMOIR XVII.] CHEMICAL KAYS AND RADIANT HEAT. 243 
 
 sively sensitive, because it absorbs all the chemical rays 
 that can disturb it, while the lavender is insensitive, be- 
 cause it reflects them. Under this point of view, sensi- 
 tiveness therefore is directly as absorption and inversely 
 as reflection. 
 
 The superiority of Daguerre's preparation over com- 
 mon sensitive paper may now be readily understood. It 
 absorbs all the rays that can affect it, but the chloride 
 of silver, spread upon paper, reflects many of the active 
 rays. The former, when placed in the camera, gives rise 
 to no reflections that can be injurious; the latter fills it 
 with active light, and stains the proof all over. Hence 
 the daguerreotype has a sharpness and mathematical ac- 
 curacy about its lines, and a depth in its shadows, which 
 is unapproachable by the other. Moreover, the translu- 
 cency of the white chloride of silver, as well as its high 
 reflecting power, permits of particles lying out of the 
 lines of light being affected, the light becoming diffused 
 in the paper. 
 
 The fact, therefore, that a given compound remains un- 
 changed even in the direct rays of the sun is no proof 
 that light cannot decompose it ; it may reflect or trans- 
 mit the active rays as fast as it receives them. It results 
 from this that optical conditions can control and even 
 check the play of chemical affinities. While thus it ap- 
 pears that there are points of analogy between this chem- 
 ical agent and radiant heat, we must not too hastily infer 
 that the laws which regulate the one obtain exclusively 
 also with the other. As is well known, there are strik- 
 ing analogies between radiant heat and lisjht. but there 
 
 o o o / 
 
 are also points of difference, the convertibility of heat of 
 one degree of refrangibility to another does not occur 
 with light ; there are also dissimilitudes in the phenom- 
 ena of radiation and its consequences. 
 
 From the phenomena of the interference of these rays, 
 
244 CHEMICAL BAYS AND RADIANT HEAT. [MEMOIR XVII. 
 
 of the sensitiveness or non sensitiveness of the same 
 chemical compound being determined merely by the fact 
 of its thickness or thinness, these, and many other similar 
 results obviously depending upon mechanical principles, 
 it seems to me that very powerful evidence may be 
 drawn against the materiality of light, and its entering 
 into chemical union with ponderable atoms. Those phi- 
 losophers who have adopted the undulatory theory will 
 probably find in studying these subjects evidence in 
 favor of their doctrines. 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 245 
 
 MEMOIR XVIII. 
 
 DESCRIPTION OF THE CHLOR-HYDROGEN PHOTOMETER. 
 
 From the American Journal of Science and Art, Vol. XL VI. ; Philosophical Mag- 
 azine, December, 1843. 
 
 CONTENTS : Properties of a mixture of chlorine and hydrogen. It is 
 acted upon by lamplight, an electric spark at a distance, etc. The 
 gases unite in proportion to the amount of light. Mode of measuring 
 out known quantities of radiations. The maximum action is in the 
 indigo space. Construction of the instrument. The gases are evolved 
 by electricity and combined by light. Theoretical conditions of equi- 
 librium. Preliminary adjustment. Method of continuous observa- 
 tion. Method of interrupted observation. Remarkable contraction 
 and expansion. 
 
 I HAVE invented an instrument for measuring the 
 force of the chemical rays found at a maximum in the 
 indigo space, and which from that point gradually fade 
 away to each end of the spectrum. The sensitiveness, 
 speed of action, and exactitude of this instrument will 
 bring it to rank as a means of physical research with the 
 thermo-multiplier of Melloni. 
 
 The methods hitherto available in optics for measuring 
 intensities of light by a relative illumination of spaces 
 or contrast of shadows are admitted to be inexact. The 
 great desideratum in that science is a photometer which 
 can mark down effects by movements over a graduated 
 scale. With those optical contrivances may be classed 
 the methods hitherto adopted for determining the force 
 of the chemical rays by stains on daguerreotype plates 
 or the darkening of sensitive papers. As deductions 
 drawn in this way depend on the opinion of the observ- 
 
246 THE CHLOK-HYDKOGEN PHOTOMETER. [MEMOIR XVIII. 
 
 er, they can never be perfectly satisfactory, nor bear any 
 comparison with thermometric results. 
 
 Impressed with the importance of possessing for the 
 study of the properties of the chemical rays some means 
 of accurate measurement, I have resorted in vain to many 
 contrivances; and, after much labor, have obtained at 
 last the instrument which it is the object of this Memoir 
 to describe. 
 
 This photometer consists essentially of a mixture of 
 equal measures of chlorine and hydrogen gases, evolved 
 from and confined by a fluid which absorbs neither. 
 This mixture is kept in a graduated tube, so arranged 
 that the gaseous surface exposed to the rays never varies 
 in extent, notwithstanding the contraction that may be 
 going on in its volume, and the hydrochloric acid result- 
 ing from its union is removed by rapid absorption. 
 
 The theoretical conditions of the instrument are, there- 
 fore, sufficiently simple ; but, when we come to put them 
 into practice, obstacles appearing at first sight insur- 
 mountable are met with. The means of obtaining chlo- 
 rine are all troublesome ; no liquid is known which will 
 perfectly confine it; it is a matter of great difficulty to 
 mix it in the true proportion with hydrogen, and have 
 no excess of either. Nor is it at all an easy affair to 
 obtain pure hydrogen speedily, and both these gases 
 diffuse with rapidity through water into air. 
 
 Without dwelling further on the long catalogue of 
 difficulties thus to be encountered, I shall first give an 
 account of the capabilities of the instrument in the form 
 now described, which will show to what an extent all 
 those difficulties are already overcome. In a course of 
 experiments on the union of chlorine and hydrogen, 
 some of which were read at the last meeting of the Brit- 
 ish Association, I found that the sensitiveness of their 
 mixture had been greatly underrated. The statement 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 247 
 
 made in the books of chemistry, that artificial light will 
 not affect it, is wholly erroneous. The feeblest gleams 
 of a taper produce a change. No further proof of this 
 is required than the tables given in this communication, 
 in which the radiant source was an oil-lamp. For speed 
 of action no compound can approach it : a light which 
 perhaps does not endure the millionth part of a second 
 affects it energetically, as will be hereafter shown. 
 
 Proofs of the sensitiveness of the instrument. The fol- 
 lowing illustrations will show that this instrument is 
 promptly affected by rays of the feeblest intensity and 
 of the briefest duration. 
 
 When, on the sentient tube, the image of a flame 
 formed by a convex lens is caused to fall, the liquid in- 
 stantly begins to move over the scale, and continues its 
 motions as long as the exposure is continued. It does 
 not answer to expose the tube to the direct emanations 
 of the lamp without first absorbing the radiant heat, 
 or the calorific effect will mask the true result. By the 
 interposition of a lens this heat is absorbed, and the 
 chemical rays alone act. 
 
 If the photometer be exposed to daylight coming 
 through a window, and the hand or a shade of any kind 
 be passed in front of it, its movement is in an instant ar- 
 rested ; nor can the shade be passed so rapidly that the 
 instrument will fail to give the proper indication. 
 
 The experimenter may further assure himself of the 
 extreme sensitiveness of this mixture by placing the in- 
 strument before a window and endeavoring to remove 
 and replace its screen so quickly that it shall fail to give 
 any indication: he will find that it cannot be done. 
 
 Charge a Leyden-jar, and place the photometer at a 
 little distance from it, keeping the eye steadily fixed on 
 the scale ; discharge the jar, and the rays from the spark 
 
248 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 will be seen to exert a very powerful effect, the move- 
 ment taking place and ceasing in^an instant. 
 
 This remarkable experiment not only serves to prove 
 sensitiveness, but also brings before us new views of the 
 powers of that extraordinary agent electricity. That 
 energetic chemical effects can thus be produced at a 
 distance by an electric spark in its momentary passage, 
 effects which are of a totally different kind from the 
 common manifestations of electricity, is thus proved ; 
 these phenomena being distinct from those of induction 
 or molecular movements taking place in the line of dis- 
 charge, they are of a radiant character; and we are led at 
 once to infer that the well-known changes brought about 
 by passing an electric spark through gaseous mixtures, 
 as when oxygen and hydrogen are combined into w r ater, 
 or chlorine and hydrogen into hydrochloric acid, arise 
 from a very different cause than those condensations and 
 percussions by which they are often explained a cause 
 far more purely chemical in its kind. If chlorine and 
 hydrogen can be made to unite silently by an electric 
 spark passing outside the vessel which contains them, at 
 a distance of several inches, there is no difficulty in un- 
 derstanding why a similar effect should take place with 
 a violent explosion when the discharge is made through 
 their midst, nor how a great many mixtures may be 
 made to unite under the same treatment. A flash of 
 lightning cannot take place, nor an electric spark be dis- 
 charged, without chemical changes being brought about 
 by the radiations emitted. 
 
 Proofs of the exactness of the indications of this pho- 
 tometer. The foregoing examples may serve to illustrate 
 the extreme sensitiveness of this instrument. I shall 
 next furnish proofs that its indications are exactly pro- 
 portional to the quantities of light incident on it. 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 
 
 249 
 
 As it is necessary, owing to the variable force of day- 
 light, to resort to artificial means of illumination, it will 
 be found advantageous to employ the following method 
 of obtaining a flame of suitable intensity. 
 
 Let A B, Fig. 40, be an Argand oil-lamp of which the 
 
 i<r. 40. 
 
 wick is C. Over the wick, at a distance of half an inch 
 or thereabouts, place a plate of thin sheet-copper, three 
 inches in diameter, perforated in its centre with a circu- 
 lar hole of the same diameter as the wick, and concentric 
 therewith. This piece of copper is represented at d d\ 
 it should have some contrivance for raising or depressing 
 it through a small space, the proper height being deter- 
 mined by trial. On this plate the glass cylinder, 0, an 
 inch and three quarters in diameter and eight or ten 
 inches long, rests. 
 
 When the lamp is lighted, provided the distance be- 
 tween the plate d d and the top of the wick be properly 
 adjusted, on putting on the glass cylinder the flame in- 
 stantly assumes an intense whiteness; by raising the 
 wick it may be elongated to six inches or more, and 
 becomes exceedingly brilliant. Lamps constructed on 
 these principles may be purchased in the shops. I have, 
 however, contented myself with using a common Argand 
 study -lamp, supporting the perforated plate d d at a 
 proper height by a retort stand. It will be easily un- 
 derstood that the great increase of light arises from the 
 circumstance that the flame is drawn violently through 
 
250 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 the aperture in the plate by the current established in 
 the cylinder. 
 
 As much radiant heat is emitted by this flame, in 
 order to diminish its action and also to increase the 
 chemical effect I adopt the following arrangement: Let 
 A B (Fig. 40) be the lamp ; the rays emitted by it are 
 received on a convex lens, D, four inches and three quar- 
 ters in diameter, that which I use being the large lens of 
 a lucernal microscope. This, placed at a distance of 
 twenty-one inches from the lamp, gives an image of the 
 flame at a distance of thirteen inches, which is received 
 on the sentient tube F ; between it and the lens there is 
 a screen, E. 
 
 Things being thus arranged, and the lamp lighted so 
 as to give a flame about three inches and a half long, 
 the experiments may be proceeded with. It is conven- 
 ient always to work with the flame at a constant height, 
 which may be determined by a mark on the glass cylin- 
 der. At a given instant, by a seconds watch, the screen 
 E is removed, and immediately the liquid begins to de- 
 scend. When the first minute is elapsed the position on 
 the scale is read off and registered ; at the close of the 
 second minute the same is done, and so on with the 
 third, etc. And now, if these numbers be compared, 
 casting aside the first, they will be found equal to one 
 another, as the following table of experiments, made at 
 different times and with different instruments, shows. 
 
 From this it will be perceived that, taking the first 
 experiment as an example, if at the end of 30 s the pho- 
 tometer has moved 7.00, at the end of 60 s it has moved 
 8.00 more; at the end of 90 s , 7.50 more; at the end of 
 120 s , 7.75 more; the numbers set down in the vertical 
 column representing the amount of motion for each 30 s . 
 And, when it is recollected that the readings are all 
 made with the instrument in motion, the differences 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 
 
 251 
 
 between the numbers do not greatly exceed the possible 
 errors of observation. It may be remarked that the 
 third and fourth experiments were made with a different 
 lamp. 
 
 TABLE I. 
 
 Showing that when the radiant source is constant, the amount of move- 
 ment in the photometer is directly proportional to the times of exposure. 
 
 Time. 
 
 Experiments. 
 
 1. 
 
 2. 
 
 3. 
 
 4. 
 
 5. 
 
 s. 
 30 
 60 
 90 
 120 
 150 
 180 
 210 
 
 7.00 
 8.00 
 7.50 
 7.75 
 
 7.75 
 
 7.00 
 7.75 
 8.00 
 7.75 
 7.25 
 
 10.25 
 11.50 
 11.50 
 11.50 
 
 1L75 
 
 13.00 
 12.00 
 
 5.25 
 6.50 
 6.25 
 6.00 
 6.00 
 6.00 
 6.00 
 
 
 
 
 
 Mean. . . 
 
 7.60 
 
 7.55 
 
 11.19 
 
 12.25 
 
 6.00 
 
 Though a Certain amount of radiant heat from a source 
 so highly incandescent as that here used will pass the 
 lens, its effects can never be mistaken for those of the 
 chemical rays. This is easily understood when we re- 
 member that the effect of such transmitted heat would 
 be to expand the gaseous mixture, but the chemical ef- 
 fect is to contract it. 
 
 Next, the indications of the photometer are strictly 
 proportional to the quantity of rays that have impinged 
 upon it ; a double quantity producing a double effect, a 
 triple quantity a threefold effect, etc. 
 
 A slight modification in the arrangement (Fig. 40) 
 enables us to prove this in a satisfactory way. The lens, 
 D, being mounted in a square wooden frame, can easily 
 be converted into an instrument for delivering at its 
 focal point, where the sentient tube is placed, measured 
 quantities of the chemical rays, and thus becomes an 
 invaluable auxiliary in those researches which require 
 known and predetermined quantities of radiations to be 
 
252 
 
 THE CHLOK-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 measured out. The method of doing this will be de- 
 scribed in a subsequent part of this Memoir. 
 
 In order, therefore, to prove that the indications of 
 the photometer are proportional to the quantity of im- 
 pinging rays, place this measuring lens in the position 
 D, setting its screens at an angle of 90. Remove the 
 screen E, and determine the effect on the photometer for 
 one minute. At the close of the minute, and without 
 loss of time, turn one of the screens so as to give an 
 angle of 180, and now the effect will be found double 
 what it was before, as in the following table : 
 
 TABLE II. 
 
 Showing that the indications of the photometer are proportional to the 
 quantity of incident rays. 
 
 Quantities. 
 
 Experiment 1. 
 
 Experiment 2. 
 
 Observed. 
 
 Calculated. 
 
 Observed. Calculated. 
 
 o 
 
 90 
 180 
 270 
 360 
 
 2.18 
 4.27 
 6.70 
 
 8.90 
 
 2.22 
 4'. 45 
 6.67 
 8.90 
 
 2.69 
 5.75 
 8.25 
 11.00 
 
 2.75 
 5.50 
 
 8.25 
 11.00 
 
 I have stated in the commencement of this paper that 
 the action upon the photometer is limited to a ray which 
 corresponds in refrangibility to the indigo, or, rather, that 
 in the indigo space its maximum action is found. The 
 table on the following page serves at once to prove 
 this fact, and also to illustrate the chemical force of the 
 different regions of the spectrum. 
 
 In this table the spaces are equal ; the centre of the 
 red, as insulated by cobalt blue glass, is marked as 
 unity ; the centre of the yellow, insulated by the same, 
 being marked 3 ; the intervening region being divided 
 into two equal spaces, and divisions of the same value 
 carried on to each end of the spectrum. 
 
 As instruments will no doubt be hereafter invented 
 for measuring the phenomena of different classes of rays, 
 
MEMOIR XVIII. ] THE CHLOR-HYDROGEN PHOTOMETER. 
 
 253 
 
 TABLE III. 
 
 Showing that the maximum for the photometer is in the indigo space of 
 
 the spectrum. 
 
 Space. 
 
 Ray. 
 
 Force. 
 
 Space. 
 
 Ray. 
 
 Force. 
 
 o 
 
 
 33 
 
 8 
 
 Blue-indigo. 
 
 204 00 
 
 1 
 
 Red 
 
 50 
 
 9 
 
 Indigo 
 
 240 00 
 
 2 
 
 
 75 
 
 10 
 
 Violet 
 
 121 00 
 
 3 
 
 
 2 75 
 
 11 
 
 Violet 
 
 72 00 
 
 4 
 
 
 10 00 
 
 12 
 
 Violet 
 
 48 00 
 
 5 
 
 6 
 
 Green-blue 
 Blue 
 
 54.00 
 108.00 
 
 13 
 14 
 
 Violet 
 Extra-spectral. . 
 
 24.00 
 12.00 
 
 7 
 
 Blue 
 
 144 00 
 
 
 
 
 
 
 
 
 
 
 it may prove convenient to designate the precise ray to 
 which they apply. Perhaps the most simple mode is to 
 affix the name of the ray itself. Under that nomencla- 
 ture the instrument described in this paper would take 
 the name of indigo-photometer. 
 
 There is no difficulty in adapting this instrument to 
 the determination of questions relating to absorption, 
 reflection, and transmission. Thus I found that a piece 
 of colorless French plate-glass transmitted 866 rays out 
 of 1000. 
 
 Description of the Instru- 
 ment, first, of the Glass 
 fart. The chlor- hydrogen 
 photometer consists of a glass 
 tube bent into the form of a 
 siphon, in which chlorine and DJJJ 
 hydrogen can be evolved 
 from hydrochloric acid con- 
 taining; chlorine in solution 
 
 o 
 
 by the agency of a voltaic 
 current. It is represented by 
 Fig. 41, where a b c is a clear 
 and thin tube four tenths of 
 an inch in external diameter, 
 
254 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 closed at the end a. At d, a circular piece of metal an 
 inch in diameter, which may be called the stage, is 
 fastened on the tube, the distance from d to a being 
 2.9 inches. At the point #?, which is two inches and a 
 quarter from d, two platinum wires, x and y, are fused 
 into the glass, and entering into the interior of the tube, 
 are destined to furnish the supply of chlorine and hy- 
 drogen ; from the stage d to the point Z>, the inner bend 
 of the tube, is 2.6 inches, and from that point to the top 
 of the siphon c the distance is three inches and a half. 
 Through the glass at 3, three quarters of an inch from c, 
 a third platinum wire is passed ; this wire terminates in 
 the little mercury-cup ?', and x and y in the cups p and 
 q respectively. 
 
 A stout tube, six inches long and one tenth of an inch 
 interior diameter, ef, is fused on at c. Its lower end 
 opens into the main siphon tube; its upper end is turned 
 over at/, and is narrowed to a fine termination so as 
 barely to admit a pin, but is not closed. This serves 
 to keep out dust, and in case of a little acid passing out, 
 it does not flow over the scale and deface the divisions. 
 At the back of this tube a scale is placed, divided into 
 tenths of an inch, being numbered from above down- 
 wards. Fifty of these divisions are as many as will be 
 required. Fig. 2 shows the termination of the narrow 
 tube bent over the scale. 
 
 From a point one fourth of an inch above the stage J, 
 downwards beyond the bend, and to within half an inch 
 of the wire 2, the whole tube is carefully painted with 
 India-ink, so as to allow no light to pass; but all the 
 space from a fourth of an inch above the stage d to the 
 top of the tube a is kept as clear and transparent as 
 possible. This portion constitutes the sentient part of 
 the instrument. A light metallic or pasteboard cap, A D, 
 closed at the top and open at the bottom, three inches 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 255 
 
 long and six tenths of an inch in diameter, blackened on 
 its interior, may be dropped over this sentient tube; it 
 being the office of the stage d to receive the lower end 
 of the cap when it is dropped on the tube so as to shut 
 out the light. 
 
 The foot of the instrument, Jc I, is of brass ; it screws 
 into the block m, which may be made of hard wood or 
 ivory ; in this three holes, p, q, r, are made to serve as 
 mercury-cups ; they should be deep and of small diam- 
 eter, that the metal may not flow out when it inclines 
 for the purpose of transferring. A brass cylindrical cov- 
 er, L M, L M, may be put over the whole when it is de- 
 sirable to preserve it in total darkness. 
 
 Things being thus arranged, the instrument is filled 
 with its fluid, prepared as will presently be described; 
 and as the tubes a Z>, b c are not parallel to each other, 
 but include an angle of a few degrees, in the same way 
 that lire's eudiometer is arranged, there is no difficulty 
 in transferring the liquid to the sealed side. Enough is 
 admitted to fill the sealed tube and the open one par- 
 tially, leaving an empty space to the top of the tube at 
 c of two and three quarter inches. 
 
 Secondly, of the Fluid Part. The fluid from which 
 the mixture of chlorine and hydrogen is evolved, and by 
 which it is confined, is yellow commercial hydrochloric 
 acid, holding such a quantity of chlorine in solution that 
 it exerts no action on the mixed gases as they are pro- 
 duced. From the mode of its preparation it always con- 
 tains a certain quantity of chloride of platinum, which 
 gives it a deep golden color, a condition of considerable 
 incidental importance. 
 
 When hydrochloric acid is decomposed by voltaic 
 electricity its chlorine is not evolved, but is taken up in 
 very large quantity and held in solution ; perhaps a bi- 
 chloride of hydrogen results. If through such a solution 
 
256 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 hydrogen gas is passed in minute bubbles, it removes 
 with it a certain proportion of the chlorine. From this, 
 therefore, it is plain that hydrochloric acid thus decom- 
 posed will not yield equal measures of chlorine and hy- 
 drogen unless it has been previously impregnated with 
 a certain volume of the former gas. Nor is it possible 
 to obtain that degree of saturation by voltaic action, no 
 matter how long the electrolysis is continued, if the hy- 
 drogen be allowed to pass through the liquid. 
 
 Practically, therefore, to obtain the photometric liquid 
 we are obliged to decompose commercial hydrochloric 
 acid in a glass vessel, the positive electrode being at the 
 bottom of the vessel and the negative at the surface of 
 the liquid. Under these circumstances, the chlorine as 
 it is disengaged is rapidly taken up, and the hydrogen 
 being set free without its bubbles passing through the 
 mass, -the impregnation is carried to the point required. 
 
 Although this chlorinated hydrochloric acid cannot of 
 course be kept in contact with the platinum wires with- 
 out acting on them, the action is much slower than 
 might have been anticipated. I have examined the 
 wires of photometers that had been in active use for 
 four months, and could not perceive the platinum sen- 
 sibly destroyed. It is well, however, to put a piece of 
 platinum foil in the bottle in which the supply of chlo- 
 rinated hydrochloric acid is kept ; it communicates to it 
 slowly the proper golden tint. 
 
 The liquid being impregnated with chlorine in this 
 manner until it exhales the odor of that gas is to be 
 transferred to the siphon a b c of the photometer, and its 
 constitution finally adjusted as hereafter shown. 
 
 Thirdly, of the Voltaic Battery. The battery which 
 will be found most applicable for these purposes con- 
 sists of two Grove's cells, the zinc surrounding the plati- 
 num. 
 
Q 
 
 MEMOIR XVJII.] THE CHLOR-HYDROGEN PHOTOMETRIC 
 
 The following are the dimensions of the 
 use. The platinum plate is half an inch wide 
 inches long; it dips into a cylinder of porous bis 
 ware of the same dimensions, which contains nitric 
 Outside this porous vessel is the zinc, which is a 
 der one inch in diameter, two inches long, and two tenths 
 thick; it is amalgamated. The whole is contained in a 
 cup, two inches in diameter and two deep, which also re- 
 ceives the dilute sulphuric acid. 
 
 The force of this battery is abundantly sufficient both 
 for preparing the fluid originally and for carrying on 
 the photometric operations. It can decompose hydro- 
 chloric acid with rapidity, and will last with ordinary 
 care a long time. 
 
 Before passing to the mode of using this photometer, 
 it is absolutely necessary to understand certain theoret- 
 ical conditions of its equilibrium. These in the next 
 place I shall describe. 
 
 Theoretical Conditions of Equilibrium. This photom- 
 eter depends for its sensitiveness on the exact proportion 
 of the mixed gases. If either one or the other is in ex- 
 cess a great diminution of delicacy is the result. The 
 comparison of its indications at different times depends 
 on the certainty of evolving the gases in exact, or, at all 
 events, known proportions. 
 
 Whatever, therefore, affects the constitution of the sen- 
 tient gases alters at the same time their indications. Be- 
 tween those gases and the fluid that confines them cer- 
 tain relations subsist the nature of which can be easily 
 traced. Thus, if we had equal measures of chlorine and 
 hydrogen, and the liquid not saturated with the former, 
 it would be impossible to keep them without change, for 
 by degrees a portion of chlorine would be dissolved and 
 an excess of hydrogen remain ; or if the liquid was over- 
 
 R 
 
258 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 charged with chlorine, an excess of that gas would ac- 
 cumulate in the sentient tube. 
 
 It is absolutely necessary, therefore, that there should 
 be an equilibrium between the gaseous mixture and the 
 confining fluid. 
 
 As has been said, when hydrochloric acid is decom- 
 posed by a voltaic current, all the chlorine is absorbed 
 by the liquid and accumulates therein; the hydrogen 
 bubbles, however, as they rise withdraw a certain pro- 
 portion, and hence pure hydrogen passed up through the 
 photometric fluid becomes exceedingly sensitive to the 
 light. 
 
 There are certain circumstances connected with the 
 constitution and use of the photometer which continu- 
 ally tend to change the nature of its liquid. The plat- 
 inum wires immersed in it by slow degrees give rise to 
 a chloride of platinum. It is true that this takes place 
 very gradually, and by far the most formidable difficulty 
 arises from a direct exhalation of chlorine from the nar- 
 row tube efj for each time that the liquid descends, a 
 volume of air is introduced, which receives a certain 
 amount of chlorine which with it is expelled the next 
 time the battery raises the column to zero; and this 
 going on time after time finally impresses a marked 
 change on the liquid. I have tried to correct it in 
 various ways, as by terminating the end f with a bulb ; 
 but this entails great inconvenience, as may be discov- 
 ered by any one who will reflect on its operation. 
 
 When by the battery we have raised the index to its 
 zero point, if the gas and liquid are not in equilibrium 
 that zero is liable to a slight change. If there be hydro- 
 gen in excess, the zero will rise; if chlorine, the zero 
 will fall. 
 
 In making what will be termed " interrupted experi- 
 ments," we must not too hastily determine the position 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 259 
 
 of the index on the scale at the end of a trial. It is to 
 be remembered that the cause of movement over the 
 scale arises from a condensation of hydrochloric acid, but 
 that condensation, though very rapid, is not instanta- 
 neous. Where time is valuable and the instrument in 
 perfect equilibrium, this condensation may be instanta- 
 neously effected by simply inclining the instrument so 
 that its liquid may pass down to the closed end #, but 
 not so much as to allow gas to escape into the other 
 side the inclination of the two sides to each other 
 makes this a very easy manipulation and the gas thus 
 brought into contact with an extensive liquid surface 
 yields up its hydrochloric acid in an instant. 
 
 Directions for using the Photometer. Preliminary ad- 
 justment. Having transferred the liquid to the sealed 
 end of the siphon, and placed the cap on the sentient ex- 
 tremity, the voltaic battery being prepared, the operator 
 dips its polar wires into the cups^, </, which are in con- 
 nection with the wires a?, y. Decomposition immediately 
 takes place, chlorine and hydrogen rising through the 
 liquid and gradually depressing it, while, of course, a 
 corresponding elevation takes place in the other limb. 
 This operation is continued until the liquid has risen to 
 the zero. It takes but a few seconds for this to be ac- 
 complished. 
 
 The polar wires having been disengaged, the photom- 
 eter is removed opposite a window, care being taken 
 that the light is not too strong. The cap is now lifted 
 off the sentient extremity a d, and immediately the liquid 
 ascends. This exposure is allowed to continue, and the 
 liquid suffered to rise as much as it will to the end a. 
 And now, if the gases have been properly adjusted, an 
 entire condensation will take place, the sentient tube a d 
 filling completely. In practice this precision is not how- 
 
260 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 ever obtained, and if a bubble as large as a pepper-corn 
 be left, the operator will be abundantly satisfied with 
 the sensitiveness of his instrument. Commonly, at first, 
 a large residue of hydrogen gas, occupying perhaps an 
 inch or more, will be left. It is to be understood that 
 even this large surplus will disappear in a few hours by 
 absorbing chlorine. But this is not to be waited for ; 
 as soon as no further rise takes place, in a minute or two, 
 the siphon is to be inclined to one side, and the residue 
 turned into the open side. 
 
 Now, recurring to what has been said on the equilib- 
 rium, it is plain that this excess of hydrogen arises from 
 a want of chlorine in the photometric liquid. A proper 
 quantity must therefore be furnished by proceeding as 
 follows : 
 
 The sentient tube being filled with the liquid by in- 
 clination, connect the polar wires with p, q, as before. 
 These may be called generating wires. Allow the liquid 
 to rise in b c until the third platinum wire 2, which may 
 be called the adjusting wire, is covered an eighth of 
 an inch deep. Then remove the negative wire from the 
 cup p into the cup r, and now the conditions for saturat- 
 ing the liquid are complete ; hydrogen escaping from 
 the surface of the liquid at 2, and chlorine continually 
 accumulating and dissolving between x and d. This 
 having been carried on for a short time, the gas in a d 
 is to be turned out by inclination and the instrument 
 recharged. That a proper quantity is evolved is easily 
 ascertained by allowing total condensation to take place, 
 and observing that only a small bubble is left at a. 
 
 It will occasionally happen in this preliminary adjust- 
 ment that an excess of chlorine may arise from continu- 
 ing the process too long. This is easily discovered by 
 its greenish-yellow tint, and is to be removed by inclin- 
 ing the instrument and turning it out. 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 261 
 
 Thus adjusted, everything is ready to obtain measures 
 of any effect, there being two different methods by 
 which this can be done: 1st, by continuous observa- 
 tion ; 2d, by interrupted observation. 
 
 Of the method of continuous observation, This is best 
 described by resorting to an example. Suppose, there- 
 fore, it is required to verify Table L, or, in other words, 
 to prove that the effect on the photometer is propor- 
 tional to its time of exposure. 
 
 Put on the cap of the sentient tube a d, connect the 
 polar wires with p, q, and raise the liquid to zero. 
 
 Place the photometer so that its sentient tube will re- 
 ceive the rays properly. 
 
 At a given instant, marked by a seconds watch, re- 
 move the cap A D, and the index-liquid at once begins 
 to descend. At the end of the first minute read off the 
 division over which it is passing. Suppose it is 7. At 
 the end of the second do the same: it should be 14 ; at 
 the end of the third, 21, etc. This may be done until 
 the fiftieth division is reached, which is the terminus of 
 the scale. 
 
 Recharge the tube by a momentary application of the 
 polar wires; but it is convenient first to remove any 
 excess of hydrochloric -acid gas in the sentient tube by 
 allowing it time for condensation ; or, if that be inadmis- 
 sible, by inclining a little to one side, so as to give an 
 extensive liquid contact. 
 
 Of the method of interrupted observation. It frequent- 
 ly happens that observations cannot be had during a 
 continuous descent, as when changes have to be made in 
 parts of apparatus or arrangements. We have then to 
 resort to interrupted observations. 
 
 This method requires that the gas and liquid should 
 be well adjusted, so that no change can arise in volume 
 when extensive contact is made by inclination. 
 
262 THE CHLOR-HYDKOGEN PHOTOMETER. [MEMOIR XVIII. 
 
 The photometer being charged, place it in a proper 
 position. At a given instant remove its cap, and the 
 liquid descends. When the time marked by a seconds 
 watch has elapsed, drop the cap on the sentient tube. 
 The liquid simultaneously pauses in its descent, but does 
 not entirely stop, for a little uncondensed hydrochloric 
 acid still exists, which is slowly disappearing in the sen- 
 tient tube. Now, incline the instrument for a moment 
 to one side, so that the liquid may run up to the end #, 
 but not so much as to let any gas escape. Restore it to 
 its position and read off on the scale. It is then ready 
 for a second trial. 
 
 The difference between continuous and interrupted ob- 
 servation is this, that in the latter we pause to wash out 
 the hydrochloric acid, and though this is effected by the 
 simplest of all possible methods, continuous observations 
 are always to be preferred when they can be obtained. 
 
 I have extended this Memoir to so great a length that 
 many points on which remarks might have been made 
 must be passed over. It is scarcely necessary to say 
 that the sentient tube must be uniformly and perfectly 
 clean. As a general rule also, the first observation may 
 be cast aside, for reasons which I will presently give. 
 Further, it is to be remarked, as it is an essential prin- 
 ciple, that during different changes of volume of the gas 
 its exposed surface must never vary in extent, the liquid 
 is not to be suffered to rise above the blackened portion 
 at d. If the measures of the different parts be such as 
 have been here given, this cannot take place, for the 
 liquid will fall below the fiftieth division before its 
 other surface rises above d. 
 
 The same original volume of gas in a d will last for 
 a long time, as we keep replenishing it as often as the 
 fiftieth division is reached. 
 
MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 263 
 
 The experimenter cannot help remarking that, on sud- 
 denly exposing the sentient tube to a bright light, the 
 liquid for an instant rises on the scale, and on dropping 
 the cap in an instant falls. This important phenomenon, 
 which is strikingly seen under the action of an electric 
 spark, I shall consider hereafter. 
 
 In conclusion, as to comparing the photometric indi- 
 cation at different times, if the gases have the same con- 
 stitution the observations will compare; and if they 
 have not, the value can from time to time be ascertained 
 by exposure to a lamp of constant intensity. To this 
 method I commonly resort. 
 
 From the space occupied in this description the reader 
 might be disposed to infer that this photometer is a 
 very complicated instrument and difficult to use. He 
 would form, however, an erroneous opinion. The pre- 
 liminary adjustment can be made in five minutes, and 
 with it an extensive series of measures obtained. These 
 long details have been entered into that the theory of 
 the instrument may be known, and optical artists con- 
 struct it without difficulty. Though surprisingly sensi- 
 tive to the action of the indigo ray, it is as manageable 
 by a careful experimenter as a common differential ther- 
 mometer. 
 
 UNIVERSITY OF NEW YORK, Sept. 26, 1843. 
 
 NOTE. (From Harpers Magazine, No. 328.) Pro- 
 fessors Bunsen and Roscoe, in their photo-chemical re- 
 searches, made at the University of Heidelberg, and 
 communicated to the Royal Society of London, 1856, 
 say : " The first and only attempt which has been 
 made to refer the chemical action of light to a stand- 
 ard measure is to be found in the researches of Draper. 
 
264 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 
 
 The description of his instrument and mode of observa- 
 tion employed by him was published in 1843. Even 
 with this instrument, which, as we shall show, is in many 
 respects defective, Draper has succeeded in establishing 
 experimentally some of the most important relations of 
 the chemical action of light. In these experiments Dra- 
 per collected hydrogen, evolved by electrolysis over hy- 
 drochloric acid saturated with chlorine, and to this hy- 
 drogen he admitted so much chlorine, either by diffusion 
 from the saturated acid or by electrolysis, that the mixt- 
 ure consisted of nearly equal volumes of the two gases, 
 and entirely or almost entirely disappeared on exposure 
 to light. The alteration in the volume of the gaseous 
 mixture arising from the absorption of the hydrochloric 
 acid formed by the action of the light was read off on a 
 scale, and being within certain limits proportional to the 
 time of exposure, served as a measure of the chemical 
 rays." 
 
 Professors Bunsen and Roscoe, having modified this 
 instrument to suit the objects they had in view, accord- 
 ingly used it in their very exhaustive and important 
 series of researches. 
 
 The measuring lens referred to in previous paragraphs 
 is constructed upon this principle : If half the surface of 
 a convex lens be screened by an opaque body, as a piece 
 of blackened card-board, of course only half the quantity 
 of rays will pass which would have passed had the 
 screen not been interposed ; if one fourth of the lens be 
 left uncovered, only one fourth of the quantity will pass. 
 But in all these instances the focal image remains of the 
 same size as at first. Therefore by adjusting upon the 
 frame of the lens two screens, the edges of which pass 
 through its centre and are capable of rotation thereupon, 
 we shall cut off all light when the screens are applied 
 
MEMOIR XVIII.] THE FERRIC-OXALATE PHOTOMETER. 
 
 265 
 
 edge to edge opposite each other. We shall have 90 
 when they are rotated so as to be at right angles, and 
 180 when they are superposed with their edges coin- 
 ciding, or one of them be taken away. Thus, by setting 
 them in different angular positions, we can have all 
 quantities, from up to 180, and by removing them 
 entirely, reach 360. The lens will thus give an image 
 of a visible object always of the same size, its brilliancy 
 or intensity varying at pleasure in a known proportion. 
 
 In Fig. 42, A, B, B, D is a double convex lens set in a 
 wooden frame, F. Its face can be cov- 
 ered by two semicircles of blackened 
 card -board, one of which revolves on the 
 centre at c. In the figure they are rep- 
 resented as set at right angles, and the 
 quarter of the lens at A is uncovered. 
 
 Fig. 42. 
 
 To the foregoing description of the 
 chlor-hydrogen photometer I may add a 
 reference to another which I have very 
 advantageously used when extreme sen- 
 sitiveness was not required. It depends 
 on the employment of an aqueous solu- 
 tion of ferric oxalate. This substance, 
 which is of a golden-yellow color, may be kept for many 
 years without undergoing any change, if in total dark- 
 ness; but on exposure to a lamp or the daylight it 
 decomposes, carbonic acid escaping, and lemon -yellow 
 ferrous oxalate precipitating. If set in the sunshine, it 
 actually hisses through the escape of the gas. The ray 
 which chiefly affects it is the indigo, the same which 
 affects the chlor-hydrogen photometer and the silver 
 compounds used in photography. This ray, to produce 
 its effect, undergoes absorption, as might be anticipated 
 from what has been previously said in this Memoir, and 
 
266 THE FERRIC-OXALATE PHOTOMETER. [MEMOIR XVIII. 
 
 as is easily proved by causing a sunbeam to pass through 
 two parallel strata of the oxalate, when it will be found 
 that the light which has gone through the first portion 
 is inoperative on the second. 
 
 Other properties which this solution of ferric oxalate 
 possesses strongly recommend it as a photometric agent. 
 Unlike solution of chlorine, it may be very conveniently 
 confined in glass tubes by mercury. In its use there are 
 two points which must be attended to: (1) the lemon- 
 yellow ferrous oxalate must not be permitted to incrust 
 the side of the glass exposed to the light, and thereby 
 injure its transparency ; (2) the ferric solution must be 
 kept nearly at a constant temperature, for its color 
 changes with the heat. At the freezing of water it is 
 of an emerald-green tint ; at the boiling, of a brownish- 
 yellow. With these variations of tint its absorptive ac- 
 tion varies, and therefore its liability to be changed. In 
 an extensive series of experiments made with it, but 
 which I have not yet published, I found that it is great- 
 ly improved by the addition of an aqueous solution of 
 ferric chloride. 
 
 It may be remarked that the oxalate is an excellent 
 photographic substance. A piece of tissue-paper, made 
 yellow by being dipped into a neutral solution of it, 
 when dried in the dark is very sensitive. Its invisible 
 impression may be developed by a weak solution of 
 nitrate of silver, two grains dissolved in an ounce of 
 water answering very well. A weak solution of chlo- 
 ride of gold is a still more sensitive developer. I have 
 in my possession photographs made by both these meth- 
 ods more than thirty years ago, which have undergone 
 apparently no deterioration. 
 
 In the application of ferric oxalate to photometry sev- 
 eral methods may be followed. The course I have most 
 commonly taken has been to determine the quantity of 
 
MEMOIR XVIII.] THE FERRIC-OXALATE PHOTOMETER. 267 
 
 carbonic acid produced, sometimes by volume, sometimes 
 by weight. It is to be understood that before any car- 
 bonic acid can be disengaged the solution must become 
 saturated therewith, and that before we can correctly 
 measure the quantity of light by the quantity of acid 
 produced this dissolved portion must be ascertained. 
 In one of my photometers the expulsion of the dissolved 
 gas is accomplished by exposure to a bath of boiling 
 water, in another by a stream of hydrogen. Both yield 
 satisfactory results. 
 
 But this method by the determination of fche produced 
 carbonic acid is only one of numerous plans; for in- 
 stance, we miorht use the weight of certain metals which 
 
 O O 
 
 the solution after exposure will precipitate. Thus a por- 
 tion which has been made and kept in the dark may be 
 mixed with chloride of gold without any action ensuing, 
 but if it has been illuminated, the weight of metallic 
 gold precipitated is in proportion to the incident light. 
 On this principle I commenced an attempt to determine 
 the hourly and diurnal illumination of a certain locality. 
 At the bottom of a metal tube, arranged as a polar axis, 
 was placed a bulb containing a standard solution of the 
 iron salt, and at the close of the proposed periods the 
 weight of gold it could reduce was ascertained. There 
 is something fascinating in determining the quantity of 
 light which the sun yields by the quantity of gold it 
 can produce. Upon the whole, however, I would rec- 
 ommend those who are disposed to renew these at- 
 tempts to select a method depending on the volume of 
 carbonic acid, for it is always easier to make an obser- 
 vation than an experiment. 
 
 Among the important results which may be expected 
 from these new modes of photometry are the hourly, 
 diurnal, and annual quantities of sunlight. These are 
 important not only in a meteorological point of view, 
 
268 THE FERRIC-OXALATE PHOTOMETER. [MEMOIR XVIII. 
 
 but also as respects physical geography and the great 
 interests of agriculture. The sum of vegetable organi- 
 zation is in all climates and localities a function of the 
 light distributed thereto. And so far as heat is con- 
 cerned, it is not the intensity only, but the absolute 
 quantity, which is to be measured. To each plant, from 
 the moment of its germination to the moment of its 
 maximum development and the completion of its life, a 
 definite quantity of heat and of light must be given. As 
 respects the heat in such inquiries, it is not only the 
 thermometer but the calorimeter which must be consid- 
 ered; and as to the light, the photometers herein de- 
 scribed determine its quantity but not its brilliancy, 
 and therefore answer the indications required. And 
 since it is not merely the temperature of a locality, but 
 also the light of the sun, which is the effective condition 
 of vegetable growth, we see how important even in agri- 
 culture itself these proposed determinations really are. 
 
 To those who would devote themselves to such inqui- 
 ries I recommend as a photometric means a mixture of 
 chlorine and hydrogen where great sensitiveness is re- 
 quired, and in other cases ferric oxalate. 
 
 The chlor-hydrogen and the ferric oxalate photometers 
 act by selective absorption, on the principle of the calo- 
 rimeter; that is to say, they measure the quantity of the 
 radiations they select. 
 
 I may, perhaps not inappropriately, close this Memoir 
 with a brief allusion to an instrument I formerly used 
 very much. It measures the intensity of the radiations 
 it is made to select, and these radiations may be varied 
 at pleasure. I will describe it first as adjusted for the ra- 
 diations of which chlor-hydrogen and ferric oxalate take 
 charge. The description is extracted from a paper I pub- 
 lished in the Philosophical Magazine, August, 1844. 
 
MEMOIK XVIH.] THE SELECTIVE PHOTOMETER. 
 
 269 
 
 Fig. 43. 
 
 Let a wooden box, A B (Fig. 43), six inches long, two 
 wide, and two deep, with perforations at A and B in its 
 ends, be provided; in the centre of its top an aperture 
 three quarters of an inch in diameter is to be made. 
 The box must be blackened interiorly, and a rectangular 
 prism of wood, C, be placed in it, with its right angle in 
 such a position that its edge bisects as a diameter the 
 circular aperture ; over this wooden prism a piece of 
 white paper is pasted, care being taken that where it 
 bends over the right angle of the prism it is folded 
 sharp. So far the reader will recognize in this Ritchie's 
 photometer. Upon the aperture in the top of the box 
 a glass trough, g 7^, is placed ; it is made by cutting a 
 circular hole an inch in diameter in a piece of plate-glass 
 one third of an inch thick, and laying on each side of it 
 a thin piece of plate-glass. This forms a circular trough, 
 in which a strong solution of sulphate of copper and 
 ammonia may be enclosed. Over the trough a tube, d, 
 eight or ten inches long, is placed so that the eye may 
 see distinctly through the aperture in the top of the box 
 the disk of paper, and more especially its dividing di- 
 ameter. 
 
270 THE SELECTIVE PHOTOMETER. [MEMOIR XVIII. 
 
 Lights, E, F, set at the opposite ends of the box, A B, 
 may therefore be compared as regards their photographic 
 intensity, the calculations being made by the common 
 photometric law. And by changing the liquid in the 
 absorbing trough, g h, any radiations may be selected 
 for examination. The instrument may therefore be des- 
 ignated as " the selective absorption photometer." 
 
 From the facts presented in this and the preceding 
 paper, as conveying the modern conception of the rela- 
 tion of luminous and calorific radiations, it may be con- 
 cluded that the thermometer with a blackened bulb is 
 an absolute photometer, and that, in accordance with the 
 principles set forth, many other selective photometers and 
 thermometers may be devised. 
 
MODIFIED CHLORINE. 271 
 
 MEMOIR XIX. 
 
 ON MODIFIED CHLOKINE. 
 
 From the Philosophical Magazine, July, 1844. 
 
 [This Memoir was read at the meeting of the British Association held at Cork, 
 1843. The concluding paragraphs were subsequently added.] 
 
 CONTENTS : Description of the experiment. The change in the chlorine 
 is not transient. There are two stages in the phenomenon. Rays are 
 absorbed in producing this change. The indigo ray is absorbed. The 
 action is positive from end to end of the spectrum. The indigo ray 
 forms hydrochloric acid and also produces the preliminary modifica- 
 tion. Change in other elementary bodies. Verification of these results 
 with the chlor-hydrogen photometer. 
 
 CHLOKINE gas which has been exposed to the daylight 
 or to sunshine possesses qualities not possessed by chlo- 
 rine made in the dark. 
 
 This is shown by the circumstance that chlorine which 
 has been exposed to the sunshine has obtained from 
 that exposure the property of speedily uniting with hy- 
 drogen gas, a property not possessed by chlorine made 
 and kept in the dark. 
 
 This quality gained by the chlorine arises from its 
 having absorbed chemical rays corresponding in refran- 
 gibility to the indigo. It is not a transient, but appa- 
 rently a permanent property, the rays so absorbed be- 
 coming latent, and the effect lasting for an unknown 
 period of time. The facts here presented will be inter- 
 esting to chemists, because they plainly lead us to sus- 
 pect that the descriptions we have of the properties of 
 all elementary and compound bodies are either inaccu- 
 rate or confused. These properties are such as bodies 
 
272 MODIFIED CHLORINE. [MEMOIR XIX. 
 
 exhibit after they have been exposed to light; we still 
 require to know what are the^ properties they possess 
 before exposure to such influences. 
 
 Natural philosophers will also find an interest in 
 these phenomena, for they finally establish for the chem- 
 ical rays two important facts : 1st, that those rays are 
 absorbed by ponderable bodies; and, 2d, that they be- 
 come latent after the manner of heat. Some years ago 
 I endeavored to prove that these things held for a com- 
 pound substance, the iodide of silver (Phil. Mag., Sep- 
 tember, 1841). 
 
 For reasons which will be obvious as the description 
 proceeds, I shall speak of chlorine which has been ex- 
 posed to the beams of the sun as modified chlorine. 
 
 I. Description of the ^Experiment. 
 
 In two similar glass tubes place equal volumes of 
 chlorine, made from peroxide of manganese and hydro- 
 chloric acid by lamplight, and carefully screened from 
 access of daylight. Expose one of the tubes to the full 
 sunbeams for some minutes, or, if the light be feeble, for 
 a quarter of an hour : the chlorine in it becomes modi- 
 fied. Keep the other tube during this time carefully in 
 a dark place ; and now, by lamplight, add to f both equal 
 volumes of hydrogen gas. These processes are best car- 
 ried on in a small porcelain or earthenware trough, filled 
 with a saturated solution of common salt, which dis- 
 solves chlorine slowly; and to avoid explosions operate 
 on limited quantities of the gases. Tubes that are eight 
 inches long and half an inch in diameter will answer 
 very well. The two tubes now contain the same gaseous 
 mixture, and only differ in the circumstance that one is 
 modified and the other not. Place them, therefore, side 
 by side before a window, through which the entrance of 
 daylight can be regulated by opening the shutter; and 
 
MEMOIR XIX.] MODIFIED CHLORINE. 273 
 
 now, if this part of the process be conducted properly, 
 it will be seen that the modified chlorine commences to 
 unite with the hydrogen and the salt water rises in that 
 tube. But the unmodified chlorine shows no disposition 
 to unite with its hydrogen, and the liquid in its tube 
 remains motionless for a long time. Finally, as it be- 
 comes slowly modified by the action of the daylight im- 
 pinging on it, union takes place. From this, therefore, 
 we perceive that chlorine which has been exposed to 
 the sun will unite promptly and energetically with hy- 
 drogen ; but chlorine that has been made and kept in 
 the dark shows no such property. 
 
 As I doubt not this remarkable experiment will be 
 repeated by chemists, I will add that the only point to 
 which attention in particular is to be given is in the 
 final exposure to the light. This must not be too feeble, 
 or the action will be tedious; but the direct sunbeam 
 must be sedulously excluded or an explosion will result. 
 A room illuminated by one small window looking to the 
 north answers very well. It need scarcely be added that 
 care must be taken that both tubes are illuminated alike. 
 
 II. The Change in the Chlorine is not Transient. 
 
 Now it might be supposed that this apparent exalta- 
 tion of the electro-negative properties of the chlorine is 
 only a transient thing, which would speedily pass away, 
 the gas reverting to its original condition. 
 
 To show that this is not so, modify some chlorine in a 
 tube as before. Place it for an hour or two in the dark 
 along with the tube of unmodified chlorine with which 
 it is to be compared, then to both add hydrogen. Ex- 
 pose them as in the former experiment to the daylight, 
 and the result will turn out as before; the modified 
 chlorine forming hydrochloric acid at once, and the un- 
 
 modified refusing to do so. 
 
 S 
 
274 MODIFIED CHLORINE. [MEMOIR XIX 
 
 This, therefore, shows that the change which the sun- 
 beams impress upon chlorine .is to a certain extent a 
 permanent change, and, unlike a calorific effect, it does 
 not spontaneously and rapidly pass away. 
 
 III. There are Two Stages in the Phenomenon. 
 
 Let us now make inquiry into the nature of the 
 change thus impressed on the chlorine. This, I shall 
 show, rests in the circumstance of the absorption of rays 
 which correspond in refrangibility to the indigo, and ap- 
 pear to become latent 
 
 In a tube, over salt water, mix together equal volumes 
 of unmodified chlorine and hydrogen gas. Expose it to 
 the daylight, marking the time at which the exposure 
 commences. Watch the level of the liquid in the tube 
 narrowly, and, though stationary for a considerable time, 
 after a certain period has elapsed it will be seen on a 
 sudden to start and commence rising. Observe now 
 hoAV fiir it will rise during a period equal to the time 
 that elapsed between the first exposure and the begin- 
 ning of the rise, and it will be seen that one fourth or 
 half the gases will disappear. 
 
 It is obvious that from the first moment of exposure 
 the rays must have been exerting their influences on the 
 mixture. As will presently be proved, absorption has 
 been all along taking place. There are, therefore, two 
 distinct phenomena exhibited by this experiment. There 
 is a period during which, though large quantities of the 
 dark rays are disappearing, no visible change is pro- 
 duced; there is a second period, during which absorp- 
 tion is accompanied by a remarkable chemical effect 
 the production of hydrochloric acid. From these things 
 we gather that a definite amount of the chemical rays 
 must disappear and become latent before hydrochloric 
 acid can form. The phenomenon is not unlike that of the 
 
MEMOIR XIX.] MODIFIED CHLORINE. 275 
 
 disappearance of a definite quantity of heat in the pas- 
 sage of ice into the condition of water. 
 
 A mixture of chlorine and hydrogen does not, therefore, 
 instantly give rise to the production of hydrochloric acid 
 on exposure to the light, but as a preliminary condition 
 a certain definite amount of absorption must take place. 
 
 Now if this were a mere molecular disturbance, such 
 as might be brought about by the action of heat, we 
 should expect to find it transient and speedily passing 
 away. Such, however, is far from being the case. As 
 with simple chlorine, so with this mixture : after it has 
 been modified it loses its quality very slowly. I have 
 observed that after a week or more has elapsed since it 
 was first exposed to the light, it commences to contract 
 when placed in a feeble gleam. 
 
 IV. Hays are Absorbed in Producing this Change. 
 
 I have thus far assumed that the rays which bring 
 about these changes are absorbed ; the following is the 
 proof which I have to offer : 
 
 Over a tube half an inch in diameter and six inches 
 long, closed at its upper extremity and open at its lower, 
 invert a jar of the same length and one inch and a half 
 in diameter. Fill the tube and the jar at the salt-water 
 trough, about two thirds full, with the same mixture of 
 chlorine and hydrogen. Expose them to diffuse day- 
 light. Now it is clear that no rays can gain access to 
 the tube except after having passed through the gaseous 
 mixture in the jar. After a certain space of time the 
 level of the liquid in the jar commences to rise, but that 
 in the tube will remain much longer wholly stationary. 
 
 It therefore appears that a beam which has passed 
 through a mixture of chlorine and hydrogen has lost, to 
 a great extent, the quality of bringing about the union 
 of a second portion of the mixed gases through w r hich it 
 
276 MODIFIED CHLORINE. [MEMOIR XIX. 
 
 may be caused to traverse. The active rays have been 
 absorbed; they disappear from .the beam and are lost in 
 producing their first effect. 
 
 A beam of light loses its energy in producing a chem- 
 ical effect ; the beam, as well as the medium on which it 
 acts, becomes changed. I have a series of results which 
 proves that this takes place for a great variety of com- 
 pound bodies. 
 
 V. It is the Indigo Hay which is Absorbed. 
 
 As has been said, it is a ray corresponding in refrangi- 
 bility to the indigo which produces these results. 
 
 In a small porcelain trough I inverted, side by side, 
 ten tubes, each of which was three inches long and one 
 third of an inch in diameter, the trough being filled with 
 salt water. I passed into each tube a certain quantity 
 of unmodified chlorine and hydrogen. A beam of the 
 sun, being directed by a heliostat into a dark room, was 
 dispersed horizontally by a flint-glass prism, and the 
 trough with its tubes so placed as to offer an exposure 
 to the different colored rays. The aperture admitting 
 the beam was about half an inch in diameter. For a 
 while no movement was observed in any of the tubes ; 
 but as soon as the preliminary absorption previously de- 
 scribed was over, the level of the liquid began to rise. 
 In the red and in the orange no movement could be 
 perceived, in the violet only after a time ; but first of all 
 the tube that was immersed in the indigo light was in 
 action, and exhibited finally a very rapid rise ; this was 
 soon followed by the tube that was in the space where 
 the indigo and violet joined, then by that in the violet 
 and that in the blue ; the tube in the green was next in 
 order. The following table gives the numerical results 
 obtained by observing the time which elapsed before 
 movement took place in each tube : 
 
MEMOIR XIX.] 
 
 MODIFIED CHLORINE. 
 
 277 
 
 TABLE I. 
 
 Name of ray. 
 
 Time. 
 
 Name of ray. 
 
 Time. 
 
 Extreme red . .. 
 
 * 
 
 Indigo 
 
 1 50 
 
 Red and orange. ... 
 Yellow and green 
 
 + 100.00 
 
 r>2 oo 
 
 Indigo and violet. . 
 Violet 
 
 2.00 
 9 95 
 
 Green and blue 
 
 4 00 
 
 Violet 
 
 5 00 
 
 Blue 
 
 2 33 
 
 Extreme violet 
 
 5 50 
 
 
 
 
 
 Many years ago M. Berard made experiments on the 
 explosion of chlorine and hydrogen, and concluded from 
 his results that it was brought about by the violet ray. 
 This was at a time when the methods of making these 
 experiments were less exactly known. It is a very easy 
 matter to prove that in reality the indigo is the active 
 ray, and that from a maximum point which is in. the in- 
 digo, but towards the blue, the effect gradually dimin- 
 ishes to each end of the spectrum. 
 
 The following table gives the calculated approximate 
 intensity of the chemical force for each ray, deduced 
 from the foregoing experiment : 
 
 TABLE II. 
 
 Name of ray. 
 
 Force. 
 
 Name of ray. 
 
 Force. 
 
 
 
 Indigo 
 
 66 60 
 
 Red and orange.... 
 Yellow and green 
 
 1.00 
 1.90 
 
 Indigo and violet. . 
 Violet 
 
 f>0.00 
 44 40 
 
 
 95 oo 
 
 Violet 
 
 20 00 
 
 Blue 
 
 42.90 
 
 Extreme violet 
 
 18.10 
 
 There is a great advantage which experiments con- 
 ducted in this way possess over those depending for 
 their indication on the stains impressed on daguerre- 
 otype plates or sensitive papers. In those cases we ob- 
 tain merely a comparative contrast for different regions 
 of the spectrum; in this we have absolute measures de- 
 termined by a definite chemical effect and the rise of a 
 
 * Even after the longest exposure I had the means of giving it, no movement took 
 place in the tube in the extreme red, and I am doubtful about that in the red and 
 orange. 
 
278 MODIFIED CHLORINE. [MEMOIR XIX. 
 
 liquid in a graduated tube; and from this we gain just- 
 er views of the true constitution of the spectrum. On 
 studying the numbers in the foregoing table, or, better 
 still, if we project them, it will appear what an enormous 
 difference there is in the chemical force of the different 
 rays. In the experiment from which I have deduced 
 this table it appears that the force of the indigo ray 
 exceeds that of the orange in a greater ratio than 66 to 
 1 ; and from the circumstances under which the experi- 
 ment is made this difference must be greatly underrated. 
 There is always diffused light in the room coming from 
 the intromitted beam, and this accelerates the rise in the 
 less refrangible tubes ; then, again, it is impossible that 
 the tube which gives the greatest elevation shall coin- 
 cide mathematically with the maximum point and ex- 
 press the maximum effect. 
 
 From some estimates I have made I am led to believe 
 that in point of chemical force, for this mixture of chlo- 
 rine and hydrogen, the indigo ray exceeds the red in a 
 higher ratio than 500 to 1. 
 
 VI. The Action is Positive from Mid to End of the 
 
 Spectrum. 
 
 M. Becquerel found that for an iodized silver plate the 
 red, the orange, and the yellow rays possess the quality 
 of continuing the action begun by the more refrangible 
 colors ; he therefore names these " rayons continuateurs? 
 For the same compound I found that those rays, acting 
 conjointly with the diffused daylight, exerted a negative 
 agency. It is therefore desirable to understand whether, 
 with respect to the gases now under consideration, the 
 less refrangible rays exert anything in the way of an ac- 
 tion of depression or hindrance to union. By direct ex- 
 periment I found that this was not the case, the action 
 being positive from end to end of the spectrum. This 
 
MEMOIR XIX.] MODIFIED CHLORINE. 279 
 
 can be shown by removing the tubes, after they have 
 been in the spectrum for an hour or two, into the gleams 
 of daylight. One by one they exhibit after a time a 
 rise, the order being the green first, then the yellow and 
 the orange, and at last the red. And if at the same time 
 a tube which has been kept in the dark be exposed 
 along with them, they will all rise before it, showing 
 that modification had set in and been going on in them 
 all; that it had been more active in the green than in 
 the yellow, in the yellow than in the orange, in the or- 
 ange than in the red; and, had the exposure to the spec- 
 trum been long enough, the liquid in every one of the 
 tubes would have risen. 
 
 VII. The Indigo Ray forms the Hydrochloric Acid as 
 well as produces the Preliminary Modification. 
 
 It only remains now to inquire whether the rays caus- 
 ing the production of the hydrochloric acid are those 
 which effect the modification of the chlorine; in other 
 words, whether the first stage of the process is brought 
 about by the same agent which carries on the second. 
 The experiment I have just described shows that modifi- 
 cation is most actively produced by the indigo ray, and 
 it is easy to show that it is the same ray which carries 
 on the second part of the process ; for, if before placing 
 the tubes in the prismatic spectrum we expose them to 
 the daylight, so that the liquid has just commenced to 
 rise in each, and then to the spectrum, it will be found 
 that the liquid of the tube in the indigo rises most rap- 
 idly, and the others in the order stated before. There- 
 fore we perceive that the same ray commences, carries 
 on, and completes the process. 
 
 Few substances can exceed in sensitiveness to light a 
 
 o 
 
 mixture of chlorine and hydrogen previously modified. 
 Brought into the obscure daylight of a gloomy chamber, 
 
230 MODIFIED CHLORINE. [MEMOIR XiX. 
 
 it is remarkable how promptly the level of the liquid in 
 the tube rises; how, when the shutters are successively 
 thrown open, the action becomes more and more ener- 
 getic; and how, in an instant, it stops when the instru- 
 ment is shaded by a screen. 
 
 I have not recorded in this communication a multi- 
 tude of experiments of detail supporting the conclusions 
 here drawn. It has been my object on this occasion to 
 call attention to the fact that chlorine, an elementary 
 body, undergoes a change after exposure to the light ; a 
 change which appears to produce an exaltation of its 
 electro-negative properties, as is shown by its power of 
 uniting more energetically with hydrogen. This change 
 must not be confounded with those transient elevations 
 of activity due to increased temperature, inasmuch as 
 this is more permanent in its character. It arises from 
 the absorption of rays existing most abundantly in the 
 indigo space of the spectrum. That the phenomenon is 
 due to a true absorption is fully shown by the circum- 
 stance that a beam which has produced this effect has 
 lost the quality of ever after producing a similar result. 
 This is borne out by what we observe to take place 
 when a feeble light falls on a mixture of chlorine and 
 hydrogen prepared in the dark. A certain space of 
 time elapses before any formation of hydrochloric acid 
 occurs, during which the absorption in question is going 
 on ; and when that is completed, and the mixture is 
 modified, union of the gases begins and hydrochloric 
 acid forms. From end to end of the spectrum the action 
 is positive, and differs only in intensity; but this differ- 
 ence in intensity opens before us new views of the con- 
 stitution and character of the solar beam, 
 
 UNIVERSITY OF NEW YORK, June 20, 1813. 
 
MEMOIR XIX.] MODIFIED CHLORINE. 
 
 Paragraphs subsequently added. ^ 
 
 Chlorine is not the only elementary 
 which the radiations produce a change. In his 
 on phosphorus, Berzelius remarks: "Light produces in^ifcv 
 (phosphorus) a peculiar change, of which the intimate 
 nature is unknown; and which, so far as we can judge 
 at present, does not alter its weight. It makes it take a 
 red tint. This phenomenon occurs not only in a vac- 
 uum, even in that of a barometer, but also in nitrogen 
 gas, in carburetted hydrogen, under water, alcohol, oil, 
 and other liquids. When we expose to the sunlight 
 phosphorus dissolved in ether, oil, or hydrogen gas, it in- 
 stantly separates under the form of red phosphorus ; it 
 undergoes very rapidly this modification in violet liglit, 
 or in glass vessels of a violet color. The light of the 
 sun makes it easily enter into fusion in nitrogen gas, but 
 it does not melt in hydrogen, and in the torricellian 
 vacuum it sublimes in the form of brilliant red scales " 
 (Berzelius, Traite, torn, i., p. 258). 
 
 Again, when speaking of phosphuretted hydrogen, he 
 says: "Exposed to the influences of the direct solar light 
 this gas is decomposed, a part of the phosphorus sep- 
 arates under the form of red phosphorus, and is depos- 
 ited on the interior surface of the glass. If we cover 
 the vessel which contains the gas imperfectly, no phos- 
 phorus is deposited on the covered spaces " (Ib., torn, i., 
 p. 265). 
 
 As Berzelius does not give these experiments as his 
 own, and I do not know to whom we are indebted for 
 them, I repeated some of them. Among other corrobo- 
 rative results it appeared that a piece of phosphorus of 
 a pale or whitish color, in a vessel filled with pure and 
 dry carbonic -acid gas, placed in the sunshine, rapidly 
 exhibited the phenomenon in question. Eventually the 
 
282 
 
 MODIFIED CHLORINE. 
 
 [MEMOIR XIX. 
 
 phosphorus became of a deep blood -red color, and on 
 the sides of the glass towards the light feathery crystals 
 formed, the tint of which bore a close resemblance to 
 that of the red prussiate of potash. 
 
 Since the invention of the chlor-hydrogen photometer 
 I have been able to observe more closely the habitudes 
 of chlorine. In the description given of that instrument 
 it is recommended to cast aside the first observation, be- 
 cause it never gives an accurate estimate of the true 
 effect. When a mixture of chlorine and hydrogen is 
 exposed, hydrochloric acid does not immediately form ; 
 but a preliminary absorption is necessary, and then at the 
 end of a certain period contraction begins to take place. 
 
 Such a photometer exposed to the daylight is much 
 too powerfully affected to allow the successive stages 
 of change to be distinctly made out; the preliminary 
 modification is accomplished so rapidly that the indica- 
 tions of it are merged and lost in the contraction which 
 instantly follows. It is necessary therefore that we 
 should operate with a small lamp-flame. 
 
 To such a flame I exposed a mixture of chlorine and 
 hydrogen, and marked the number of seconds which 
 elapsed before contraction, arising from the production 
 of hydrochloric acid, took place. The first indications 
 of movement occurred at the close of 600 seconds. 
 
 The index then moved through the first degree in 480 seconds ; 
 
 second 
 
 third 
 
 fourth 
 
 fifth 
 
 sixth 
 
 105 
 
 130 
 
 95 
 
 93 
 
 93 
 
 and continued to move with regularity at the same rate. 
 
 These observations, therefore, prove that a very large 
 amount of radiant matter is absorbed before chemical 
 combination takes place, and that in the case of chlorine 
 and hydrogen the total action is divisible into two 
 
MEMOIR XIX.] MODIFIED CHLORINE. 283 
 
 periods: the first during which a simple absorption is 
 taking place without a chemical effect, the second during 
 which absorption is attended with the production of 
 hydrochloric acid. 
 
 The facts which I am endeavoring to set forth promi- 
 nently in this Memoir are 1st, the preliminary modifi- 
 cations just discussed; and, 2d, the persistent character 
 of the change impressed upon chlorine when it has been 
 exposed to the sun, an effect wholly unlike a calorific 
 effect, which would soon disappear. 
 
 By resorting to the chlor-hydrogen photometer we ob- 
 tain information equally distinct upon the second, point, 
 that the preliminary modification is not a transient effect 
 which at once passes away, but is, on the contrary, a per- 
 sistent change. 
 
 I modified the chlorine and hydrogen contained in the 
 instrument, and kept it in the dark for ten hours. On 
 exposure to the lamp-rays it moved after a few seconds, 
 showing, therefore, that the change impressed on the 
 chlorine was not lost. In the former case 600 seconds 
 had elapsed before any movement was visible. 
 
 When, however, we remember that the invisible im- 
 ages on daguerreotype plates, and even photographic im- 
 pressions on surfaces of resin, and probably all other 
 similar changes, are slowly effaced, it would be prema- 
 ture to conclude that modified chlorine does not. revert 
 to its original condition. I have sometimes thought 
 that there were in several of niy experiments indications 
 that this was taking place, but would not be understood 
 as asserting it positively. Whether it be so or not, one 
 thing is certain, that the taking on of this condition and 
 the loss of it is a very different affair from any transient 
 exaltation of action due to a temporary elevation of tem- 
 perature, or the contrary effect produced by cooling. 
 
 April 26, 1844. 
 
284 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 MEMOIR XX. 
 
 ON THE ALLOTROPISM OF CHLORINE AS CONNECTED WITH 
 THE THEORY OF SUBSTITUTIONS. 
 
 From the American Journal of Science and Art, Vol. XLIX. ; Philosophical Mag- 
 azine, Nov., 1845. 
 
 CONTENTS : Chlorine exists in two states, active and passive. Decompo- 
 sition of water by it in the sunlight. Facts connected with this de- 
 composition. The relations of chlorine and hydrogen. The allotro- 
 pism of chlorine. Connection of these facts with the theory of substi- 
 tutions. 
 
 THE researches of M. Dumas on chemical types have 
 shown that between chlorine and hydrogen remarkable 
 relations exist, indicating that the electrical characters 
 of elementary atoms are not essential, but rather inci- 
 dental properties. The extension of these researches 
 has given much weight to the opinion that the electro- 
 chemical theory may be regarded as failing to account 
 for the replacement of such bodies as hydrogen by chlo- 
 rine, bromine, oxygen, etc. I do not know that as yet 
 any direct evidence has been offered that the electrical 
 character of an atom is not an essential quality, but one 
 that changes with circumstances. It appears to be 
 rather a matter of inference than of absolute demonstra- 
 tion. 
 
 It is the object of this Memoir to furnish such direct 
 evidence, and to show that chlorine, the substance which 
 has given rise to the discussions connected with the 
 theory of substitutions, under the very circumstances 
 contemplated, has its electro-chemical relations changed. 
 
 More than two years ago I brought before the British 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 035 
 
 Association some of the facts. The connection of these 
 experiments with the discussion between the theory of 
 substitutions and the electro-chemical theory is obvious. 
 
 Very recently M. Berzelius has published an impor- 
 tant paper on the allotropism of simple bodies, the ob- 
 ject of which is to point out that many of those bodies 
 can assume different qualities by being subjected to cer- 
 tain modes of treatment. Thus carbon furnishes three 
 forms charcoal, plumbago, and diamond. 
 
 To a certain extent these views coincide with those 
 which have offered themselves to me from the study of 
 the properties of chlorine. They are not, however, al- 
 together the same. M. Berzelius infers that elementary 
 bodies can, as has been said, assume under varying cir- 
 cumstances different qualities. The idea which it is at- 
 tempted to communicate in this Memoir is simply this, 
 that a given substance, such as chlorine, can pass from a 
 state of high activity, in which it possesses all its well- 
 known properties, to a state of complete inactivity, in 
 which even its most energetic affinities disappear. And 
 that between these extremes there are innumerable in- 
 termediate points. Between the two views there is, 
 therefore, this essential difference: from the former it 
 does not appear what the nature of the newly assumed 
 properties may be ; from the latter they must obviously 
 be of the same character, and differ only in intensity or 
 degree diminishing from stage to stage until complete 
 inactivity results. 
 
 In the case of chlorine the same activity which is com- 
 municated by the indigo rays can also be communicated 
 by a high temperature, or by the action of platinum. 
 
 The points which this Memoir is intended to establish 
 are 
 
 I. That chlorine gas can exist under two forms. In 
 the same way that metallic iron can exist as active or 
 
286 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 passive iron, chlorine can assume the active or passive 
 state. 
 
 II. Having established the fact of the allotropism of 
 chlorine, I shall then show its connection with the the- 
 ory of substitutions of M. Dumas, and how the most re- 
 markable points in that theory may be easily accounted 
 for. 
 
 The time, perhaps, has not yet arrived for offering a 
 complete mechanical explanation of the assumption of 
 an active or passive state. It may be remarked that a 
 very trivial modification of our admitted views of the re- 
 lation between atoms and their properties is all that is 
 required to give a consistent explanation of every one of 
 these facts. Instead of regarding the specific qualities 
 of an atom as appertaining equally to the \vhole of it in 
 the aggregate, we have merely to assume that there is a 
 relation between its properties and its sides, and that 
 any force which can make it change its position upon its 
 own axis will throw it into the active or passive state. 
 But this is nothing more than the well-known idea of 
 the polarity of atoms. 
 
 Phenomena of the Decomposition of Water by Chlorine 
 in the Hays of the Sun. 
 
 From the various facts which might be employed as 
 offering the means of establishing the allotropism of 
 chlorine, I shall select those which arise from an exam- 
 ination of the phenomena of the decomposition of an 
 aqueous solution of chlorine by the rays of the sun. 
 
 For many years it has been known that an aqueous 
 solution of chlorine undergoes decomposition by the ac- 
 tion of the solar rays. Several of the most remarkable 
 phenomena connected with this decomposition appear to 
 have been overlooked. Among such may be mentioned 
 the singular fact that chlorine which has been thus in- 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 287 
 
 fluenced by the sun has obtained the quality of effecting 
 this decomposition subsequently, to a measured extent, 
 even in the dark. Not to anticipate what I have to 
 offer on this point, I shall now proceed in the first place 
 to establish the various facts connected with the decom- 
 position in question. 
 
 Having provided a number of small glass vessels, con- 
 sisting of a bulb and neck of the capacity of from 1.5 to 
 2 cubic inches, I filled them with a solution of chlorine 
 in recently boiled w^ater, and inverted them in small 
 glass bottles containing the same solution, as shown ^_^ 
 in Fig. 44. With these bulbs the following exper- (~ \ 
 inients were made : 
 
 I. An aqueous solution of chlorine does not de- 
 compose in the dark. 
 
 One of the bulbs was shut up in a dark closet, 
 
 and kept there for a week, being examined from Fi s- 44 - 
 time to time. No decomposition was perceptible, for no 
 gas collected in the upper part of the bulb. 
 
 II. An aqueous solution of chlorine decomposes in the 
 light. 
 
 One of the bulbs was placed in a beam of the sun re- 
 flected into the room by a heliostat. For sixteen min- 
 utes no change was perceptible, then small bubbles of 
 gas made their appearance; they increased in quantity 
 for a time, but finally the speed of decomposition became 
 uniform. On analysis by explosion with hydrogen, after 
 washing out any chlorine contained in it, this gas was 
 found to contain 97 per cent, of oxygen. 
 
 III. The rapidity of this decomposition depends on 
 the quantity of the rays and on the temperature. 
 
 In various repetitions of these experiments on differ- 
 ent days I soon convinced myself that the rate of evo- 
 lution of the oxygen depended on the quantity of the 
 rays. Among other proofs I may mention this: After 
 
288 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 ascertaining the rate of decomposition in the reflected 
 beam, if the bulb be set in the direct sunshine the bub- 
 bles increase in number; the total quantity of oxygen 
 evolved becoming greater in the same space of time, an 
 effect obviously due to the difference of intensity of the 
 reflected and incident beams. When a certain point is 
 gained, apparently no farther increase of effect takes 
 place on increasing the brilliancy of the light, as I found 
 by employing a convex lens. 
 
 With respect to the influence of temperature. If, 
 while one of the bulbs is actively evolving gas in the 
 sun-rays, it be warmed by the application of a spirit- 
 lamp, the amount of gas thrown off becomes very much 
 greater. A difference of a few degrees produces a strik- 
 ing effect. As an illustration of this I placed in the sun- 
 shine two bulbs which were nearly alike, except that 
 one of them was painted black with India-ink on that 
 portion which was farthest from the sun. The rays 
 coming through the transparent part had access to the 
 solution, and then, impinging on the dark side, raised its 
 temperature. On measuring the quantity of gas col- 
 lected, it was found 
 
 In the transparent bulb 3.46 
 
 In the half-blackened bulb 6.19 
 
 IV. The decomposition of water, once begun in the 
 sunbeams, goes on afterwards in the dark. 
 
 1st. This very important fact may be established in a 
 variety of ways. Thus, if a bulb be removed from the 
 sunshine w r hile it is actively evolving gas, and be placed 
 in the dark after all the gas has been turned out of it, a 
 slow evolution continuously goes on, the gas collecting 
 in the upper part of the bulb. 
 
 2d. A bulb, A, Fig. 45, having a neck, , the end of 
 which was bent at c upwards at an angle of about 45 
 degrees, was employed. After exposure to the sun, by 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 289 
 
 inverting the bulb, and with one finger closing the ex- 
 tremity c, the gas disengaged could be trans- ^-^ 
 ferred to a graduated vessel and measured. A ( ) 
 I satisfied myself by several variations of this 
 arrangement that the small quantity of water 
 introduced from time to time when the gas 
 bubble escaped from the end of the tube c 
 exerted no essential influence on the phenom- Fi =- 45 - 
 enon. The following table shows the amount of gas 
 evolved in the dark during the periods indicated. 
 
 The bulb having been exposed to the sunshine, in ten 
 minutes the evolution of gas commenced, and in an hour, 
 0.107 cubic inch having collected, this was thrown away 
 and the arrangement placed in the dark. To prevent 
 the undue escape of the chlorine, the flat piece of glass 
 d was laid on the open end of the tube c. In each suc- 
 cessive hour the quantity of gas given in the following 
 table was then evolved: 
 
 First hour... ,. 0.0162 
 
 Second 
 
 . 0159 
 
 Third 
 
 0086 
 
 Fourth 
 
 .". 0.0060 
 
 Fifth 
 
 0038 
 
 Sixth 
 
 .. 0.0031 
 
 And for four days afterwards gas was collecting in the 
 bulb in diminished quantities. 
 
 V. This evolution of gas in the dark is not merely a 
 gradual escape of oxygen, originally formed while the 
 solution was exposed to the sun, but is traceable to an 
 influence continuously exerted by the chlorine arising in 
 properties it has acquired during its exposure to the 
 rays. 
 
 If a bulb which has been exposed to the sun be raised 
 by a spirit-lamp to such a temperature that its gaseous 
 constituents are rapidly evolved, its extremity dipping 
 beneath some of the solution in the bottle, after allowing 
 
 T 
 
290 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 a sufficient space of time for the disengaged chlorine to 
 be redissolved, and the oxygen be turned out of the 
 bulb, it will be found on keeping the arrangement in 
 the dark that oxygen will slowly disengage as before. 
 
 Now there is every reason to believe that any small 
 amount of oxygen dissolved in the liquid would be ex- 
 pelled with the chlorine at a high temperature. We 
 therefore have to infer that the chlorine after this treat- 
 ment still retains the quality of causing the decompo- 
 sition to go steadily forward. 
 
 The oxygen which thus accumulates in the course of 
 time in the dark, after an exposure to the sun, does not 
 arise from any portion of that gas held in a state of 
 temporary solution, nor from peroxide of hydrogen, nor 
 from chlorous acid in the liquid undergoing partial de- 
 composition. From any of these states a high temper- 
 ature would disen^a^e it. 
 
 o o 
 
 VI. The evolution of gas is not of the nature of a 
 fermentation ; for when it once sets in, the molecular 
 motion is not propagated from particle to particle, but 
 affects only those originally exposed to the rays. 
 
 Let a bulb be filled with chlorine- water which has 
 been exposed to the sun, and in a second bulb place a 
 quantity of the same liquid equal to about one third of 
 its capacity. Fill up the remaining two thirds with 
 chlorine -water which has been made and kept in the 
 dark; and after keeping both bulbs in obscurity for 
 some days, measure the volumes of gas they contain. If 
 the qualities of chlorine w r hich has been changed by 
 exposure were communicable by contact or close prox- 
 imity from atom to atom, we might expect that both the 
 bulbs would yield the same quantity of gas; but this is 
 far from being the case, and in such an experiment I 
 found that the bulb containing the mixture gave only 
 one fourteenth of the gas found in the other. 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 291 
 
 VII. The quantity of gas thus collecting in the dark 
 depends on the intensity of the original disturbance, 
 which in its turn depends on the time of exposure to 
 the rays, to their intensity, and other such conditions. 
 In other words, the rays are perfectly definite in their 
 action a long exposure giving a larger amount of sub- 
 sequent decomposition, and short exposure a less amount. 
 
 On exposing a bulb filled with chlorine-water to the 
 rays until bubbles of gas began to appear, and a second 
 one until the decomposition had been actively going on 
 for a quarter of an hour, and then transferring both to 
 the dark and measuring the oxygen collected at the end 
 of a day, I found in the former one twelfth of what was 
 collected in the latter. 
 
 VIII. In a given quantity of chlorine -water, the de- 
 composition in the dark corresponding to a given expos- 
 ure to the light having been performed, and the proper 
 quantity of oxygen evolved and the phenomenon ended, 
 it can be re-established from time to time, as long as 
 any chlorine is found in the liquid, by a renewed expos- 
 ure to the sun. 
 
 In a glass vessel, like Fig. 46, which, indeed, was noth- 
 ing more than one of Liebig's dry- 
 ing apparatus, I placed a sufficient 
 quantity of chlorine -water to fill 
 the larger vessel, and the vertical 
 tubes half full. After exposing 
 this to the light for a certain time, Fi - 46 - 
 
 until decomposition had fairly set in, I placed it in the 
 dark and found that for several days it gave off gas 
 the quantity continually diminishing. Finally, no more 
 gas was evolved. But the liquid still contained free 
 chlorine, as was shown by its color. I therefore again 
 exposed it to the sun, and, repeating the former observa- 
 tion, found that it evolved gas for several days in the 
 
292 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 dark. A third exposure was followed by the same re- 
 sult. 
 
 The form of this vessel renders it very convenient for 
 . these experiments ; because when sufficient gas has col- 
 lected for the purpose of observation, it is easily removed 
 by inclining the instrument, without the necessity of 
 introducing fresh quantities of liquid. 
 
 Having found, as has been said, that the rapidity of 
 the decomposition depended to a certain extent on the 
 temperature, it seemed desirable to determine whether 
 heat alone could bring about the change. 
 
 IX. The decomposition of water by chlorine is not 
 brought about by mere elevation of temperature when 
 the liquid is set in the sunbeam : although heat accel- 
 erates, it does not give rise to the phenomenon. 
 
 1st. I raised by a spirit-lamp the temperature of one 
 of the bulbs nearly to its boiling-point, until so much 
 gas was given off that all the liquid was expelled from 
 the tube to the bottle beneath. If at this temperature, 
 which probably was higher than 200 Fahr., chlorine 
 had been able to decompose water, an equivalent quan- 
 tity of oxygen would have been produced ; but on al- 
 lowing the apparatus to cool, all the gas was reabsorbed 
 with the exception of a small bubble, amounting in vol- 
 ume to y^Vr of the water. This bubble, which was left 
 after the chlorine was recondensed, I found in three dif- 
 ferent experiments contained 32, 33, and 36 per cent, of 
 oxygen, the remainder being nitrogen ; but this being 
 nearly the constitution of the gas dissolved in ordinary 
 water, the source from which the small bubble came 
 was inferred to be the water used in these experiments. 
 
 2d. One of the bulbs was painted black all over with 
 India-ink. Its temperature now rose much higher than 
 in former experiments when it was set in the sun, but 
 not a bubble of oxygen appeared. 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLOKINE. 293 
 
 X. When chlorine-water has been exposed to the sun, 
 the oxygen accumulated in it is readily expelled by rais- 
 ing the temperature. 
 
 Having exposed one of the bulbs used in the last ex- 
 periment until it was actively evolving gas, I raised its 
 temperature with the spirit-lamp until the bulb was full 
 of gas. But on cooling this gas did not all condense as 
 in the last instance, a large quantity remained behind. 
 This was oxygen. 
 
 These ninth and tenth facts are of further interest, as 
 bearing upon a question much discussed by chemists 
 the nature of the bleaching compounds of chlorine. The 
 chloride of lime, and other such substances, probably 
 have the same theoretical constitution as chlorine- water. 
 Berzelius and Balard suppose that in this solution chlo- 
 rous or hypochlorous acid exists. It might be inquired, 
 if this be the condition of things, why does not an ex- 
 posure to heat alone evolve oxygen, for chlorous acid is 
 exceedingly liable to decomposition by slight elevation 
 of temperature, and we should be justified in inferring 
 that if any of this acid is to be found in chlorine-water 
 it would be decomposed at the boiling-point. M. Mil- 
 Ion adopts the view that the bleaching compounds are 
 metallic chlorides analogous to the corresponding per- 
 oxides. But the ninth fact seems incompatible with this 
 view. If chlorine -water be analogous to peroxides of 
 hydrogen, and this last be what its name imports, and 
 not merely oxygenated water, it is difficult to understand 
 why, when chlorine-water is thus boiled, oxygen is not 
 given off. If the atom of chlorine and the atom of oxy- 
 gen in this body are placed under the same relations to 
 the atom of hydrogen, it seems necessary that the chlo- 
 rine atom at 212 Fahr. should expel the oxygen atom 
 and hydrochloric acid form. It is probable, indeed, that 
 the two oxygen atoms in peroxide of hydrogen are re- 
 
294 THE ALLOTKOPISM OF CHLORINE. [MEMOIR XX. 
 
 lated to their hydrogen atom with different degrees of 
 affinity, and that one of them is retained far more loosely 
 than the other. But this would correspond to our ideas 
 of oxygenized water and not of peroxide of hydrogen, 
 and lead us to the conclusion that the solution employed 
 in this Memoir is strictly a solution of chlorine in water. 
 
 XL The decomposition of chlorine-water, when placed 
 in the sunbeam, does not begin at once, but a certain 
 space of time intervenes, during which the chlorine is 
 undergoing its specific change. 
 
 I need quote no further instance of the truth of this 
 than the experiment given in support of the second fact. 
 This is the same phenomenon which takes place when 
 chlorine and hydrogen are exposed together; they do 
 not begin to unite at once, but a certain space of time 
 elapses, during which the preliminary absorption is tak- 
 ing place, and when that is over union begins. 
 
 On the Relations of Chlorine and Hydrogen. 
 
 We have thus traced the cause of the decomposition 
 of water, in the case before us, to a change impressed on 
 the chlorine by exposure to the rays of the sun. In this 
 decomposition three elementary bodies are involved 
 chlorine, oxygen, and hydrogen. 
 
 We can therefore reduce the problem under discussion 
 to simple conditions, and study the relations of each of 
 these substances to each other and to the solar rays suc- 
 cessively. 
 
 When a mixture of oxygen and hydrogen gases, in the 
 proportion to form water, is exposed to the most brilliant 
 radiation converged upon it by convex lenses, union does 
 not ensue ; the reason being, as I have shown, that those 
 gases are perfectly transparent to the rays, and do not 
 possess either real or ideal coloration. For the same 
 cause, water exposed alone for any length of time to the 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 295 
 
 sun, or to the influence of a large convex lens, does not 
 decompose. It is transparent, and cannot absorb any of 
 the rays. 
 
 But, as is well known, a mixture of chlorine and hy- 
 drogen unites, under the same circumstances, with an 
 explosion. I have formerly proved that this depends on 
 the absorption of the indigo rays. For in the indigo 
 space of the spectrum the action goes on with the great- 
 est activity. 
 
 If, therefore, this phenomenon be due to absorption 
 taking place by the mixture, it is easy to determine 
 the function discharged by each of its ingredients. 
 
 I transmitted a ray of light through hydrogen gas 
 contained in a tube seven inches long, the ends of which 
 were terminated by pieces of flat glass; and then, dis- 
 persing the ray by a flint-glass prism, received the result- 
 ing spectrum on a daguerreotype plate. Simultaneously, 
 by the side of it, I received the spectrum of a ray which 
 had not gone through hydrogen, but through a similar 
 tube filled with atmospheric air. On comparing the im- 
 pressions together, I could find no difference between 
 them. 
 
 I therefore infer that hydrogen gas does not exert 
 any absorptive action on the solar rays. 
 
 In one of the foregoing tubes I placed dry chlorine 
 gas, the other containing atmospheric air as before, and 
 receiving the two spectra side by side on the same da- 
 guerreotype plate, I found that a powerful absorption 
 had been exercised by the chlorine. All the chemical 
 rays between the fixed line H and the violet termination 
 of the spectrum were removed, and no impression cor- 
 responding to their place was left upon the plate. On 
 repeating this experiment so as to determine with pre- 
 cision the rays which had been absorbed, I found that 
 chlorine absorbs all the rays of the spectrum included 
 
296 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 between the fixed line i and the violet termination, and 
 is probably affected by all those waves the lengths of 
 which are between 0.00001587 and 0.00001287 of a 
 Paris inch ; and inasmuch as it absorbs luminous rays 
 included between the same limits, it is to this absorption 
 that its yellow color is due. 
 
 In these Memoirs the same result is established by me 
 in another way. I found that a ray which had passed 
 through a given thickness of a mixture of equal volumes 
 of chlorine and hydrogen lost by absorption just half as 
 much of its original intensity as when it passed through 
 the same thickness of pure chlorine gas ; a result which 
 obviously leads to the conclusion that when chlorine 
 and hydrogen unite under the influence of the sun, they 
 discharge different functions the chlorine an active, and 
 the hydrogen a passive function. The primary action 
 or disturbance takes place upon the chlorine, and a dis- 
 position is communicated to it enabling it to unite read- 
 ily with the hydrogen. 
 
 By arranging in the spectrum a series of tubes con- 
 taining a mixture of these gases, it was found that 
 the gases placed in the indigo space went into union 
 first. 
 
 These various experiments enabling me thus to trace 
 to the chlorine the source of disturbance, I have next to 
 remark that chlorine which has been exposed to the rays 
 of the sun has gained thereby a tendency to unite with 
 hydrogen not possessed by chlorine made and kept in 
 the dark. In proof of this fact I may cite an experiment 
 from Memoir XIX. : 
 
 " In two similar glass tubes place equal volumes of 
 chlorine made from peroxide of manganese and hydro- 
 chloric acid by lamplight, and carefully screened from 
 access of daylight Expose one of the tubes to the full 
 sunbeams for some minutes, or, if the light be feeble, for a 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 297 
 
 quarter of an hour: the chlorine in it becomes modified. 
 Keep the other tube during this time carefully in a dark 
 place ; and now, by lamplight, add to both equal volumes 
 of hydrogen gas. These processes are best carried on 
 in a small porcelain or earthenware trough, filled with a 
 saturated solution of common salt, which dissolves chlo- 
 rine slowly, and to avoid explosions operate on limited 
 quantities of the gases. Tubes that are eight inches 
 long and half an inch in diameter will answer very well. 
 The two tubes now contain the same gaseous mixture, 
 and only differ in the circumstance that one is modified 
 and the other not. Place them, therefore, side by side 
 before a window, through which the entrance of daylight 
 can be regulated by opening the shutter; and now, if 
 this part of the process is conducted properly, it will be 
 seen that the modified chlorine commences to unite with 
 the hydrogen, and the salt water rises in that tube. But 
 the unmodified chlorine shows no disposition to unite 
 with its hydrogen, and the liquid in its tube remains 
 motionless for a long time. Finally, as it becomes slow- 
 ly modified by the action of daylight impinging on it, 
 union takes place. From this, therefore, we perceive 
 that chlorine which has been exposed to the sun will 
 unite promptly and energetically with hydrogen ; but 
 chlorine which has been made and kept in the dark 
 shows no such property." 
 
 This form of experiment may be supposed imperfect, 
 since the chlorine is in a moist condition and confined 
 by water. I have therefore made the following vari- 
 ation : 
 
 I took a tube, A, Fig. 47, six inches long and half an 
 inch in diameter, closed at one end and open at the 
 other, and cemented its open end on a piece of flat plate- 
 glass, M N, one inch wide and two long, ground on both 
 sides, and having a hole,j9, one sixth of an inch in diam- 
 
298 
 
 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 Fig. 4T. 
 
 eter perforated through it. This hole was 
 not in the centre of.the glass, but towards 
 N , one side, as shown in the figure. The interior 
 [ of the tube was perfectly clean and dry. 
 
 A second tube, B, consisting, as shown in 
 Fig. 47, of two portions ; a wide portion, B, 
 and a narrower tube, <?, was cemented on an- 
 other piece of ground plate -glass, similar to 
 the foregoing in all respects. The tube c 
 was open at its lower extremity, and the entire capacity 
 of B and c conjointly was adjusted so as to be equal to 
 the capacity of A. 
 
 Next I filled A with dry chlorine, and B c with dry 
 hydrogen, and kept them from mixing until the proper 
 time by operating in the following way: I 
 placed the ground glasses face to face, as 
 shown in Fig. 48, with a small quantity of 
 soft tallow between them, arranging them 
 in such a way that the aperture which led 
 to the interior of A was open. 
 
 Through this aperture dry chlorine was 
 conveyed. It was generated by a mixture 
 of peroxide of manganese and hydrochloric 
 acid in the flask D, Fig. 49, and passed along a tube, E, 
 filled with chloride of calcium. A slender glass tube,/, 
 
 conveyed it to the bottom of 
 A, which was then filled by 
 displacing the atmospheric 
 air. When A was supposed 
 to be full of chlorine, it was 
 slowly lowered so as to bring 
 the tube out of the aperture, 
 and as soon as it was disen- 
 gaged the glass plates were 
 moved in such a manner by 
 
 Fig. 48. 
 
 Fig. 49. 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 
 
 299 
 
 sliding them on one another that the aperture leading 
 into A was shut, but that leading into B was open. 
 The vessel A was thus filled with dry chlorine and se- 
 curely closed. 
 
 In the next place I filled B with dry hydrogen, which 
 was done as follows : To a bottle, Gr, Fig. 50, containing 
 dilute sulphuric acid and zinc, a dry ing- tube, 
 K, of chloride of calcium was adjusted, and at 
 its upper end a cork, A, arranged so as to re- 
 ceive tightly the tube c. In a short time, 
 therefore, B became full of dry hydrogen, the 
 surplus escaping through the open aperture^?. 
 The two ground-glass plates were now moved 
 on one another in such a manner that they 
 mutually closed one another. The vessel A 
 was therefore filled with dry chlorine, and the 
 vessel B c with an equal volume of dry hydro- 
 gen, without communicating for the present 
 with one another. 
 
 I had provided two sets of these tubes as 
 nearly alike as they could be made, and operated with 
 them in the following manner: 
 
 In a dark room I filled the tube A of each of them 
 with dry chlorine in the manner just described, and con- 
 fined it by sliding the plates. One of the tubes was 
 retained in the dark room and kept carefully screened 
 from the light, but the other was set for half an hour in 
 the sunbeams. The chlorine which was in it underwent 
 the specific change the object of this Memoir to de- 
 scribe. 
 
 After restoring this tube to the dark room and wait- 
 ing a few minutes for it to gain the same temperature 
 as the other, the tubes B c of each set were filled with 
 dry hydrogen in the manner described. In each in- 
 stance, as soon as the plates were moved on each other 
 
 Fig. 50. 
 
300 
 
 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 so as to confine the hydrogen, and they were released 
 from the cork h of the drying-tube K, Fig. 50, the lower 
 extremity of each was dipped beneath the surface of 
 some water contained in a saucer, P, Fig. 51 ; the two 
 
 sets of tubes being held 
 steadily in a proper posi- 
 tion by the aid of a wooden 
 frame, Q R. The tubes now 
 differed from one another 
 in nothing but the circum- 
 stance that the chlorine of 
 one had been exposed to 
 the sun, and that of the 
 Fi s- 51 - other had not. 
 
 The gases were now brought in contact. This was 
 easily done by sliding each pair of ground glasses until 
 their apertures coincided, as shown at p in Fig. 51. The 
 hydrogen now rose through the hole into the upper ves- 
 sel, the chlorine descending through it, mutual and per- 
 fect diffusion of the two gases rapidly taking place. 
 This was done by lamplight in the dark room. And 
 now it could be ascertained that the gases were at the 
 same temperature in the different tubes, and that the 
 experiment had thus far been carried on successfully, by 
 the water retaining its level at the same point in the 
 tubes c of both sets. If that which had been in the 
 sunshine was warmer than the other, as soon as the 
 apertures coincided a bubble of gas would have escaped 
 through the water, or at all events the level would have 
 changed. 
 
 It remained now to open the shutter of the dark room, 
 the tubes having been previously set in such a position 
 that the light would fall equally on both. As soon as 
 this was done, the chlorine which had been exposed to 
 the sun united at once with its hydrogen, and the water 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 30} 
 
 rose in the tube c. But in the other, which had not 
 been exposed to the sun, no movement took place until 
 the gases had had time to be modified by the light com- 
 ing through the open shutter. 
 
 When care has been taken to have the gases made 
 quite dry, and, owing to the narrowness of the tube , no 
 aqueous vapor has had time to contaminate the gas in B, 
 so that no water is present to condense the hydrochloric 
 acid as it forms, a little delay may be occasioned in the 
 liquid rising in the tube, the chlorine of which was ex- 
 posed to the sun. But after a time a mist arises in the 
 neighborhood of the water in the narrow tube, due to 
 the hydrochloric acid condensing, and then the process 
 goes forward with regularity. 
 
 It appears, therefore, that chlorine by exposure to the 
 sun contracts a tendency to unite with hydrogen which 
 is not possessed by chlorine which has been kept in the 
 dark. 
 
 On the Allotropism of Chlorine, or its Passive and Act- 
 ive States. 
 
 In what, then, does this remarkable change impressed 
 by indigo rays upon chlorine consist ? This is the ques- 
 tion immediately arising from the phenomena we have 
 had under consideration. 
 
 To this I answer that when chlorine has been thus in- 
 fluenced its electro-negative properties are exalted, and 
 it has passed from an inactive to an active state. 
 
 It is now fully established that a great number of the 
 elementary bodies undergo similar modifications. Many 
 of them can exist in no less than three different states, 
 and these peculiarities are impressed on the compounds 
 to which they give rise. To these peculiarities Berze- 
 lius directed the attention of chemical philosophers 
 in his Memoir " On the Allotropism of Simple Bodies, 
 
3Q2 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 and its Relation with Certain Cases of Isoraerisrn in 
 their Combinations." He shows that of the elementary 
 bodies now known, many undoubtedly exist in several 
 allotropic states, and infers that all are liable to anal- 
 oomis modifications. He indicates that the isomerism 
 
 o 
 
 of compound bodies is due sometimes to the different 
 modes in which the atoms of which their constituent 
 molecules consist are grouped, and sometimes to the dif- 
 ferent allotropic states in which one or the other of those 
 elements is found. Thus, as M. Millon has remarked, 
 the intrinsic difference between carburetted hydrogen 
 gas (CH) and ottar of roses (CH), which are isomeric 
 bodies, may perhaps consist in this, that in the former 
 the carbon is under the form of common charcoal, and in 
 the latter under the form of diamond. 
 
 The following instances from Berzelius may serve as 
 examples of these allotropic states : 
 
 Carbon is known under three forms charcoal, plum- 
 bago, and diamond. They differ in specific gravity, in 
 specific heat, and in their conducting power as respects 
 caloric and electricity. In their relations to light, the 
 first perfectly absorbs it, the second reflects it like a 
 metal, the third transmits it like glass. In their rela- 
 tions with oxygen they also differ surprisingly ; there 
 are varieties of charcoal that spontaneously take fire in 
 the air, but the diamond can only be burned with diffi- 
 culty at a high temperature in pure oxygen gas. The 
 second and third varieties do not belong to the same 
 crystalline form. 
 
 Silicon exists also under two forms. In its first it 
 burns with facility in the air under a slight elevation of 
 temperature. But if it be previously exposed to a strong 
 red heat it changes into the second variety and becomes 
 incombustible, so that it will not oxidize when placed 
 with nitrate of potash in the hottest part of a blowpipe 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 3Q3 
 
 flame. As is well known, there are two forms of silicic 
 acid: one soluble in water and hydrochloric acid, but 
 passing into the insoluble state by being previously 
 made red-hot. The silicon therefore carries in its com- 
 bination the same properties that it exhibits in the free 
 state. 
 
 In the same manner it might be shown that sulphur, 
 selenium, phosphorus, titanium, chromium, uranium, tin, 
 indium, osmium, copper, nickel, cobalt, and a variety of 
 other bodies, exist under several different forms, with 
 distinctive properties that are often well marked. In 
 several of them the influence of this allotropic condition 
 is plainly carried into the compounds, as is well shown 
 in the two varieties of arsenic which give rise to the two 
 arsenious acids. 
 
 The passage from one allotropic state to another takes 
 place commonly through the agency of apparently very 
 trivial causes, such as a slight elevation of temperature 
 and the contact of certain bodies. Thus iron, which is 
 so easily oxidized under ordinary circumstances, appears 
 to lose its affinity for oxygen after it has been touched 
 under the surface of nitric acid by a piece of platinum. 
 It then puts on the attributes of a noble metal, and sim- 
 ulates the properties of platinum and gold. 
 
 This remarkable instance of the passage from an act- 
 ive to a passive state, as Berzelius remarks, may lead to 
 a conjecture respecting the true condition of certain 
 gases. No one can reflect on the inactivity of nitrogen 
 gas under ordinary circumstances, contrasted with its 
 equally extraordinary activity as a constituent of organ- 
 ic bodies, without being struck with the apparent con- 
 nection of that phenomenon with these of allotropism. 
 And though Berzelius with his customary caution mere- 
 ly insinuates that nitrogen can exist under two forms, 
 the facts here developed in relation to chlorine appear to 
 
304 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 show that that opinion rests on something more solid 
 than conjecture. The habitudes of many of the gaseous 
 bodies strengthen this conclusion. Oxygen refuses to 
 unite when mixed with hydrogen precisely in the man- 
 ner of chlorine, and it requires a certain modification to 
 be made in the electro- negative element before water or 
 hydrochloric acid can result. 
 
 Just, therefore, in the same manner that so many ele- 
 mentary bodies can put on under the influence of exter- 
 nal causes an active or passive condition, I infer, as the 
 result of the experiments brought forward in this Mem- 
 oir, that chlorine is one of these allotropic bodies, having 
 a double form of existence. That, as commonly prepared, 
 it is in its passive state; but that on exposure to the 
 indigo rays or other causes it changes and assumes an 
 active form. That, in this latter state, its affinity for 
 hydrogen becomes so great that it decomposes water 
 without difficulty, as in the experiment which this Mem- 
 oir is designed to illustrate. 
 
 On the Relation of the Preceding Conclusions with the 
 Theory of Substitutions. 
 
 Having thus explained the facts which appear to indi- 
 cate the allotropism of chlorine, I shall now offer some 
 considerations on its connection with the theory of sub- 
 stitutions of M. Dumas. 
 
 Admitting the fact that the electro-negative qualities 
 of chlorine are exalted upon its exposure to the indigo 
 rays, and that the resulting effect is not a temporary 
 change, but one lasting for a considerable period of time, 
 we can give a very plain and simple account of the de- 
 composition of water by this gaseous substance under 
 the influence of sunshine. 
 
 On the same principle that a mixture of chlorine and 
 hydrogen may be kept in the dark without union for a 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 3Q5 
 
 long time, so may a solution of chlorine in water be pre- 
 served. The chlorine is in an inactive state. 
 
 But if anything be done to make the chlorine take on 
 its other form and pass to the active condition if it be, 
 for example, set in the sunshine its affinity for hydro- 
 gen is exhibited and decomposition is the result. 
 
 The qualities thus communicated to the chlorine not 
 being of a transient kind, but remaining for a length of 
 time, we see how it is that after an exposure to the sun 
 decomposition is subsequently carried forward in the 
 dark. 
 
 The indisposition of chlorine to unite with carbon, 
 which has been regarded as a singular quality, is not 
 more remarkable than its indisposition to unite with hy- 
 drogen in the dark. 
 
 If the power assumed by chlorine of uniting with hy- 
 drogen and carbon depends on a change in its electrical 
 relations a passage from the passive to the active state 
 we might expect that those various causes which in 
 the case of other elementary bodies bring about anal- 
 ogous changes, and throw them from one allotropic con- 
 dition to another, would here also exercise a perceptible 
 action. Among such causes we may enumerate the ac- 
 tion of a high temperature and the contact or presence 
 of other bodies. 
 
 It may be remarked, in the instances to which Berze- 
 lius has referred, that exposure to a high temperature is 
 one of the most frequent causes of allotropic change. In 
 the case of chlorine the remark holds good, for, as is 
 well known, when a mixture of chlorine and hydrogen is 
 passed through a red-hot tube, hydrochloric acid forms 
 with rapidity. The high temperature, therefore, impress- 
 es on chlorine the same tendency to unite with hydrogen 
 which is communicated by the solar rays. 
 
 But the contact of other bodies frequently determines 
 
 U 
 
306 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 in a given substance an allotropic change. Thus, when 
 a piece of iron is placed in nitric acid in contact with 
 platinum, the iron becomes less electro-positive, or, what 
 is the same thing, more electro-negative, than it was be- 
 fore, and the acid can no longer oxidize it. The contact 
 of the very same substance, platinum, determines an an- 
 alogous change in chlorine giving it at once the capac- 
 ity of uniting with hydrogen. The porous condition of 
 spongy platinum is not essential to the result, for clean 
 platinum foil exhibits the same phenomenon. 
 
 In the case of iron, the action of a high temperature 
 or the contact of platinum throws the metal from the 
 active to the passive state ; in the case of chlorine the 
 same causes apparently produce the opposite result, 
 throwing the gas from the passive to the active state. 
 But the difference is rather in appearance than in real- 
 ity. In both cases it amounts to the same thing, and is 
 an exaltation of the electro-negative qualities of either 
 substance respectively. 
 
 The same causes, therefore, which produce allotropic 
 changes in other bodies produce analogous changes in 
 chlorine. 
 
 Now, among the physical facts connected with the 
 theory of types and substitutions, two are prominent : 
 1st. The union of chlorine with hydrogen, giving rise to 
 the removal of that hydrogen as hydrochloric acid. 2d. 
 The subsequent function discharged by the chlorine, 
 which has entered as an integrant portion of the mole- 
 cules, and occupies the place of the hydrogen removed. 
 This function is in many instances that of the hydrogen 
 itself, and it is this fact which is the remarkable point in 
 the phenomena of substitution that an intensely elec- 
 tro-negative body can act the part of a positive body. 
 It is this fact which is leading chemists to the conclusion 
 that the properties of compound bodies arise as much 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 397 
 
 from the mode of grouping of their constituent atoms as 
 from the qualities of those atoms themselves. 
 
 But, if it be admitted that the experiments related in 
 this Memoir establish the allotropism of chlorine, then it 
 is plain that a very different and perhaps satisfactory 
 account of the phenomena of substitution may be given. 
 
 As has been already said, no difficulty can arise in 
 accounting for the removal of hydrogen from organic 
 bodies, or for the first fact just alluded to. This remov- 
 al will ensue whenever processes are resorted to which 
 bring the chlorine into an active state. When we ex- 
 pose acetic acid and chlorine to the sun, the latter be- 
 comes active, gains the quality of uniting with hydrogen, 
 and chlor-acetic acid forms. Probably the same change 
 could be brought about by the aid of spongy platinum 
 and heat. 
 
 Upon the second fact the similarity of function dis- 
 charged by the chlorine which has replaced the hydro- 
 gen atoms with the function of those atoms themselves 
 a flood of light is thrown by other phenomena of allot- 
 ropism. If a piece of iron be dipped in hyd rated nitric 
 acid, though it may be acted on for a few moments, it 
 rapidly becomes passive. And so with the chlorine 
 atoms which have substituted the hydrogen. In the 
 circumstances in which they are placed they rapidly re- 
 vert from the active to the passive state. They are no 
 longer endued with an intense electro-negative quality 
 they have assumed the condition of inactivity. The 
 fact that chlorine in chlor-acetic acid simulates the func- 
 tions of hydrogen in acetic acid is not more remarkable 
 than that iron touched by platinum under nitric acid 
 simulates the properties of that noble metal. 
 
 Do not, therefore, these circumstances seem to point 
 out that if we admit the fact that simple substances can 
 exist in different states, in a passive and an active form, 
 
308 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 the phenomena of substitution are deprived of much of 
 their singularity. 
 
 Thus, to recall once more the example to which I have 
 before referred, and which has been so well illustrated 
 by the researches of M. Dumas, the transmutation of 
 acetic into chlor-acetic acid exhibits a double phenom- 
 enon. 1st. The existence of active chlorine, expressed 
 by the removal of hydrogen, activity having been com- 
 municated by the rays of the sun, or by some other ap- 
 propriate method. 2d. The existence of passive chlorine 
 in the particles of chlor-acetic acid. 
 
 I consider that, were no other instances known, the 
 two cases cited by Berzelius of the double forms of silicic 
 acid and arsenious acid establish the fact that a given 
 allotropic condition may be continued by an elementary 
 atom when it goes into union with other bodies. And 
 I regard the various cases in which hydrogen is replaced 
 by iodine, bromine, etc., in which, in the resulting com- 
 pound, those energetic electro-negative elements fail to 
 give any expression of their presence and activity, as 
 analogous to other common and too much overlooked 
 facts. Chlorine which is in the dark may be kept in 
 contact with hydrogen without exhibiting any of its 
 latent energies. Touched by an indigo ray, it instantly 
 assumes the active state, and a violent explosion is the 
 result. 
 
 To use, therefore, the same nomenclature to which 
 Berzelius has resorted in the case of other allotropisms, 
 we may designate the ordinary form of chlorine made 
 by the action of hydrochloric acid on peroxide of man- 
 ganese as Cl/3, and admit that this passes into the con- 
 dition Cla by the action of the solar rays, contact of plat- 
 inum, or a high temperature ; and that in any case of 
 substitution the hydrogen is removed under the con- 
 dition Cla, and the resulting compound contains Cl/3, 
 
MEMOIR XX. J THE ALLOTROPISM OF CHLORINE. 30 9 
 
 the assumption of the passive state disguising the pres- 
 ence of the electro-negative atom. 
 
 The explanation here given of the phenomena of 
 substitutions involves the position that chlorine when 
 brought in relation with carbon under certain circum- 
 
 o 
 
 stances is thrown into the passive state, the state Cl/3. 
 We naturally look for direct evidence that this is the 
 case. It seems to me that there are many w 7 ell-known 
 chemical facts tending to establish the passive condition. 
 In the first case to which we turn, the chlorides of car- 
 bon, the inactive state is established in a striking man- 
 ner. The affinity existing between chlorine and carbon 
 is apparently feeble ; yet when these bodies have once 
 united the chlorine is brought into such a condition 
 that it has lost the quality of being detected by the or- 
 dinary tests which determine its presence. How strong- 
 ly does this contrast with the case of hydrochloric acid ! 
 A feeble affinity unites carbon and chlorine, an intense 
 affinity unites hydrogen and chlorine ; yet in the former 
 case the chlorine is undiscoverable by the commonest 
 tests, in the latter it yields to them all. And the causes 
 are obvious ; in the one case it is in the passive, in the 
 other in the active condition. 
 
 I have hitherto spoken of the active and passive 
 states as though they were fixed points in elementary 
 bodies, and as though the transition from one to the 
 other were abrupt and sudden. I have done this that 
 the views here offered might be unembarrassed and dis- 
 tinct. But there are many facts which serve to show 
 that the passage from a state of complete activity to a 
 state of complete inactivity takes place through gradu- 
 al steps. Thus, in carbon itself, there are undoubtedly 
 many intermediate stages between the almost spontane- 
 ously inflammable varieties and diamond, which, under 
 common circumstances, is incombustible. Berzelius ad- 
 
310 THE ALLOTROPISM OF CHLORINE. [MEMOIR XX. 
 
 mits three allotropic conditions of this body, Ca, C/3, Cy. 
 Between the first and last term s, of this series it is prob- 
 able that several intermediate bodies besides plumbago 
 might be found, their existence establishing the gradual 
 passage from one to the other state. 
 
 For similar reasons, in this Memoir the illustrations 
 and arguments given have for the most part been re- 
 stricted to one subject, chlorine. It need scarcely be 
 pointed out, in conclusion, that if the views here offered 
 be true, very much of this reasoning may be transferred 
 to other bodies, as oxygen, nitrogen, hydrogen, sulphur, 
 etc. When oxygen and hydrogen are mixed, there is no 
 disposition exhibited by them to unite; and this does 
 not arise from their happening to have the gaseous form. 
 As in the instance we have been considering, if they are 
 exposed to a high temperature, or to the influence of 
 platinum, the active condition is assumed with prompti- 
 tude, and union takes place. 
 
 The power possessed by carbon of throwing bodies 
 into a completely passive state is for from being limited 
 to chlorine. It reappears in the case of sulphur. The 
 sulphide of carbon yields to none of the tests to which 
 we commonly resort for determining the presence of sul- 
 phur, for the simple reason that its sulphur is in an inac- 
 tive state. This substance, moreover, serves to illustrate 
 what has been said of the gradual passage of bodies 
 from a state of complete activity to one of complete in- 
 activity. Berzelius recognizes for it three different allo- 
 tropic states an alpha, beta, and gamma condition. In 
 none of these is it in that condition of absolute inactiv- 
 ity which it assumes in the sulphide of carbon.* 
 
 * For these examples, the chloride and sulphide of carbon, I am indebted to M. 
 Millon's paper, " Remarks on the Elements which Compose Organic Substances, and 
 on their Mode of Combination, "in the Comptes Rendus, t. xix., p. 799. That chem- 
 ist, however, gives a very different explanation of the phenomena involved. 
 
MEMOIR XX.] THE ALLOTROPISM OF CHLORINE. 
 
 In offering these experiments and arguments to the 
 consideration of chemists, I am fully aware of the mag- 
 nitude of the change which would be impressed on the 
 science generally, and especially on several modern theo- 
 ries, by their reception. The long -established idea of 
 the immutability of the properties of elementary bodies 
 would, to a certain extent, be sacrificed ; and it is prob- 
 able that before these results are conceded more cogent 
 evidence of the main principle will be required. In the 
 meantime, however, it is plain that the admission of 
 these doctrines throws much light on theories now ex- 
 tensively attracting attention, and for that reason they 
 commend themselves to our consideration. I have of- 
 fered no opinion here on the atomic mechanism involved 
 in these changes from an active to a passive state, 
 though it is impossible to deal with these things with- 
 out the reflection arising in our minds that here we are 
 on the brink of an extensive system of evidence con- 
 nected with the polarity of atoms an idea which, under 
 a variety of forms, is now occurring in every department 
 of natural philosophy. 
 
 UNIVERSITY OF NEW YORK, July 29, 1845. 
 
INFLUENCE OF LIGHT UPON CHLORINE. [MEMOIR XXI. 
 
 MEMOIR XXI. 
 
 ON THE INFLUENCE OF LIGHT UPON CHLORINE, AND SOME 
 REMARKS ON ALCHEMY. 
 
 From the Philosophical Magazine, November, 1857. 
 
 CONTENTS : Modification of chlorine by the sun-rays. The modification 
 is not transient. Alchemical attempts to modify metals. Exposure 
 of silver chloride to a burning-lens. The resulting silver is not acted 
 upon by nitric acid. 
 
 SEVERAL years ago I observed that when a mixture 
 of chlorine and hydrogen is exposed to light, union does 
 not occur at once, but that a certain time must elapse 
 during which absorption takes place, the combination 
 then proceeding in a uniform manner. 
 
 It is by the chlorine that this absorptive agency is ex- 
 ercised, the indigo ray being mainly concerned. And 
 not only is it that ray which is thus absorbed : to it 
 must be attributed also the subsequent combination. 
 
 Among several other facts connected with this subject, 
 which may be found in the Philosophical Magazine 
 (July, 1844), The American Journal of Science, Vol. 
 XLIX., and other publications of that time, there is one 
 to which I would particularly direct attention. Chlo- 
 rine which has been exposed to the sun has obtained 
 properties not possessed by chlorine which has been 
 made and kept in the dark, and the change is by no 
 means transient. It lasts for many hours, and even days. 
 
 In their recent examination of this fact, Professor Bun- 
 sen and Dr. Roscoe do not appear to regard the modifi- 
 cation in question as being of so permanent a nature. 
 
MEMOIR XXL] INFLUENCE OF LIGHT UPON CHLORINE. 3^3 
 
 Perhaps it may have been that the insolation to which 
 they submitted the chlorine was not continued sufficient- 
 ly long, or perhaps the light was not sufficiently intense. 
 My opinion was founded on three different conditions of 
 the experiment 1st. On the behavior of chlorine itself 
 confined over salt water ; 2d. On the effect of a mixture 
 of chlorine and hydrogen in equal volumes as disengaged 
 from hydrochloric acid by a voltaic current; 3d. On the 
 action of a solution of pure chlorine in distilled water. 
 In each of these instances, the active properties imparted 
 to the chlorine by exposure to light were plainly per- 
 ceptible for a long time after. Indeed, I infer from the 
 experiments of those chemists that they found the effects 
 to continue for a certain brief period. If they do so 
 continue, though only in a momentary manner, after the 
 light has been shut off, I do not see in what other man- 
 ner we are to explain the result than on the principle of 
 a change in the relations of the chlorine. In this inter- 
 pretation it is very well known that Berzelius coincided 
 in his account of my experiments in the Annual Report 
 for 1847. 
 
 At first I thought that there was a general analogy 
 between the case of chlorine thus thrown into an active 
 state and that of iron in its passive condition. An iron 
 wire which has been made passive will quickly revert 
 to the condition of activity if submitted to any jarring, 
 vibration, or other trivial disturbance; its passive state 
 being in one sense permanent, though very easily lost. 
 But subsequently I found many reasons for supposing 
 that the impression is of a much more lasting nature, 
 and resembles that on phosphorus after a similar expos- 
 ure to the indigo rays. As an illustration of what is 
 here meant, I may relate that having obtained a thin 
 stratum of perfectly white phosphorus between two 
 pieces of glass, I exposed it to a motionless solar spec- 
 
314 INFLUENCE OF LIGHT UPON CHLORINE. [MEMOIK XXI. 
 
 trum, and found that it turned to a dark-brown color in 
 those spaces on which the more refrangible rays fell, the 
 effect reaching a maximum under the indigo ray. The 
 fixed lines of Fraunhofer were very prettily depicted, as 
 white streaks, particularly the large ones at H. I kept 
 the sample of phosphorus for several years without its 
 showing any disposition to resume the active state. 
 
 Professor Bunseu and Dr. Roscoe dwell very appro- 
 priately on the disturbing effects of minute quantities of 
 extraneous gases mingled with chlorine on photo-chem- 
 ical induction. No one who has used a chlor-hydrogen 
 photometer can have failed to make a similar remark. 
 My attention has been directed to that subject in its 
 more general aspect, and I will ingenuously confess that 
 I have made several attempts at the transmutation of 
 metals, on the principle of compelling them, by the aid 
 of solar light, to be disengaged from states of combina- 
 tion in the midst of resisting or disturbing media. 
 
 The following is a description of one of these alchem- 
 ical attempts: In the focus of a burning -lens, twelve 
 inches in diameter, was placed a glass flask, two inches 
 in diameter, containing nitric acid diluted with its own 
 volume of water. Into the nitric acid were poured alter- 
 nately small quantities of a solution of nitrate of silver 
 and of hydrochloric acid, the object being to cause the 
 chloride of silver to form in a minutely divided state, so 
 as to produce a milky liquid, into the interior of which 
 the brilliant converging cone of light might pass, and the 
 currents generated in the flask by the heat might drift 
 all the chloride successively through the light. The 
 chloride, if otherwise exposed to the sun, blackens mere- 
 ly upon the surface, the interior parts undergoing no 
 change ; this difficulty I therefore hoped to avoid. The 
 burning-glass promptly brings on a decomposition of the 
 salt, evolving on the one hand chlorine, and disengaging 
 
MEMOIR XXL] INFLUENCE OF LIGHT UPON CHLORINE. 
 
 a metal on the other. In one experiment the exposure 
 lasted from 1 1 A.M. to 1 P.M. ; this was, therefore, equal 
 to a continuous mid-day sun of seventy-two hours. The 
 metal was disengaged readily. But what is it ? It 
 cannot be silver, since nitric acid has no action upon it. 
 It burnished in an agate mortar, but its reflection is not 
 like the reflection of silver: it is yellow. The light 
 must, therefore, have so transmuted the original silver 
 as to enable it to exist in the presence of nitric acid. In 
 1837 I published some experiments on the nature of 
 this decomposition in the Journal of the Franklin In- 
 stitute. 
 
 Though this experiment, and several modifications of 
 it which I might relate, fail to establish any permanent 
 change in the metal under trial, in the sense of an actual 
 transmutation, it does not follow that we should despair 
 of final success. It is not likely that Nature has made 
 fifty elementary substances of a metallic form, many of 
 them so closely resembling each other as to be with dif- 
 ficulty distinguished. Moreover, chlorine and other ele- 
 mentary substances can be changed by the sunlight in 
 some respects permanently; and if silver has not thus far 
 been transmuted into a more noble metal, as platinum 
 or gold, it has at all events been made transiently into 
 something which is not silver. Those who will reflect 
 a little on the matter cannot fail to observe that the 
 sun-rays really possess many of the powers once fabu- 
 lously imputed to the powder of projection and the phi- 
 losopher's stone. 
 
316 THE ACTION OF GLASS AND QUARTZ. [MEMOIR XXII. 
 
 MEMOIR XXII. 
 
 ON THE ACTION OF GLASS AND QUARTZ ON THE RADI- 
 ATIONS THAT PRODUCE PHOSPHORESCENCE. 
 
 From the Philosophical Magazine, Aug., 1844. 
 
 CONTENTS : Phosphorescence by rays from a Leyden spark through 
 quartz. From the voltaic arc. It is occasioned by the more refran- 
 gible rays. Imperviousness of glass. Professor Henry's experiments. 
 Comparison of the chemical and phosphorogenic rays. 
 
 AT the distance of six inches from the terminations of 
 two blunt wires, between which the spark from a Ley- 
 den-jar was caused to pass, I placed a lens of quartz, the 
 focus of which for parallel rays was six inches, and then 
 intercepted the resulting beam by a diaphragm, with a 
 circular aperture in it one third of an inch in diameter. 
 I had caused an equiangular prism of quartz to be cut 
 and polished from a large and faultless crystal; it was 
 cut transverse to the axis. This prism I placed in such 
 a way that, in dispersing the beam coining through the 
 circular aperture, I got rid of double refraction and ob- 
 tained only one spectrum ; this was received on a metal 
 plate, which, having been washed over with gum-water 
 and sulphide of lime dusted on it, offered a uniform 
 phosphorescent surface which might be set in a vertical 
 plane. When the spark passed, I saw that the plate 
 was phosphorescing on those portions where the more 
 refrangible rays had fallen. 
 
 But the transient light of a Leyden spark did not last 
 long enough, nor was the phosphorescence it produced 
 powerful enough to enable me to conduct the experiment 
 
MEMOIR XXII. ] THE ACTION OF GLASS AND QUARTZ. 
 
 in a way entirely satisfactory. I resorted, therefore, to 
 the brilliant light obtained when a piece of metal, or, 
 what is better, the hard variety of carbon obtained from 
 gas-retorts, is lowered upon mercury entirely filling a 
 very small open porcelain cup, and the continuous dis- 
 charge of a voltaic battery passed. The battery used 
 contained fifty pairs of Grove's cells, but a smaller num- 
 ber would have been amply sufficient. 
 
 As soon as the light was emitted, I marked on the 
 lime sulphide the beginning of the red, the centre of the 
 yellow, and the termination of the visible violet ray. 
 Then, stopping the current, I examined on what parts 
 the plate was phosphorescing. The commencement of 
 the glow was between the indigo and the blue ; towards 
 the blue it extended far beyond the visible boundaries 
 of the spectrum. I could not see any divisions or points 
 of maximum in it. The surface of the plate shone all 
 over except in the region of the less refrangible rays, 
 and there were traces of the negative action which M. 
 Becquerel has illustrated in the cases of the solar ema- 
 nations rays which, however, were first observed in the 
 last century. 
 
 It is necessary to remark that the rays from the voltaic 
 discharge resemble those from an electric spark in their 
 inability to pass through glass. On this observation all 
 the value of the foregoing experiment depends. 
 
 But it can nevertheless be easily proved that although 
 glass is impervious to the phosphorogenic emanation 
 coming from the voltaic deflagration of any metallic bod- 
 ies, the observation applies to transient discharges only. 
 A voltaic light which lasts but a moment fails to cause 
 phosphorescence through glass in the same way that an 
 electric spark does; but if the discharge be continued 
 the surface presently begins to glow, and if maintained 
 for several minutes it shines as brightly as though a 
 piece of quartz had been used. 
 
318 THE ACTION OF GLASS AND QUARTZ. [MEMOIR XXII. 
 
 The inability of an electric spark to cause phosphor- 
 escence is connected with its transient duration. The 
 voltaic light enables us at pleasure to imitate the effects 
 of an electric spark or those of the sun. The phosphor- 
 ogenic rays, whether they originate in an electric spark 
 or from the sun, occupying thus the same place in the 
 spectrum, and even exhibiting the same peculiarities as 
 the chemical rays on iodide of silver, we have next to de- 
 termine whether this is an apparent or a positive identity. 
 
 Professor Henry, of Princeton, read a paper before the 
 American Philosophical Society, in May, 1843, in which 
 he discussed all the leading mechanical properties of the 
 phosphorogenic rays, and, among other important exper- 
 iments, made some with a view of determining this par- 
 ticular question. A daguerreotype plate and some lime 
 sulphide were simultaneously exposed to the sky: the 
 plate was stained, but no effect was produced on the 
 lime. A daguerreotype plate and some lime sulphide 
 were exposed to the light of an electric spark : the lime 
 was observed to glow, but no impression was produced 
 on the plate. When the plate was exposed to a succes- 
 sion of sparks for ten minutes with a sheet of mica inter- 
 posed, an impression was made. Lime exposed to the 
 moon did not phosphoresce, but a sensitive plate under 
 the circumstances is said to be stained. In. view of 
 these different facts, Professor Henry observes : " These 
 experiments, although not sufficiently extensive, appear 
 to indicate that the phosphorogenic emanation is distinct 
 from the chemical, and that it exists in a much greater 
 quantity in the electric spark than either the luminous 
 or chemical radiation." 
 
 From Wilson's experiments it appears that he was 
 aware that when the phosphorescent surface is warmed 
 so as to hasten the disengagement of light, the moon- 
 beams may be found to have left traces of action upon 
 
MEMOIR XXIL] THE ACTION OF GLASS AND QUARTZ. 
 
 it, feeble, it is true, but nevertheless very apparent. We 
 have seen, also, that the peculiarity of an electric spark 
 is due to its transient duration. Before, therefore, a 
 final decision can be obtained on this point, we are re- 
 quired to examine the effects of the chemical rays and 
 phosphorogeuic emanations under circumstances which 
 are precisely similar as to intensity and time. 
 
 For the transient rays of an electric spark, quartz is 
 transparent and glass is nearly opaque. Having pre- 
 pared a bromo-iodized silver plate so as to be exceeding- 
 ly sensitive, I set in front of it at the distance of about one 
 third of an inch a disk of quartz and one of crown-glass 
 of equal thickness, and between a pair of copper wires, 
 the interval of which was three eighths of an inch, I 
 passed the spark of a Ley den-jar fifteen times ; the dis- 
 tance between this spark and the sensitive plate was 
 about two inches. On mercurializing this plate, it was 
 deeply whitened all over, equally so through the glass, 
 through the quartz, and on the uncovered spaces ; but a 
 spot of sealing-wax which I had put on the glass left its 
 shadow on the plate beautifully depicted, so also were 
 the edges of the glass and the quartz. The two disks 
 overlapped one another to a certain extent, but the cor- 
 responding portion of the silver plate was as deeply 
 stained there as anywhere else. 
 
 Next I put a surface of lime sulphide in the place of 
 the daguerreotype plate, everything else remaining as be- 
 fore. On passing fifteen sparks the lime phosphoresced 
 powerfully under the quartz, but not under the glass, so 
 that the difference between its shadow and that of the 
 spot of wax could not be distinctly seen. 
 
 For these reasons, therefore, I adopt the view ex- 
 pressed by Professor Henry, that the phosphorogenic 
 emanation and the chemical rays are distinct. Under 
 the same circumstances glass is transparent to the one, 
 to the other it is opaque. 
 
320 RAYS OF INCANDESCENT LIME, ETC. [MEMOIR XXIII. 
 
 MEMOIR XXIII. 
 
 ON A REMARKABLE DIFFERENCE BETWEEN THE RAYS OF 
 
 INCANDESCENT LIME AND THOSE EMITTED BY AN 
 
 ELECTRIC SPARK. 
 
 From the Philosophical Magazine, December, 1845. 
 
 CONTENTS : Non-permeability of glass to spark radiations. Permeabil- 
 ity to calcium-light radiations. Different refrangibility of the spark 
 and the calcium -light radiations. Shadows imbedded in Canton's 
 phosphorus. Evolution of old shadows in an order of succession. 
 
 SOME years ago M. Becquerel discovered that the rays 
 of an electric spark, if transmitted through a screen of 
 glass, could not excite the phosphorescence of lime sul- 
 phide. 
 
 To make this experiment, wash a metallic plate over 
 with gum-water. Dust upon it from a fine sieve a quan- 
 tity of Canton's phosphorus (oyster-shells calcined with 
 sulphur), and allow the plate to dry. A uniform surface 
 is thus obtained suitable for these purposes. Place be- 
 fore that surface a piece of glass and a piece of polished 
 quartz, and discharge a Leyden-jar a few inches off, so 
 that the rays of its spark may fall on the plate. It will 
 be found that under the quartz the phosphorus will 
 shine as much as on the spaces that have not been cov- 
 ered, but under the glass it will remain almost entirely 
 dark. 
 
 Last winter I observed the curious fact that when this 
 experiment is made with a piece of lime incandescing in 
 a stream of oxygen directed through the flame of a spirit- 
 lamp, the glass, so far from being unable to transmit the 
 
MEMOIR XXIIL] KAYS OF INCANDESCENT LIME, ETC. 321 
 
 rays, appears to be as transparent to them as quartz or 
 atmospheric air. 
 
 A screen of glass is opaque to the phosphorogenic 
 rays of an electric spark, but it is quite transparent to 
 those of incandescent lime. 
 
 It might be supposed that the very brief duration of 
 an electric spark has something to do with this phenom- 
 enon, but the voltaic arc passing between charcoal points 
 gives the same results. I caused its rays to impinge on 
 a plate for thirty seconds, and observed the obstructing 
 effect of glass in a very satisfactory manner. That a 
 certain portion of the rays passes through may be 
 shown by continuing the light for a few minutes, when 
 the phosphorus will begin to shine under the glass. 
 
 I came to the conclusion, also, that the transient dura- 
 tion of the light had nothing to do with the phenomenon, 
 because the lime light occasions phosphorescence through 
 glass in the space of a single second, but in that time 
 the rays from a voltaic arc could not traverse a piece of 
 glass so as to produce a sensible effect ; the phosphorus 
 beneath it appearing quite dark, and yet this light is in- 
 comparably brighter than the lime light. 
 
 The blue light emitted when a platinum wire in con- 
 nection with one pole of the voltaic battery is brought 
 down upon some mercury in connection with the other, 
 and the green light obtained when the copper wires are 
 the medium of discharge, appear to produce the same 
 effect as charcoal points. 
 
 It is therefore neither the color nor the duration of 
 the light that determines this result. It seems to depend 
 on a peculiarity of the electric discharge. 
 
 Some time ago I determined the refrangibility of the 
 rays of an electric spark which excite phosphorescence 
 in lime sulphide ; they are found at the violet extremity 
 of the spectrum. I have made attempts to ascertain the 
 
 X 
 
322 KAYS OF INCANDESCENT LIME, ETC. [MEMOIR XXIII. 
 
 position of the active rays of incandescent lime. They 
 cannot pass through the blue solution of ammonio-sul- 
 phate of copper, but through the red solution of sulpho- 
 cyanide of iron, and also through a strong solution of 
 bichromate of potash, they pass ; in the latter case al- 
 most as copiously as through atmospheric air. 
 
 The phosphorogenic rays of an electric spark are in 
 the violet space, but those of incandescent lime are at 
 the other extremity of the spectrum. 
 
 An Argand lamp, when made to burn very brightly, 
 emits phosphorogenic rays which traverse glass. As 
 has been proved long ago, the sun-rays possess the same 
 property. 
 
 Thus, therefore, the rays of incandescent lime, of an oil- 
 lamp, and of the sun can excite phosphorescence through 
 glass, and differ from those of an electric spark or voltaic 
 discharge, in which that peculiarity is deficient. 
 
 I have also remarked some curious cases of spectral 
 appearances. They are analogous to those instances to 
 which I first drew public attention in 1840, and which 
 at a later date M. Moser brought before the British As- 
 sociation. They are interesting as affording an ocular 
 proof of secondary radiation. The following experiments 
 may serve as illustrations : 
 
 Place a key or any other opaque object before a sensi- 
 tive phosphorescent surface, and having made that sur- 
 face glow intensely by a voltaic discharge between char- 
 coal points continued for two or three minutes, on re- 
 moving the key an image of it will of course be seen. 
 This image in a short time will disappear. Then shut 
 the plate in a dark place, where no light can have access 
 to it in the daytime. If in a day or two the surface be 
 carefully inspected in the dark, no trace of anything will 
 be visible upon it ; but if it be laid on a piece of warm 
 iron, a spectral image of the key is suddenly evolved. 
 
MEMOIR XXIIL] RAYS OF INCANDESCENT LIME, ETC. 323 
 
 It is still more curious that a number of these latent 
 images may co-exist on the same surface. Provide a 
 phosphorescent surface on which the latent image of a 
 key impressed a day or two before by a voltaic discharge 
 is known to exist. Take some other object, as a metal 
 ring, and setting it before the surface, discharge at a 
 short distance a Leyden-jar. The phosphorus shines all 
 over, save on those portions shaded by the ring; it ex- 
 hibits, therefore, an image of that body. This image 
 soon fades away and totally disappears. Set the plate 
 now upon a piece of warm iron ; it soon begins to glow, 
 and the image of the ring is first reproduced, and as it 
 declines away the spectral form of the key gradually un- 
 folds itself, and after a time it totally vanishes. 
 
 A series of spectral images may thus exist together 
 on a phosphorescent surface, and after remaining there 
 latent for a length of time, they will come forth in their 
 proper order on raising the temperature of the surface. 
 
 The idea that phosphorescence is merely the light of 
 an electric discharge from particle to particle seems to 
 me wholly incompatible with such results. 
 
324 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV. 
 
 MEMOIR XXIV. 
 
 ON THE ELECTRO-MOTIVE POWER OF HEAT. 
 
 From the Philosophical Magazine, June, 1840 ; Harper's Magazine, No. 328. 
 
 CONTENTS: Experimental arrangement to determine the electro-motive 
 power. Temperatures calculated from quantities of electricity. In- 
 crease of tension with increase of temperature. Depends on increased 
 resistance to conduction. Quantity of electricity independent of heated 
 surface. In thermo-electric piles the quantity of electricity is propor- 
 tional to the number of pairs. Best forms of construction of thermo- 
 electric pairs. 
 
 FROM the Memoir of M. Melloni "On the Polarization 
 of Heat," inserted in the second part of the first volume 
 of Taylor's " Scientific Memoirs," we learn that M. Bec- 
 querel, as well as himself, had made experiments to de- 
 termine the quantities of electricity set in motion by 
 known increments of heat. From these experiments 
 they conclude that through the whole range of the ther- 
 mometric scale those quantities are directly proportional 
 to each other. 
 
 But as thermo-electric currents are now employed in 
 a variety of delicate physical investigations, and as there 
 appears to be much misconception as to their character, 
 I propose in this Memoir to show 
 
 1st. That equal increments of heat do not set in mo- 
 tion equal quantities of electricity. 
 
 2d. That the tension undergoes a slight increase with 
 increase of temperature, a phenomenon due to the in- 
 creased resistance to conduction of metals when their 
 temperature rises. 
 
 3d. That the quantity of electricity evolved at any 
 
MEMOIR XXIV.] THE ELECTRO-MOTIVE POWER OF HEAT. 335 
 
 given temperature is independent of the amount of heat- 
 ed surface; a mere point being just as efficacious as an 
 indefinitely extended surface. 
 
 4th. That the quantity of electricity evolved in a pile 
 of pairs is directly proportional to the number of the 
 elements. 
 
 First, then, as to the comparative march of electric de- 
 velopment with the rise of temperature in the case of 
 pairs of different metals. 
 
 The experimental arrangement which I have em- 
 ployed is represented 
 in Fig. 52. A A is a 
 glass vessel, about 
 three inches in diame- 
 ter, with a wide neck, 
 through which can be 
 inserted a mercurial 
 thermometer, 5, and 
 one extremity of a pair 
 
 of electro-motoric wires. The wires I have employed 
 have generally been a foot long and one sixteenth of an 
 inch in diameter. The extremities, S, of the wires thus 
 introduced into the vessel ought to be soldered with hard 
 solder; their free extremities dip into the glass cups d, 
 d, filled with mercury, and immersed in a trough, 0, con- 
 taining w r ater and pounded ice. By means of the cop- 
 per wires f f, one sixth of an inch thick, communication 
 is established with the mercury-cups of the galvanom- 
 eter. The coil of this galvanometer is of copper wire 
 one eighth of an inch thick, and making twelve turns 
 only round the needles, which are astatic. The devi- 
 ations were determined by the torsion of a glass thread. 
 
 It is surprising to those who have never before seen 
 the experiment with what promptitude and accuracy a 
 
326 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV. 
 
 copper and iron wire soldered thus together will indi- 
 cate temperatures. 
 
 In the arrangement now described, when an experi- 
 ment has to be made, the vessel A A is to be filled 
 two thirds full of water, the bulb of the thermometer 
 being so adjusted as to be in its middle, the soldered 
 extremity S of the two wires being placed in contact 
 with it, and a small cover with suitable apertures ad- 
 justed on the top of the vessel, so that the steam as it 
 is generated may rush up alongside of the tube of the 
 thermometer and bring the mercurial column in it to 
 a uniform temperature. If the extremity of the thermo- 
 electric pair be allowed to rest on the bottom of the 
 glass vessel, no accurate results can be obtained ; the 
 pair does not then indicate the temperature of the 
 water. The commxinicating wires ff are then placed in 
 the cups, the trough e filled with water and pounded ice, 
 and carefully surrounded with a flannel cloth. The wa- 
 ter in the vessel A A is then gradually raised to the 
 boiling-point by means of a spirit-lamp, and kept at that 
 temperature until the galvanometer needles and the ther- 
 mometer are quite steady. The same plan must be fol- 
 lowed when any other temperature than 212 is under 
 trial, for the thermo-electric wires changing their temper- 
 ature more rapidly than the mercury in the thermom- 
 eter, it is absolutely necessary to continue the experi- 
 ment for some minutes to bring both to the same state 
 of equilibrium. 
 
 When a temperature higher than 212 Fahr., but un- 
 der a red heat, is required, I substitute in place of the 
 vessel A A a tabulated retort, the tubulure of which is 
 large enough to allow the passage of the bulb of the 
 thermometer and the wires. A quantity of mercury, 
 sufficient to fill the retort half full, is then introduced, 
 and the tubulure being closed by appropriate pieces of 
 
MEMOIR XXIV.] THE ELECTRO-MOTIVE POWER OF HEAT. 327 
 
 soapstone, the neck of the retort is inclined upwards, so 
 that the vapor as it rises may condense and drop back 
 again without incommoding the operator,. As in the 
 former case, it is here also necessary to continue each 
 experiment for a few minutes, to bring the thermometer 
 and thermal pair to the same condition. There is not 
 much difficulty in obtaining any required temperature, 
 by raising or lowering the wick of the lamp. 
 
 The metals I have tried were in the form of wires. 
 They were in the state found in commerce, and therefore 
 not pure ; they were obtained in the shops of Philadel- 
 phia. 
 
 TABLE I. 
 
 Names of the Pairs of Metals. 
 
 Temperatures (Fahr.). 
 
 32 F. 
 
 122 F. 
 
 212 F. 
 
 662 F. <> 
 
 Copper and iron 
 Silver and palladium 
 
 
 
 
 
 
 
 
 93 
 65 
 112 
 Jl 
 
 89 
 
 28 
 
 176 
 147 
 223 
 26 
 137 
 56 
 
 233 } 
 613/ s?a 
 631 1 2.S 
 122 p 2, 
 244 H 
 248 j ? 
 
 Iron and palladium.. 
 
 Platinum and copper 
 Iron and silver 
 
 Iron and platinum 
 
 Ill this table I have estimated the temperature of 
 boiling mercury at 662 Fahr. The quantities of elec- 
 tricity evolved, as estimated by the torsion of a glass 
 thread, are ranged in columns under their corresponding 
 temperatures. Each series of numbers is the mean of 
 four trials, the differences of which were often impercep- 
 tible, and hardly ever 
 amounted to more than 
 one degree. 
 
 Now if this table be 
 constructed, the tem- 
 peratures being ranged 
 along the axis of abscis- 
 sas, and the quantities 
 of electricity being rep- 
 resented by correspond- Fig. 53. 
 
328 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV. 
 
 ing ordinates, we shall have results similar to those given 
 in Fig. 53, in which it is to be observed that the curves 
 given by the systems of silver and iron, copper and iron, 
 and palladium and iron are concave to the axis of ab- 
 scissas ; but those given by platinum and copper, silver 
 and palladium, and platinum and iron are convex. 
 
 Let us now apply the numbers obtained by these sev- 
 eral pairs for the calculation of temperatures, which will 
 set their action in a more striking point of view. The 
 following table contains such a calculation, on the sup- 
 position that for the 90 degrees from 32 Fahr. to 122 
 Fahr. the increments of electricity are proportional to the 
 temperatures. 
 
 
 Temperatures by the Mercurial Thermometer. 
 
 32 F. 
 
 122 F. 
 
 Water boils. 
 
 Mercury boils. 
 
 
 E*s 
 
 V 1, 
 C ^ 
 
 Copper and iron 
 
 32 
 32 
 32 
 32 
 32 
 32 
 
 122 
 122 
 122 
 122 
 122 
 122 
 
 202 
 235 
 211 
 244 
 170 
 212 
 
 257 
 880 
 539 
 1030 
 279 
 829 
 
 Silver and palladium. . 
 Iron and palladium. . . 
 Platinum and copper. . 
 Iron and silver 
 
 Iron and platinum 
 
 We are therefore led to the general conclusion that in 
 these six different systems of metals the developments of 
 electricity do not increase proportionally with the temper- 
 atures, but in some with greater rapidity and in others 
 with less. 
 
 The results here given I have corroborated in a vari- 
 ety of w r ays and with a variety of wires. A pair con- 
 sisting of copper and platinum gave for the temperature 
 of tin when in the act of congealing 452 Fahr. instead 
 of 442 Fahr., the point usually taken. For the melting- 
 point of lead it gave 942^ Fahr. instead of 612 Fahr. 
 The melting-points of tin, lead, zinc, and occasionally of 
 antimony and bismuth, were in this manner employed, 
 for they allow time for the working of the torsion bal- 
 ance, and with the exception of bismuth, their tempera- 
 
MKMOIR XXIV.] THE ELECTRO-MOTIVE POWER & H^T. v<29 
 
 ture appears to be steady all the while they "' 
 granular condition before they finally solidify, 
 tion of these metals on the thermo-electric pair is easily^ 
 prevented by dipping it into a cream of pipe-clay. ' / 
 
 A pair of copper and platinum gave for a dull red 
 heat 1416 Fahr., and for a bright red 2103 Fahr. 
 
 A pair of palladium and platinum gave for a dull red 
 1850 Fahr., and for a bright red 2923 Fahr. 
 
 Some of the combinations into which iron enters as 
 an element give rise to remarkable results. Thus, if we 
 project the curve given by a system of copper and iron 
 w r e shall find it resem- 
 bling Fig. 54, where 
 the maximum ordinate 
 b occurs at a temper- 
 ature of about 650 Ficr>54 
 Fahr. ; the point c ap- 
 pears to be given between 700 and 800 degrees ; d by a 
 dull red heat ; e is very nearly the point at which an 
 alloy of equal parts of brass and silver melts, for if the 
 pair be soldered with this substance, it fuses when the 
 needles have returned almost exactly to the zero. point. 
 With harder solders or with wires simply twisted, the 
 curve may be traced on the opposite side of the axis 
 towards /, its ordinates increasing with regularity. At 
 60 Fahr., taking the length of the ordinate correspond- 
 ing to a temperature of 212 Fahr. as unity, the length 
 of the maximum ordinate at b is 1.85, very nearly. 
 
 A system of silver and iron gives also a similar curve, 
 the point b occurring at a temperature rather higher 
 than the analogous one for the preceding system, but 
 still below the boiling-point of mercury. 
 
 Now, all these things serve to show that we cannot 
 determine with accuracy unknown temperatures by the 
 aid of thermo-electric currents, on the supposition that 
 
330 THE ELECTRO-MOTIVE POWER OF HEAT. [MKMOIR XXIV. 
 
 the increments of the quantities of electricity are propor- 
 tional to the increments of temperature throughout the 
 range of the mercurial thermometer. 
 
 Let us now pass to the second proposition : "That the 
 tension undergoes a slight increase with increase of tem- 
 perature a phenomenon due to the increased resistance 
 to conduction of metals when their temperature rises." 
 
 It will be seen, on consulting the following table, that 
 pairs of different metals at the same temperature have 
 tensions which are apparently very different. 
 
 The currents the tensions ot which are here indicated 
 were generated by keeping one end of the thermal pair 
 in boiling water, the other ends being maintained at a 
 temperature of 32 Fahr. 
 
 TABLE III. 
 
 A pair of 
 
 Tension. 
 
 A pair of 
 
 Tension. 
 
 Antimony and bismuth. . 
 Copper and iron 
 
 .137 
 183 
 
 Platinum and iron 
 
 .470 
 473 
 
 Silver and lend 
 
 .307 
 
 Platinum and palladium.. 
 
 .500 
 
 Lead and palladium 
 
 313 
 
 Tin and iron 
 
 518 
 
 Silver and platinum . 
 
 380 
 
 Platinum and tin 
 
 567 
 
 
 
 
 
 We perceive, therefore, that there apparently exist 
 specific differences in the qualities of electric currents 
 derived from different sources. If, for example, we take 
 a pair of platinum and palladium and expose it to a tem- 
 perature which shall generate a current capable of de- 
 flecting the torsion balance through 1000, and then 
 obstruct it by a wire of such dimensions as to stop one 
 half, or only allow 500 to pass, and repeat the experi- 
 ment with a current generated by bismuth and antimo- 
 ny, the temperature being still so adjusted as to give a 
 deflection of 1000, on making this pass through the 
 same intercepting wire, perhaps not much more than one 
 eighth of it will go through the galvanometer. 
 
 This peculiarity of thermo-electric currents depends 
 
MEMOIR XXIV.] THE ELECTRO-MOTIVE POWER OF HEAT. 33^ 
 
 on the conducting resistance of the system that generates 
 them. It is possible to give a current a higher or a 
 lower tension by simply making use of thin or thick 
 wires to generate or to carry it. In the foregoing table 
 the current from platinum and palladium had a high 
 tension, because slender wires of those metals happened 
 to be used to generate it ; and the current from antimo- 
 ny and bismuth had a low tension because thick bars 
 of those substances were employed. In the former case, 
 the conducting resistance was greater than in the latter, 
 and hence the tension of the current was higher. 
 
 That this is strictly true will appear on examining the 
 current evolved by any number of systems under the 
 same condition of resistance to conduction. 
 
 In a great number of trials I failed in getting any 
 trustworthy results as respects tension of currents at 
 high temperatures, on account of the difficulty of main- 
 taining the thermo-electric pair at the same degree with- 
 out variation. By employing, however, a small black- 
 lead furnace, to which was adapted a covered sand-bath 
 into which the wires could be plunged, I succeeded at 
 last ; for with this arrangement a regulated temperature 
 could be kept up for a length of time. 
 
 The experiment was made with care in the case of 
 two systems of metals: 1st, copper and platinum; 2d, 
 copper and iron. 
 
 1st. At the boiling-point of water, a pair of copper and 
 platinum, the unexcited extremity of which was carefully 
 maintained at 67 Fahr., evolved as a mean of four trials, 
 three of which were absolutely identical, 123 of electric- 
 ity, of which 23 could pass a secondary wire. 
 
 Then, by the aid of the furnace and sand-bath, the 
 temperature was raised until the pair evolved 783 as a 
 mean of four trials; of these 163 could pass the second- 
 ary wire. Now, 
 
332 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV. 
 As 783 : 163 :: 123 : 25| instead of 23, 
 
 showing, therefore, a slight rise of tension. 
 
 2d. The pair of copper and ifcon gave at the boiling- 
 point of water 300, of which 57 passed the secondary 
 wire. The temperature was now raised, with the follow- 
 ing results : 
 
 400 degrees passing the primary, 95 the secondary wire. 
 
 553 " " " ' 113 " 
 
 545 " " " 112 
 
 493 " " " 110 " " 
 
 It will be understood that although the quantities of 
 electricity indicated in the first column do not regularly 
 increase, the temperatures were, notwithstanding, going 
 regularly upwards: to this peculiarity of the systems 
 into which iron enters I have already alluded. Let us 
 now compare these measures with those obtained for the 
 boiling-point of water : 
 
 As 490 : 95 
 553 : 113 
 545 : 1 1 2 
 493 : 1 10 
 
 300 : 58 instead of 57. 
 300 : 01 " 
 300 : 61 " 
 300 : 67 " 
 
 We find, therefore, that in the case of both these systems 
 of metals the tension slowly rises with increase of tem- 
 perature, being much better marked in the latter than in 
 the former instance. 
 
 The increase of tension here detected depends unques- 
 tionably on increased resistance to conduction, which the 
 wires exhibit as their temperature rises, as the following 
 experiments show. 
 
 A pair of copper and iron evolved a current at the 
 boiling-point of water, which, passing through a wire of 
 copper eight feet long, was determined at the galvanom- 
 eter to be 176. Having twisted a part of this wire into 
 a spiral, so as to go over the flame of a spirit-lamp, eight 
 inches of it were thereby brought to a red heat; the de- 
 
MEMOIR XXIV.] THE ELECTRO-MOTIVE POWER OF HEAT. 333 
 
 viation of the needle fell now to 165, being a deficit of 
 11. In this experiment care was taken that no heat 
 should be transmitted along the wire to the connecting 
 cups. 
 
 The same was repeated with an iron wire of the same 
 length and under the same circumstances. The current 
 at first being 90, as soon as the spiral was made red-hot 
 it fell to 61, being a deficit, therefore, of nearly one third 
 the whole amount. 
 
 To the increased resistance to conduction, occasioned 
 by an increased temperature, we are to impute the slight 
 rise of tension observed in thermo-electric currents. The 
 quantities are of the same order. 
 
 We have next to show " that the quantity of electric- 
 ity evolved at any given temperature is independent of 
 the amount of heated surface, a mere point being just 
 as efficacious as an indefinitely extended surface." 
 
 The quantities of electricity evolved by hydro-electric 
 pairs increase with the surfaces, but it is not so in ther- 
 mo-electric arrangements. A pair of disks of copper and 
 iron, two inches in diameter, were soldered together; 
 they had continuous straps projecting from them, which 
 served to connect them with the galvanometer cups. At 
 the boiling-point of water they gave 62; on being cut 
 down to half an inch in diameter, they still gave 62. 
 On the disk being entirely removed, and the copper 
 made to touch the iron by a mere point, its extremity 
 being roughly sharpened, the deflection was still 62. 
 
 By means of a common deflecting multiplier, I ob- 
 tained the following results: 1st. A copper wire being 
 placed in a bath of mercury, the temperature of which 
 was 240 Fahr., I dipped into it a second copper wire, 
 the temperature of which was about 60 Fahr. ; the gal- 
 vanometer needles moved through 15. 
 
334 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIK XXIV. 
 
 2d. The cold wire being sharpened to a point and 
 plunged deliberately into the mercury to the bottom of 
 the bath, the deflection was 19V 
 
 3d. But when I touched the surface of the mercury 
 with the very point of the cold wire, there was a deflec- 
 tion of 60. 
 
 Having laid a plate of tinned iron upon the surface 
 of some hot mercury, it was touched with the point of 
 the cold wire. There was a strong deflection of the 
 needles in the opposite direction to what W 7 ould have 
 been the case had the mercury been touched and not the 
 iron. The under surface of the iron was, therefore, act- 
 ing as a hot face, and the parts round the point as a cold 
 face, being temporarily chilled by the touch of the wire. 
 
 These results explain the anomalies observed by some 
 of those who investigated the course of thermo-electric 
 currents by means of small metallic fragments. 
 
 It would therefore seem that when wires of the same 
 metal are used as electromotors, the quantity of electric- 
 ity evolved depends on the quantity of caloric that can 
 be communicated in a given time. Time, therefore, under 
 these circumstances, must enter as an element of thermo- 
 electric action. In the case of a single metal, the maxi- 
 mum effect would be produced when the hot element is 
 a mass and the cold one a point. 
 
 And, lastly, " that the quantities of electricity evolved 
 in a pile of pairs are directly proportional to the number 
 of the elements." 
 
 In the first trials I made to determine the effect of in- 
 creasing the number of pairs in a pile, the results ob- 
 tained were contradictory ; by operating, however, in the 
 following way, the proposition was at last satisfactorily 
 determined. 
 
 1st. The resistance to conduction was made nearly 
 
MEMOIR XXIV.] THE ELECTRO-MOTIVE POWER OF HEAT. 335 
 
 constant by uniting all the pairs intended to be worked 
 with at once. The current, therefore, whether generated 
 by one, two, three, four, or more pairs, had always to run 
 through the same length of wire, and experienced in all 
 cases a uniform resistance. 
 
 2d. By making each individual element of consider- 
 able length, the liability of transmission from the hot to 
 the cold extremity was diminished. 
 
 Having, therefore, taken six pairs of copper and iron 
 wires, -j^ of an inch thick and each element 38 inches 
 long, I formed them, by soldering their alternate ends, 
 into a continuous battery. Then I successively immersed 
 in boiling water one, two, three, etc., of the extremities, 
 their length allowing freedom of motion, and the other 
 extremities not differing perceptibly from the temper- 
 ature of the room. 
 
 The following table exhibits one of this series of ex- 
 periments : 
 
 TABLE IV. 
 
 No. of Pairs. 
 
 Calculated Deviations. 
 
 Observed Deviations. 
 
 1 
 
 55 
 
 55 
 
 2 
 
 110 
 
 111 
 
 3 
 
 165 
 
 165 
 
 4 
 
 220 
 
 220 
 
 5 
 
 275 
 
 272 
 
 6 
 
 330 
 
 332 
 
 Hence there cannot be any doubt that the quantities 
 of electricity evolved by compound batteries at the same 
 temperature are directly proportional to the number of 
 the pairs. 
 
 With some general remarks, arising from the forego- 
 ing subjects, I shall conclude this Memoir. 
 
 1. It is of importance to remember that thermo-electric 
 currents traverse metallic masses only on account of dif- 
 ferences of temperature existing at different points. 
 
336 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV. 
 
 2. When a current of electricity, flowing from the poles 
 of a battery, is made to traverse a metallic sheet, the 
 whole of it does not pass in a straight line from one pole 
 to the other, but diffuses itself through the metal, di- 
 verging from one point and converging to the other. 
 The greater part of the current is found, however, to 
 take the shortest route. 
 
 3. Combining, therefore, the foregoing observations 
 (1, 2), we perceive that there are certain forms of con- 
 struction which will give to thermo-electric arrangements 
 peculiar advantages. For example, the surfaces united 
 
 by soldering must riot be too mass- 
 ive. Let A, Fig. 55, be a semi -ring 
 of antimony, and B a semi-ring of 
 bismuth ; let them be soldered to- 
 gether along the line C H, and at 
 the point H let the temperature be 
 raised : a current is immediately ex- 
 cited ; but this does not pass around 
 the ring A B, inasmuch as it finds a 
 shorter and readier channel through the metals, between 
 H and d, circulating therefore as indicated by the arrows. 
 Nor will the whole current pass round the ring until the 
 temperature of the soldered surface has become uniform. 
 An obvious improvement in such a combination is 
 shown at H in Fig. 56, which con- 
 sists of the former arrangement cut A 
 out along the dotted lines: here the 
 whole current so soon as it exists is 
 forced to pass along the ring. And 
 because the mass of metal has been diminished at the 
 line of junction, such a pair will change 
 its temperature very quickly. 
 
 One of the best forms for a thermo- 
 Fig.57. electric couple is given in Fig. 57, where 
 
MEMOIR XXIV.] THE ELECTRO-MOTIVE POWER OF HEAT. 33^ 
 
 A is the section of a semicylindrical bar of antimony, B 
 of one of bismuth, united together by the opposite cor- 
 ners of a lozenge- shaped piece of copper, C. From its 
 exposing so much surface, the copper becomes hot and 
 cold with the greatest promptitude, and from its good 
 conducting power it may be made very thin without 
 injury to the current. With a pair of bars three fourths 
 of an inch thick, and a circular cop- 
 per plate, as at D, Fig. 58, having both 
 surfaces blackened, I have repeated 
 the greater part of those experiments 
 which M. Melloni made with his mul- 
 tiplier. A platinum wire, as at E, Fig. 59, may sometimes 
 
 be very advantageously used. 
 
 4. The currents circulating in a steel 
 
 magnet are to all appearance perpetual. 
 Fior 59 I thought for some time it might be 
 
 possible to procure similar perpetual 
 currents by compound thermo-electric arrangements. 
 Let a, b, c be wires of three different metals soldered to- 
 gether so as to form a triangle. Now if these metals 
 were selected, so that a and b could form a more power- 
 ful thermo-electric pair than a and c, or b and e, it might 
 be expected that at all temperatures an incessant current 
 would run round the system. Such, however, will not 
 be found to be the case. In effect, any one of these three 
 serves simply as a connecting solder to the other two, 
 and hence no current is excited ; for the ends that have 
 the third metal between them, although that metal inter- 
 venes, are under exactly the same condition as the other 
 ends which are in contact. 
 
 Y 
 
338 MICROSCOPIC PHOTOGRAPHY. [MEMOIR XXV. 
 
 MEMOIR XXV. 
 
 ON MICROSCOPIC PHOTOGRAPHY. 
 
 CONTENTS : Method of making microscopic photographs by condensed 
 sunlight. Specimens of the art. 
 
 WHEN giving courses of lectures on Physiology in 
 the University of New York, I found it very desirable 
 to have photographic representations of various micro- 
 scopic objects. There are many such objects, costing 
 much time and trouble in their preparation, which it is 
 very difficult, if not impossible, to preserve. A photo- 
 graph is their best substitute. 
 
 I used as the sensitive surface daguerreotype plates, 
 for this was previous to the invention of the collodion 
 process. The daguerreotype plate is, however, much less 
 sensitive than the collodion film. My first attempts 
 were to copy the objects by lamplight, but this was 
 found to be not sufficiently intense. Even after very 
 long exposures with low powers the impressions ob- 
 tained are unsatisfactory. 
 
 On employing a beam of sunlight impressions could 
 be quickly obtained, and by concentrating the beam 
 with a condensing lens the object could be sufficiently 
 illuminated to suit any magnifying power. A fine mi- 
 croscope was used to give the image. 
 
 Two difficulties were, however, to be provided for: 
 1st. If the condensing lens be large, the heat at its focal 
 point may be so high as to injure or even destroy the 
 object. 2d. It is not easy to find the chemical focus, for 
 it does not coincide with the visual one. 
 
MEMOIR XXV.] MICROSCOPIC PHOTOGRAPHY. 339 
 
 But these difficulties may be completely overcome by 
 causing the illuminating beam to pass through a solu- 
 tion of sulphate of copper and ammonia. This transmits 
 all the radiations that affect a photographic surface, but 
 absorbs all others. With the absorbed rays the heat so 
 nearly disappears that the most delicate preparation may 
 be left at the focal point for any length of time without 
 risk. 
 
 The solution of sulphate of copper and ammonia also 
 enables us to ascertain the focal point with precision. 
 On receiving the image on the ground glass of a camera, 
 it is, of course, of a blue color, and when brought to a 
 sharp focus its photograph will be equally sharp. 
 
 The best results are obtained when the blue image 
 of the sun, formed by the condenser, coincides with the 
 object. 
 
 In Fig. 60, a a is a heliostat mirror reflecting the sun's 
 
 Fig. CO. 
 
 rays horizontally ; c a condensing lens three inches in 
 aperture ; there are adapted to it different diaphragms, 
 b, to regulate the quantity of light used, since it is not 
 well to have too bright an image on the photographic 
 plate ; d is a glass cell, formed by cutting a circular hole 
 three inches in diameter in a glass plate a quarter of an 
 inch thick ; on each face of this plate a flat piece of glass 
 is laid, thus forming a receptacle in which the sulphate 
 of copper and ammonia solution may be placed. It 
 holds the liquid without risk of leakage ; e is the object- 
 stage of the microscope,/; g, the camera with its ground 
 glass or sensitive plate. 
 
340 MICROSCOPIC PHOTOGRAPHY. [MEMOIR XXV. 
 
 In this manner I made many microscopic daguerreo- 
 types. Some of them, such as were needed, were subse- 
 quently cut in wood to serve as illustrations for my 
 Treatise on " Human Physiology." From that work I 
 extract the specimens on the opposite page. 
 
 When, the collodion process was invented, I made 
 many of these photographs in that way. Some of my 
 old original daguerreotypes w r ere exhibited in the Cen- 
 tennial Exposition at Philadelphia, as illustrations of the 
 early history of the art. Collodion photographs have, of 
 course, the great advantage over daguerreotypes in this, 
 that they may be magnified by the lantern and used for 
 demonstration before a large class. 
 
MICROSCOPIC PHOTOGRAPHY. 
 
 341 
 
 Pig. Gl. Spiracle of insect, magnified 75 diame- Fig. 62. Elliptic blood-cells of frog, magnified 
 ters. 250 diameters. 
 
 Pig. 63,-UlLimate muscular fibre, magnified 200 Fig 64 ._s p iral vessels of banana, magnified 50 
 diameters. diameters. 
 
 
 
 Pig. 65. -Woody fibre ^Tpine, magnified 50 di- Fi S- 66 - Mncous membrane of the stomach of a 
 ameters. carnivorous beetle, magnified 50 diameters. 
 
342 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 MEMOIR XXVI. 
 
 ON CAPILLARY ATTRACTION AND INTERSTITIAL MOTIONS. 
 
 THE CAUSE OF THE FLOW OF SAP IN PLANTS AND 
 
 THE CIRCULATION OF THE BLOOD IN ANIMALS. 
 
 Extracted and condensed from various Memoirs as follows : On Capillary Attrac- 
 tion, Franklin Institute Journal, Philadelphia, September, 1834. On the Tidal 
 Motions of Movable Electric Conductors, Franklin Institute Journal, January, 
 1836. Experiments on Endosmosis, Franklin Institute Journal, March, 1836. 
 On Endosmosis through Water and Soap Bubbles, Franklin Institute Journal, July, 
 1836. On Interstitial Movements, American Journal of Medical Sciences, May, 
 1836. On the Physical Theory of Capillary Attraction, American Journal of 
 Medical Sciences, February, 1 838. On the great Mechanical Force Generated by 
 the Condensing Action of Tissues, American Journal of Medical Sciences, May, 
 1838. On the Physical Theory of Endosmosis, American Journal of Medical 
 Sciences, August, 1838. Analysis of Some Coins, Silliman's American Journal, 
 Vol. XXIX., 1836. On the Circulation of the Blood, Silliman's American Journal, 
 Second Series, Vol. II., 1846. On the Constitution of the Atmosphere, London 
 and Edinburgh Philosophical Magazine, October, 1838. On Capillary Attraction, 
 London and Edinburgh Philosophical Magazine, March, 1845. On the Circula- 
 tion of the Blood, London and Edinburgh Philosophical Magazine, March, 1846. 
 A Treatise on the Forces that Produce the Organization of Plants, New York, 
 1 844. A Treatise on Human Physiology, New York, 1856. 
 
 NOTE. These Memoirs are collectively so voluminous that I could not publish them 
 in this volume in full. I made the following abridgment of them, which was 
 printed in Harper's Magazine, January, 1878. 
 
 CONTENTS: Interstitial motions of solids. Motions of metal in coins. 
 Movement of liquids in crevices. Capillary attraction. Conditions 
 for a continuous flow. Capillary attraction an electrical phenomenon. 
 Motions of liquid conductors. Dutrochefs experiment of endosmosis. 
 Passage of gases through liquids. Soap bubbles. Passage against 
 heavy pressure. Distribution of sap in plants. Circulation in plants 
 due to sunlight producing gum. Circulation of blood in animals ex- 
 plained. The systemic, the pulmonary, and the portal circulation. 
 
 IT is necessary for the life of every organized being 
 that a liquid should circulate through all its parts. In 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 343 
 
 plants that liquid is called the sap; in animals, the 
 blood. 
 
 When the distance through which such a liquid has 
 to pass is small, there seems to be little difficulty in as- 
 signing the cause of its movement; but how shall we 
 explain the rise of the sap in the great sequoia-trees of 
 California, some of which attain a height of more than 
 four hundred and thirty feet? To force a column of 
 water to such an elevation would require a pressure of 
 more than two hundred pounds on the square inch. Yet 
 we cannot doubt that the power which overcomes these 
 enormous resistances is the same as that engaged in the 
 most insignificant transudations. 
 
 Even in the case of solid mineral substances the parti- 
 cles are not at rest. Boyle, in his tract on " The Lan- 
 guid Motions of Bodies," has collected several interesting 
 instances. 
 
 He says : " But what is more extraordinary, a gentle- 
 man of my acquaintance had a turquois stone wherein 
 were several spots of different colors, which seemed to 
 him for many months to move slowly from one part of 
 the stone to the other. And having the ring wherein it 
 was set put into my custody, I drew pictures of the 
 spots at different times ; and by comparing several of the 
 draughts together, it evidently appeared that they shifted 
 their places, as if the matter whereof they consisted made 
 its way through the substance of the stone. And as far 
 as we observed, the motion of these spots was exceeding- 
 ly slow and irregular. An experienced jeweller, likewise, 
 assured me that in a few turquois stones he had ob- 
 served two different blues in different parts of the same 
 stone, and that one of these colors would, by slow and 
 imperceptible degrees, invade and at length overspread 
 that part of the stone which the other before possessed. 
 And the same gentleman who lent me the spotted tur- 
 
044 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 quois also showed me an agate baft of a knife, wherein 
 was a certain cloud which an ingenious person had for 
 some years observed to change its place in the stone." 
 
 Some years ago, having occasion to make an analysis 
 of certain Roman silver coins which had been long bur- 
 ied in the earth, I found that much of the alloying cop- 
 per had made its way to the surface, constituting the 
 green patina of antiquarians, and that the silver had be- 
 come comparatively pure. An interstitial movement in 
 these denarii must therefore have taken place a move- 
 ment so slow that it had required many centuries to 
 yield the observed result. 
 
 If one end of a porous substance, such as a sponge, be 
 dipped into water, the liquid very soon percolates in all 
 directions through the mass, which becomes charged with 
 as much as it can hold. If a piece of glass having a 
 crack in it be put into water so that the end of the 
 crack is immersed, the liquid instantly runs spontaneous- 
 ly to the other end. And if from the crack other smaller 
 ones branch forth, along these also the water rapidly 
 finds its way. 
 
 These effects have long been studied by using slender 
 glass tubes, which operate in the same manner as cracks, 
 but permit the phenomena to be observed in a more con- 
 venient and exact manner. 
 
 Water will pass through a crevice the width of which 
 is less than one half of the millionth of an inch. The 
 
 proof of this is readily 
 obtained experimentally. 
 If we take a convex lens, 
 a a (Fig. 67), of long fo- 
 cus, and place it upon a glass plane, b , there will be seen 
 at the point of apparent contact, <?, on looking down, a 
 black spot surrounded by a series of variously colored 
 concentric circles, the appearance being well known 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 345 
 
 among optical writers under the name of Newton's col- 
 ored rings. At the point c the lens and the plane are, 
 as Newton has shown, a distance apart of about one half 
 of the millionth of an inch ; and from this centre, pro- 
 ceeding outwardly, the distance between the glasses of 
 course increases. If anywhere at the outer portion a 
 drop of water be introduced, it extends itself instantly 
 across all the colored rings, reaching even across the cen- 
 tral black spot. 
 
 If a tube of such diameter that it could lift water ten 
 inches be broken off so as to be only six inches long, we 
 might inquire whether the water would overflow from 
 its top or simply remain suspended there. 
 
 Mathematical considerations, as well as direct experi- 
 ments, prove that in such a case there would be no over- 
 flow. A capillary tube under these circumstances lifts 
 the water, but does not produce a continuous current. 
 
 But if a removal of the liquid at the top of the tube 
 take place in any manner, as by evaporation or by being 
 dissolved in another liquid, a continuous current is pro- 
 duced. 
 
 As illustrating the production of such a continuous 
 flow we may cite the case of a spirit-lamp, the wick of 
 which maybe regarded as a fagot of capillary tubes. Be- 
 tween the fibres of the wick there are interspaces that 
 answer as tubes. If the cover of the lamp be taken off, 
 all the spirit will eventually pass up the wick and escape 
 from the reservoir by evaporation. Or, in an oil-lamp, 
 the wick of which becomes readily saturated with the 
 oil, but never exhibits an overflow, on the lamp being 
 lighted, the oil is burned off, a current is established, and 
 after a time the reservoir is emptied. 
 
 The phenomena of capillary tubes are connected with 
 the adhesion of surfaces. Clairaut showed that if the 
 
346 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 mutual attraction of a solid and liquid amount to less 
 than half the cohesion of the latter, the liquid will be 
 depressed in a capillary tube ma*de of the solid ; if it be 
 equal to half, the liquid will stand level in the tube ; if 
 it exceed half, the liquid will rise. 
 
 I published a paper in the Journal of the Franklin 
 Institute of Philadelphia for September, 1834, its object 
 being to show that the adhesion of surfaces, whether 
 solid or liquid, to each other, and the rise or depression 
 of liquids in capillary tubes, are strictly electrical phe- 
 nomena. 
 
 When a glass plate is laid on the surface of quicksil- 
 ver, a considerable force is required to separate them. 
 On the separation being made, if the substances be ex- 
 amined by the electroscope, the glass will be found to be 
 electrified positively, the mercury negatively. Their at- 
 traction or adhesion is, therefore, a necessary electrical 
 result. So intense is this electrical development that if 
 during the act of separation the mercury be in connec- 
 tion with a gold-leaf electroscope, the 
 gold leaves are commonly torn asun- 
 der. In Fig. 68, a a is a glass dish 
 containing mercury, c the glass plate, 
 d tine gold-leaf electroscope. 
 In like manner, if some melted sulphur be poured into 
 a conical glass and permitted to solidify, on making the 
 separation the interior of the glass and the solid sulphur 
 cone will be found to be in opposite electrical states. 
 And the same occurs when surfaces of various kinds are 
 parted from each other. There ought, therefore, 
 to be adhesion. In Fig. 69, a a is the conical 
 glass, b the solidified sulphur, with handle for 
 <* its removal attached. 
 Fig. 69. j U |f a gj ass pl a te be laid on a surface of 
 
 water, there is no apparent development of electricity on 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 
 
 separating them. And the reason is obvious, for the 
 glass has brought away with it a layer of water, and 
 there has been no true separation of the solid from the 
 liquid, but only of water from water. The force of ad- 
 hesion of the glass to the water has exceeded the cohe- 
 sion of the water for itself. 
 
 If a plate of polished zinc be laid on mercury, there 
 will, again, be no electrical development apparent on 
 separating them. For, owing to the conductibility of 
 the zinc, there is nothing to prevent the opposite elec- 
 tricities from uniting, and all electrical manifestations 
 must cease. 
 
 Whatever can disturb the electrical relations of a 
 solid and a liquid will disturb their capillarity. On 
 wetting the interior of a glass tube, so as to form a tem- 
 porary tube of water, and placing some mercury in it, 
 the mercury will be depressed below the hydrostatic 
 level. But on connecting the mercury with the negative 
 pole of a voltaic battery, and the water with the posi- 
 tive, the mercury at once rises, their mutual attraction 
 being increased. 
 
 I derived these conclusions from the following experi- 
 ments : 
 
 1. In a watch-glass (Fig. 70) place a quantity of pure 
 mercury, a &, and upon it a drop of water, c. Bring the 
 water in contact with the positive 
 platinum electrode of a voltaic bat- 
 tery, and touch the mercury with 
 the negative. The moment the con- 
 tact is made the drop of water Fig. 70. 
 loses its spherical form and spreads out into a thin cir- 
 cular disk, wetting the surface of the mercury. The di- 
 ameter of the disk seems to be greater in proportion as 
 the battery is more powerful. 
 
 Under ordinary circumstances water does not wet 
 
348 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 quicksilver. A drop of water remains on the surface of 
 quicksilver in the same manner that a drop of oil re- 
 mains on water. As soon, hoWever, as their electro- 
 chemical relations are disturbed by the aid of a voltaic 
 battery, the phenomenon of wetting at once occurs. 
 
 2. Take a tube, Fig. 71, in the form of an inverted 
 siphon, one branch of which, , is about half an inch 
 wide, and the other, Z>, not more than 
 the tenth of an inch. Fill the siphon to 
 a given height, a b, with mercury ; the 
 metal, of course, does not rise in the nar- 
 row branch, Z>, to its hydrostatic level, 
 for mercury is depressed in a capillary 
 Fiir 71 tube, inasmuch as it cannot wet glass. 
 
 Introduce a small column of water, b c. 
 The mercury may now be regarded as being in contact 
 with a tube of water, because that liquid wets the sides 
 of the glass intervening between it and the mercury. 
 
 Pass a slender platinum wire, a?, down the tube, so as 
 to touch the water ; let it be in communication with the 
 positive electrode of the voltaic battery ; with the nega- 
 tive electrode, y, touch the mercury in the wide branch 
 of the siphon, a, and in an instant the metal will rise in 
 the narrow tube, and fall again to its former position as 
 soon as the current is stopped. 
 
 If into a watch-glass fifty or sixty grains of mercury 
 be poured, and over that as much water acidulated with 
 sulphuric acid as is sufficient to cover the surface of the 
 mercury, on the mercury being placed in contact with 
 the negative pole of a battery, and the water with the 
 positive, currents are produced both in the water and 
 the mercury, as was first observed by Erman and Ser- 
 rulas. 
 
 If the wires be on opposite sides of the mercury, as 
 shown in Fig. 72, the metal instantaneously elongates, as 
 
MEMOIR XXVI. J CAPILLARY ATTRACTION, ETC. 
 
 349 
 
 Fig. 72. 
 
 Fig. 73. 
 
 indicated by the dotted line, and currents also are seen 
 playing in the water. If the neg- 
 ative wire be introduced into the ^c. 
 centre of the metallic globule, and 
 the positive be- brought on one 
 side, as in Fig. 73, the mercury will bulge out elliptically 
 
 at both sides, nearest and farthest 
 from the positive pole. If, now, 
 the negative wire be cautiously 
 raised from its position, so as to 
 be just out of contact with the surface of the metal, the 
 mercury is immediately convulsed, its whole surface be- 
 ing covered with circular waves. On lowering the neg- 
 ative wire to its former position and advancing the posi- 
 tive, the moment it conies to the edge of the mercurial 
 ellipsoid intense convulsions are produced, which increase 
 until contact of the mercury and wire takes place. 
 
 At the same time that these movements are going on 
 in the mercury, the surface of the water is ploughed by 
 gentle currents, exactly resembling those that might be 
 produced by directing a stream of air 
 from a blow-pipe slantingly across the 
 surface (Fig. 74). 
 
 The following experiment illustrates 
 the nature of these effects : 
 
 A platinum needle, a c, Fig. 75, is suspended by a 
 thread of unspun silk from a stand, b b, 
 in a cup filled with acidulated water as 
 high as d d. The needle hangs hori- 
 zontally, its ends being about one fourth 
 of an inch distant from the platinum 
 polar wires, p, n, of a battery. Now the 
 wire p being positive and n negative, 
 the extremity a of the suspended nee- 
 
 Fig. 74. 
 
 Fig. 75. 
 
 die would be negative and c positive 
 
350 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 by induction. The conjoined effect of the forces thus 
 brought to bear on the needle causes it to move on its 
 centre, and take up a position of rest between the polar 
 wires. To this, if it were turned aside, it would return 
 after a few slow oscillations. 
 
 From this it appears that though the polar wires are 
 plunged into a conducting medium and the current is 
 passing, they still act as centres of attraction. 
 
 When a globule of mercury under the surface of some 
 water is brought in presence of a point of attraction, p, 
 situated at a short distance from its surface, two tides 
 will be formed on the globule ; one, &, 
 Fig. 76, directly in front of the point 
 of attraction, the other, &, 180 from it. 
 In the quadrantal regions, <?, d, there 
 will be an ebb. If the point p be 
 moved round the globule, both tides will follow it, keep- 
 ing the same relative position that they had at first. 
 These motions imitate on a small scale the effect which 
 takes place by the action of the sun and moon in pro- 
 ducing the tides of the ocean, and the explanation is 
 the same as in the case of those tides. 
 
 If a spring-tide were formed on a spherical ocean, and 
 the sun and moon then annihilated, the elevation must 
 sink, pressing the under waters aside, and causing them 
 to rise where they were depressed. But the motion 
 would not cease when the level was reached, for the wa- 
 ter would arrive at that position with an accelerated ve- 
 locity. It would, therefore, pass that position and form 
 a high water where it had been low, and low water 
 where it had been high. And this would be repeated 
 again and again. 
 
 Now this theoretical case may be imitated with the 
 globule of mercury, for on approaching the positive wire 
 to it, a position will be reached at which contact will 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 351 
 
 take place between the protuberant tide on the mercury 
 and the wire. At that moment the cause of attraction 
 is annihilated, the whole current of electricity now pass- 
 ing along perfect conductors, and fulfilling the supposed 
 case of an annihilation of the sun and moon at the time 
 of high tide. And the same reasoning that held in one 
 case applies equally in the other the mercurial tide falls 
 with an accelerated motion, and the line which before 
 was the transverse axis of the ellipse becomes the conju- 
 gate, tides being produced at right angles to the former 
 ones. But here the strict comparison ends, for as the 
 mercury ebbs from its protuberant position the metallic 
 connection breaks, and the wire is again put in action as 
 a point of attraction. The motion of the ebbing tide is 
 checked ; it flows once more. Once more the metallic 
 contact is complete ; and when the tide falls, it is only to 
 flow again as long as the battery current passes. Tides 
 take place at right angles to each other, in a series too 
 rapid to be counted, and the whole surface of the mer- 
 cury is worked into those various and beautiful undula- 
 tions which have been before referred to. 
 
 With respect to the currents that are observed, as if a 
 gentle wind were playing over the surface, the explana- 
 tion is obvious. We have seen that when a voltaic cur- 
 rent is passed through mercury and water, the pressure 
 on the surface of contact is changed. Newton has shown 
 (" Principia," Vol. II., Bk. ii., Pr. 41) that if the particles 
 of a fluid do not lie in a right line, a pressure propagated 
 through that fluid will not be in a rectilinear direction, 
 but the particles that are obliquely 
 posited have a tendency to be urged 
 out of their position. So the particles 
 a, a, Fig. 77, pressing on the particles 
 #, d, which stand obliquely to them by Fig - 7T - 
 
 reason of the shape of the mass of mercury, M, have a 
 
352 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 tendency to be urged from their places towards e and c 
 respectively, and the motion thus produced in a fluid 
 diverges from a rectilinear progress into the unmoved 
 spaces ; and such a pressure taking place in a liquid free 
 to move continually returns the moving particles to 
 their position, after making them describe an elliptical 
 orbit. 
 
 Such is the nature of the evidence that may be brought 
 forward in proof of the hypothesis that the adhesion of 
 surfaces and capillary attraction are electrical results. 
 We may now pass to an exposition and explanation of 
 the more interesting instances of motion in the cases of 
 liquids and gases. If, in the lapse of centuries, metallic 
 copper can part itself from silver with which it has been 
 alloyed, coming forth from the interior of a coin to form 
 a new compound on the surface, such movements, it 
 might be expected, would more readily occur in liquids, 
 of which the cohesion is but small, and in gases, in which 
 cohesion perhaps does not exist at all. 
 
 And, first, as respects liquids : 
 
 The phenomena of endosmosis, first brought to general 
 notice in the case of liquid substances by M. Dutrochet, 
 may be explained as follows : If some alcohol be placed 
 in a bladder, the neck of which is tightly tied, and the 
 bladder be sunk into a vessel of water, percolation en- 
 sues, so that the bladder is distended to its utmost 
 capacity, and may even burst. Or if, instead of tying 
 the mouth of the bladder, a glass tube, open at both 
 ends and a foot or two long, be fastened into it without 
 leakage, as the water introduces itself through the pores 
 of the bladder to mingle with the alcohol the liquid rises 
 in the glass tube, and, when it has reached the top of it, 
 overflows. To express this inward passage of the water 
 the term endosmosis was introduced ; and since a little of 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 
 
 353 
 
 the alcohol simultaneously passes outward to mix with 
 the water, it is said to exhibit exosmosis. 
 
 In Fig. 78 is represented the endosmometer of Dutro- 
 chet. It consists of a small blad- 
 der, #, tightly tied to a tube, d, 
 open at both ends, and bent as 
 seen in the figure at c; the blad- 
 der being completely filled with 
 alcohol, and the tube to some 
 such point as d, the arrangement 
 is placed in a vessel of water, ee j 
 almost immediately the level of 
 the liquid will be seen to be ris- 
 ing, the bend of the tube is 
 reached, and one drop after an- Fig - 78 - 
 
 other falls from the open end into the receiver, b. And 
 this continues until the liquids inside and outside of the 
 bladder are uniformly commingled. 
 
 In these results there is nothing more than should 
 take place on the ordinary principles of capillary action. 
 The pores of a bladder are only short capillary tubes, 
 into which water readily finds its way, because it can 
 wet the substance surrounding the pores. If the blad- 
 der be distended with air and sunk under water, al- 
 though the water will fill the pores, it will not exude 
 from them and accumulate in the interior of the blad- 
 der; for, as we have seen, a capillary tube cannot estab- 
 lish a continued current or flow. But the case becomes 
 totally different when the bladder is filled with alcohol ; 
 for then as fast as the water presents itself on the inner 
 end of each pore it is dissolved away by the alcohol, and 
 the necessary condition for a continuous flow is complied 
 with. Meantime through the pore itself a little alcohol 
 passes in the opposite w r ay by infiltrating through the 
 incoming water, provided that the current be not too 
 
 Z 
 
354 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 strong, and so the endosmosis of the water and exosmo- 
 sis of tbe alcohol take place; the current of the former 
 greatly preponderates over th#t of the latter, and an ac- 
 cumulation of liquid in the interior of the bladder ensues. 
 
 That in all this there is nothing specially dependent 
 on the organic texture employed is obvious from the fact 
 that the same results arise when any inorganic porous 
 body is used. Vessels of unglazed earthenware, pieces 
 of baked slate or stucco, answer the purpose very well, 
 as will also a glass vessel with a minute fissure or crack 
 in it. 
 
 An incorrect representation of the conditions under 
 which endosmosis takes place is often made. It is said 
 to depend on the relative specific gravities of the liquid, 
 and that the lighter liquid always moves towards the 
 denser more abundantly than the denser towards the 
 lighter. But water endosmoses equally well to alcohol, 
 which is lighter than it, and to gum-water or salt-water, 
 which are heavier. 
 
 The force with which a liquid will thus pass through 
 a pore to mingle with another liquid beyond is, as we 
 shall presently see, very great. 
 
 It has sometimes been affirmed that living organic 
 textures possess a so-called selecting power, permitting 
 some substances to pass through them and refusing a 
 passage to others. But this so-called selecting power is 
 purely physical, as are the separations and apparent de- 
 compositions to which it gives rise. 
 
 If we take a glass tube, a a, Fig. 79, over the lower 
 end of which a piece of peritoneum or other delicate 
 membrane, b b, is tightly tied, and half fill it with litmus- 
 water, and then place it in a glass of alcohol, c c, the lev- 
 els of the liquids inside and outside being adjusted ac- 
 cording to their specific gravities, so that there may be 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 
 
 355 
 
 Fig. 79. 
 
 no hydrostatic pressure either one way or 
 the other through the pores of the perito- 
 neum, as soon as the arrangement is com- 
 pleted, if the observer be so placed as to 
 view it by transmitted light, he will see 
 the water descending through the pores of 
 the peritoneum in striae and streams in a 
 perfectly colorless state. The membrane 
 has, therefore, absorbed and transmitted 
 the water, but has refused to the coloring 
 matter a passage. Such illustrations may, 
 therefore, satisfy us that the selecting pow- 
 er of organic textures, like that of inorganic fj 
 ones, is dependent on simple physical cir- 
 cumstances, and for these reasons we may 
 exclude from the mechanism of animal ab- 
 sorption the influence of any vital or other mysterious 
 principles, and adopt the opinion of Abbe Hauy, that 
 "those specious causes and imaginary powers to which 
 in the Middle Ages all natural phenomena, even those of 
 an astronomical kind, were referred, but which, through 
 
 / ft O 
 
 the genius of Newton and Laplace, have been banished 
 from the celestial spaces, have taken their last refuge in 
 the recesses of organic beings, and from these retreats 
 science is preparing to expel them." 
 
 We may next turn to an examination of analogous phe- 
 nomena in the case of gases. I found that on blowing a 
 little bubble of melted shellac on the end of a glass tube, 
 and putting some reddened litmus- water in its interior, 
 ammoniacal gas, to which it was exposed, could almost 
 instantly permeate the thin texture or barrier, and, gain- 
 ing access to the inside of the bubble, turn the litmus 
 blue. 
 
 When the pores of such barriers are of sensible size, 
 
a 
 
 356 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 it is plain that the passing material is influenced by the 
 substance of which the pore consists only on those por- 
 tions that come in contact therewith. Through the cen- 
 tral parts of the pore the material will pass by mere 
 leakage. If, then, we desire to determine the physical 
 conditions under which these movements take place, we 
 must make use of barriers the pores of which are not of 
 sensible size. 
 
 I therefore closed the top of a glass tube, A, Fig. 80, 
 otherwise open at both ends, with a disk of 
 paper placed at such a distance down the tube 
 a as to permit a stratum of water, a a, one eighth 
 of an inch thick, to be laid upon it. Conveying 
 the tube to the pneumatic trough, I filled it in 
 succession with various gases, and watched their 
 passage through the water roof, for so it might 
 be called, into the air. In such an experiment 
 a column of hydrogen gas half an inch in length 
 had escaped in twenty-four hours. 
 
 But experiments of this kind may be much shortened 
 by using very thin films instead of a thick stratum. A 
 glass bell-jar, a a, Fig. 81, was filled with 
 hydrogen gas, and by the side of it was 
 placed a small bottle containing atmos- 
 pheric air. A finger dipped in soap- 
 water was passed over the mouth of the 
 bottle, so as to close it with a thin film, 
 and the glass bell -jar of hydrogen was 
 then placed over it, as at b. In the course of two min- 
 utes the film, which was at first horizontal, had become 
 convex, and eventually swelled into a large spherical 
 bubble, c. In sixteen minutes it had become so thin 
 that it was of a dark metallic lustre. 
 
 But the action is much more speedy if, instead of these 
 horizontal films, soap-bubbles are used. Such films are 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 
 
 357 
 
 Fig. 82. 
 
 at first too thick, they expose too small a surface to the 
 atmosphere to which they are subjected, and it is not 
 until the close of the experiment that the action becomes 
 rapid. But a bubble at once exposes a large surface, 
 and by using proper precautions there is no difficulty in 
 preserving it for an hour, or even much longer. 
 
 The quickness of these motions is very well illustrated 
 by the following experiment. If 
 a glass flask, a a, be rinsed out 
 with ammonia, and then by means 
 of a bent glass tube, b b, narrowed 
 at its projecting extremity, a soap- 
 bubble, <?, be blown therein, the air 
 from the bubble being immediate- 
 ly drawn into the mouth without 
 a moment's delay, the strong taste 
 of the ammonia will be perceived. 
 Or if a rod dipped in hydrochloric 
 acid be presented to the projecting end of the glass tube, 
 as the bubble slowly collapsing presses out its contents, 
 copious white fumes arise. Care must be taken that no 
 drop of water obstructs the egress of the gas from the 
 bubble, and that the cork fits the flask with a slight 
 leakage. This, therefore, shows that va- 
 pors will pass through barriers having no 
 proper pores, the transit taking place in- 
 stantaneously. 
 
 I constructed an apparatus (Fig. 83) for 
 exposing gases to each other with the in- 
 tervention of a soap-bubble, and subse- 
 quently measuring and analyzing them. 
 a a is a tin saucer about three inches in 
 diameter and half an inch deep; into it 
 water can be poured ; it also serves as a 
 Fig. S3. platform to support a small bell-glass, b. 
 
358 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 Through its centre at c passes a glass tube,/, one eighth 
 of an inch in diameter, the upper extremity of which is 
 cemented into a hole of the same size in a round, thin 
 piece of copper, d, about half an inch in diameter ; the 
 other end of the pipe opens into another small bell-glass, 
 &, through a perforation in its top, the communication be- 
 ing capable of being cut off by means of a stop-cock, g. 
 The apparatus is used as follows : The upper bell, being 
 taken off the platform, is filled with any gas to be tried 
 oxygen, for instance and is placed aside on the shelf 
 of a pneumatic trough. The lower bell-glass is then filled 
 with water by depressing it in the trough ; and the stop- 
 cock being closed, five hundred measures of hydrogen, for 
 instance, are thrown into it. After seeing that the cop- 
 per plate d is free from moisture, a drop of water ren- 
 dered viscid by soap is placed upon it, exactly where the 
 orifice of the tube f opens. The upper glass containing 
 the oxygen is now placed upon the tin saucer platform 
 as in the figure. The lower glass is next depressed in 
 the pneumatic trough, and as soon as the cock is opened 
 a bubble of hydrogen containing five hundred measures 
 expands, the spare oxygen escaping from the edge of the 
 upper glass through the water in the tin saucer. The 
 cock is next closed, and the apparatus placed on the 
 trough shelf as long as the operator desires the experi- 
 ment to continue. Keeping that position, when the cock 
 is once more opened the gas passes into the lower bell 
 until the bubble has entirely collapsed, when the cock is 
 again closed, the contents of the bubble being now ready 
 for measurement and analysis. As the gas was passing 
 from the bubble into the lower bell, the water rose from 
 the tin saucer into the upper bell, confining the gas that 
 was outside of the bubble. This, by the common mode 
 of manipulation, is to be transferred from the tin plat- 
 form to the shelf of the trough for inspection. 
 
MEMOIR XXVI.] CAPILLAltY ATTRACTION, ETC. 359 
 
 By this apparatus it was found that one thousand 
 measures of atmospheric air exposed to atmospheric air 
 underwent no change either in volume or composition. 
 The exposure in some cases lasted an hour. 
 
 One thousand measures of hydrogen, in the bubble, 
 were exposed to atmospheric air in the bell. In five 
 minutes there remained only four hundred and seventy- 
 two. (It will be understood that the numbers quoted 
 in this and other succeeding experiments are for the pur- 
 pose of illustrating the general principle. They change 
 w r ith the relative proportion of gases inside and outside 
 of the bubble.) 
 
 A reverse action ensues when nitrogen is substituted 
 for hydrogen. The bubble swells instead of diminish- 
 ing. Thus one hundred measures of nitrogen in half an 
 hour became one hundred and seven and a half. 
 
 Oxygen decreases in bulk. Two hundred and fifty 
 measures in ten minutes became one hundred and fifty- 
 three. This gas passes more rapidly through the bub- 
 ble than nitrogen. 
 
 Carbonic acid passes through the bubble very rapidly. 
 When five hundred measures were used, the bubble col- 
 lapsed almost as fast as it had expanded. Under a 
 water roof half an inch thick and two inches in diame- 
 ter, five thousand measures escaped into the air in forty- 
 eight hours. In its place there were found two hundred 
 measures of atmospheric air, which had passed in the op- 
 posite direction through the water roof. 
 
 By this apparatus it was proved that these motions 
 through soap-bubbles continue until the gases on both 
 sides of the bubble have the same chemical composition. 
 
 It has long been known that liquids and gases pass 
 through porous structures though resisted by consider- 
 able force. Thus, if over the mouth of a cylindrical jar 
 Z>, Fig. 8-4, a thin sheet of India-rubber be tied, and the 
 
360 
 
 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 Fig. 84. 
 
 jar exposed to an atmosphere of 
 carbonic - acid gas, that gas will 
 force its .way through the rubber 
 and mingle with the atmospheric 
 air in the jar, the rubber will be 
 pressed outward, as at &, and even- 
 tually may be burst. I found 
 that carbonic acid would thus 
 force its way against a pressure of 
 ten pounds on the square inch. If 
 the conditions be reversed, the jar being filled with car- 
 bonic acid and then exposed to the atmosphere, the In- 
 dia-rubber will be depressed as at #, and stretch so as 
 almost to sink to the bottom. Dr. Mitchell, of Philadel- 
 phia, has shown that this percolation will take place 
 though resisted by many inches of mercury. 
 
 The same holds good as respects liquids. Thus water 
 will readily pass through animal membrane to mix with 
 alcohol against a pressure of fifteen pounds on the square 
 inch. 
 
 In making experiments for determining the effect of 
 such pressures it is to be borne in mind that there are 
 certain disturbing circumstances which may vitiate the 
 results. Among these is that general leakage which 
 happens through the open pores of all tissues. Thus in 
 the experiment first referred to (Fig. 78) it might be 
 supposed that the force with which water passes through 
 animal membrane into alcohol is not greater than one 
 atmosphere, whereas in truth it is much more; but as 
 soon as the pressure within the vessel had amounted to 
 about one atmosphere, the alcohol escaped from the ves- 
 sel by general leakage from the whole surface of the 
 membrane as rapidly as the water entered. It is obvi- 
 ous that in a pore of sensible size those parts alone of a 
 passing liquid in contact with its substance are subjected 
 
V 
 
 O 
 
 MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 
 
 to its influence, and those situated in its centra 
 
 are ready to be influenced by any extraneous pressur^ 
 
 In an experiment made on the passage of ammonia 7 , 
 into atmospheric air through India-rubber, it was found 
 that, though the passage of the gas was resisted by a 
 pressure of seventy-five inches of mercury, or upwards of 
 two atmospheres and a half, it took place apparently as 
 readily as if no such resistance had been opposed. The 
 question at once arises, Whence is this powerful impul- 
 sive force derived ? Clearly not from the action of one 
 gas on the other. To the porous tissue or barrier alone 
 we must refer the seat of this power. 
 
 It is well known that porous solids of all kinds and 
 fluids absorb gaseous matter very readily, in volumes 
 varying according to circumstances. Water, for exam- 
 ple, absorbs its own volume of carbonic acid, and four 
 hundred and eighty times its volume of hydrochloric- 
 acid gas. In the latter case, therefore, an extremely 
 great condensation takes place. So, too, a fragment of 
 porous charcoal absorbs nearly ten times its volume of 
 oxygen, and ninety times its volume of ammonia. These 
 gases, therefore, exist in the absorbing substance in a 
 state of very high compression. And the reasoning 
 which here applies, applies also in the case of two gases 
 separated by a tissue. If, for example, we separate by a 
 medium of this kind a certain volume of ammonia from 
 a like volume of nitrogen gas, though at the outset of 
 the experiment both the gases might be existing under 
 the same pressure, this equality would very rapidly be 
 lost. Ammonia, being absorbed more rapidly than ni- 
 trogen, would be presented to this latter gas not under 
 an equal pressure, but in a state of great condensation. 
 Under such circumstances, the transit of a gas is not 
 analogous to the case in which it flows under common 
 
 O 
 
 pressure into a vacuum, or into another gas ; but the tis- 
 
362 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 sue, continually acting as a perpetual condensing engine, 
 brings the two media in contact with each other under 
 extremely different conditions the one in a compressed 
 state, but ready to exert the whole of its elastic force ; 
 the other in a state perhaps little varying from its nor- 
 mal condition. 
 
 Moist membranes and films of water, by reason of 
 their affinity for gaseous substances and their 
 consequent condensing action, become the origin 
 of great mechanical power. I have seen car- 
 bonic acid pass into atmospheric air through 
 India-rubber against a pressure of ten atmos- 
 pheres, and sulphuretted hydrogen against a 
 pressure of twenty-five atmospheres. 
 
 The apparatus with which these results were 
 obtained may be thus described. A strong 
 glass tube, a , Fig. 85, seven inches or more 
 in length and half an inch in diameter, is her- 
 metically closed at one end, through which a 
 pair of platinum wires, &, <?, pass to the interior, 
 parallel but not touching. The other end, a a, 
 has a lip or rim turned on it. Between the 
 platinum wires a gauge tube, d, is dropped. 
 On the top of the gauge tube a small test glass,/, is 
 placed, to contain a reagent suited to the gas under trial, 
 as lime-water for carbonic acid, acetate of lead for sul- 
 phuretted hydrogen, litmus water for sulphurous acid. 
 Sometimes, instead of this test tube, a piece of paper 
 soaked in the proper reagent was employed. The large 
 tube was then filled with water to the height e e, and 
 over its lip a thin sheet of India-rubber was tightly tied, 
 and over this again, to give strength, a very stout piece 
 of silk. Everything being thus arranged, the projecting 
 wires b y c, were connected with a voltaic battery ; decom- 
 position of the water ensued, oxygen and hydrogen be- 
 
MEMOIR XXVL] CAPILLARY ATTRACTION, ETC. 353 
 
 ing disengaged, and a condensed mixture of atmospheric 
 air and those gases accumulated in the space a a, e e, the 
 gauge tube showing the extent to which the condensa- 
 tion had gone. Now if the little cup/ had been filled 
 previously with lime-water, and the whole arrangement 
 introduced into a jar of carbonic-acid gas, the upper part 
 of the lime-water presently became milky, and after a 
 time a copious precipitate of carbonate of lime subsided. 
 This would readily take place when the gauge was indi- 
 cating a pressure of ten atmospheres. In like manner, 
 when a piece of paper covered with carbonate of lead 
 had been introduced, and a pressure of twenty-four and 
 a half atmospheres accumulated, on introducing the in- 
 strument into a vessel of sulphuretted hydrogen the pa- 
 per quickly became brown. So sulphuretted hydrogen 
 can pass through a sheet of India-rubber, and diffuse 
 into an atmosphere of oxygen, hydrogen, and atmos- 
 pheric air beyond, though it be resisted by a pressure 
 equal to that of eight hundred feet of water. 
 
 The method of condensation here employed, because 
 of its freedom from mechanical concussions, enabled me 
 to continue these researches up to pressures of fifty at- 
 mospheres, without leakage, in comparatively slender 
 tubes, and even under these circumstances gaseous diffu- 
 sion seemed to take place without any restraint. 
 
 In passing, it may be remarked that when water is 
 enclosed hermetically in a vessel, and a voltaic current 
 passed through it, decomposition ensues, a portion of the 
 gases making their appearance in the gaseous form, fill- 
 ing the small space occupied by the decomposed water, 
 and the remainder being absorbed by that liquid as fast 
 as it is given off. When the pressure is high, the di- 
 mensions of the vessel become sensibly greater, and the 
 little bubble of air accumulated exceeds in bulk the vol- 
 ume of the decomposed water. 
 
3(54 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 If, as it thus appears, no pressure we can command be 
 sufficient to restrain one gas from passing into another, 
 we next inquire what obstacle the condensed gas pre- 
 sents. There is abundant evidence to show that this 
 medium bears the same relation to the percolating gas 
 that a vacuum would do, inasmuch as the rate of dis- 
 charge into it is the same as it is into a vacuum. If the 
 particles of different gases possess no repulsive tendency 
 as respects each other, if the presence of one makes no 
 difference in, nor produces any retardation in, the par- 
 ticles of the other, then it is immaterial how many of 
 such particles are condensed together in a given space. 
 The vacuum is not less a vacuum because it is contained 
 under smaller dimensions, any more than a torricellian 
 vacuum is less perfect when the mercury is made to rise 
 nearly to the top of a barometer tube than it was when 
 there was a vacant space several inches in length. This 
 would therefore indicate that these diffusions will take 
 place under all pressures, provided the gaseous condition 
 subsists; and this conclusion is abundantly borne out 
 by the experiments herein detailed. 
 
 The explanation we thus give of the action of con- 
 densing barriers rests upon a fundamental principle of 
 dynamics, that when the moving force and the matter 
 to be moved vary in the same proportion, the resulting 
 velocity will always be the same. Thus, if a cylinder 
 filled with air and fitted with a piston communicate with 
 a vacuum through an aperture, it is immaterial whether 
 the air be allowed to flow into the void without any 
 pressure, or whether it be urged by a direct action on 
 the piston its velocity, as it goes into the void, will be 
 the same in both cases ; for if it be compressed, the ac- 
 tion of the piston is to reduce the air to such a density 
 that its elasticity is equal to the compressing force ; and 
 because the elasticity varies as the density, the density 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 355 
 
 of the air increases with the impelling force. The mat- 
 ter to be moved is increased, therefore, in the same pro- 
 portion as the pressure, and therefore the final velocity 
 is the same, and the same takes place in the case of a 
 tissue which is compressing a gas. 
 
 Such is the case while the gases are engaged with 
 each other in the tissue; but as soon as they pass from 
 it, the condensed gas, being no longer under its compres- 
 sion, expands freely, and, when measured, gives a result 
 differing from that which it would have been had not 
 the tissue compressed it. 
 
 Accordingly, when carbonic acid and air are separated 
 by a screen of stucco, which absorbs each to a very small 
 extent, they diffuse, according to the law of the square 
 roots of their density, one volume of air replacing 0.8091 
 of carbonic acid, the volume on that side of the screen 
 where the carbonic acid was increasing in quantity. 
 But if a thin sheet of India-rubber be used as a screen, 
 since it can condense one atmosphere of carbonic acid 
 while it does not act upon the air, though the same rate 
 of exchange ensues, there is a diminution of the gaseous 
 matter on the side containing the acid, and one volume 
 of air replaces 1.6182 of the acid. 
 
 In every plant two prominent operations are carried 
 forward: (1) the production of organic matter; (2) its 
 distribution through the various parts of the vegetable 
 system. 
 
 In 1844 I published a work under the title of "A 
 Treatise on the Forces that Produce the Organization of 
 Plants." It \vas a monograph chiefly devoted to the il- 
 lustration or investigation of those operations, and com- 
 bated the existence of the Vital Force of physiologists 
 as a homogeneous and distinct power. 
 
 The progress of science shows plainly that living 
 
3(36 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 structures, far from being the products of one such homo- 
 geneous power, are rather the resultants of the action 
 of a multitude of material forces. Gravity, cohesion, 
 elasticity, the agency of the imponderables, and all other 
 powers which operate both on masses and atoms, are 
 called into action, and hence it is that the very evolution 
 of a living form depends on the condition that all these 
 various agents conspire. Organized beings and organ- 
 ized bodies spring forth in those positions only to which 
 the rays of the sun have access. They are, therefore, 
 limited to the atmosphere, the sea, and the surface of the 
 earth. 
 
 If we expose some spring -water to the sunshine, 
 though it may have been clear and transparent at first, 
 it presently begins to assume a greenish tint, and after a 
 while flocks of green matter collect on the sides of the 
 vessel in which it is contained. On these flocks, when- 
 ever the sun is shining, bubbles of gas may be seen, 
 which, if collected, prove to be a mixture of oxygen and 
 nitrogen, their proportion being variable. Meantime 
 the green matter rapidly grows, its new parts, as they 
 are developed, being all day long covered with air- bells, 
 which disappear as soon as the sun is set. If these ob- 
 servations be made on a stream of water the current of 
 which runs slowly, it will be found that the green mat- 
 ter serves as food for thousands of aquatic insects which 
 make their habitations in it. These insects are indued 
 with powers of rapid locomotion, and possess a highly 
 organized structure. In their turn they fall a prey to 
 the fishes which frequent such streams. 
 
 Thus by the influence of the sunlight organic matter 
 is added to vegetable systems, the action being accom- 
 panied by a variety of chemical decompositions and in- 
 terstitial diffusions. The substances arising are such as 
 are necessary for the uses of the plant ; and in order to 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 
 
 distribute them, mechanical motion has to take place. 
 This, in the more highly organized plants, goes under 
 the designation of the flow of the sap. 
 
 The flow of the sap in plants and the circulation of 
 the blood in animals are probably due to the same 
 physical cause. And bringing into view the experi- 
 ments already related respecting capillary attraction, I 
 considered, in the work above referred to, the conditions 
 necessary for producing a continual flow, such as evap- 
 oration, decomposition, solution, developing the general 
 law of those movements, and illustrating the great force 
 with which they are accomplished. I showed that these 
 motions depend on this physical principle: "That if two 
 liquids communicate with one another in a capillary 
 tube, or in a porous or parenchymatous structure, and 
 have for that tube or structure different chemical affin- 
 ities, movement will ensue; that liquid which has the 
 most energetic affinity will move with the greatest veloc- 
 ity, and may even drive the other liquid entirely before 
 it ;" and that this is due to common capillary attraction, 
 which, in its turn, is due to electric excitement. 
 
 Applying this principle to the case of plants, the 
 liquid of which the ascending sap is constituted is de- 
 rived from the ground by the action of the spongioles, 
 and consists of water holding in solution the different 
 saline bodies necessary to the plant, along with carbonic 
 acid, etc. This passes upwards by the woody fibre and 
 ducts of the alburnum, making its way to the leaf, on 
 the upper surface of which, in common cases, a change in 
 its chemical constitution occurs through the influence of 
 the sunlight. It obtains a quantity of carbon. This 
 elaborated sap, or latex, now returns to the bark, and 
 descends through its cellular tissue and intercellular 
 spaces, finding its way by the route of the medullary 
 rays to all parts of the plant. During its descent the 
 
368 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 different vegetable principles necessary for the economy 
 of the plant are removed from it, and a certain quantity 
 goes down to the roots, partly to aid in their growth, 
 and partly to throw new quantities of ascending sap 
 into the tree. In this descent the elaborated sap moves 
 through a system of vessels which anastomose with one 
 another in the same manner as the capillary vessels of 
 animals. 
 
 There are, therefore, two points in this circulation 
 which require attentive consideration the spongiole 
 and the leaf. The spongioles are nothing but the young 
 succulent extremities of the roots, which have been re- 
 cently formed from portions of the descending sap, and 
 that sap is itself a species of mucilaginous solution. Pre- 
 cisely, therefore, as water will pass through the tissue of 
 a bladder the interior of which is filled with gum-water, 
 so will moisture from the ground flow through the spon- 
 giole. There is no difficulty in thus accounting for the 
 rise of the ascending sap on the principles of capillary 
 attraction, and indeed this is the explanation generally 
 received by vegetable physiologists. 
 
 Guided by the principle above laid down, I offered 
 the following as an explanation of the action of the leaf. 
 The ascending sap, which we may assume to be a weak, 
 watery solution, rises to the upper face of the leaf. It 
 there obtains carbonic acid from the air ; of this the sun- 
 light effects the decomposition, with the production of 
 gum, the result being a change from water to a muci- 
 laginous solution. In the tissue of the leaf we have, 
 therefore, two liquids engaged water and a mucilagi- 
 nous solution. On the principle above indicated, the 
 water will drive the mucilaginous solution before it, and 
 force it back along its proper vessels into the stem. 
 
 What, then, is the reason that the light of the sun 
 controls the rapidity with which the ascending current 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 359 
 
 comes ? Because it controls the amount of carbonic acid 
 reduced, and, therefore, the amount of elaborated sap 
 formed. Why is it that the upward flow diminishes 
 when changes are befalling the leaves, and why does it 
 stop in the winter ? Because the mucilaginous solution 
 made by light diminishes in quantity, or ceases to be 
 formed altogether. 
 
 There are, therefore, two sources of force in a flower- 
 ing plant the spongiole and the leaf; and they derive 
 their power from ordinary physical principles. What- 
 ever has been said respecting the movements of sap in 
 exogenous plants applies also to the case of endogenous, 
 and indeed to flowerless plants too. 
 
 It has been clearly established by the researches of 
 comparative anatomists that the presence of a circula- 
 tory mechanism is determined by the centralization of 
 the nutritive and respiratory apparatus. In exogenous 
 and endogenous plants, from the circumstance that liquid 
 and solid materials are introduced at distant points, chan- 
 nels of communication from one to the other, and indeed 
 to every part, are required, and hence the introduction 
 of a circulatory apparatus. In the lower tribes of vege- 
 table life, where the separation of function does not ex- 
 ist, the circulatory mechanism is correspondingly absent. 
 Sea- weeds absorb on their whole surface, and nutrition 
 is directly carried forward at the points of reception. 
 In lichens there is the first appearance of a transfusory 
 mechanism, arising from the circumstance that on those 
 parts which are shaded from the light, absorption most 
 rapidly takes place : here, probably, however, the chan- 
 nels of movement are the interspaces between the cells, 
 and the cause simple capillary attraction. In mush- 
 rooms there is a closer approximation to the mechanism 
 more fully developed in the higher plants, for in them 
 the rootlets absorb nutrient matter from the soil, from 
 
 A A 
 
370 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 which it passes by capillary action to every part of the 
 system. 
 
 The cause of the movement of the sap in flowering 
 plants, both of the rise of the crude sap and the descent 
 of the elaborated sap, is the light of the sun, which ef- 
 fects the decomposition of carbonic acid. 
 
 From this explanation of the causes of the movement 
 of sap in plants, I turn to the circulation of the blood in 
 animals. 
 
 In man there are three chief circulations the sys- 
 temic, the pulmonary, the portal. Bearing the above- 
 mentioned general principle in mind, I presented the 
 following explanation of these circulations: 
 
 I., The Systemic. The arterial blood, which moves 
 along; the various aortic branches, and is distributed to 
 
 O 7 
 
 every part of the system, contains oxygen which it has 
 received during its passage through the lungs. Its color 
 is crimson. As soon as it has reached its destination in 
 the minute capillary vessels, it begins to carry on its 
 proper process of oxidation, attacking in a measured 
 way the tissues through which it is flowing. The di- 
 rect result of this operation is an evolution of heat. 
 But while this chemical change in the tissues is going 
 forward, the arterial blood itself is also suffering a 
 change in giving up its oxygen and gaining in exchange 
 the results of combustion. From being crimson, it turns 
 dark ; from being arterial, it changes into venous blood. 
 Now, under these circumstances, what must take place 
 in every capillary or each small portion of a porous struct- 
 ure ? On the arterial side, we have the crimson arterial 
 blood ; on the venous side, dark venous blood two dif- 
 ferent liquids. What, then, is the relation that obtains 
 between each of these liquids and the walls of the tube, 
 or the substance of the parenchyma in which they are 
 
MEMOIR XXVL] CAPILLARY ATTRACTION, ETC. 
 
 placed ? Must it not be that the arterial blood bearing 
 its oxygen has an intense affinity for those structures, 
 but, those affinities being satisfied, that which was arte- 
 rial passes into the condition of venous blood ? The af- 
 finities it had for the structures with which it was in 
 contact are satisfied and have come to an end. The 
 arterial blood presented a highly energetic force, which 
 in the venous has diminished to zero. Under these cir- 
 cumstances, in accordance with the general principle, the 
 arterial blood must press the venous blood before it, and 
 the flow must be from the artery to the vein. 
 
 2. The Pulmonary. In this the venous blood pre- 
 sents itself in the air-cells to receive oxygen. The sys- 
 temic circulation deoxidized arterial blood, the pulmo- 
 nary oxidizes venous. The latter, therefore, is the con- 
 verse of the former. The venous blood has an affinity 
 for the oxygen dissolved in the tissues with which it is 
 in contact, and the arterial blood has none. Movement, 
 therefore, must ensue ; but as the conditions of the affin- 
 ity are reversed, so also is the direction of the motion, 
 for now the venous blood drives the arterial before it, 
 and drives it to the heart. 
 
 3. The Portal Circulation. In this the same physical 
 principles apply. The blood which flows towards the 
 liver along the portal vein has been obtained by that 
 vein from the chylopoietic viscera ; it has, therefore, the 
 same relation to the blood furnished from the different 
 and corresponding aortic branches as has the general 
 systemic venous blood. The arterial blood, therefore, 
 drives it before it in the same way that the general sys- 
 temic circulation takes place ; and, passing along the por- 
 tal vein, it is now distributed to the liver. In this or- 
 gan it also receives the blood which has been brought 
 by the hepatic artery. 
 
 The process of biliary secretion now takes place, and 
 
CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 compounds of carbon and hydrogen with soda are sepa- 
 rated as bile, and pass along the biliary tubes. In its 
 final effect, therefore, the chemioal action of the liver close- 
 ly resembles the chemical action of the lungs. Compared 
 with the blood which passes along the branches of the 
 hepatic veins and finds its way into the ascending vena 
 cava, the portal blood differs by containing the elements 
 of bile. 
 
 Two systems of forces now conspire to drive the por- 
 tal blood out of the liver into the ascending cava. 
 
 First, the blood which is coming along the capillary 
 portal veins and that which is receding by the hepatic 
 veins, compared together as to their affinities for the 
 substance of the liver, obviously have this relation : the 
 portal blood is acted upon by the liver, and there are 
 separated from it the constituents of the bile ; the affin- 
 ities that have been at work in producing the result 
 have all been satisfied ; and the residual blood, over 
 which the liver can exert no action, constitutes that 
 which passes into the hepatic veins. Between the por- 
 tal blood and the substance of the liver there is an ener- 
 getic affinity, indicated by the circumstance that a chem- 
 ical decomposition takes place and bile is separated ; but, 
 that change once completed, the residue, which is no 
 longer acted upon, forms the venous blood of the he- 
 patic veins, and hence the portal blood drives before it 
 the inert blood which is in those veins. 
 
 But, in addition to this, the blood of the hepatic ar- 
 tery, after serving for the economic purposes of the liver, 
 is thrown into the portal plexus. Hence arises a second 
 force, which, conspiring in its resultant with the former, 
 produces movement in the same direction. The pres^ 
 ence of the arterial blood in the hepatic capillaries is not 
 only sufficient to give a force towards that in the capil- 
 laries of the portal veins, but also to give it a pressure 
 
MEMOIR XXVI.] CAPILLARY ATTRACTION, ETC. 373 
 
 towards that in the hepatic veins. No regurgitation can 
 take place backwards through the portal vein upon the 
 blood arising from the chylopoietic viscera, because along 
 that channel there is a pressure propagated in the oppo- 
 site direction arising from the arterial blood of the aortic 
 branches. This pressure conspires with that of the por- 
 tal blood, and both together join in giving rise to mo- 
 tion towards the ascending cava. 
 
 On the same principle we may explain the circulation 
 of the blood in other types of life ; for example, in the 
 case of the model adopted in fishes, the aorta of which 
 has Ions; been recognized as bearing a strong; resem- 
 
 o o o o 
 
 blance to the portal vein of the mammalia. To any one, 
 however, who reflects on the principles here laid down, 
 there will arise no difficulty in explaining the circula- 
 tion in any particular case, if this plain precept be con- 
 stantly kept in mind : that, in consequence of the phys- 
 ical principle which has been assigned, a pressure will 
 always be exerted by the liquid which is ready to un- 
 dergo a change upon that which has already undergone 
 it a pressure which, as there is no force to resist it, will 
 always give rise to motion in a direction from the chang- 
 ing to the changed liquid. 
 
 I then, in the work referred to and in my " Physiol- 
 ogy," continued the investigation to a determination of 
 the uses and action of the heart (heretofore considered 
 as the sole cause of the circulation), the action in as- 
 phyxia, the case of obstructed trachea, local inflamma- 
 tion, etc. 
 
 By regarding the affinity between the blood and the 
 tissues with which it is in contact as the primary cause 
 of the circulation, we assign a reason for those various 
 phenomena which cannot be accounted for by Harvey's 
 doctrine : the motions in the embryo ; the periodic and 
 local variations; the portal circulation; the changes in 
 
374 CAPILLARY ATTRACTION, ETC. [MEMOIR XXVI. 
 
 the current as seen under the microscope ; the movement 
 in the capillaries after the heart is cut out; the empty 
 condition of the arteries after death ; the phenomena of 
 acardiac monsters; local inflammations and congestions; 
 the gangrene of parts while their capillaries are pervi- 
 ous; the retardation of the current on the application 
 of cold or of carbonic -acid gas; the results of asphyxia 
 and death by drowning or hanging ; the changes of press- 
 ure in the arteries and veins respectively during a check 
 on the respiration ; the vis a tergo of the veins ; the ef- 
 fects of a ligature on those vessels ; the action of irres- 
 pirable gases when breathed, and the opposite condi- 
 tions when oxygen gas or protoxide of nitrogen is used. 
 A doctrine which accounts with simplicity for such a 
 long list of miscellaneous facts commends itself to our 
 attention at once. There are, however, considerations of 
 a still weightier character which must compel us to 
 adopt it. The affinity between the blood and the parts 
 with which it is in contact is a chemical fact beyond 
 contradiction. The pressures and motions I have been 
 speaking of follow as the inevitable consequences of that 
 affinity. We therefore cannot gainsay their existence in 
 the living mechanism, and the only doubt we can enter- 
 tain is as to whether they are of competent power to 
 produce all the effects before us. But after what has 
 already been said respecting the energy of endosmotic 
 movements against pressures of many atmospheres, we 
 may abandon those doubts; and since we have here a 
 force of universality enough and intensity enough, and 
 in every instance acting in the right direction, it would 
 be unphilosophical to look farther, since such a force 
 must, under these conditions, exist in the physical neces- 
 sity of the case. 
 
MEMOIK XXVII.] THE EFFECTS OF ALLOTROPISM. 375 
 
 MEMOIR XXVII. 
 
 ON THE EXISTENCE AND EFFECTS OF ALLOTROPISM ON 
 THE CONSTITUENTS OF LIVING BEINGS. 
 
 From the Philosophical Magazine, April, 1849. 
 
 CONTENTS : Allotropism of elementary substances. FranJcenheim* s no- 
 menclature. Brought about by light , heat, electricity, and by the ner- 
 vous principle. Explanation of inflammation and congestion. 
 
 IT has been completely established for the majority 
 of elementary substances that there are several forms 
 under which each may occur forms differing entirely 
 both in their physical and chemical relations. 
 
 Thus, in the case of carbon, many such forms are 
 known. To three of them Berzelius has directed at- 
 tention : 1st, ordinary charcoal; 2d, plumbago; 3d, dia- 
 mond. They are three distinct modifications of the same 
 element. They differ in specific gravity, in specific heat, 
 and in conducting power, both for electricity and caloric. 
 In their relations to light, the first perfectly absorbs it, 
 and is black; the second reflects it like a metal; the 
 third is transparent like glass. When crystallized, plum- 
 bago and diamond do not belong to the same system : 
 their chemical relations are also strikingly different. 
 Charcoal takes fire with facility, and some varieties of it 
 are even spontaneously combustible in the air ; but cru- 
 cibles and furnaces are made of plumbago because of its 
 incombustibility ; and the diamond with difficulty is set 
 on fire in pure oxygen gas. 
 
 It seems immaterial to what class elementary bodies 
 belong, whether electro - negative or positive: they pre- 
 
376 THE EFFECTS OF ALLOTROPISM. [MEMOIR XXVII. 
 
 sent analogous results. Silicon, sulphur, selenium, phos- 
 phorus, titanium, chromium, uranium, tin, iridium, osmi- 
 um, copper, nickel, cobalt, iron, oxygen, chlorine, are cases 
 in point; and the instances which appear as exceptions 
 are rapidly diminishing in number. 
 
 As is well known, to these singular modifications 
 Berzelius gave the designation of allotropic forms, and 
 the whole phenomenon passes conveniently under the 
 designation of allotropism. He shows that the peculiar- 
 ity assumed is often of such a persistent nature that it is 
 not lost, even though the substance affected should go 
 into combination with others. Thus, there are two forms 
 of silicon one combustible, and the other remarkably 
 incombustible. Each, by uniting with oxygen, gives rise 
 to silicic acid ; the acid in one case being soluble in 
 water and in hydrochloric acid, and in the other the re- 
 verse. And, in like manner, metallic arsenic, which ex- 
 hibits the same duality of condition, gives rise to two 
 different arsenious acids. Of phosphorus there are at 
 least two modifications ; and, accordingly, we have two 
 compounds of that body with hydrogen, one of which is 
 spontaneously inflammable, and the other not ; and at 
 least two oxygen acids, the monobasic and tribasic, in 
 which the essential difference rests in the state of the 
 phosphorus they contain. 
 
 It is to be remarked that, so far as observation ex- 
 tends, the most common cause of producing these singu- 
 lar differences is the action of that class of agents which 
 we term imponderable substances. In very many cases, 
 change of temperature brings about allotropic change; 
 in others it is the agency of light, as in chlorine and 
 phosphorus ; and again, in others, association with for- 
 eign bodies, which apparently establish new voltaic rela- 
 tions. Heat, light, and electricity seem to be the general 
 modifying agents. 
 
MEMOIR XXVIL] THE EFFECTS OF ALLOTROPISM. 3^ f 
 
 Berzelius, following the suggestion of Frankenheim, 
 proposes a nomenclature for pointing out the peculiar 
 form referred to in any special case. It depends on the 
 use of Greek letters. Thus, we have the three forms 
 of carbon just alluded to designated on these principles 
 by Ca, C/3, Cy. But in a Memoir republished in this 
 work, p. 285, it is remarked that we may often with 
 greater convenience use the simple expressions "active" 
 and "passive." Thus, active chlorine is that which will 
 decompose water in the dark, passive chlorine failing to 
 do so. In this Memoir the same expressions will be em- 
 ployed. 
 
 Hitherto, allotropism has only been considered as af- 
 fecting inorganic states of matter, but its influence can 
 be plainly traced in / the far more interesting case of or- 
 ganic beings ; and this, when placed in a proper point 
 of view, yields a remarkable explanation of some of the 
 most obscure but important facts in physiology and 
 pathology. These explanations I propose now to point 
 out. 
 
 In the Philosophical Magazine (March, 1846, p. 178) 
 there is a paper by me explanatory of the causes of the 
 circulation of the blood in the capillary vessels. It is 
 merely an abridgment of a lecture which for eight years 
 past has been delivered in this university. The doctrine 
 there set forth has been generally received in America, 
 and introduced into some of the standard works on 
 physiology published in England. The principle on 
 which it essentially depends, and which has been abun- 
 dantly confirmed by direct experiment, is briefly this 
 that if there be two fluids occupying a capillary tube, or 
 a porous structure of any kind, under the condition that 
 one of them has a stronger chemical affinity for the sub- 
 stance of that tube or structure than the other, a move- 
 ment of the liquids will at once ensue, that which has 
 
378 THE EFFECTS OF ALLOTKOPISM. [MEMOIR XXVII. 
 
 the stronger affinity driving the other before it. On this 
 principle a clear account of the systemic circulation of 
 animals may be given ; for the. arterial blood, an oxidiz- 
 ing liquid, having a stronger affinity for the soft tissues 
 with which it is in contact, than the venous blood, the 
 affinities of which have been satisfied, and therefore no 
 longer exist, necessarily exerts such a pressure that mo- 
 tion must ensue, the arterial blood forcing the venous 
 before it. 
 
 An application of the same principles shows that in 
 the pulmonary circulation the motions must necessarily 
 be in the opposite direction, or from the venous to the 
 arterial side, as is actually the case. It also explains 
 clearly the conditions of the portal circulation, in which 
 the direct action of the heart could hardly be expected 
 to be felt. With the generality which ought to belong 
 to a true theory, it meets all the cases which occur in the 
 lower orders of animal life, such as the greater circula- 
 tion in fishes, in which there is no systemic heart ; the 
 movements which take place in the vascular system of 
 insects ; and even the extreme case of the rise and de- 
 scent of sap in plants. 
 
 In this doctrine everything depends on the relation- 
 ship between the nutritive fluid, or blood, and the solid 
 parts with which it is brought in contact ; and whatever 
 changes that relationship must impress a corresponding 
 change on the circulation itself. 
 
 From experiments which I made some time ago, I have 
 been led to suppose that the arterialization of the blood, 
 as it takes place on the cell-walls of the lungs, bears a 
 strong analogy to the oxidation of white indigo. The 
 loose hold which the coloring matter of the blood retains 
 on the oxygen, coupled but not combined with it, is not 
 unlike what is witnessed in other nitrogenized coloring 
 matters, such as indigo, w T hich oxidizes and deoxidizes 
 
MEMOIR XX VII.] THE EFFECTS OF ALLOTROPISM. 379 
 
 with the utmost facility. Charged with the oxygen it 
 has thus obtained, the arterial blood passes to all parts 
 of the system ; and now arises that striking but all-im- 
 portant physiological fact, that it does not attack indis- 
 criminately all those parts of the soft solids which it first 
 encounters, but, proceeding in a measured w r ay, exerts its 
 action on such particles alone as have become effete, and, 
 accomplishing the great process of interstitial death, re- 
 solves those particles into other forms, so that they can 
 be eliminated from the system by the lungs, the kidneys, 
 and the skin. 
 
 Now why is it that things proceed in this way ? What 
 is it that regulates this interstitial death ? Why is one 
 atom preserved and another surrendered ? 
 
 It is upon these obscure points that the recent discov- 
 eries in allotropism shed a flood of light. 
 
 The three leading neutral nitrogenized bodies, fibrin e, 
 albumen, and caseiue, are characterized by exhibiting al- 
 lotropism in a most remarkable degree, and that in a 
 double sense. 1st. Though so different from one another 
 in their physical and chemical relations, it is admitted 
 on all hands that they are mutually convertible : the al- 
 bumen of the egg, during incubation, gives rise to fibrine 
 and other allied bodies; from caseine in the milk with 
 which the young mammalia are nourished, the albumi- 
 nous and fibrinous constituents of their systems arise; the 
 nurse fed on fibriue and albumen secretes caseine from 
 the mammary gland. Indeed, there is no more. reason 
 to regard these three bodies as essentially distinct sub- 
 stances than there is to apply the same conclusion to 
 charcoal, plumbago, and diamond. Between the two 
 cases there is the most complete analogy; and if char- 
 coal, plumbago, and diamond are merely allotropic forms 
 of one substance, the same holds good for fibrine, albu- 
 men, and caseine. But, 2d, each of these three com- 
 
380 THE EFFECTS OF ALLOTKOPISM. [MEMOIK XXVII. 
 
 pounds betrays a disposition under trivial causes to as- 
 sume new forms ; as with silicic acid so with fibrine, 
 there are two varieties one soluble in water, the other 
 not. A difference of a few degrees of heat turns trans- 
 parent albumen into the porcellanous variety, and analo- 
 gous observations might be made respecting caseine. 
 
 It may therefore be asserted that these proteine bod- 
 ies exhibit a propensity to allotropism in a far more re- 
 markable manner than any other substances known ; not 
 only passing indiscriminately into one another, but also 
 exhibiting special variations under the influence of the 
 most trivial causes. 
 
 And now we may recall the fact that, of the agents 
 which in the inorganic kingdom bring about these 
 changes, the so-called imponderable principles are pre- 
 eminent. I transfer this observation to the case of or- 
 ganized beings, and infer that the nervous system has 
 the power of throwing organized atoms into the active 
 or passive state ; that this is the fundamental fact on 
 which all the laws of interstitial death depend ; and that 
 upon this principle its existing allotropic condition 
 an organized molecule either submits to the oxidizing 
 influence of arterial blood, or successfully resists that 
 action. 
 
 But it has been stated that there are certain patho- 
 logical conditions which upon these views meet with 
 a clear explanation conditions which, though long and 
 laboriously studied by physicians, remain involved in 
 contradictions and obscurity. The conditions to which 
 I refer are those known as inflammation and congestion. 
 
 It is agreed among chemists that during the preva- 
 lence of these conditions the urine assumes a peculiar 
 constitution. In inflammatory actions the relative quan- 
 tity of urea and sulphuric acid is much above the nor- 
 mal standard, while in congestive cases the reverse holds 
 
MEMOIR XXVII.] THE EFFECTS OF ALLOTROPISM. 381 
 
 good, and the urea and sulphuric acid are below the 
 standard. What is the interpretation of these remark- 
 able facts? We shall find they are very significant. 
 
 The quantity of urea and sulphuric acid in the urine 
 undoubtedly expresses the quantity of proteine matter 
 that has undergone oxidation in the system. In all 
 cases where that quantity is above the normal standard, 
 the destruction of proteine matter has been correspond- 
 ingly accelerated ; and where it is deficient, the destruc- 
 tion has been reduced. The result of inflammations cor- 
 responds to the first of these cases, and of congestions to 
 the second. 
 
 Recalling now what has been said respecting the cause 
 of the capillary circulation, we see how all these appar- 
 ently disconnected facts group themselves together in 
 the attitude of dependent effects. In inflammation there 
 has been that allotropic change in the soft solids in- 
 volved that they have assumed a disposition for rapid 
 oxidation they are active. Their relations with arterial 
 blood have become highly exalted ; and the blood flows, 
 on the principles I have set forth, to the affected part 
 with energy. Redness of that part and a higher temper- 
 ature are the result. Oxidation goes on with prompti- 
 tude, and urea and sulphuric acid begin to accumulate 
 in the urine. 
 
 But in congestive cases it is the reverse ; the parts af- 
 fected are thrown into a more passive state. Oxidation 
 goes on in a reluctant way, the amount of tissue meta- 
 morphosed diminishes, the urea and sulphuric acid di- 
 minish in the urine ; and, on the principles which I have 
 endeavored to explain respecting the capillary circula- 
 tion, we perceive that an immediate action must be ex- 
 erted on the flow of the blood : the passive condition of 
 the tissues and diminished capacity for oxidation re- 
 strain the flow from the arteries, and, there being now 
 
332 THE EFFECTS OF ALLOTROPISM. [MEMOIR XXVII. 
 
 less pressure on K the contents of the veins, engorgement 
 of those vessels is the result ; and this condition of 
 things is what a physician designates as congestion. 
 
 In this manner, if we admit the existence of allotro- 
 pism in organic atoms, we can give a very clear explana- 
 tion of the condition of the circulation in the patholog- 
 ical states of inflammation and congestion, and also of 
 the peculiarities which in those states belong to the con- 
 stitution of the urine. 
 
 UNIVERSITY OF NEW YORK, Feb. 17, 1849. 
 
MEMOIR XXVIIL] DISTRIBUTION OF HEAT IN THE SPECTRUM. 333 
 
 MEMOIR XXVIIL 
 
 ON THE DISTRIBUTION OF HEAT IN THE SPECTRUM. 
 
 From the American Journal of Science and Arts for 1872, Third Series, Vol. CIV., 
 No. 161 ; Philosophical Magazine, August, 1872. 
 
 CONTENTS : Early experiments seeming to prove that the maximum of 
 heat is in the less refrangible spaces. Comparison of the dispersion 
 and diffraction spectra. Effect of compression in the less refrangible 
 regions and of dilatation in the more refrangible. Measure of heat in 
 the two halves of the visible dispersion spectrum. Description of the 
 apparatus employed. The different colored spaces are equally warm. 
 
 MANY experimenters, at various times, have occupied 
 themselves with the problem of the Distribution of Heat 
 in the Spectrum. At first it was supposed that there is 
 a coincidence between the luminous and calorific radia- 
 tions, and that the maximum of intensity in both occurs 
 at the same point that is, in the yellow space. This 
 view was abandoned on the publication of the well- 
 known experiments of Sir W. Herschel, who showed that 
 in certain cases the maximum is below the red. Subse- 
 quently Melloni, having discovered the singular heat- 
 transparency of rock-salt, proved that when a prisrn of 
 that substance is used, the maximum in question is as far 
 below the red as the red is below the yellow ; but that if 
 the light has passed through flint-glass, the maximum ap- 
 proaches the red; if through crown-glass, it passes into 
 the red ; if through water or alcohol, it enters the yellow. 
 
 In the case of the sun's spectrum the distribution of 
 heat was more closely examined by Professor Miiller, 
 whose results in a general manner confirmed the views 
 then held, that the invisible radiation below the red 
 
384 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 greatly exceeds that in the visible spectrum ; and still 
 more recently Dr. Tyndall, examining the spectrum of 
 the electric light through rock - salt, showed that the 
 curve indicating the distribution " in the region of the 
 dark rays beneath the red shoots suddenly upward in a 
 steep and massive peak, a kind of Matterhorn of heat, 
 which quite dwarfs by its magnitude the portion of the 
 diagram representing the visible radiation." These in- 
 vestigations were made under unexceptionable circum- 
 stances ; the beam of electric light had practically under- 
 gone no atmospheric absorption, and the optical refract- 
 ing train was of rock-salt. 
 
 Sir J. Herschel had shown in 1840 that when the 
 sun's rays are dispersed by a flint-glass prism, the dis- 
 tribution of the heat towards the less refrangible region 
 is not continuous, but there are three maximum points. 
 These points, as shown by Dr. Tyndall, do not exist in 
 the spectrum of electric light, the decline of which is 
 continuous; they are therefore to be attributed to the 
 absorptive disturbance which the sun's rays have under- 
 gone. Quite recently (1871), M. Lamansky has succeed- 
 ed in identifying these interruptions by the aid of the 
 thermo- multiplier. In his Memoir he states that, with 
 the exception of Foucault and Fizeau, in their well- 
 known experiments on the interference of heat, no one 
 has made reference to these lines, and that all experi- 
 menters describe the heat -curve as a continuous one 
 (Philosophical Magazine, April, 1872). 
 
 I may therefore be excused for remarking at this point 
 that the three lines in question were not only observed 
 by me nearly thirty years ago, but that an engraving 
 of them was published in that journal, in a memoir an- 
 nouncing the discovery of fixed lines in the invisible 
 portions of the spectrum (May, 1843). It will be seen, 
 from an inspection of that engraving, that these lines 
 
MEMOIR XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 335 
 
 are marked a, )3, 7, They were impressed on daguerre- 
 otype plates by resorting to the well-known processes 
 for obtaining photographs of the less refrangible regions 
 of the spectrum. The paper is republished in the pres- 
 ent volume, Memoir III. 
 
 In view of the preceding statement and others that 
 might be given, it may, I think, be affirmed that the 
 general opinion held at the present day as to the consti- 
 tution of the spectrum is this, that there exists a heat 
 spectrum in the less refrangible regions, a light spectrum 
 in the intermediate, and a spectrum producing chemical 
 action in the more refrangible regions. An experimental 
 attempt to correct this view, and to introduce a more ac- 
 curate interpretation, will not be without interest, espe- 
 cially as it is necessarily and directly connected with the 
 important subject of photometry. In this Memoir I shall 
 offer some experiments and suggestions respecting the 
 heat of the spectrum, and in another, shortly to be pub- 
 lished, shall consider the distribution of the so-called 
 chemical rays. Among the numerous problems of actino- 
 chemistry, there are none more important than these. 
 
 All the experiments hitherto made on the heat of the 
 spectrum have been conducted on the principle of ex- 
 posing a thermometer in the differently colored spaces, 
 Such was Sir W. Herschel's method. Leslie used a dif- 
 ferential with small bulbs. Melloni, Miiller, Tyndall, a 
 thermo-electric pile, the form preferred being the linear. 
 This was advanced successively through all the radia- 
 tions, and the deflections of the multiplier noted. 
 
 Is not this method essentially defective I Does it not 
 necessarily lead to incorrect results ? 
 
 " There is an inherent defect in the prismatic spectrum 
 a defect originating in the very cause which gives rise 
 to that spectrum itself unequal refrangibility. Of two 
 groups of rays compared together, one taken in the red, 
 
 BB 
 
386 DISTRIBUTION OF HP: AT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 the other in the violet region, it is clear that in the same 
 spectrum, from the very circumstance of their greater re- 
 frangibility, those in the violet will be relatively more 
 separated from each other than those in the red. The 
 result of this increased separation in the more refrangi- 
 ble regions is to give an apparent dilatation to them, 
 while the less refrangible are concentrated. The relative 
 position of the colors must also vary : the fixed lines 
 must be placed at distances greater than their true dis- 
 tances as the violet end is approached." I am quoting 
 from the fifth chapter of a work " On the Forces which 
 Produce the Organization of Plants," published by me in 
 1844. In this chapter, one of the chief points insisted on 
 is the necessity of using wave-lengths in the measure- 
 ment and discussion of spectrum results a suggestion 
 which, I believe, I was the first to make, and which I re- 
 newed in a memoir in the Philosophical Magazine (June, 
 1845). 
 
 The importance of these remarks respecting the pecul- 
 iarities of the prismatic or dispersion spectrum may per- 
 haps be most satisfactorily recognized on examining such 
 a spectrum by the side of a diffraction or interference 
 one. By the aid of Fig. 86 this may be done. 
 
 Regarding the space between the fixed lines D and E 
 as representing the central region, in each the fixed lines 
 
 D and E are made 
 
 A BC D E F G H 
 
 coincident. The 
 
 J _ I _ I _ II ,, r 
 
 other lines are 
 
 laid off in the 
 prismatic as they 
 
 Fig. 86. ,1 i 
 
 appear through 
 
 the flint-glass prism of the spectroscope; those of the 
 diffraction are arranged according to their wave-lengths. 
 It thus appears that in the prismatic, from the fixed line 
 D to A, the yellow, orange, and red regions occupy but 
 
MEMOIR XXVIIL] DISTRIBUTION OF HEAT IN THE SPECTRUM. 
 
 little more than half the space they do in the diffraction ; 
 while the green, blue, indigo, and violet, from the fixed 
 line E to H, occupy nearly double the space in the pris- 
 matic that they do in the diffraction spectrum. The gen- 
 eral result is, that in the prismatic the less refrangible 
 regions are much compressed, and the more refrangible 
 much dilated. And it is plain that the same will hold 
 good in a still greater degree for any invisible rays that 
 are below the red and above the violet respectively. 
 
 Now, if a thermometer of any kind were carried in 
 succession from the greatly dilated, more refrangible re- 
 gions to the greatly condensed, less refrangible, could the 
 measures obtained be accepted as expressing the true 
 distribution ? The thermometric surface being invariable, 
 would it not receive in the less refrangible spaces more 
 than its proper amount of heat, and in the more refrangi- 
 ble less than its proper amount ? 
 
 If we should admit that the distribution of heat in a 
 correctly formed spectrum is uniform, it is plain that 
 measures made by the use of a prism would not substan- 
 tiate that admission. The concentration to which I have 
 alluded as taking place in the less refrangible region 
 would give an exaggerated, an increased heat for that 
 region ; and, on the contrary, the dilatation of the more 
 refrangible would give an exaggerated diminution of 
 heat for that space. But if it were possible to make sat- 
 isfactory heat measures on the diffraction spectrum, in 
 which the colored spaces and fixed lines are arranged ac- 
 cording to their wave-lengths, the admission would be 
 substantiated. 
 
 In view of these facts, I did attempt, many years ago, 
 to make heat measures on the diffraction spectrum. But 
 so small is the heat that, as may be seen in the Philo- 
 sophical Magazine (March, 1857), the results were un- 
 satisfactory. More recently I have tried another meth- 
 
388 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 od of investigation, on principles which I will now ex- 
 plain. 
 
 For the sake of clearness, restricting our thoughts for 
 the moment to the more familiar case of the visible spec- 
 trum, if we desired to ascertain the true distribution of 
 heat, would not the proper method be to collect all the 
 more refrangible rays into one focal group, and all the 
 less refrangible into another focal group, and then meas- 
 ure the heat that each gave ? If the view currently re- 
 ceived be correct, would not nearly all the heat observed 
 be found in the latter of these foci, and little, if indeed 
 any, be found in the former ? But if all the various re- 
 gions of the spectrum possess equal heat-giving powers, 
 would not the heat in each of these foci be the same? 
 
 Let us give greater precision to this idea. Using Ang- 
 strom's wave-lengths the length at the line A is 7604, 
 and that at H 2 3933, and these lines are not very far 
 from the less and more refrangible ends of the visible 
 spectrum respectively. The middle point of this spec- 
 trum is at 5768, which may therefore be called its optical 
 centre. This is a little beyond the sodium line D, which 
 is 5892. Now, if by suitable means we reunite all the 
 rays between 7604 and 5768 into one focus, and all the 
 rays between 5768 and 3933 into another focus, are we 
 not in a position to determine the true distribution of 
 the heat ? Should the heat at these two foci be sensibly 
 the same, must not the conclusion at present held be 
 abandoned ? 
 
 If in these investigations the rays of the sun be used, 
 it is necessary to restrict the examination to the visible 
 spectrum, excluding the invisible red and invisible violet 
 radiations. On these the earth's atmosphere exerts not 
 only a very powerful but a very variable action, and, 
 what is still more, an action the result of which we can- 
 not see, so that we are literally working in the dark. 
 
MEMOIR XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 339 
 
 There are days on which, owing to the excessive absorp- 
 tion taking place among the ultra-red rays, a rock-salt 
 train has no advantage over one of glass. But if it is 
 the visible spectrum alone that we are using, and the 
 prisms are of a material colorless to the eye, we may be 
 certain that they are exerting no selective absorption on 
 any of the radiations of that spectrum, and that the in- 
 dications they are giving are reliable. 
 
 This variable absorptive action of the atmosphere de- 
 pends partly on changes in the amount of water vapor, 
 and partly on the altitude of the sun. At midday and 
 at midsummer it is at a minimum. The disturbance 
 is not merely a therniochrose, for both ends of the spec- 
 trum are attacked. It is a matter of common observa- 
 tion that the horizontal sun has but little photographic 
 power, owing to atmospheric absorption of the ultra-vio- 
 let rays, and under the same circumstances his heating 
 power is diminished, owing to the absorption of the ultra- 
 red rays. But if the day be clear, and the sun's altitude 
 sufficient, the visible spectrum may be considered as un- 
 affected. 
 
 It should be borne in mind that the envelopes of the 
 sun himself exert an absorptive action, which is power- 
 fully felt in the ultra-violet region, as is indicated by 
 the numerous fixed lines crowded together in that region. 
 The force of this remark will be appreciated on examin- 
 ing the plate above referred to in the Philosophical Mag- 
 azine for May, 1843. 
 
 It seems, then, that all the conditions necessary for the 
 solution of this problem will be closely approached if we 
 make use of prisms constituted of any substance which is 
 completely colorless to the eye, and confine our measures to 
 the visible spectrum, collecting all the radiations between 
 the fixed line A and the centre of that spectrum just be- 
 vond D into one focus, and all the radiations between 
 
390 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 that centre and H 2 into another focus, and by the thermo- 
 pile, or any other suitable means, measuring the heat of 
 these foci. 
 
 Such is the method I have followed in obtaining the 
 measures now to be presented : but before giving them, 
 there are certain preparatory facts which I wish to sub- 
 mit to the consideration of the reader. 
 
 (1.) In the mode of experiment hitherto adopted, no 
 special care has been taken to ascertain with accuracy the 
 position of the " extreme red," yet that is held to be the 
 point from which, on one side, we are to estimate the in- 
 visible, and, on the other, the visible spectrum. Differ- 
 ent persons', perhaps because of a different sensitiveness 
 of their eyes, will estimate that position differently. The 
 red light shades off gradually it is almost impossible 
 to tell when it really comes to an end. A linear thermo- 
 pile, such as is commonly used, is liable, under these cir- 
 cumstances, to give deceptive results, and any error in its 
 indications counts in a double manner. It not only di- 
 minishes the value of one spectrum, but it adds that dim- 
 inution to the value of the. other. The force of this re- 
 mark will be understood by considering the best experi- 
 ments hitherto made on this subject, those of Dr. Tyndall, 
 as related in his "Heat a Mode of Motion" (London edi- 
 tion, 1870, p. 420, etc.). In the case of the electric light, 
 the result yielded by those experiments was that the 
 heat in the invisible is eight times that of the visible 
 region. But had there been an error in estimating the 
 position of the extreme red by only two millimeters, so 
 much would have been taken from the invisible and add- 
 ed to the visible that they would have been brought to 
 equality, and then the slightest turn of the screw that 
 carried the pile towards the dark space would have 
 given a preponderance to the visible. It is obvious, 
 therefore, that there cannot be certainty in such meas- 
 
MEMOIR XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 39 1 
 
 ures, unless the fixed lines be resorted to as standard 
 points. 
 
 (2.) A ray which has passed through a solution of 
 sulphate of copper and ammonia possesses no insignifi- 
 cant heating power. I took a stratum of a solution of 
 that salt, of such strength that it only permitted waves 
 to pass which are of less length than 4860. Seen in the 
 spectroscope, the colors transmitted through it com- 
 menced with a thin green fringe, followed by blue, in- 
 digo, violet. It therefore gave rays in which, according 
 to the accepted views, little or no heat should be detect- 
 ed. Yet I found that such rays produced one ninth of 
 the heat of the solar beam. Does not this indisputably 
 show that the more refrangible rays have a higher ca- 
 lorific power than is commonly imputed to them ? 
 
 (3.) Again, by the use of the apparatus presently to 
 be described, I found no difficulty in recognizing heat in 
 the violet region. But in the mode of conducting the 
 experiment heretofore resorted to, it could not be detect- 
 ed in rays more refrangible than the blue. It was this 
 result which gave so much weight to the conclusion, that 
 
 O O ' 
 
 in the more refrangible regions the calorific power is re- 
 placed by chemical force, and strengthened the idea com- 
 monly entertained that the solar radiations consist of 
 three distinct principles heat, light, and actinism. In 
 the Memoir above referred to, as soon to be published, I 
 shall present some facts which apparently make this view 
 indefensible. 
 
 (4.) If waves of light falling upon an absolutely black 
 surface, and becoming extinct thereby, be transmuted 
 into heat, if the warming of surfaces by incident light 
 be nothing more than the conversion of motion into 
 heat an illustration of the modern doctrine of the corre- 
 lation of forces, heat itself being only " a mode of mo- 
 tion" it would seem extraordinary that the conversion 
 
392 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 should cease in the green or blue or in any middle ray. 
 On the contrary, calorific effects ought to be traceable 
 throughout the entire length of the spectrum. These 
 views on the transference of motion from the ether to the 
 particles of ponderable bodies, and conversely, I endeavor- 
 ed to explain in detail in a memoir on Phosphorescence 
 inserted in the Philosophical Magazine, February, 1851, 
 p. 98, etc. (Memoir VIII.). I had previously indicated 
 them in the same journal, February, 1847 (Memoir V.). 
 A given series of waves of red light impinging upon an 
 extinguishing surface will produce a definite amount of 
 heat, and similar series of violet waves should produce 
 the same amount ; for though an undulation of the lat- 
 ter may have only half the length of one of the former, 
 and therefore only half its vis viva, yet, in consequence 
 of the equal velocity of waves of every color, the impacts, 
 or impulses, of the violet series will be twice as frequent 
 as those of the red. The same principle applies to any 
 intermediate color, and hence it follows that every color 
 ought to have the same heating power. 
 
 Description of the Apparatus employed. 
 
 The optical arrangement I have employed for carrying 
 the foregoing suggestions into practice is represented by 
 Fig. 87, and in a horizontal section by Fig. 88. 
 
 A ray of sunlight reflected by a Silberman's heliostat 
 comes into a dark room through a slit, a, one millimeter 
 wide. It then passes through a prism, b. On the front 
 face of this prism is a black paper screen, c c, having a 
 rectangular opening, just sufficient to permit the light of 
 the slit to pass. After refraction the dispersed rays fall 
 as a spectrum on a concave metallic mirror, dd, nine inch- 
 es in aperture and eleven in focus for parallel rays. I 
 have sometimes used one of speculum metal, but more 
 frequently one silvered on its front face. In front of this 
 
MEMOIR XXVIIL] DISTRIBUTION OF HEAT IN THE SPECTRUM. 393 
 
 mirror there are therefore three foci. At a distance of 
 eleven inches there is one, e, Fig. 88, giving a spectrum 
 image of the sun. Still farther there is a second,/, which 
 is a spectrum image of the slit, #, in which, if the prism 
 be at its angle of minimum deviation, and the other ad- 
 justments be correctly made, will be seen the Fraunhofer 
 lines. Again, still farther off, at </, is a focal image of 
 the rectangular opening of the black paper, c c, on the 
 front face of the 
 prism. This im- 
 age, arising from 
 the recombination 
 of all the dispersed 
 rays, is consequent- 
 ly white. The sec- 
 ond and third foci 
 are at distances 
 from the mirror de- 
 pending on the dis- 
 tance of the slit, a, 
 and the black pa- 
 per, cc, respectively. 
 
 With the intention of being certain that the light com- 
 ing through the slit, #, is falling properly on the rectangu- 
 lar opening in the prism screen, c c, a small looking-glass 
 is placed at^>. The experimenter, sitting near the mul- 
 tiplier, m, can then see distinctly the reflected image of 
 that opening. 
 
 At the place where the second focal image, with its 
 Fraunhofer lines, forms, two screens of white pasteboard, 
 \ i, are arranged. By suitably placing the former of 
 these, /, the more refrangible rays may be intercepted, 
 and in like manner by the other, i, the less refrangible. 
 In using these screens, and particularly A, care must be 
 taken that no rays passing from the prism to the mirror 
 
 Pig. 87. 
 
394 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 be obstructed, a remark that applies especially to the in- 
 visible rays of less refrangibilifcy than the red. For this 
 reason the mirror, d d, must be placed at such an obliquity 
 to its incident rays as to throw the focal images sufficient- 
 ly on one side. Yet this obliquity must riot be greater 
 than is actually necessary for that purpose, or the purity 
 of the second spectrum, with its Fraunhofer lines, will be 
 interfered with. At the place of the third focus, arising 
 
 from the reunion of the 
 dispersed rays, is the ther- 
 mopile, g, connected by its 
 wires, & &, with the multi- 
 plier, m. 
 
 Whenever any of the 
 visible rays of the Fraun- 
 hofer spectrum are inter- 
 cepted by advancing either 
 of the screens A, i, the im- 
 age on the face of the pile 
 ceases to be white. It be- 
 comes of a superb tint, an- 
 swering to the combination of the non-intercepted rays. 
 A slip of white paper placed for a moment in front of the 
 pile will satisfy the experimenter how magnificent these 
 colors are. It is evident, therefore, that by this arrange- 
 ment the pile will enable us to measure the heat of any 
 particular ray, or of any selected combination of rays. 
 The screens can be arranged so as to reach any desig- 
 nated Fraunhofer line. 
 
 The pile I have used is of the common square form ; 
 a linear pile would not answer. The focal image on the 
 pile is of very much greater width than the slit a, on ac- 
 count of the obliquity of the front face of the prism. 
 
 By removing the screen li, and placing the screen i so 
 that its edge coincides with the line A of the Fraunhofer 
 
 \ 
 
 Fig. 88. 
 
MEMOIR XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 395 
 
 spectrum, all the invisible heat radiations of less refran- 
 gibility than the red are cut off, except the contaminat- 
 ing ones arising from the general diffusion of light by 
 the substance of the prism. Under these circumstances 
 the image on the pile will be white, and the multiplier 
 will give a deflection representing the heat of the visible 
 and the extra-violet regions. If then the screen be ad- 
 vanced still farther, until it has intercepted all the less 
 refrangible regions up to the sodium line D or a little 
 beyond that is, to the optical centre of the spectrum 
 the tint on the face of the pile will be greenish -blue, 
 and the multiplier will give a measure of the heat of 
 the more refrangible half of the visible spectrum, to- 
 gether with that of the ultra-violet rays : the latter por- 
 tion may, however, be eliminated by properly using the 
 other screen, Ji. 
 
 Besides the error arising from stray heat diffused 
 through the spectrum in consequence of the optical im- 
 perfection of the prism, there is another which may be 
 recognized on recollecting the relative positions of the 
 prism, the concave mirror, and the face of the pile. It is 
 evident that the prism, considered as a warm or a cool 
 mass, is a source of disturbance, for the mirror reflects its 
 image that is, the image of the prism itself to the pile. 
 After the intromitted sunbeam has passed through the 
 prism for a short time, the temperature of that mass has 
 risen, and the heat from this source has become inter- 
 mingled with the proper spectrum heat. But this error 
 is very easily eliminated. It is only necessary to put a 
 screen, n, in the path of the incoming ray, between the 
 slit and the prism, and note the deflection of the multi- 
 plier. Used as we are here supposing, the multiplier 
 has two zeros. The first, which may be termed the 
 magnetic, is the position in which the needles will stand 
 when no current is passing through the coil. The scale 
 
396 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 of the instrument should be set to this. The other, 
 which may be termed the working zero, is formed by 
 coupling the pile and the multiplier together, and intro- 
 ducing the screen n between the intromitting slit and 
 the prism. On doing this it will probably be found 
 that the index will deviate a few divisions. Its po- 
 sition should be accurately marked at the beginning 
 and close of each set of measures, and the proper correc- 
 tion for them made. The disturbing influences of the 
 mass of the prism, of the mirror, and of the pile itself, 
 are thus eliminated. As respects the last, it should not 
 be forgotten that it may be affected by changes in the 
 position of the person of the experimenter himself. 
 
 With the intention of diminishing these errors, I have 
 usually covered the upper and lower portions of the con- 
 cave mirror, dd, with pieces of black paper, so arranged as 
 to leave a band of sufficient width to receive and reflect 
 the entire spectrum, a a, Fig. 89, is the upper paper, b b 
 the lower, c c the uncovered reflecting 
 band, receiving the spectrum, r v. Had 
 the spaces thus covered been permitted to 
 reflect, they would have rendered more in- 
 tense the image of the prism with its ex- 
 Fj g- 89 - traneous heat. 
 
 As regards the multiplier, care must be taken to avoid 
 disturbance from aerial currents. I have one of these 
 instruments of French construction, which could not be 
 used in these delicate researches until proper arrange- 
 ments were applied. It was covered with a glass shade. 
 The slightest cause occasioned currents in its included 
 air, which perpetually drifted and disturbed the needles. 
 For this reason, and also for more accurate reading, it is 
 best to view the position of the index through a small 
 telescope. 
 
 The combination of needles being nearly astatic, at- 
 
MEMOIR XXVIIL] DISTRIBUTION OF HEAT IN THE SPECTRUM. 
 
 tention must be paid to their magnetic perturbations, 
 whether arising from local or other causes; and, since 
 the vibrations are very slow, ample time must be given 
 before the reading is ascertained. 
 
 The condition of the face of the pile is of importance. 
 It must be such as to extinguish as completely as possible 
 all the incident rays. To paint it with lamp-black, mix- 
 ed with gum, will not answer the surface so produced 
 is too glossy and reflecting. The plan I have found best 
 is to take a glass tube half an inch in diameter and six 
 inches long, open at both ends, and use it as a chimney. 
 A piece of camphor being set on fire at the lower end, 
 and the face of the pile to be blackened being held for a 
 moment at the upper, it is covered with a dense black 
 film, without any risk of injury to the pile. Even at the 
 best, when this has been done, there is an unavoidable 
 source of error in the want of perfect blackness of the 
 lamp-black. It is sufficient to inspect the face of the pile 
 when receiving rays from the concave mirror to be satis- 
 fied how large a portion of light is reflected. The ex- 
 periments of Dr. Tyridall show that this substance also 
 transmits a considerable percentage of the heat falling 
 on it. Its quality of transmitting light is well known 
 to every one who has looked at the sun through a 
 smoked glass. 
 
 The galvanometer I have used is calibrated according 
 to the usual method. The numbers given in this Memoir 
 do not represent the angles of deflection, but their corre- 
 sponding forces. 
 
 The proper position of the intercepting screens h,i, can 
 often be verified with precision by looking through blue 
 cobalt glass. This glass insulates a definite red, an 
 orange, and a yellow ray in the less refrangible regions, 
 and then, commencing with the green, gives a continuous 
 band to the end of the violet. Its red ray begins at the 
 
398 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 less refrangible end of the spectrum, and ends near C ; 
 it includes the fixed lines A, B, C. Its orange ray lies 
 wholly between C and D, including neither of those lines. 
 Its yellow ray begins near 5894, and ends about 5581 ; 
 the line D is therefore near its point of beginning. The 
 remaining continuous band begins about 5425 ; it there- 
 fore includes the lines E, &, F, G, H. I have found this 
 glass of much use in determining how far the screen i 
 has been pushed. It is convenient to select a light kind 
 of it, and by looking through one, two, or three pieces, 
 the depth of color can be regulated at pleasure. 
 
 The optical train which has acted on the sunbeam un- 
 der examination is, therefore, (1) the sun's atmosphere ; 
 (2) the earth's atmosphere ; (3) the heliostat mirror of 
 speculum metal ; (4) the prism ; (5) the concave mirror 
 of silvered glass ; (6) the blackened face of the thermo- 
 pile. 
 
 Results obtained by the Apparatus. 
 
 We are now ready to examine the results which this 
 optical apparatus yields, it having been of course pre- 
 viously ascertained that the reflecting band of the con- 
 cave mirror, d d, is sufficient to receive all the radiations 
 coming from the prism, and that none are escaping past 
 its edges. 
 
 The operations required are as follows : 
 
 The heliostat is to be set, and its reflected ray brought 
 into the proper position. The optical train is adjusted, 
 the prism being at its minimum deviation, and the con- 
 cave mirror giving a white image on the face of the 
 pile. 
 
 The screen h is then to be placed so that, without in- 
 tercepting any rays coming from the prism to the mirror, 
 it cuts off all the Fraunhofer spectrum above H 2 . 
 
 The screen i is so placed as to cut off all rays less re- 
 frangible than the sodium line D. More correctly, the 
 
MEMOIR XXVIII. ] DISTRIBUTION OF HEAT IN THE SPECTRUM. 399 
 
 screen should be a little beyond D. The light on the 
 face of the pile will now be greenish-blue. 
 
 The screen n is then placed so as to intercept the in- 
 troraitted beam. When the needles of the multiplier 
 come to rest, they give the working zero, which must be 
 noted. 
 
 The intromitting screen n being now removed, the mul- 
 tiplier will indicate all the heat of the more refrangible 
 rays ; that is, from a little beyond D up to H 2 . The force 
 corrected for the working zero is to be noted. 
 
 The screen i is then removed to the line A, so as to give 
 all the radiations between the lines A and H 2 . The light 
 on the face of the pile is white, and the multiplier gives 
 the whole heat of the visible spectrum. By subtracting 
 the foregoing measure from this, we have the heat of the 
 less refrangible region ; that is, from A to the centre of 
 the spectrum. 
 
 As a matter of curiosity the experimenter may now, if 
 he please, remove the screens h, i ; the light on the face 
 of the pile will still be white, and the multiplier will 
 give the force of the entire radiations, except so far as 
 they are disturbed by the thermochrose of the media. 
 These measures, as they do not bear upon the prob- 
 lem under consideration, I do not give in the following 
 tables. 
 
 Instead of advancing the screen i from the less tow- 
 ards the more refrangible regions, I have very frequently 
 moved li from the more refrangible to the less. When it 
 is brought down from H 2 to the centre of the spectrum, 
 the light on the face of the pile is of an intense orange- 
 red it might perhaps be called a bromine-red. I need 
 not give further details of this mode of experimentation, 
 as I did not find that its results differed in any impor- 
 tant degree from those obtained as just described. 
 
 The variation in different experiments may generally 
 
400 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 be traced to errors in placing the screen i with exactness 
 on the centre of the spectrum and on the line A. 
 
 For the sake of more convenient comparison, I have re- 
 duced all the different sets of experiments to the stand- 
 ard of 100 for the whole visible spectrum. 
 
 I have made use of four prisms: (1) rock-salt; (2) 
 flint-glass ; (3) bisulphide of carbon ; (4) quartz, cut out 
 of the crystal so as to give a single image. 
 
 All the observations here recorded were made on davs 
 
 / 
 
 when there was a cloudless sky. 
 
 TABLE I. Distribution of Heat by RocJc-salt. 
 
 Series I. Series II. 
 
 (1 ) Heat of the whole visible spectrum 100 100 
 
 (2) more refrangible region 53 51 
 
 (3) " less " " 47 49 
 
 In this table the column marked Series I. gives the 
 mean of four sets of measures, and that marked II. of 
 three. At the beginning of each set the rock-salt was 
 repolished. 
 
 TABLE II. Distribution of Heat by Flint-glass. 
 
 Series I. Series II. 
 
 (1) Heat of the whole visible spectrum 100 1 00 
 
 (2) " more refrangible region 49 52 
 
 (3) " less " " 51 48 
 
 Series I. gives the mean of ten sets of measures, Series 
 II. of eight. 
 
 TABLE III. Distribution of Heat by Bisulphide of Carbon. 
 
 Series I. Series II. 
 
 (1) Heat of the whole visible spectrum, 100 100 
 
 (2J more refrangible region 52 49 
 
 (3) less " " 48 51 
 
 The sulphide employed was devoid of any yellowish 
 tinge ; it was quite clear. Series I. is the mean of eight 
 experiments, Series II. of ten. 
 
MEMOIR XXVIIL] DISTRIBUTION OF HEAT IN THE SPECTRUM. 49 1 
 
 TABLE IV. Distribution of Heat by Quartz. 
 
 Series I. Series II. 
 
 (1) Heat of the whole visible spectrum 100 100 
 
 (2) " more refrangible region 49 53 
 
 (5) " less " " , 51 47 
 
 Series I. represents twenty -seven experiments, Series 
 II. twelve. In the former two quartz prisms were used 
 to increase the dispersion ; in the latter only one was 
 employed. 
 
 Perhaps it may not be unnecessary for me to say that 
 I have repeated these experiments many hundred times 
 during a period of several months, including the winter 
 and the summer, varying the conditions as to the hour of 
 the day, arrangement of the apparatus, etc., as much as I 
 could, and present the foregoing tables as fair examples 
 of the results. Apprehending that the heliostat mirror, 
 which was of speculum metal, might exert some disturb- 
 ing influence on account of its faint reddish tinge, I re- 
 placed it with one of glass silvered on the front face, but 
 could not detect any substantial diiference in the results. 
 
 The important fact clearly brought into view by these 
 experiments is, that if the visible spectrum be divided 
 into two equal portions, the ray having a wave-length of 
 5768 being considered as the optical centre of such a 
 spectrum, these portions will present heating powers so 
 nearly equal that we may impute the differences to errors 
 of experimentation. Assuming this as true, it necessarily 
 follows that in the spectrum any two series of undula- 
 tions will have the same heating power, no matter what 
 their wave-lengths may be. 
 
 But this conclusion leads unavoidably to a most im- 
 portant modification of the views now universally held 
 as regards the constitution of the spectrum. When a 
 ray falls on an extinguishing surface, heat is produced ; 
 but that heat did not pre-exist in the ray. It arose from 
 the stoppage of ether waves, and is a pure instance of 
 
 Co 
 
402 DISTRIBUTION OF HEAT IN THE SPECTRUM. [MEMOIR XXVIII. 
 
 the conversion of motion into heat an illustration of 
 the modern doctrines of the conservation and transmu- 
 tation of force. 
 
 From this point of view, the conception that there ex- 
 ist in an incident ray various principles disappears alto- 
 gether. We have to consider an incident ray as consist- 
 ing solely of ethereal vibrations, which, when they are 
 checked by an extinguishing substance, lose their vis viva. 
 The effect that ensues depends on the quality of the sub- 
 stance. The vibrations imparted to it may be manifest- 
 ed by the production of heat, as in the case of lamp- 
 black, or by chemical changes, as in the case of many of 
 the salts of silver. In the parallel instance of acoustics 
 clear views have long ago been attained, and are firmly 
 held. No one supposes that sound is one of the ingre- 
 dients of the atmosphere, and it would not be more in- 
 correct to assert that it is something emitted by the 
 sounding body than it is to affirm that light or heat, or 
 actinism, is emitted by the sun. 
 
 The progress of actino-chemistry would be greatly ac- 
 celerated if there could be steadfastly maintained a clear 
 conception of the distinction between the mechanism of 
 a ray and the effects to which that ray may give rise. 
 The evolution of heat, the sensation of light, the produc- 
 tion of chemical changes, are merely effects manifesta- 
 tions of the motions imparted to ponderable atoms; and 
 these in their turn can give rise to converse results, as 
 when we gradually raise the temperature of a substance 
 the oscillating movements of its molecules are imparted 
 to the ether, and waves of less and less length are suc- 
 cessively engendered. 
 
 In the title of this Memoir I have employed the phrase 
 "distribution of heat" in accordance with general usage; 
 but if the conclusion arrived at be true, it is plain that 
 this should be exchanged for " production of heat." The 
 
MEMOIR XXVIII.] DISTRIBUTION OF HEAT IN THE SPECTRUM. 493 
 
 heat observed did not pre-exist in the incident rays : it is 
 the result of their extinction. 
 
 The remark has been made that these results are es- 
 sentially connected with photometry. In fact, any ther- 
 mometer is converted into a photometer if its ball or 
 other receiving surface be coated with a perfectly opaque 
 non-reflecting substance. 
 
404 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 MEMOIR XXIX. 
 
 ON THE DISTRIBUTION OF CHEMICAL FORCE IN THE 
 SPECTRUM. 
 
 From the American Journal of Science and Arts for December, 1872, and January, 
 1873; Philosophical Magazine, December, 1872. 
 
 CONTENTS: The curves of the calorific, luminous, and chemical spec- 
 tra. Their errors. Inappropriateness of the term actinic rays. 
 There is no localization of chemical effects. Every radiation can pro- 
 duce some specific effect. Case of the silver compounds. Bitumens 
 and resins. Carbonic acid. Colors of flowers. Law of Grotthus 
 Chlorine and hydrogen. Bending of stems of plants. Absorption es- 
 sential to chemical action. Decomposition of silver iodide. Union of 
 chlorine and hydrogen. Angstrom's law. General conclusions. 
 Chemical force exists in every portion of the spectrum. Each radia- 
 tion exercises chemical influences proper to itself. 
 
 WITH scarcely an exception, the most recent works on 
 the chemical action of radiations and spectrum analysis 
 describe a tripartite arrangement of the spectrum, illus- 
 trated by an engraving of three curves, exhibiting the 
 supposed relations of the calorific, the luminous, and the 
 chemical spectra. This view, which by a mass of evi- 
 dence may be shown to be erroneous, is exerting a very 
 prejudicial effect on the progress of actino-chemistry. 
 
 I propose now to present certain facts which may aid 
 in correcting this error. For this purpose it is necessary 
 to show that chemical effects decompositions and com- 
 binations may take place in any part of the spectrum. 
 The points to be established may be thus distinctly 
 stated : 
 
 1st. That so far from chemical influences bein^ re- 
 
 o 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 495 
 
 stricted to the more refrangible rays, every part of the 
 spectrum, visible arid invisible, can give rise to chemical 
 changes, of modify the molecular arrangement of bodies. 
 
 2d. That the ray effective in producing chemical or 
 molecular changes in any special substance is determined 
 by the absorptive property of that substance. 
 
 I may here remark that both these propositions were 
 maintained by me many years ago : an example of the 
 first will be found in the Philosophical Magazine (Dec., 
 1842), and of the second in a paper in the same journal, 
 " On some Analogies between the Phenomena of the 
 Chemical Rays and those of Radiant Heat " (Sept., 1841), 
 Memoir XVII. 
 
 The opinion commonly held respecting the distribu- 
 tion of chemical force in the spectrum is mainly founded 
 on the behavior of some of the compounds of silver. 
 These darken when exposed to the more refrangible 
 rays, and, unless correct methods of examination be re- 
 sorted to, seem to be unaffected by the less refrangible. 
 Hence it has been supposed that in the higher parts of 
 the spectrum a special principle prevails, to which the 
 designation of " actinic rays " is often applied an inap- 
 propriate iteration. In these pages I use the derivatives 
 of CIKTIQ not in this restricted sense, but as expressive of 
 radiations of every kind. This is their proper significa- 
 tion. 
 
 Every part of the spectrum, no matter what its re- 
 frangibility may be, can produce chemical changes, and 
 therefore there is no special localization of force in any 
 limited region. Out of a large body of evidence that 
 might be adduced, I select a few prominent instances. 
 
 1st. Case of the Compounds of Silver. 
 
 Silver is the basis of the most important photographic 
 sensitive substances. Its iodide, bromide, and chloride, 
 
406 CHEMICAL FORCE IN THE SPECTKUM. [MEMOIR XXIX. 
 
 darkening with rapidity under the influence of the more 
 refrangible rays, have mainly been the cause of the mis- 
 conception above alluded to respecting the tripartite con- 
 stitution of the spectrum. It is necessary, therefore, to 
 determine what are really the habitudes of these sub- 
 stances. 
 
 (1.) If a spectrum be received on iodide of silver, 
 formed on the metallic tablet of the daguerreotype, and 
 carefully screened from all access of extraneous light, 
 both before and during the exposure, on developing 
 with mercury vapor an impression is evolved in all the 
 more refrangible regions. This stain corresponds in 
 character and position to the blackening effect which 
 under like circumstances would be found on any com- 
 mon sensitive silver paper. It is this which has given 
 rise to the opinion that the so-called actinic rays exist 
 only in the upper part of the spectrum. If, however, the 
 action of the light be long continued, a white stain 
 makes its appearance over all the less refrangible re- 
 gions. It has a point of maximum to which I shall 
 again presently refer. 
 
 (2.) But if the metallic tablet during its exposure to 
 the spectrum be also receiving diffused light of little in- 
 tensity, as the light of day or of a lamp, it will be found 
 on developing that the impression obtained differs strik- 
 ingly from the preceding. Every ray that the prism can 
 transmit, from below the extreme red to beyond the ex- 
 treme violet, has been active. The ultra-red lines a, /3, y 
 are present. It must be borne in mind that the impres- 
 sion of these lines is a proof of proper spectrum action, 
 and distinguishes it from that of diffused light, arising 
 either from the atmosphere or from the imperfect trans- 
 parency of the prism a valuable indication. The result- 
 ing photograph shows two well-marked regions or phases 
 of action. On its general surface, which, having con- 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 
 
 densed the mercury vapor, has the aspect of the high 
 lights of the daguerreotype, and forms, as it were, the 
 basis for the spectrum picture, there is in the region of 
 the more refrangible rays a bluish or olive-colored im- 
 pression, the counterpart of the result described in the 
 foregoing paragraph. But in the region of the less re- 
 frangible rays no mercurial deposit has occurred, the 
 place of those rays being depicted in metallic silver, 
 dark, and answering to the shadows of the daguerreo- 
 type. This protected portion, which stands out in bold 
 relief from the white background, reaches from a little 
 below G to beyond the extreme red, and encloses the 
 heat lines above named. They are in the form of white 
 streaks. Though I speak of them as single lines, they 
 are in reality groups, or perhaps bands. 
 
 The general appearance of the photograph at once 
 suggests that the less refrangible rays can arrest the ac- 
 tion of the daylight and protect the silver iodide from 
 change. A close examination shows that there are three 
 points the extreme red, the centre of the yellow, and the 
 extreme violet which apparently can hold the daylight 
 in check. There are also two intervening ones in which 
 the actions conspire. The point of maximum protection 
 corresponds to the point of maximum action referred to 
 above in paragraph (1). 
 
 (3.) If the metallic tablet, previously to its exposure 
 to the spectrum, be submitted for a few moments to a 
 weak light, so that, were it developed, it would at this 
 stage whiten all over, the action of the spectrum upon 
 it will be the same as in the last case (2). But this 
 change in the mode of the experiment leads to a very 
 important conclusion. The less refrangible rays can re- 
 verse or undo the change, in whatever it may consist, 
 that light has already impressed on the iodide of silver. 
 
 Now, bearing in mind the fact that the photographic 
 
408 CHEMICAL FOKCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 action of diffused light on this iodide is mainly due to 
 the more refrangible rays it contains, we are brought by 
 these experiments to the following conclusions : 
 
 1st. Every ray in the spectrum acts on silver iodide. 
 
 2d. The more refrangible rays apparently promote the 
 action of daylight on that substance ; the less refrangible 
 apparently arrest it. 
 
 3d. For the display of this arresting or antagonizing 
 effect, it is not necessary that the less and more refran- 
 gible rays should be acting simultaneously. An interval 
 may elapse, and they may act successively. Hence the 
 effect is not due to the contemporaneous interference of 
 waves of different periods of vibration with one another 
 the material particles of the changing substance of 
 the silver iodide are involved. 
 
 I abstain for the moment from giving further details 
 of these spectrum impressions. That has been very 
 completely done by Herschel in the case of one I sent 
 him many years ago. His examination of it, illustrated 
 by a lithograph, may be found in the Philosophical Mag- 
 azine (Feb., 1843). I shall have to return to the subject 
 of the behavior of silver iodide in presence of radiations 
 on a subsequent page of this Memoir. 
 
 The main point at present established is this, that the 
 iodide under proper treatment is affected by every ray 
 that a flint-glass prism can transmit, and therefore it is 
 altogether erroneous to suppose that chemical force is 
 restricted to the more refrangible portions of the spec- 
 trum. 
 
 2d. Case of Bitumens and Hesins. 
 
 These substances are of special interest in the history 
 of photography, since in the hands of Niepce they prob- 
 ably were the first on which impressions in the camera 
 were obtained and fixed. Their use has been abandoned 
 in consequence, as it seems to me, of an incorrect opinion 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 499 
 
 of their want of sensitiveness. Properly used, they are 
 scarcely inferior to chloride of silver. 
 
 The theory of their use is very simple. Alcohol, ether, 
 and various volatile oils, respectively dissolve certain por- 
 tions of these substances. If such a solution be spread 
 in a thin film upon glass, as in the collodion operation, 
 and parts of the surface be then exposed to light, the 
 portions so exposed become insoluble in the same men- 
 struum. They may therefore be developed by its use. 
 Practically, care has to be taken to moderate the solvent 
 action and to check it at the proper time. The former 
 is accomplished by dilution with some other appropriate 
 liquid, the latter by the affusion of a stream of water. 
 
 The substance I have used is West India bitumen dis- 
 solved in benzine, and developed by a mixture of ben- 
 zine and alcohol. 
 
 The bitumen solution being poured on a glass plate 
 in a dark room, and drained off as in the operation of 
 collodion, leaves a film sufficiently thin to be iridescent. 
 This is exposed to the spectrum for five minutes, and 
 then developed. 
 
 The beginning of the impression is below the line A, 
 its termination beyond H. Every ray in the spectrum 
 acts. The proof is continuous except where the Fraun- 
 hofer lines fall. A better illustration that the chemical 
 action of the spectrum is not restricted to the higher 
 rays, but is possessed by all, could hardly be adduced. 
 
 3d. Case of Carbonic Acid. 
 
 The decomposition of carbonic acid by plants under 
 the influence of sunshine is undoubtedly the most im- 
 portant of all actino-chemical facts. The existence of the 
 vegetable world, and indeed it may be said the existence 
 of all living beings, depends upon it. 
 
 I first effected this decomposition in the solar spec- 
 
410 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 tram, as may be seen in Memoir X. The results obtain- 
 ed by me at that time from the direct spectrum experi- 
 ment, that the decomposition of* carbonic acid is effected 
 by the less, not by the more refrangible rays, have been 
 confirmed by all recent experimenters, who differ only 
 as regards the exact position of the maximum. In the 
 discussions that have arisen this decomposition has often 
 been incorrectly referred to the green parts of plants. 
 Plants which have been caused to germinate and grow 
 to a certain stage in darkness are etiolated. Yet these, 
 when brought into the sunlight, decompose carbonic 
 acid, and then turn green. The chlorophyl thus pro- 
 duced is the effect of the decomposition, not its cause. 
 Facts derived from the visible absorptive action of chlo- 
 rophyl do not necessarily apply to the decomposition of 
 carbonic acid. The curve of the production of chlorophyl, 
 the curve of the destruction of chlorophyl, the curve of 
 the visible absorption of chlorophyl, and the curve of the 
 decomposition of carbonic acid, are not all necessarily co- 
 incident. To confound them together, as is too frequent- 
 ly done, is to be led to incorrect conclusions. 
 
 Two different methods may be resorted to for deter- 
 mining the rays which accomplish the decomposition of 
 carbonic acid: 1st, the place of maximum evolution of 
 oxygen gas in the spectrum may be determined ; 2d, 
 the place in which young etiolated plants turn green. 
 
 I resorted to both these methods, and obtained from 
 them the same results. The rays which decompose car- 
 bonic acid are the same which turn etiolated plants 
 green. They may be designated as the yellow with the 
 orange on one side and a portion of green on the other. 
 Though the form of experimentation does not admit of a 
 close reference to the fixed lines, I think we are almost 
 justified in supposing that the point of maximum action 
 is in the yellow. It must be borne in mind that the 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 
 
 rapidly increasing concentration of the rays occasioned 
 by the peculiarity of prismatic dispersion towards the 
 red end will give a deceptive preponderance in that di- 
 rection. Without entering further into this discussion, 
 it is sufficient for my present purpose to understand that 
 the decomposition in question is accomplished by rays 
 between the fixed lines B and F. 
 
 The two absorptive media, potassium bichromate and 
 cupro-ammonium sulphate, so often and so usefully em- 
 ployed in actino - chemical researches, corroborate this 
 conclusion. Plants cannot decompose carbonic acid, nor 
 can they turn green in rays that have passed through a 
 solution of the latter salt. They accomplish both those 
 results in rays that have passed through the former. 
 
 The decomposition of carbonic acid, and the produc- 
 tion of chlorophyl by the less refrangible rays of the 
 spectrum, afford thus a striking illustration that chem- 
 ical changes may be brought about by other than the 
 so-called chemical rays. 
 
 . Case of tlie Colors of Flowers. 
 
 / / 
 
 The production and destruction of vegetable colors by 
 the agency of light have, of course, long been a matter of 
 common observation. Little has, however, been done in 
 the special examination of the facts, and that little for 
 the most part by Herschel. 
 
 We have only to examine his Memoir in the Philo- 
 sophical Transactions (Part II., 1842) to be satisfied that 
 nearly every radiation can produce effects. Thus the 
 yellow stain imparted by the Corcliorus Japonica to pa- 
 per is whitened by the green, blue, indigo, and violet 
 rays. The rose-red of the Ten-weeks-stock is in like man- 
 ner changed by the yellow, orange, and red. The rich 
 blue tint of the Viola odorata, turned green by sodium 
 carbonate, is bleached by the same group of rays; that is, 
 
412 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 by those less refrangible than the yellow. The green 
 (chlorophyl) of the Elder leaf is changed by the extreme 
 red. 
 
 It is needless to extend this list of examples. The 
 foregoing establish the principle that every part of the 
 spectrum displays activity, some vegetable colors being 
 affected by one, others by other rays. It is, however, 
 desirable that the general principle at which Herschel ar- 
 rived viz., that the luminous rays are chiefly effective 
 should be more closely examined. Some important phys- 
 iological explanations turn on that principle. These so- 
 called luminous rays are such as can impress the retina, 
 which, like organic colors, is a carbon compound. There 
 are strong reasons for inferring that carbon is affected 
 mainly by rays the wave-lengths of which are between 
 those of the extreme red and extreme violet, the maxi- 
 mum being in the yellow. 
 
 It is, however, to a former experimenter, Grotthus, that 
 we owe the discovery of the law under which these de- 
 compositions of the colors of flowers take place. This 
 law in repeated instances was verified by Herschel, and 
 more recently by myself. It may be thus expressed : 
 " The rays which are effective in the destruction of any 
 given vegetable color are those which by their union 
 produce a tint complementary to the color destroyed." 
 Even the partial establishment of this law, already ac- 
 complished, is sufficient to prove that chemical effects 
 are not limited to the more refrangible portions of the 
 spectrum, but can be occasioned by any ray. 
 
 6th. Case of the Union of Chlorine and Hydrogen. 
 
 In Memoir XVIII. may be found the description of 
 an actinometer invented by me, depending for its indi- 
 cations on the combination of chlorine and hydrogen, 
 these gases having been evolved in equal volumes from 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPEQTR^M. 
 
 hydrochloric acid by a small voltaic battery;/ Thi^ip- 
 strument, modified to suit their purposes, was tf^^4 ky?> 
 Professors Bunsen and Roscoe in their photometrica^re- 
 searches. Many of my experiments were repeated 05^ 
 them (Transactions of the Royal Society, 1856, 1857). 
 
 In Table III. of my Memoir above referred to, it is 
 shown that this mixture is affected by every ray of the 
 spectrum ; but by different ones with very different en- 
 ergy. The maximum is in the indigo, the action there 
 being more than 700 times as powerful as in the ex- 
 treme red. 
 
 th. Case of the Sending of the Stems of Plants in the 
 
 Spectrum,. 
 
 It is a matter of common observation that plants tend 
 to grow towards the light. Dr. Gardner was, however, 
 the first to examine the details of this phenomenon in 
 the spectrum. His Memoir is in the Philosophical Mag- 
 azine (Jan., 1844). When seeds are made to germinate 
 and grow for a few days in darkness, they develop verti- 
 cal stems, very slender and some inches in length. These, 
 on being placed so as to receive the spectrum, soon ex- 
 hibit a bending motion. The stems in other parts of 
 the spectrum turn towards the indigo; those in the in- 
 digo bend to the approaching ray. Removed into dark- 
 ness, they recover their upright position. These move- 
 ments are the most striking of all actinic phenomena. I 
 have often witnessed them with admiration. 
 
 Dr. Gardner's experiments were repeated and con- 
 firmed by M. Dutrochet, who, in a report to the French 
 Academy of Sciences (Oomptes - Rendus, No. 26, June, 
 1844), added a number of facts respecting the bending 
 of roots from the light, which he found to be occasioned 
 by all the colored rays of the spectrum. 
 
 In Dr. Gardner's paper there are also some interesting 
 
4X4 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 facts respecting the bleaching or decolorization of chlo- 
 rophyl by light. He used an ethereal solution of that 
 substance : 
 
 "The first action of light is perceived in the mean red 
 rays, and it attains a maximum incomparably greater at 
 that point than elsewhere. The next part affected is in 
 the indigo, and accompanying it there is an action from 
 + 10.5 to +36.0 of the same scale (Herschel's) beginning 
 abruptly in Fraunhofer's blue. So striking is this whole 
 result that some of my earlier spectra contained a per- 
 fectly neutral space, from 5.0 to +20.5, in which the 
 chlorophyl was in no way changed, while the solar 
 picture in the red was sharp and of a dazzling white. 
 The maximum in the indigo was also bleached, pro- 
 ducing a linear spectrum, as follows : 
 
 in which the orange, yellow, and green rays are neutral. 
 These, it will be remembered, are active in forming 
 chlorophyl. Upon longer exposure, the subordinate 
 action along the yellow, etc., occurs, but not until the 
 other portions are perfectly bleached. 
 
 "In Sir J. Herschel's experiments there remained a 
 salmon color after the discharge of the green. This is 
 not seen when chlorophyl is used, and is due to a color- 
 ing matter in the leaf, soluble in water, but insoluble in 
 ether." 
 
 I have quoted these results in detail because they il- 
 lustrate in a striking manner the law that vegetable col- 
 ors are destroyed by rays complementary to those that 
 have produced them, and furnish proof that rays of every 
 refrangibility may be chemically active. 
 
 At this point I abstain from adding other instances 
 showing that chemical changes are brought about in 
 every part of the spectrum. The list of cases here pre- 
 sented might be indefinitely extended, if these did not 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 
 
 suffice. But how is it possible to restrict the chemical 
 force of the spectrum to the region of the more refrangi- 
 ble rays, in face of the fact that compounds of silver 
 such as the iodide, which have heretofore been mainly 
 relied upon to support that view, and, in fact, originated 
 it, are now proved to be affected by every ray from the 
 invisible ultra -red to the invisible ultra-violet; how, 
 when it is proved that the decomposition of carbonic 
 acid, by far the most general and most important of the 
 chemical actions of light, is brought about not by the 
 more refrangible, but by the yellow rays ? The delicate 
 colors of flowers, which vary indefinitely in their tints, 
 originate under the influence of rays of many different 
 refrangibilities, and are bleached or destroyed by spec- 
 trum colors complementary to their own, and, therefore, 
 varying indefinitely in their refrangibility. Towards 
 the indigo ray the stems of plants incline ; from the red 
 their roots turn away. There is not a wave of light 
 that does not leave its impress on bitumens and resins, 
 some undulations promoting their oxidation, some their 
 deoxidation. These actions are not limited to decompo- 
 sition ; they extend to combinations. Every ray in the 
 spectrum brings on the union of chlorine and hydrogen. 
 
 The conclusion to which these facts point is, then, 
 that it is erroneous to restrict the chemical force of the 
 spectrum to the more refrangible, or, indeed, to any 
 special region. There is not a ray, visible or invisible, 
 that cannot produce a special chemical effect. The dia- 
 gram so generally used to illustrate the calorific, lumi- 
 nous, and chemical parts of the spectrum serves only to 
 mislead. 
 
 While thus we find that chemical action may take 
 place throughout the entire length of the spectrum, the 
 remarks that have been made in a previous Memoir 
 (XXVI.) respecting the differences of calorific distribu- 
 
416 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 tion in dispersion and diffraction, apply likewise to the 
 chemical force. To be satisfied of this it is only neces- 
 sary to compare photographic, impressions given by a 
 prism and a grating ! 
 
 I published engravings of such photographs in 1844. 
 They are referred to in Memoir VI. As they were ob- 
 tained on silver plates made sensitive by iodine, bro- 
 mine, and chlorine, they do not extend to the line F. 
 
 I had found that certain practical advantages arise 
 from the use of a reflected instead of a transmitted 
 spectrum. The ruled glass was therefore silvered upon 
 its ruled face with tin amalgam, copying the surface per- 
 fectly. Of the series of spectra I used the first. 
 
 The fixed lines were beautifully represented in the 
 photographs. They were, however, so numerous and so 
 delicate that I did not attempt to do more than to mark 
 the prominent ones. These were, I believe, the first dif- 
 fraction photographs that had ever been obtained. The 
 wave-lengths assigned were according to Frauuhofer's 
 scale, which represents parts of a Paris inch. 
 
 The length of the photographic impression given by the 
 prism I was then using, from the line H to the ultra-violet 
 end of the spectrum, was about three times that from H 
 to G ; but in the spectrum by the grating, though the 
 exposure was in one instance continued for a whole hour, 
 the impression beyond H was not more than 1J times 
 the length of that to G. In more moderate exposures 
 the last fixed line in the photograph was about as far 
 from H on one side as G was on the other. This, there- 
 fore, showed very clearly the difference of distribution in 
 the diffraction and prismatic spectra. 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTKUM. 
 
 OF THE CHEMICAL ACTION OF RADIATIONS ON SUBSTANCES. 
 
 Having offered the foregoing evidence in support of 
 the first proposition considered in this Memoir, which 
 was to the effect 
 
 " That so far from chemical influences being restricted 
 to the more refrangible rays, every part of the spectrum, 
 visible and invisible, can give rise to chemical changes, 
 or modify the molecular arrangement of bodies," I now 
 pass to the second, which is 
 
 " That the ray effective in producing chemical or mo- 
 lecular changes in any special substance is determined 
 by the absorptive property of that substance." 
 
 This involves the conception of selective absorption, 
 as I have formerly shown (Philosophical Magazine, Sept., 
 1841), Memoir XVII. A ray which produces a maxi- 
 mum effect on one substance may have no effect on an- 
 other. Thus the rays which change chlorophyl are not 
 those which change silver iodide. 
 
 In the examination of this subject I shall select two 
 well-known instances, presenting the fewest elements and 
 the simplest conditions. They are, 1st, the decomposition 
 of silver iodide, the basis of so many photographic prepa- 
 rations ; 2d, the production of hydrochloric acid by the 
 union of its two constituents, chlorine and hydrogen a 
 mixture of these gases being exceedingly sensitive to 
 liht. 
 
 
 1st. Of the Decomposition of Silver Iodide. 
 
 There are two forms in which the silver iodide has 
 been used for photographic purposes : 1st, when prepared 
 by the action of the vapor of iodine on metallic silver, as 
 in the daguerreotype tablet; 2d, when nitrate of silver 
 is decomposed by iodide of potassium, or other metallic 
 iodides. These preparations differ strikingly in their ac- 
 
 DD 
 
418 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 tinic behavior, the former furnishing by far the most in- 
 teresting series of facts. 
 
 When a polished surface of- silver is exposed at com- 
 mon temperatures to the vapor of iodine, it speedily 
 tarnishes, a film of silver iodide forming. This passes 
 through several well-marked tints as the exposure con- 
 tinues and the thickness increases. They may be thus 
 enumerated in the order of their occurrence : 1, lemon- 
 yellow; 2, golden -yellow; 3, red; 4, blue; 5, lavender; 
 6, metallic ; 7, deep yellow ; 8, red ; 9, green. 
 
 All these films are sensitive. Under the influence of 
 radiations they exhibit two phases of modification : 1st, 
 an invisible modification, which, however, can be made 
 apparent or developed, as Dagtierre discovered, by ex- 
 posure to the vapor of mercury, the iodide turning white 
 by the condensation of mercury upon it wherever it has 
 been exposed to light, but remaining unacted upon in 
 parts that have been in shadow ; 2d, a visible modifica- 
 tion, which arises under a longer exposure, the iodide 
 passing through various shades of olive and blue, and 
 eventually becoming dark gray. 
 
 But though all the variously tinted films of silver 
 iodide are impressionable, they differ greatly in relative 
 sensitiveness when compared with each other. This may 
 be very satisfactorily shown by producing on one silver 
 tablet bands of all the above-named colors an effect 
 readily accomplished by suitably unscreening successive 
 portions of the tablet during the process of iodizing, and 
 then exposing all at the same time to a common radiation. 
 It will be found, on developing with mercury vapor, that 
 the bands of a yellow color have been the most sensitive, 
 those of a metallic aspect have been scarcely acted on, 
 and those of other tints intermediately. It is to be par- 
 ticularly remarked that the second yellow, numbered 7 
 in the above series, is equally sensitive with the first 
 yellow, numbered 2. 
 
MEMOIK XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 
 
 419 
 
 From this it appears that the sensitiveness of this form 
 of iodide depends not merely on its chemical constitution, 
 but also on its optical properties. The explanation of 
 this different sensitiveness in different films of iodide be- 
 comes obvious when we cause a tablet prepared, as just 
 described, with tinted bands to reflect the radiations fall- 
 ing on it to another tablet iodized to a yellow color, and 
 placed in a camera. After due exposure and develop- 
 ment of both with mercury, it will be found that the im- 
 age of the first tablet formed on the second consists of 
 bands of different shades of whiteness. The yellow parts 
 of the first tablet have scarcely affected the second, but 
 its metallic and blue parts have acted very powerfully. 
 On comparing the first plate and its image on the second 
 together, it will be perceived that the parts that have 
 been affected on the one are less affected on the other. 
 
 It may therefore be inferred that the yellow films are 
 sensitive because they absorb the incident radiation, and 
 the metallic and blue are insensitive because they re- 
 flect it. 
 
 The effect, in whatever it may consist, which occurs 
 during the invisible modification is not durable : it grad- 
 ually passes away. If tablets that have received im- 
 pressions be kept for a time before developing, the im- 
 ages upon them gradually disappear. On these tablets 
 there is na lateral propagation of effect, nothing answer- 
 ing to conduction. 
 
 On examining the operation of a radiation continuous- 
 ly applied to one of these sensitive films, it will be dis- 
 covered that a certain time elapses that is, a certain 
 amount of the radiation is consumed before there is any 
 perceptible effect. When that is accomplished, the radi- 
 ation affects the film to a degree proportional to its quan- 
 tity, until a second stage is reached ; there is then an- 
 other pause, followed by the second stage, in which vis- 
 
420 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 ible modification or chemical decomposition sets in. The 
 film begins to darken ; it passes through successive tints 
 brown, red, olive, blue and eventually becomes dark 
 gray. 
 
 I have described in some of the foregoing paragraphs 
 the action of the spectrum on silver iodide as presented 
 on the tablet of the daguerreotype, showing the differ- 
 ence in the impression obtained: 1st, when extraneous 
 light has been excluded ; and, 2d, when it has been per- 
 mitted simultaneously or previously to act. 
 
 In the latter case, in all that region of the spectrum 
 from the more refrangible extremity to somewhat below 
 the line G, the usual darkening effect manifested by silver 
 compounds is observed ; but beyond this, and to the ex- 
 treme less refrangible rays, with certain variations of in- 
 tensity, the action of the extraneous and simultaneously 
 acting light is checked, and the effect of previously act- 
 ing light is destroyed. 
 
 It happened that in 1842 I obtained two very fine 
 specimens of the latter spectra; one of these I sent to 
 Sir J. Herschel, the other is still in my possession. 
 
 In the Philosophical Magazine (Feb., 1843), Herschel 
 gave a detailed description of these spectrum impressions. 
 He was disposed to refer the appearance they present to 
 the phenomena of thin films, but at the same time point- 
 ed out the difficulties in the way of that explanation. 
 He also sent me three proofs he had obtained on ordi- 
 nary sensitive paper, darkened by exposure to light, then 
 washed with a solution of iodide of potassium, and placed 
 in the spectrum. He described them as follows : 
 
 (1.) "Blackened paper from which excess of nitrate of 
 silver has not been abstracted, under the influence of an 
 iodic salt. Produced by a November sun. N.B. View 
 it also transparently against the light." 
 
 (2.) "Blackened paper under the influence of an iodic 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 421 
 
 salt when no excess of nitrate of silver exists in the 
 paper." 
 
 (3.) "Action of spectrum under iodic influence when 
 very little nitrate of silver remains in excess in the pa- 
 per. To be viewed also transparently." 
 
 These paper photographs I still preserve. They are as 
 perfect as when first made. The different colored spaces 
 of the spectrum are marked upon them with pencil. The 
 appearances they respectively present are as follows: (1) 
 is bleached by the more refrangible rays, and blackened 
 deeply from the yellow to the ultra-red ; (2) is bleached 
 from the ultra invisible red to the ultra-violet (a maxi- 
 mum occurs abruptly about the blue) ; (3) has the same 
 upper spectrum as the others, a bleached dot in the cen- 
 tre of the yellow, and a darkened space on the extreme 
 red. The action has reached from the ultra-red to the 
 ultra-violet. 
 
 In Herschel's opinion, these effects in the less refran- 
 gible region are connected with the drying of the paper. 
 It is well known that paper in a damp condition is more 
 sensitive than such as is dry. But obviously this con- 
 dition does not obtain in the case of the daguerreotype 
 operation, which is essentially a dry process. 
 
 In 1846 MM. Foucault and Fizeau, having repeated 
 the experiment originally made by me, presented a com- 
 munication to the French Academy of Sciences to the. 
 effect that when a silver tablet which has been sensitized 
 by exposure to iodine and bromine, and then impressed 
 by light, is exposed to the spectrum, the effect is greatly 
 increased in all the region above the line C, and is neu- 
 tralized in all that below C. They remarked the dis- 
 tinctness with which the atmospheric line A comes out, 
 and saw the ultra spectrum heat-rays a, ]3, y described by 
 me some years previously. 
 
 The interpretation given by them is, that the more re- 
 
422 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 frangible rays promote the previous action of light ; the 
 less neutralize it. The curve representing the chemical 
 intensity of the different rays would cross the axis of 
 abscissas about the boundary of the red and orange ; be- 
 low that point to the ultra-red the ordinates would have 
 negative values ; above it to the ultra-violet those values 
 would be positive (Comptes-Rendus, No. 14, tome xxiii.). 
 Hereupon M. Becquerel communicated to the same 
 Academy a criticism on this interpretation, the opinion 
 maintained by him being that while the more refrangi- 
 ble rays excite sensitive surfaces, the less refrangible, far 
 from neutralizing, continue the action so becmn. To the 
 
 O' O 
 
 former he gave the designation "rayons excitateurs," to 
 the latter "rayons continuateurs " (Comptes-Hendus^o. 
 17, tome xxiii.). 
 
 In 1847 M. Claudet communicated a paper to the 
 Royal Society, subsequently published in the Philosoph- 
 ical Magazine (Feb., 1848), on this subject. His atten- 
 tion had been drawn to it by observing that the red im- 
 age of the sun, during a dense fog, had destroyed the 
 effect previously produced on a sensitive silver surface, 
 and that this destruction could be occasioned at pleasure 
 by the use of red and yellow screens. A surface which 
 has been impressed by daylight, and the impression then 
 obliterated by the less refrangible rays, had recovered its 
 primitive condition. It was ready to be impressed again 
 by daylight, and again the resulting effect might be de- 
 stroyed. Claudet found that this excitation and neutral- 
 ization might be repeated many times, the chemical con- 
 stitution of the film remaining unchanged to the last. 
 
 These facts seem to be inconsistent with Herschel's 
 opinion, that positive and negative pictures may succeed 
 each other by the continued action of a radiation, on the 
 principle of Newton's rings. 
 
 On a collodion surface such negative neutralizing or 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 423 
 
 reversing actions cannot be obtained by the less refran- 
 gible rays. The spectrum impression developed in the 
 usual manner by an iron salt presents a sudden maxi- 
 mum about the line G, and continues thence to the high- 
 est limit of the spectrum. In the other direction it ex- 
 tends below F. From E to the ultra-red not a trace of 
 action can be detected. The lines o,j3, 7 cannot be ob- 
 tained on collodion. There is, therefore, a difference be- 
 tween its behavior under exposure to light and that of 
 a daguerreotype tablet. 
 
 The reversals that are obtained on collodion by the 
 use of certain haloid compounds are altogether different 
 from the reversals on the thin films of a silver tablet. 
 They are produced by the more refrangible rays. 
 
 On exposing a collodion surface (prepared in the usual 
 manner) to daylight long enough to stain it completely, 
 then washing off the free nitrate, and in succession dip- 
 ping the plate into a weak solution of iodide of potas- 
 sium, exposing it to the spectrum, washing, again dip- 
 ping it into the nitrate bath, and finally developing, a re- 
 versed action is obtained. The daylight is perfectly neu- 
 tralized, but not after the manner in a daguerreotype. 
 In the region about G, the place of maximum action in 
 collodion, the impression of the light is totally removed 
 by an exposure of five seconds. In twelve seconds the 
 protected space is much larger; in thirty seconds it has 
 spread from F to H. It is, however, to be particularly 
 remarked that the less refrangible rays show no action. 
 
 The results are substantially the same when, instead 
 of iodide of potassium, the chloride of sodium, corrosive 
 sublimate, bromide of potassium, or fluoride of potassium 
 is used. In all these the reversing action is from F to 
 H, and has its maximum somewhere about G. That is, 
 the reversing action coincides with the direct action : 
 there is no protection in the lower portion of the spec- 
 
424 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 trum, as in the daguerreotype. The effect is altogether 
 due to the change of composition of the sensitive film. 
 Ordinarily it contains free nitrate; now it contains free 
 iodide, chloride, etc. 
 
 The silver compounds of collodion absorb the radia- 
 tions falling on them which are capable of producing a 
 photographic effect. Yet, sensitive as it is, collodion is 
 very far from having its maximum sensitiveness, as is 
 shown by the following experiment, which is of no small 
 interest to photographers: I took five dry collodion 
 plates, prepared by what is known as the tannin process, 
 and, having made a pile of them, caused the rays of a gas- 
 flame to pass through them all at the same time. On 
 developing, it was found that the first plate was strongly 
 impressed, and the second, which had been behind it, ap- 
 parently quite as much. Even the fifth was considerably 
 stained. From this it follows that the collodion film, as 
 ordinarily used, absorbs only a fractional part of the 
 rays that can affect it. Could it be made to absorb the 
 whole, its sensitiveness would be correspondingly in- 
 creased. 
 
 Radiations that have suffered complete absorption can 
 bring about no further change. Partial absorption, aris- 
 ing from inadequate thickness, may leave a ray possessed 
 of a portion of its power. There must be a correspond- 
 ence between the intensity of the incident ray and the 
 thickness of the absorbing medium to produce a maxi- 
 mum effect. 
 
 Though the silver iodide is affected by radiations of 
 every refrangibility, it is decomposed so that a subiodide 
 results only by those of which the wave-length is less 
 than 5000. If in presence of metallic silver, as on the 
 daguerreotype tablet, the iodine disengaged unites with 
 the free silver beneath. The rays of high refrangibility 
 occasion in it chemical decomposition ; those of less re- 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 425 
 
 frangibility, physical modification. In the language of 
 the older theories of actino-chernistry, this substance may 
 be said to exert a selective absorption. In this it illus- 
 trates the general principle, that it depends on the nat- 
 ure of the ponderable material presented to radiations 
 which of them shall be absorbed. 
 
 2d. Of the Union of Chlorine and Hydrogen. 
 
 ^An interesting experiment, illustrating the fact that 
 chlorine gas absorbs the radiations which bring about 
 its combination with hydrogen, may be made by cover- 
 ing a test-tube containing an explosive mixture of equal 
 volumes of thofee gases with a large jar filled with chlo- 
 rine. This arrangement may be exposed in the open 
 daylight without risk of exploding the mixture ; but if 
 the experiment be made with a covering jar containing 
 atmospheric air instead of chlorin,e, the gases immediate- 
 ly unite, and commonly with an explosion. 
 
 I placed a mixture of equal volumes of chlorine and 
 hydrogen in a vessel made of plate-glass, the edges of 
 the pieces being cemented together. This vessel was so 
 arranged on a small porcelain trough, containing a satu- 
 rated solution of common salt, that it could be used as a 
 gas-jar. The radiations of a lamp were caused to pass 
 through it, so as to be submitted to the selective absorp- 
 tion of the mixture. They were then received on a chlor- 
 hydrogen actinometer. 
 
 Successive experiments were then made : 1st, with the 
 radiations of a lamp after passing through the absorption 
 vessel ; 2d, with the same radiations after the vessel had 
 been removed. 
 
 Two facts were now apparent: 1st, the mixture of 
 chlorine and hydrogen in the absorption vessel began to 
 unite under the influence of the rays of the lamp ; 2d, 
 the rays which had passed through that mixture had lost 
 
426 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 very much of their chemical force. It was not totally 
 extinct, but the actinometer showed that it had under- 
 gone a very great diminution. 
 
 From this it follows that, on its passage through a 
 mixture of chlorine and hydrogen, the radiation had suf- 
 fered absorption, and as respects the mixture under trial 
 had become deactinized. Simultaneously the mixture 
 itself had been affected, its constituent gases uniting. 
 And thus it appears that the radiation had undergone a 
 change in producing a change in the ponderable matter. 
 
 The following modification of this experiment shows 
 the part played by the chlorine and hydrogen respective- 
 ly when they are in the act of uniting : 
 
 (a) The glass absorption vessel above described was 
 filled with atmospheric air, and the chemical force of the 
 radiation passing from the lamp through it was deter- 
 mined. It was measured by the time required to cause 
 the index of the actinometer to descend through one di- 
 vision. This was 12 seconds. 
 
 (&) The absorption vessel was now half filled with 
 chlorine, obtained from hydrochloric acid and peroxide 
 of manganese. The chemical force of the ray after pass- 
 ing through it was determined as before. It was now 
 represented by 25^ seconds. 
 
 (c) To the chlorine an equal volume of hydrogen was 
 added, the absorption vessel being consequently full of 
 the mixture. The radiation was now passing through a 
 stratum of chlorine diluted with hydrogen, and the point 
 to be determined was whether it had undergone the 
 same, or a greater, or less loss than in the preceding case, 
 since the chlorine was now uniting with the hydrogen. 
 On measuring the force, it was found to be represented 
 by 19 seconds. 
 
 (cl) Lastly, the first (ct) of these measures was re- 
 peated with a view of ascertaining whether the inten- 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 427 
 
 sity of the lamp had changed. It gave 12 seconds as 
 before. 
 
 From these observations it may be concluded that the 
 addition of hydrogen to chlorine does not increase its 
 absorptive power. Moreover, it is obvious that the ac- 
 tion of the radiation is expended primarily on the chlo- 
 rine, giving it a disposition to unite with the hydrogen, 
 and that the functions discharged by the chlorine and 
 by the hydrogen respectively are altogether different. 
 The ray itself also undergoes a change ; it suffers absorp- 
 tion and loss of a part of its vis viva. 
 
 As to the ray which is thus absorbed. In 1835 I 
 found that a radiation which had passed through a solu- 
 tion of potassium bichromate failed to accomplish the 
 union of chlorine and hydrogen ; but one which had 
 passed through ammonia sulphate of copper could do it 
 energetically. This indicates that the effective rays are 
 among the more refrangible. On exposing these gases 
 in the spectrum, the maximum action takes place in the 
 indigo rays (Philosophical Magazine, Dec., 1843), Memoir 
 XVIII. 
 
 Recently (1871) some suggestions have been made by 
 M. Budde respecting the action of light upon chlorine. 
 Admitting the correctness of the theorem that the mole- 
 cules of most elementary gases consist of two atoms, he 
 conceives that the effect of light on chlorine is to tend 
 to divide, or actually to divide, its molecule into isolated 
 atoms. These atoms, if the gas be kept in the dark, 
 may reunite into molecules. 
 
 The chlorine molecule cannot unite with hydrogen; 
 the chlorine atom can ; hence insolation brings on com- 
 bination. But if the chlorine be unmixed, there will, as 
 a consequence of insolation, be a certain proportion of 
 uncombined atoms; and from this, together with Avo- 
 gadro's theorem, is drawn the conclusion that this gas 
 
428 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 through insolation increases in specific volume. More- 
 over, as the reunion of the chlorine atoms probably 
 produces heat, rays of high refrangibility will cause 
 chlorine to expand ; but it will contract to its original 
 volume when no longer under the influence of light. 
 
 In corroboration of this conclusion, Budde found that 
 a differential thermometer filled with chlorine showed a 
 certain expansion when placed in the red or yellow rays, 
 but it gave an expansion six or seven times greater 
 when in the violet rays. With carbonic acid and ether 
 no such effect took place. 
 
 It should not be forgotten, however, in considering 
 the bearing of these experiments, that chlorine merely 
 because it is yellowish -green will absorb rays of a com- 
 plementary that is, of an indigo and violet color and 
 become heated thereby. 
 
 It has next to be determined whether the points of 
 maximum action that is, the points of maximum ab- 
 sorption correspond to the rays of emission of either or 
 both these gases, as they apparently ought to do under 
 Angstrom's law: "A gas when luminous emits rays of 
 light of the same refrangibility as those which it has the 
 power to absorb." 
 
 Of the four rays characteristic of hydrogen there is 
 one the wave-length of which is 4340. It is in the in- 
 digo space. 
 
 Pliicker gives for chlorine a ray nearly answering to 
 this. Its wave-length is 4338, and also another, 4346, the 
 latter being one of the best marked of the chlorine lines. 
 
 There are, therefore, rays in the indigo which are ab- 
 sorbed both by hydrogen and by chlorine. The place 
 of these rays in the spectrum corresponds to that in 
 which the gases unite the place of maximum action for 
 their mixture. 
 
 But the absorptive action of chlorine is not limited 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 409 
 
 to a few isolated lines. The gas removes a very large 
 portion of the spectrum. Subsequent experiments must 
 determine whether each of these lines of absorption is 
 also a line of maximum chemical action. 
 
 The chlor- hydrogen actinometer furnishes the means 
 of ascertaining many facts respecting the combination of 
 those substances advantageously, since it gives accurate 
 quantitative measures. 
 
 By referring to my papers in the Philosophical Mag- 
 azine (Dec., 1843; July, 1844; Nov., 1845; Nov., 1857) it 
 will be found that chlorine and hydrogen do not unite 
 in the dark at any ordinary temperature or in any 
 length of time ; but if exposed to a feeble radiation, such 
 as that of a lamp, they are strongly affected. The phe- 
 nomena present two phases: 1st, for a brief period there 
 is no recognizable chemical effect, a preliminary actiniza- 
 tion, or, as Professors Bunsen and Roscoe subsequently 
 termed it, photo - chemical induction, taking place (it is 
 manifested by an expansion and contraction of the mixt- 
 ure) ; 2d, the combination of the gases begins, it steadily 
 increases, and soon acquires uniformity. In obtaining 
 measures by the use of these gases we must, therefore, 
 wait until this preliminary actiuization is completed. 
 That accomplished, the hydrochloric acid arising from 
 the union of the gases is absorbed so quickly that the 
 movements of the index-liquid over the graduated scale 
 give trustworthy indications. 
 
 As regards the duration of the effect produced on the 
 gases by this preliminary actinization, I found that it 
 continued some time several hours (Philosophical Mag- 
 azine, July, 1844), Memoir XIX. Professors Bunsen and 
 Roscoe, however, in their Memoir read before the Royal 
 Society, state that it is quite transient (Transactions 
 of the Royal Society, 1856). 
 
 This preliminary actinization completed, the quantity 
 
430 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 of hydrochloric acid produced measures the quantity of 
 the acting radiation. This I proved by using a gas- 
 flame of standard height, and a measuring lens (page 
 265) consisting of a double convex, five inches in diame- 
 ter, sectors of which could be uncovered by the rotation 
 of pasteboard screens upon its centre, the quantity of hy- 
 drochloric acid produced in a given time being propor- 
 tional to the area of the sector uncovered. The same was 
 also proved by using a standard flame and exposing the 
 gases during different periods of time. The quantity 
 of hydrochloric acid produced is proportional to the time. 
 The following experiment illustrates the phenomena 
 arising during the actinization of a mixture of chlorine 
 
 o o 
 
 and hydrogen, and substantiates several of the foregoing 
 statements. 
 
 The diverging rays of a lamp were made parallel by a 
 suitable combination of convex lenses. In the resulting 
 beam a chlor - hydrogen actinometer was placed, there 
 being in front of it a metallic screen, so arranged that it 
 could be easily removed or replaced, and thus permit the 
 rays of the lamp to fall on the actinometer or intercept 
 them. 
 
 On removing the screen and allowing the rays to fall 
 on the sensitive mixture in the actinometer, an expansion 
 amounting to half a degree was observed. In 60 sec- 
 onds this expansion ceased. 
 
 The volume of the mixture now remained stationary, 
 no apparent change going on in it. At length, after the 
 close of 270 seconds, it was beginning to contract and 
 hydrochloric acid to form. 
 
 At the end of 45 seconds more a contraction of half a 
 degree had occurred: the volume of the mixture was 
 therefore now the same as when the experiment began, 
 this half degree of contraction compensating for the half 
 degree of expansion. 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 
 
 The rate of contraction of the gaseous mixture that 
 is, the rate at which its constituents were uniting was 
 then ascertained. 
 
 From these observations it appeared that when chlo- 
 rine and hydrogen unite, under the influence of a radia- 
 tion, there are four distinct periods of action : 
 
 1st. For a brief period the mixture expands. 
 
 2d. For a much longer period it then remains station- 
 ary in volume, though still absorbing rays. 
 
 3d. Contraction arising from the production of hydro- 
 chloric acid begins. At first it goes on slowly, then 
 more and more rapidly. 
 
 4th. After that contraction is fully established, it pro- 
 ceeds with uniformity, equal quantities of hydrochloric 
 acid being produced in equal times by the action of 
 equal quantities of the rays. 
 
 The prominent phenomena exhibited by a mixture of 
 chlorine and hydrogen are a preliminary absorption and 
 a subsequent definite action. 
 
 It may be remarked, since a similar preliminary ab- 
 sorption occurs in the case of other sensitive substances, 
 that there is in practical photography an advantage, both 
 as respects time and correctness in light and shadow, 
 gained by submitting a sensitive surface to a brief ex- 
 posure in a dim light, so as to pass it through its pre- 
 liminary stage. 
 
 The expansion referred to as taking place during the 
 first of these periods may be advantageously observed 
 when the disturbing radiation is very intense. It is well 
 seen when a Leyden-jar is discharged in the vicinity of 
 the actinometer. Though this light lasts but a very 
 small fraction of a second, it produces an instantaneous ex- 
 pansion, followed by an instantaneous contraction. Not 
 unfrequently the gases unite with an explosion. I have 
 had several of these instruments destroyed in that manner. 
 
432 
 
 CHEMICAL FOKCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 It might be supposed that this instantaneous expansion 
 is due to a heat disturbance arising from the absorption 
 of rays that are not engaged in producing the chemical 
 effect. But this interpretation seems to be incompatible 
 with the instantaneously following contraction. Though 
 it is admissible that heat should be instantaneously dis- 
 engaged by the preliminary actinization, it is difficult to 
 conceive how it can so instantaneously disappear. 
 
 When the radiation is withdrawn and the hydrochloric 
 acid absorbed, there is no after-combining. The action is 
 perfectly definite. For a given amount of chemical ac- 
 tion, an equivalent quantity of the radiation is absorbed. 
 
 The instances I have cited in this discussion of the 
 mode of action of radiations are, one of decomposition, in 
 the case of the silver iodide ; and one of combination, in 
 the case of hydrochloric acid. I might have introduced 
 another the dissociation of ferric oxalate, which I have 
 closely studied but it would have made the Memoir of 
 undue length. From the facts herein considered the fol- 
 lowing deductions may be drawn : 
 
 When a radiation impinges on a material substance, it 
 imparts to that substance more or less of its vis viva, and 
 therefore undergoes a change itself. The substance also 
 is disturbed. Its physical and chemical properties de- 
 termine the resulting phenomena. 
 
 (1st.) If the substance be black and undecomposable, 
 the radiation establishes vibrations among the molecules 
 it encounters. We interpret these vibrations as radiant 
 heat. The molecules of the medium do not lose the vis 
 viva they have acquired at once, since they are of greater 
 density than the ether. Each becomes a centre of agita- 
 tion, and heat-radiation and conduction in all directions 
 are the result. The undulations thus set up are com- 
 monly of longer waves ; and as the movements gradually 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 433 
 
 decline, the shorter waves of these are the first to be ex- 
 tinguished, the longer ones the last. This, therefore, is 
 in accordance with what I found to be the case in the 
 gradual warming of a solid body, in which the long waves 
 pertain to a low temperature, the short ones arising as 
 the temperature ascends (Memoir I.). 
 
 In some cases, however, instead of the disturbing un- 
 dulation giving rise to longer waves, it produces shorter 
 ones, as is shown when a platinum wire is put into a 
 hydrogen flame, or by Tyndall's experiments, in which 
 invisible undulations below the red give rise to the igni- 
 tion of platinum. 
 
 (2d.) If the substance be colored and undecomposable, 
 it will extinguish rays complementary to its own tint. 
 The temperature will rise correspondingly. 
 
 (3d.) If the substance be decomposable, those portions 
 of the radiation presented to it which are of a comple- 
 mentary tint will be extinguished. The force thus dis- 
 appearing will not be expended in establishing vibrations 
 in the arresting particles, but in breaking down the union 
 of those which have arrested them from associated par- 
 ticles. No vibrations, therefore, are originated ; no heat 
 is produced ; there is no lateral conduction. 
 
 In actinic decompositions the effects may be conven- 
 iently divided into two phases : 1st, physical ; 2d, chemi- 
 cal. The physical phase precedes the chemical. It con- 
 sists in a preliminary disturbance of the group of mol- 
 ecules about to be decomposed. Up to a certain point 
 the dislocation taking place may be retraced or reduced, 
 and things brought back to their original condition. 
 But that point once gained, decomposition ensues, and 
 the result is permanent. 
 
 I may perhaps illustrate this by a familiar example. 
 If a sheet of paper be held before a fire, its surface will 
 
 EE 
 
434 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 
 
 gradually warm ; and if the exposure be not too long or 
 the fire too hot, on removing .it the paper will gradually 
 cool, recovering its former condition without any perma- 
 nent change. One could conceive that the laws of ab- 
 sorption and radiation might not only be studied, but 
 again and again illustrated by the exposure and removal 
 of such a sheet. But a certain point of temperature or 
 exposure gained, the paper scorches that is, undergoes 
 chemical change and then there is no restoration, no re- 
 covery of its original condition. Hence it may be said 
 of such a sheet of paper that it exhibits two phases, in 
 the first of which a return to the original condition is 
 possible ; in the second such a return is impossible, be- 
 cause of the supervening of the chemical change. 
 
 An investigation of the facts produced by a ray pre- 
 sents, then, these two separate and distinct phases the 
 physical and the chemical. 
 
 General Conclusions. 
 
 The facts presented in the preceding and the present 
 Memoir suggest the following conclusions: 
 
 1st. That the concentration of heat heretofore observed 
 in the less refrangible portion of the prismatic spectrum 
 arises from the special action of the prism, and would not 
 be perceived in a diffraction spectrum. 
 
 2d. From the long-observed and unquestionable fact 
 that there is in the prismatic spectrum a gradual dimi- 
 nution in the heat-measures from a maximum below the 
 red to a minimum in the violet, coupled with the fact 
 now presented by me (that the heat of the upper half of 
 the spectrum is equal to that of the lower half), it follows 
 that the true distribution of heat throughout the spaces 
 of the spectrum is equal. In consequence of the equal ve- 
 locity of ether-waves, they will, on complete extinction by 
 a receiving surface, generate equal quantities of heat, no 
 
MEMOIR XXIX.] CHEMICAL FORCE IN THE SPECTRUM. 435 
 
 matter what their length may be, provided that their 
 extinction takes place without producing any chemical 
 effect. 
 
 3d. That it is incorrect to restrict to the upper portion 
 of the spectrum the property of producing chemical 
 changes. Such changes may be produced by waves of 
 any refrangibility. 
 
 4th. That every chemical effect observed in the spec- 
 trum is in consequence of the absorption of a specific 
 radiation, the absorbed or acting radiation being deter- 
 mined by the properties of the substance undergoing 
 change. 
 
 5th. That the figure so generally employed in works 
 on actino-chemistry to indicate the distribution of heat, 
 light, and actinism in the spectrum serves only to mis- 
 lead. The heat curve is determined by the action of the 
 prism, not by the properties of calorific radiations; the 
 actinic curve does not represent any special peculiarities 
 of the spectrum, but the habitudes of certain compounds 
 of silver. 
 
436 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 MEMOIR XXX. 
 
 ON BURNING GLASSES AND MIRRORS THEIR HEATING AND 
 CHEMICAL EFFECTS. 
 
 Collected and condensed from the Philosophical Magazine, May, 1851 ; Harper's 
 Monthly Magazine, No. 329. 
 
 CONTENTS : Can concentrated rays produce new chemical decompositions? 
 Effects of amplitude, frequency, and direction of vibration in the 
 ether-waves. Clock lenses for long exposures. Decomposition of water 
 by chlorine. Attempt to decompose it by bromine and iodine. Use of 
 absorbing troughs. Dry silver iodide not decomposed by light. De- 
 compositions under water. Decompositions in a spherical concave. 
 Effect of extraneous mixtures. They do not make collodion more sen- 
 sitive. Antagonization of radiations. Case of electric spark. Effects 
 of polarized light. Attempts to polarize light by an electro-magnet. 
 Mechanical cause of decompositions by light. 
 
 WHEN, many years ago, I commenced an experimental 
 examination of the chemical action of light, I entertained 
 great expectations of what might be accomplished by the 
 use of burning-glasses. It seemed reasonable to suppose 
 that if the direct sun -rays could occasion so many de- 
 compositions, their chemical force would be incompara- 
 bly greater when their brilliancy was exalted by a mirror 
 or a lens. Of the two, a concave metallic mirror should 
 produce a more characteristic effect, since it returns the 
 rays as it receives them, but a special and very impor- 
 tant portion of them is absorbed by the selective action 
 of the lens. 
 
 I had not, however, at that time the means of making 
 
 77 O 
 
 these experiments in a satisfactory manner, and, though 
 very much disappointed with the result, postponed the 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 437 
 
 prosecution of them to a more favorable opportunity. 
 Obtaining from time to time several isolated facts, I was 
 led, in meditating upon them, to what seemed to be some 
 general conclusions respecting the chemical action of ra- 
 diations. Several of these were published, in a desultory 
 manner, in various periodicals ; but it was not until May, 
 1851, that they were collected in the Philosophical Mag- 
 azine, under the title of a " Memoir on the Chemical Ac- 
 tion of Light." Of this, the following is an abstract. 
 
 The general discussion of the problem of the chemical 
 action of a ray involves the following considerations : 
 
 1. In what manner does the ray act, and what are the 
 changes it undergoes ? 
 
 2. What is the nature of the impression made on the 
 material group, the decomposition of which ensues ? 
 
 Many facts justify the supposition that the parts of all 
 material substances are in a state of incessant vibration. 
 To each particular thermometric degree there belongs a 
 particular frequency of vibration. As soon as these mo- 
 tions approach four hundred billions in a second, red 
 light is emitted, and the temperature is near 1000 Fahr. 
 As the frequency increases, rays of a higher refrangibility 
 are in succession evolved, and the temperature correspond- 
 ingly rises. On the other hand, when these oscillatory 
 movements decline, the temperature of the body falls. 
 
 These principles lead to a ready explanation of the 
 nature of the exchanges of heat and the cause of the 
 equilibrium of temperature. The vibratory molecular 
 motions are necessarily propagated to the ether, through 
 which medium they are again transferred to the particles 
 of other bodies, on which the ethereal waves impinge, as 
 a vibrating string excites undulations in the air, and 
 these, in their turn, can give birth to analogous motions 
 in other strings at a distance. 
 
 There is an analogy between the relations of a hot 
 
438 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 and cold body and those of two strings, one of which is 
 emitting a musical sound and compelling the other to 
 execute synchronous movements. The ether in the one 
 case and the air in the other are the media through which 
 their motions pass. 
 
 Equilibrium of temperature takes place when the mol- 
 ecules of the substances concerned are in synchronous 
 and equal vibration. A hot body in presence of a cold 
 one compels the latter to hasten its rate of motion, its 
 own rate all the time declining, and this continues until 
 both have the same frequency ; then equilibrium of tem- 
 perature results. The theory of the exchanges of heat is, 
 therefore, only an expression for the exchanges of vibra- 
 tions through the ether. 
 
 But temperature in thermotics is the equivalent term 
 for brilliancy in optics. Both refer to compound quali- 
 ties, depending not only on frequency of vibration, but 
 also on its amplitude. As the degree of heat of a mass 
 rises, the mass expands, the increase in its volume indi- 
 cating that not only do its parts vibrate more swiftly, 
 but also that their individual excursions are increased. 
 It follows, therefore, that every mass will have a deter- 
 minate volume for every degree of heat, the volume in- 
 creasing as the temperature rises. On this view the ex- 
 planation of the expansion of bodies by heat is that their 
 parts are not only vibrating more quickly, but also that 
 the individual excursions are greater. 
 
 The atoms of the chemical elements differ in weight. 
 We therefore should not expect that the ethereal vibra- 
 tions would throw them into movement with equal facil- 
 ity, but that some would yield more readily than others. 
 Is not this what we express in chemistry by the term 
 specific heat ? a body, the capacity of which is great, re- 
 quiring a prolonged application of ethereal pulses before 
 a consentaneous motion is reached, and in its turn im- 
 
MEMOIR XXX.] ON BURNING GLASSES AND M1HRORS. 439 
 
 pressing on the ether during cooling a correspondingly 
 prolonged series of motions. And is not this the cause 
 of that remarkable relation between the atomic weights 
 of elementary bodies and their specific heats, discovered 
 by Dulong and Petit ? 
 
 These considerations may lead us to inquire whether 
 the general cause of the decomposition of compound bod- 
 ies by radiations is due to the circumstance that all the 
 atoms of which their molecules are composed take on the 
 vibratory motion with unequal facility. Thus if a cer- 
 tain compound molecule be submitted to the influence of 
 an intense radiation, some of its constituent particles may 
 vibrate consentaneously at once, and others more tardily. 
 Under these circumstances, the continued existence of 
 the group may become impossible, and decomposition en- 
 sue in the necessity of the case. 
 
 In entering upon the experimental analysis of the ac- 
 tion of a ray upon a decomposable body, there are three 
 different points to be considered, so far as the ray itself 
 is concerned : 1. To what extent and in what manner is 
 the result affected by the intensity of the ray, or by the 
 amplitude of the vibrating excursions? 2. How is it af- 
 fected by the frequency of the pulsatory impressions? 
 and, 3. How by the direction in which the vibrations 
 are made, as involved in the idea of polarization? I 
 shall now examine these in succession. 
 
 1. To what extent and in what manner is the decompo- 
 sition of a compound body affected by the INTENSITY of a 
 ray or the amplitude of the vibrating excursions ? 
 
 If the different degrees of facility with which atoms 
 receive the impression of ethereal vibrations be the true 
 cause of decomposition by light, we should expect that 
 many such changes would become possible under the in- 
 fluence of a burning-lens which are not so in the direct 
 rays of the sun. 
 
440 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 This idea is favored by what we find in the case of heat. 
 The burning-glass has long had celebrity in that respect, 
 and in former times w^as the most powerful means of 
 reaching a high temperature. 
 
 The effect of the glass is due to the rapidity with 
 which it can supply caloric, contrasted with the loss by 
 conduction, radiation, etc. Thus an object of any kind 
 exposed to the sun receives heat at a certain rate ; but 
 it is simultaneously experiencing a loss by conduction, 
 radiation, and currents in the air. Exposed to the focus 
 of a lens, the supply becomes, in a given time, greater 
 than before ; and the temperature rising, great effects are 
 the necessary result. 
 
 But changes brought about by light are in a different 
 predicament. Here conduction is entirely absent, as is 
 also loss by currents in the air. The cumulative effects 
 of a long exposure give the same action as a highly con- 
 centrated ray furnishes in a brief period of time. In this 
 case, therefore, everything will depend on the absorptive 
 power of the substance. 
 
 When a piece of polished silver is placed in the focus 
 of a burning -lens, it remains quite cold, because of its 
 high reflecting power; but if blackened, it melts in an 
 instant. And so with chemical changes. A body which, 
 like chlorine, can exert an absorptive action on the ray 
 becomes modified, and induces changes; but if, like oxy- 
 gen, it has not that property, it will remain indifferent 
 and unaffected by the most intense radiation. 
 
 Considering, however, that the calorific effects of the 
 converged solar rays are so striking, we may reasonably 
 inquire whether, in like manner, the chemical action can 
 be increased. There is a very general impression that 
 the intense radiation of tropical climates accomplishes 
 changes which cannot be imitated by the feebler light 
 of higher latitudes, and perhaps decompositions may be 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 
 
 441 
 
 brought about by a large convex lens which the direct 
 rays of the sun are wholly inadequate to produce. 
 
 A very brilliant beam may possibly break up a given 
 combination, which a far greater quantity of light, acting 
 through a long period, might be inadequate to touch. 
 Sir R. Kane states that he, with M. Dumas, could remove 
 two atoms of hydrogen from acetone by the action of 
 chlorine in the sunshine at Paris, but in Dublin only 
 one. 
 
 In Fig. 90, a is a convex burning -lens supported in 
 ribbed frame, b b ; there is at c a second lens to hasten 
 
 Pig. 90. 
 
 the convergence ; dd, a cir- 
 cular arc for directing the 
 lens towards the sun ; e g, 
 a stand on which objects 
 may be exposed to the fo- 
 cal point, f. It is carried 
 by a stout bar, m n, attached to the frame. 
 
 By this instrument I endeavored to collect a series 
 of facts which might set this part of the question in 
 its true light. The lens a was of very fine and thin 
 French plate -glass, twelve inches in diameter in the 
 clear. Its goodness was such that on a fine day plat- 
 inum might be melted in its focus. It was ground 
 
442 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 and polished for me by the late Mr. Fitz, whose skill 
 was shown in the large and excellent telescopic objec- 
 tives that he made. He mounted it on a suitable sup- 
 port ; it required, however, to be guided by the hand as 
 the sun moved. When the college building of the Med- 
 ical Department of the University of New York was de- 
 stroyed by fire in 1865, 1 had to regret the loss of this 
 instrument, with much other apparatus, and many docu- 
 ments that were of un appreciable value to me. Mounted 
 as the lens was, its use was attended with considerable 
 risk to the eyes, on account of the excessive brilliancy of 
 the focus. Screens and dark spectacles were found to be 
 very unsatisfactory, and an illness which I consequently 
 contracted admonished me either to abandon the subject 
 or pursue it in some other way. 
 
 The following experiments were made with a smaller 
 glass, consisting of a combination of two similar lenses, 
 their diameter being five inches and focal distance eight. 
 It was, in fact, the large lens of an old-fashioned lucernal 
 microscope, such as was made in London a century ago. 
 I had it fixed on a polar axis, as shown in Fig. 91, and 
 by the aid of a clock it could follow the motion of the 
 sun with such accuracy that, when once set in the morn- 
 ing, an object might be exposed in its focus, if desirable, 
 for a whole day. It had a contrivance on the frame car- 
 rying the lens for supporting small crucibles, glass mat- 
 rasses (Fig. 92), charcoal supports, etc., at the proper 
 point, which might be either at the focus or at any other 
 distance from the lens, as the circumstances of the exper- 
 iment required. Among these instruments were ther- 
 mometers, blackened or otherwise so arranged as to ex- 
 ercise any desired selective absorption. At the outset 
 of any experiment, the whole face of the lens could be 
 covered with a blackened pasteboard screen, with a hole 
 half an inch in diameter. Through this a sufficient 
 
MEMOIK XXX.] ON BURNING GLASSES AND MIRRORS. 
 
 443 
 
 Fig. 91. 
 
 amount of light could be transmitted to enable one to 
 arrange the various details of the proposed experiment ; 
 and when everything was ready, the screen was re- 
 moved, and in the concentrated and brilliant focus the 
 action went on. I found that this simple contrivance 
 was an invaluable relief to the eyes. 
 
 In Fig. 91, a a is the heliostat clock; Z>, its polar axis; 
 d d, a frame carrying the lens, <?, and 
 having an arrangement at /for support- 
 ing flasks, crucibles, or other apparatus. 
 This turns on a double joint at <?, so 
 that the lens may be directed to the 
 sun. 
 
 In Fig. 92, a is a small flask receiving 
 the convertnncr rays, b, at their focus, 
 
 O O t/ I 7 7 i 
 
 The lens being five inches in di- 
 ameter, and the space covered by 
 the solar focal image, owing to 
 want of achromaticity and spherical aberration, one fifth 
 of an inch, the multiplying effect 
 would be 625 times, if the glass were 
 perfectly transparent, and there were 
 no loss by reflection from its sur- 
 faces. On a summer day of average 
 brightness, with the thermometer at 
 68 in the shade, and the bulb, not 
 being blackened, at 108 in the sun, 
 this lens could fuse copper instantly, the bead oxidizing 
 only superficially, and cutting readily after fusion. Black 
 oxide of copper in a little crucible of platinum foil melt- 
 ed into a slaty-looking substance at once. Wrought iron 
 did not melt alone; but if exposed on a charcoal support 
 in a globule of microcosmic salt, previously fused by the 
 lens, it gave a clear, round bead, which readily extended 
 when beaten upon an anvil. The globule of flux turned 
 
 Fig. 92. 
 
444 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 black. The specimen employed was cut from a piece of 
 good iron wire ; and though it might be thought that ex- 
 posure on the charcoal would tend to turn it into cast 
 iron, its subsequent complete malleability seems to dis- 
 prove this. Spongy platinum did not melt alone, nor 
 even if enclosed in a globule of fused microcosmic salt. 
 We may therefore estimate the working power of this 
 lens on a substance placed in its focus as somewhat 
 above the point of fusion of wrought iron, and lower 
 than the point of fusion of platinum. This refers to tem- 
 perature only. The power of the lens as to light must 
 be enormously greater. 
 
 We may now examine the chemical effects produced 
 by this lens. 
 
 Two small glass matrasses, the bulbs of which were 
 about half an inch in diameter, were filled with chlorine 
 water, the one being exposed to the direct rays of the 
 sun, the other to the converging rays of the lens. De- 
 composition of the water occurred in both, but with far 
 more activity in that placed in the focal point. The dif- 
 ference was at once so striking to the eye that I made no 
 attempt to measure it. It is plain that the greater the 
 quantity of incident light, the more rapid the decomposi- 
 tion ; though, after the first moment of action, the solu- 
 tions being no longer the same in constitution, the quan- 
 tities of gas disengaged are no longer proportional to the" 
 incident light. 
 
 There is thus no difficulty in effecting rapidly the de- 
 composition of water by chlorine under the influence of 
 the sun, but under the same circumstances iodine and 
 bromine are inadequate to produce such an effect. 
 
 A solution of bromine in water was prepared, the wa- 
 ter being first boiled to expel the air contained in it. It 
 was placed in a half-inch matrass (Fig. 93), and exposed 
 to the focus of the lens. As the temperature rose rap- 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 445 
 
 idly, the water was depressed in the bulb by the steam 
 and bromine vapor which occupied the upper part, the 
 bulb being placed uppermost, and the tube dipping into 
 a phial which served as a reservoir. After the exposure 
 had continued for two hours and a half, the matrass was 
 removed from the lens and suffered to cool. There re- 
 mained uncondensed a little bubble, measuring about yJ-Q 
 of a cubic inch ; but this was probably nothing more 
 than the atmospheric air which had found access to the 
 water, for on submitting the same specimen to another 
 exposure for three hours, after the gas had been decanted 
 from it, a little bubble, the diameter of which was esti- 
 mated at one fiftieth of an inch, was all that could be 
 procured. 
 
 In Fig. 93, a is the flask containing the bromine water ; 
 &, a phial serving as a reservoir. It is half filled 
 w 7 ith the same water. 
 
 In like manner I endeavored to decompose wa- 
 ter by iodine, and with the same negative result, 
 even when the exposure to the focal point lasted 
 four hours. When proper care had been taken 
 
 to remove from the solution all traces of air, no Fig. 93. 
 gas was evolved. 
 
 To reduce the heating effect of the lens, and allow the 
 more refrangible rays alone to act, there was interposed 
 between the lens and its focus a stratum of a solution of 
 sulphate of copper and ammonia one third of an inch 
 thick, included between two flat plates of glass, suitably 
 arranged and carried along with the other parts by the 
 movement of the clock. The cone of solar rays now 
 passed through this absorbent medium (Fig. 94). 
 
 In Fig. 94, #, 1) is the flask and reservoir, but the con- 
 verging rays pass through an absorbent trough, c c, 
 shown in front view at d, e being the circular cell con- 
 taining the blue solution. 
 
446 
 
 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 (I 
 
 Fig. 94. 
 
 In the focus of blue light thus formed there was ex- 
 posed for two and a half 
 hours (from 7| to 10J A.M., 
 June 13, 1848) an inverted 
 half- inch bulb containing 
 iodine water, with a few 
 particles of iodine. Tem- 
 perature in the shade, 64 ; 
 in the sun, 86. At the 
 end of that time there was 
 found an insignificant bub- 
 ble of air, estimated at one 
 thirtieth of an inch in diameter. It could, of course, be 
 nothing but atmospheric air.- 
 
 The absorbing medium was now removed, and the 
 full rays of the sun permitted to converge on the mat- 
 rass. The* temperature of the water quickly ran up to 
 the boiling-point, and the bulb was filled with steam 
 and the purple vapor of iodine. Everything seemed 
 favorable for the decomposition of the water to take 
 place, if the iodine could accomplish it under so intense 
 a radiation. At first I thought that the experiment had 
 succeeded, for the color of the bulb became paler a 
 result that ought to have ensued if hydriodic acid 
 was forming and oxygen being eliminated. The action, 
 therefore, was kept up for four hours ; but as soon as 
 the sun was screened from the lens and the bulb began 
 to cool, the water returned and filled it almost entirely. 
 This, therefore, shows that under a most intense radia- 
 tion iodine cannot decompose water. 
 
 A similar experiment was tried with bromine, and 
 with the same result. It failed to decompose water. 
 
 Some silver chloride, carefully purified, was exposed 
 in a little crucible of platinum foil (Fig. 95) so inclined 
 that the cone of rays could come in at its mouth. The 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 447 
 
 absorbing trough was not used. Though the sun's rays 
 were not brilliant, the chloride at once melted, forming a 
 reddish -looking liquid. It was kept in that condition 
 all day. When cool it proved to be in the state of 
 horn-silver, easily cut by a knife. When the rays first 
 touched it a fume was disengaged, due probably to the 
 escape of vapor of water. It seems, therefore, that this 
 substance when perfectly dry is not decomposable by 
 sunlight, though so sensitive at common temperatures 
 when moist. 
 
 In Fig. 95, a is the platinum crucible ; &, the place of the 
 material experimented upon, re- 
 ceiving at the focus the converg- 
 ing rays, c. 
 
 I must refer to the original 
 Memoir for the detail of numer- 
 ous experiments on many metal- 
 lic compounds, the general re- 
 sult of these being that, no mat- 
 ter how brilliant a ray may be, it cannot carry a decom- 
 position farther than a feeble one acting for a corre- 
 sponding longer period of time could do. Compounds 
 that can resist the force of an ordinary ray cannot be 
 broken down by the intense illumination of the focal 
 point of a burning -lens. That instrument cannot do 
 what the voltaic pile has done effect decompositions 
 which had never been effected before. 
 
 To reduce the disturbing effect of heat as far as pos- 
 sible, and give every advantage to the condensed lumi- 
 nous focus, I received the cone of rays coming from the 
 twelve -inch burning -lens on a glass globe (Fig. 96) six 
 inches in diameter, filled with water. This increased 
 the converging of the rays, and brought them more 
 quickly to a focus. Then through the neck of the globe 
 was introduced to the focus, in a matrass, spoon, or other 
 
448 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 suitable support, the substance to be experimented upon. 
 The mass of water kept the temperature down, and in 
 some cases the hot water was removed by an aspirator 
 and cold water introduced below. A spoon could be 
 used when powders were employed of so great a specific 
 gravity as not to drift too high from the focus in the 
 ascending current of hot water. 
 
 In Fig. 96, a a is a matrass filled with water, through 
 
 which come in the converging 
 rays, c. Through the neck at d 
 A ^ a spoon, b, may be passed down 
 
 to the focal point,/. 
 
 The result was, however, the 
 same as before. The focus of a 
 burning-lens cannot cause any 
 chemical change which the un- 
 
 converged sun-rays are incompetent to produce. It mere- 
 ly hastens the effect. 
 
 Upon the whole, we may therefore conclude that it is 
 not the intensity of a beam which determines its decom- 
 posing power, and that we cannot produce greater chem- 
 ical effects by the action of converging mirrors and lenses 
 than we can by the application of the simple sunbeam, 
 continued for an equivalent period of time. 
 
 In estimating the influence of light on different solu- 
 tions, we should constantly bear in mind that the maxi- 
 mum effect is never produced unless complete absorption 
 has taken place. When the color of a solution is pale, 
 it may require considerable thickness before complete 
 absorption is accomplished. Thus if two equal tubes, 
 containing equal quantities of the same solution of chlo- 
 rine in water, be exposed to the rays, they will evolve 
 equal quantities of oxygen gas; but if behind one of 
 them a piece of looking-glass be placed, the effect is im- 
 mediately increased. The rays that have passed through 
 
MEMOIR XXX.] ON BURNING GLASSES AND MlRIipll) /- 4.49 /^ 
 
 V^t/ ^ */ 
 
 the solution are compelled to cross it agaiif^ an4<if n$j 
 already exhausted, thus to act once more. The wfapvt- /' 
 ing illustrations are examples of the kind : 
 
 Two small bulbs of equal size containing chlorine/^ 
 water were exposed to the rays of the sun; behind one 
 of them a concave hemispherical mirror was placed, so 
 that the rays which had crossed the solution were com- 
 pelled to cross it again. The amount of oxygen set free 
 in this bulb was about one fourth greater than that in 
 the other. 
 
 The same was repeated, the exposure being to the sky 
 light instead of the sun-rays. The quantity of oxygen 
 set free in the two bulbs was as 18 to 55, 
 
 It might be supposed that part of this increased effect 
 is due to the rise of temperature, from the mirror ob- 
 structing radiation. To exert a cooling action the fol- 
 lowing modification was therefore tried. In a glass jar 
 (Fig. 97) full of quicksilver a half-inch bulb containing 
 chlorine water was placed in such a way that a small 
 portion of its surface, about one eighth of an inch in di- 
 ameter, projected above the surface of the liquid metal. 
 On this part the solar focus from a burning-glass was 
 thrown. The rays therefore gained access to the interior 
 of the bulb, and were thrown about in all directions, 
 crossing and recrossing the liquid in every way by the 
 numerous reflections they underwent the mercury, as it 
 applied itself to the outer surface of the glass, acting like 
 a spherical concave mirror, and, from its mass and high 
 conducting power, effectually keeping the temperature 
 down. The quantity of oxygen emitted in a given time 
 was measured. The same experiment was then repeated 
 with the bulb removed from the mercury. After the 
 close of the same time, on measuring the oxygen set free, 
 it was found that the reflecting action of the mercury 
 had nearly tripled the effect. 
 
 FF 
 
450 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX, 
 
 In Fig. 97, a a is the vessel filled with mercury ; I d, the 
 glass flask immersed in it, but having 
 at its upper part a small portion un- 
 covered, through which the converging 
 rays, <?, may come in. 
 
 The power of a ray thus depending 
 on the degree of absorption exerted 
 upon it, I was led to inquire whether, 
 by admixture with other suitable sub- 
 stances in a solution undergoing de- 
 composition, the effect could be increased. Chlorine wa- 
 ter decomposes more rapidly as its yellow tint is deeper. 
 Four equal bulbs were therefore taken a, containing 
 chlorine water; &, the same, deepened with chloride of 
 gold ; <?, chlorine water, with commercial hydrochloric 
 acid of a yellow tint; d, chlorine water, with tincture of 
 iodine. These were all exposed together to the sun. It 
 was at once obvious that a was giving off most oxygen, 
 arid eventually it was found that b yielded a much 
 smaller quantity, and c and d none at all. The presence 
 of these bodies, therefore, exerted a prejudicial effect. 
 
 A system of vibrating molecules will solicit an adja- 
 cent one to execute similar motions through the medium 
 of the intervening ether. A rise of temperature is due 
 to an increased rapidity or intensity of the oscillations 
 of the groups of vibrating molecules, but chemical de- 
 composition is due to the dislocation of their parts. It, 
 of course, by no means follows that when a compound 
 molecule is undergoing entire disruption, those in the 
 neighborhood should be compelled to pass into a similar 
 state. For the very reason that chemical decomposition 
 takes place is because the group that receives the pro- 
 voking ray cannot vibrate consentaneously with it ; and 
 if that group cannot assume the motion in question, how 
 can it possibly transmit it to any other? 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 45 1 
 
 Any artificial coloration by the addition of extraneous 
 bodies does not increase the rate of decomposition, but 
 retards it. This is precisely what ought to be expected. 
 A compound atom has its grouping destroyed by the 
 action of light upon its own parts, and is in no manner 
 concerned in what is taking place in other atoms around. 
 They therefore cannot increase the effect on it; but, on 
 the contrary, they may greatly diminish the action on 
 the mass by exerting a special absorption themselves. 
 Thus the chloride of gold retards the decomposition of 
 chlorine water, when mixed therewith, in the same man- 
 ner as if it were placed in a trough in front of the water, 
 and intercepted the impinging beam. 
 
 Experiments similar to the foregoing were made with 
 a solution of ferric oxalate mixed with alcohol, ammonia 
 citrate of iron, tincture of turmeric, sodic chloride, etc. 
 In every instance it was clear that the action of the light 
 is strictly molecular ; that it is impressed on the group of 
 atoms, and not on the mass; and that when various bod- 
 ies are conjointly exposed to the sun, each one undergoes 
 its own specific change, independently of and unaifected 
 by all the rest. 
 
 These experiments, with others of a like kind, made 
 many years ago, have an important bearing on some 
 recently published by Professor Vogel, Captain Abney, 
 Captain Waterhouse, and others on imparting increased 
 sensitiveness to collodion by mixing it with variously 
 colored substances. I repeated their experiments as care- 
 fully as I could, and should have thought that my want 
 of success was due to unskilfulness had I not borne in 
 mind the foregoing considerations. 
 
 2. We may next inquire, To what extent and m what 
 manner is the decomposition of a compound body affected 
 by the FREQUENCY of vibration of a ray ? 
 
452 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 From the beginning of optical chemistry, investigations 
 have been made for the purpose of determining the ac- 
 tion of rays of different refrangibHities. Almost a hun- 
 dred years ago it had been shown, in special cases, that 
 there is an antagonism between the opposite ends of the 
 spectrum. Thus the phosphorescence excited in Can- 
 ton's preparation by the violet end of the spectrum is ex- 
 tinguished by the red. As respects colored compounds, 
 Grotthus showed that the active ray is very commonly 
 of the tint complementary to that which it destroys. 
 
 More recently this branch of the subject has been ex- 
 amined to a great extent, and the behavior of all kinds 
 of substances in the solar spectrum made known. The 
 general result is this, that on wave-length or, what is 
 the same thing, frequency of vibration, the number of 
 impulses it can communicate in a given period of time 
 depends the power of a ray to break down the union of 
 any group of atoms. A compound that may resist a 
 slowly recurring motion may be unable to maintain it- 
 self when the impulses increase in frequency. 
 
 So numerous and well known are the photographic 
 and other changes brought on by light that I need not 
 occupy space with a description of them here. I shall 
 only refer to some curious instances of antagonism or in- 
 terference, the details of which will be found in the orig- 
 inal Memoir. Hitherto they have been very much over- 
 looked. 
 
 Two rays may be so placed in relation to each other 
 that their motions may conspire or may antagonize ; and 
 as one or other of these conditions ensues, the chemical 
 result will correspond. 
 
 When iodine vapor is permitted to have access to a 
 surface of polished silver, the silver tarnishes, the tar- 
 nished film increasing in thickness as the exposure to 
 the iodine is prolonged. It assumes in succession colors 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 453 
 
 which undoubtedly arise from the interference of the in- 
 cident light with the light reflected from the metal at 
 the back of the film. They are the colors of thin plates, 
 like those of a soap-bubble. 
 
 Now, there is a great difference in the action of light 
 upon these differently colored films, though chemically 
 they are the self-same silver iodide. Some have been un- 
 acted upon ; in them the effect of the incident light has 
 been destroyed or reversed by the effect of the light re- 
 flected from the back of the film. Some have been pow- 
 erfully acted upon ; in them the chemical effect is at a 
 maximum the incident and reflected rays have con- 
 spired. 
 
 If any proof were required that these maxima and 
 minima of chemical effect arise from the superposition of 
 similar or contrary motions, it is found in the relative 
 thickness of the films which have been acted or unacted 
 upon. Those in which there has been maximum action 
 have thicknesses as 2:1; that showing the minimum 
 action is 1J. 
 
 If the daylight and simple spectrum rays be permitted 
 to act together on a daguerreotype plate, the rays of 
 which the times of vibration are as 1,2, etc., aid the day- 
 light; but those of wiiich the times of vibration are as 
 i, 1|, 2|, etc., interfere with it and destroy its effect. 
 
 In these numbers we may discern the suggestion of 
 some very important facts. 
 
 One of the most striking instances of this positive and 
 negative action I discovered in the case of the electric 
 spark. Let there be placed over a daguerreotype plate 
 (Fig. 98) two metal balls, connected respectively with 
 the inside and outside of a Leyden-jar in such a way that 
 the discharge may pass from one of the balls at about 
 half an inch distance to the sensitive plate, and from the 
 sensitive plate to the other ball at about the same dis- 
 
454 
 
 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 tance. One spark is sufficient. The experiment should 
 
 be made in a dark room. 
 
 In Fig. 98, a a, the metal photographic plate ; Z>, <?, the 
 
 brass balls connected with 
 the Leyden - phial. The 
 spark passes between them 
 an( ^ *ke met al plate. At 
 dj e, the effect is shown, and 
 again more plainly in Fig. 
 99. 
 
 On developing it will be 
 
 Fig. 99. 
 
 found that on the point 
 which received the spark 
 there is a blue-white spot about one fortieth of an inch 
 in diameter (Fig. 99). Immediately around this an an- 
 nular space which is perfectly black, 
 the rays of the spark having there had 
 no action ; then follows a white ring, 
 and then another black one. Finally 
 succeeds a whitish stain of an indistinct 
 circular form, which can be traced by 
 inclining the plate as having a diame- 
 ter of about 1| inches. 
 
 That part of the plate from which the spark escaped 
 shows a repetition of the same phenomenon. 
 
 How shall we account for the production of these al- 
 ternate white and black spaces rings of action and in- 
 action ? Some persons might at first be led to suppose 
 that this is only an interesting form of Priestley's exper- 
 iment of u the fairy rings," formed by receiving the shock 
 of a battery on a polished steel surface, when, by the ox- 
 idation that ensues, a film is formed of variable thickness, 
 and giving the colors of thin plates. But a little con- 
 sideration will show that this is impossible, and the facts 
 are only to be explained on the principles of interference. 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 455 
 
 3. In loliat manner is tlw decomposition of a compound 
 body affected by tlie condition of POLARIZATION of the dis- 
 turbing ray ? 
 
 A beam of light passing through a circular aperture 
 one inch in diameter was received on the achromatic 
 lens of a camera-obscura, and then fell on a doubly re- 
 fracting prism, so placed as to give on the ground glass 
 two circular images of the aperture, one third of an inch 
 in diameter, and overlapping each other to a small ex- 
 tent. In these images the light was, of course, polarized 
 at right angles respectively. 
 
 When paper rendered sensitive by being washed with 
 ferric oxalate was placed so as to receive them the 
 light permitted to act nine minutes, and its effect devel- 
 oped by chloride of gold the images (Fig. 100) were 
 found of equal blackness, and the lenticular space formed 
 by their overlapping of greater depth. This was repeat- 
 ed with several different photographic compounds, and 
 ahvays with the same result. It shows that plane polar- 
 ized light acts precisely like common light, and with a 
 rapidity proportional to its intensity, 
 
 In Fig. 100, #, #, the disks of plane polarized light, po- 
 larized at right angles to each other; 
 at <?, the place of overlapping. 
 
 While thus attempting to detect a 
 difference in the decomposing action 
 of common and polarized light, I made 
 some inquiries as to the possibility of polarizing light by 
 a magnet. 
 
 A great many experiments have been made at differ- 
 ent times for the purpose of producing disturbance on a 
 ray of light by magnets. There are two methods which 
 may be resorted to. The one hitherto followed has been 
 to intercept the ray in its course, and submit it to mag- 
 netic action ; but the principle on which my attempts 
 
 100. 
 
456 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 have been founded is to attack it at its origin, and at- 
 tempt to produce an impression on the shining body. 
 These methods are essentially distinct. To borrow an 
 illustration from acoustics, it is one thing to try to 
 modify a sound on its passage through the air, and a 
 very different thing to exert an influence on the sound- 
 ing body. 
 
 When Bancalari's experiment on the influence of the 
 poles of a powerful magnet on a flame was first publish- 
 ed, I repeated it at once, expecting that the oscillations 
 of the shining particles were constrained to take place in 
 one plane by the magnetism, and that the light emitted 
 would be polarized. The result, however, did not seem 
 to prove this. 
 
 A similar experiment was then made with the electric 
 spark from the prime conductor of a machine. It was 
 compelled to cross between the poles of a powerful elec- 
 tro-magnet. But when the magnetism was on it did not 
 seem that the light was polarized. 
 
 De la Eive has shown that the voltaic arc between 
 charcoal points is greatly disturbed when it passes be- 
 tween the poles of a powerful electro-magnet. In the 
 hope that this would produce the expected disturbance, 
 I examined an arc formed between points of copper, plat- 
 inum, and gas carbon ; but though the sounds emitted 
 were strong, resembling the sudden tearing of a piece of 
 cloth, I could not perceive that the light was polarized. 
 
 In like manner the induction spark from a contact- 
 breaker and the phosphorescent light from fluor-spar were 
 tried without success. I still think, however, that with 
 better means than those thus employed the experiment 
 would succeed. 
 
 At the commencement of this Memoir it was stated 
 that we should consider, 1st, the manner in which a ray 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 45 f 
 
 of light acts in bringing about decomposition, and the 
 changes it undergoes ; 2d, the nature of the impression 
 made on the material group, the decomposition of which 
 ensues. The observations I proposed offering in relation 
 to the former of these points being completed, I may pass 
 to some remarks respecting the latter. 
 
 An examination of many cases of the decomposition 
 of bodies by light has led me to the conclusion that its 
 cause is to be attributed to the inability of the group of 
 molecules affected to withstand the periodic impulses 
 communicated to them. Of those molecules some, per- 
 haps, take on a vibratory motion more readily than the 
 others, and the continuance of a given group becoming 
 impossible, a rearrangement ensues. 
 
 But in other cases the mechanism of decomposition is 
 undoubtedly different : a change is impressed on one of 
 the elements acted upon, which weakens its affinity for 
 the others. Thus, under the influence of the sunshine, 
 plants can decompose many bodies, such as carbonic, sul- 
 phuric, and phosphoric acids. 
 
 The nature of these changes may be best illustrated by 
 tracing the complete course through which any one of 
 these substances passes. The chief facts may be seen in 
 the case of phosphorus. This substance, when freshly 
 made, commonly exhibits a white waxy appearance, but 
 when exposed to the sunshine, it turns to a deep mahog- 
 any-red. If the exposure has been long continued, or the 
 effect hastened by the action of a burning-lens, the change 
 of aspect is very striking. It is analogous to that which 
 sulphur exhibits when heated to 400 or 500. I have a 
 specimen which has been kept for many years in an at- 
 mosphere of dry carbonic acid ; the sides of the vessel 
 are incrusted with crystals, which have slowly sublimed, 
 and which in color resemble the ferrid cyanide of po- 
 tassium. 
 
458 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 The chemical properties of these two varieties of phos- 
 phorus are very different; indeed, there is scarcely a 
 point in which they may not be said to be unlike. The 
 common kind shines in the dark ; the red does not. The 
 common is soluble in a variety of menstrua, which do not 
 act on the other : thus one of the methods of preparing 
 red phosphorus is to expose a solution of the common in 
 sulphuric ether to light a red powder, the substance in 
 question, precipitates. Compared together, the one dis- 
 plays a range of affinity which the other does not ; nor do 
 these properties seem to leave them when they are united 
 with other bodies. Thus the active or white phosphorus, 
 when united with hydrogen, yields a gas which is spon- 
 taneously combustible in the air; the red or passive va- 
 riety yields a hydrogen compound of the same constitu- 
 tion, but devoid of the property of spontaneous combus- 
 tibility. 
 
 It should be understood that though other agents as 
 a high temperature can impress this remarkable change 
 upon phosphorus, none can do it with more energy or 
 more completely than the solar rays. I found by expos- 
 ing a surface of white or active phosphorus to the pris- 
 matic spectrum that it is the more refrangible rays that 
 are the most effective. Thus the rays most efficient in set- 
 ting oxygen free from the bodies with which it is united 
 have also the quality of impressing such a change on 
 those bodies that they oxidize subsequently with diffi- 
 culty. It follows that the true cause of such decompo- 
 sitions is the impression which the light makes on the 
 elementary substance. Thus if phosphoric acid be de- 
 composed by the solar rays, the decomposition is owing 
 to the phosphorus being thrown into the red or passive 
 state a state in which its affinity for oxygen has almost 
 entirely disappeared. 
 
 These considerations enable us to explain what takes 
 
MEMOIR XXX.] ON BURNING GLASSES AND MIRRORS. 459 
 
 place in the economy of plants. The water of the soil is 
 always charged with carbonic acid, which communicates 
 to it the quality of dissolving bone-earth. The solution 
 passing through the spongioles goes to the leaves as as- 
 cending sap. Here it is exposed to light, the effect of 
 which is, aided by the cell-growth there taking place, to 
 set the phosphoric acid free, and turn its phosphorus into 
 the passive state. Its continued union with oxygen as 
 an acid compound thus becomes impossible, and it is 
 now associated with the proteine and oily bodies form- 
 ing in the plant. Nor does it again unite with oxygen 
 until it has passed into the systems of animals as a con- 
 stituent of their nervous and muscular tissues. At the 
 moment of activity of these, and especially of the former, 
 it is oxidized, the change being apparently an immediate 
 consequence of that activity, and, reverting to the acid 
 state, it is finally dismissed from the system under the 
 form of phosphate of soda and ammonia. 
 
 In the same manner might be explained the decompo- 
 sition of carbonic acid by plants in the sunshine ; for car- 
 bon, like phosphorus, and, indeed, like all other elemen- 
 tary bodies, has its active and passive states, as is exem- 
 plified in the contrast between diamond and lamp-black. 
 The sunlight enables the leaves of plants to bring the 
 carbon into the inactive state, and decomposition ensues 
 as a secondary result. The carbon compounds arising 
 form the food of various animals; nor does this element 
 recover its active state until it has given rise to the proc- 
 esses of life, when it suddenly unites with oxygen brought 
 by the arterial blood, and the compounds it then forms 
 are dismissed from the system by the lun^s and kidneys 
 
 / / V 
 
 conjointly. It might seem that the mechanism of decom- 
 position by vibratory movement is essentially different 
 from that by these allotropic changes, but a more detail- 
 ed examination will show that this is not necessarily so. 
 
460 ON BURNING GLASSES AND MIRRORS. [MEMOIR XXX. 
 
 In this Memoir I have endeavored to examine how 
 far the decomposing action of a radiation is dependent 
 on the amplitude, the frequency, or the direction of its 
 vibrations. The result arrived at is that decompositions 
 are not determined by amplitude that is, brilliancy 
 since a faint light continued long enough can produce 
 precisely the same effect as the more concentrated ray of 
 a burning-lens applied for a shorter time. Nor does the 
 direction of motion, as involved in the idea of polariza- 
 tion, whether plane or circular, exert any effect, but it is 
 the frequency of the periodic impulses that is the sole 
 determining cause. And the phenomena of interference 
 from the superposition of such small motions occur ex- 
 actly as might have been predicted. 
 
 The immediate cause assigned for such decompositions 
 is that a ray forcing the material particles on which it 
 falls into a state of rapid vibration, it comes to pass in 
 many compound molecules that their constituent atoms 
 can no longer exist together as the same group, because 
 of the impossibility of their being animated by consenta- 
 neous or conspiring motions, and dislocation, rearrange- 
 ment, or decomposition is the result. 
 
 In this Memoir I have spoken of heat and light as 
 though they were distinct agencies, and considered such 
 facts as conductibility, etc., displayed by the one and not 
 by the other. But if we recall what has been said in 
 preceding Memoirs to the effect that these are only modes 
 of motion, and that the difference of the effects they dis- 
 play turns on the character of the receiving surface or 
 substance, there will be no difficulty in translating this 
 commoner language into terms that are more exact, and 
 in presenting the phenomena in question under a more 
 rigidly scientific point of view. Familiar expressions 
 very frequently convey to the mind clearer ideas than 
 others which, perhaps, may be more strictly correct. 
 
r AWARDED ^ 
 BY THE ^ 
 
 ^'AMERICAN ACADEMY Vj 
 
 J OF \ 
 
 ^ARTS AND SCIENCES^! 
 
 TO O.|| 
 
 i JOHN W f LLiAM DRAPiR, 31 
 
 ^ FO R ^W 
 
 RESEARCHES 01* 
 
 L '&! 
 
 THE KUMFORD MEDAL. 
 
APPENDIX. 
 
 " In 1796, Count Rumford presented to the Royal Society of London 
 1000, the interest of which was to be spent in striking two medals, 
 both in the same die, one of gold and one of silver, of the value of the 
 interest of the donation for two years, and to be given biennially for the 
 most important discovery or improvement relating to light and heat that 
 had been made during the preceding two years in any part of Europe." 
 
 At the same time Count Rumford made " a corresponding donation 
 to the American Academy of Arts and Science of $5000, the interest of 
 which was to bo used in like manner as regards American discoveries. 
 It was provided that if this term passed without any discovery or im- 
 provement being made that should be deemed worthy of the award, the 
 accruing interest was to be added to the principal, and the augmented 
 income thus arising was to be added to the medals when the next award 
 was made." 
 
 The Royal Society accepted the trust, and made the first award to 
 Rumford himself in 1802; in 1804, to Leslie; in 1806, to Murdock ; 
 in 1810, to Malus; in 1814, to Wells; in 1816, to Davy; in 1818, to 
 Brewster; in 1824, to Fresnel; in 1834, to Melloni; in 1838, to Forbes; 
 in 1840, to Biot ; in 1842, to Fox-Talbot ; in 1846, to Faraday ; in 1848, 
 to Regnault ; in 1850, to Arago ; in 1852, to Stokes ; in 1854, to Arnott; 
 in 1856, to Pasteur; in 1858, to Jamin ; in 1860, to Clerk-Maxwell; in 
 1862, to Kirchhoff; in 1864, to Tyndall ; in 1866, to Fizeau; in 1868, 
 to Balfour Stewart, etc. 
 
 These awards covered the most important discoveries that had been 
 made in relation to heat and light the production of heat by friction, 
 the correlation of forces, the effects of surfaces on radiation, the polari- 
 zation of light, the theory of the formation of dew, the transverse vi- 
 brations of light, the application of the thermopile to the study of radi- 
 ant heat, the refraction of dark rays, the invention of photography, the 
 discovery of fluorescence, the fixed lines of the spectrum, the distribution 
 of heat in the spectrum, the velocity of light, etc. 
 
 The American Academy permitted several years to pass without mak- 
 ing any award. Meantime the fund had so greatly increased that the 
 
464 APPENDIX. 
 
 Academy had to apply to the Legislature for authority to depart from 
 the strict letter of the endowment, and use the funds with more freedom 
 in the interest of advancing knowledge. In 1839, the Academy gave 
 from the interest of the fund the sum of $600 to Hare for the invention 
 of the oxyhydrogen blowpipe and improvements in galvanic apparatus. 
 In 1862, it awarded the medal to Ericsson for his caloric engine; in 
 1865, to Treadwell for improvements in the management of heat; in 
 1867, to Alvan Clark for improvements in the lenses of refracting tele- 
 scopes ; in 1870, to Corliss for improvements in steam-engines. The fund 
 had now reached $42,000. It will be seen that the American awards 
 had been mainly for inventions ; the English for discoveries. 
 
 At the six hundred and eighty-ninth meeting of the American Acad- 
 emy, held March 8, 1876, the chairman of the Rumford committee in- 
 troduced the special business of the evening, and handed to the presi- 
 dent, Hon. Charles Francis Adams, the Rumford medals (in gold and 
 silver), on each of which had been engraved the following inscription : 
 " Awarded by the American Academy of Arts and Science to John 
 William Draper for his Researches in Radiant Energy, May 25, 1875." 
 
 In presenting the medals, the president gave a brief history of the 
 fund, and announced that, " after a careful review of the services of Pro- 
 fessor Draper in this great field of inquiry, the committee having the 
 subject in their charge have, for reasons given by them, recommended 
 through their chairman that the medals prescribed in the deed of trust 
 should be presented to him, as having fully deserved them." 
 
 The president then recapitulated some of these reasons : 
 
 "In 1840, Dr. Draper independently discovered the peculiar phenom- 
 ena commonly known as Moser's images, which are formed when a medal 
 or coin is placed upon a polished surface of glass or metal. These im- 
 ages remain, as it were, latent, until a vapor is allowed to condense upon 
 the surface, when the image is developed and becomes visible. 
 
 " At a later period he devised the method of measuring the intensity 
 of the chemical action of light, afterwards perfected and employed by 
 Bunsen and Roscoe in their elaborate investigations. This method con- 
 sists in exposing to the source of light a mixture of equal volumes of 
 chlorine and hydrogen gases. Combination takes place more or less 
 rapidly, and the intensity of the chemical action of the light is measured 
 by the diminution in volume. No other known method compares with 
 this in accuracy, and most valuable results have been obtained by its 
 use. 
 
 "In an elaborate investigation, published in 1847, Dr. Draper estab- 
 lished experimentally the following important facts : 
 
APPENDIX. 465 
 
 " 1. All solid substances, and probably liquids, become incandescent at 
 the same temperature. 
 
 "2. The thermometric point at which substances become red-hot is 
 about 977 Fahr. 
 
 " 3. The spectrum of an incandescent solid is continuous ; it contains 
 neither bright nor dark fixed lines. 
 
 "4. From common temperatures nearly up to 977 Fahr., the rays 
 emitted by a solid are invisible. At that temperature they are red, and 
 the heat of the incandescing body being made continuously to increase, 
 other rays are added, increasing in refrangibility as the temperature 
 rises. 
 
 " 5. While the addition of rays, so much the more refrangible as the 
 temperature is higher, is taking place, there is an increase in the in- 
 tensity of those already existing. Thirteen years afterwards Kirchhoff 
 published his celebrated memoir on the relations between the coeffi- 
 cients of emission and absorption of bodies for light and heat, in which 
 he established mathematically the same facts, and announced them as 
 new. 
 
 " 6. Dr. Draper claims, and we believe with justice, to have been the 
 first to apply the daguerreotype process to taking portraits. 
 
 " 7. Dr. Draper applied ruled glasses and specula to produce spectra 
 for the study of the chemical action of light. The employment of 
 ruled metallic specula for this purpose enabled him to avoid the ab- 
 sorbent action of glass and other transparent media, as well as to es- 
 tablish the points of maximum and minimum intensity with reference to 
 portions of the spectrum defined by their wave-lengths. He obtained 
 also the advantage of employing a normal spectrum in place of one 
 which is abnormally condensed at one end and expanded at the other. 
 
 " 8. We owe to him valuable and original researches on the nature of 
 the rays absorbed in the growth of plants in sunlight. These researches 
 prove that the maximum action is produced by the yellow rays, and they 
 have been fully confirmed by more recent investigations. 
 
 " 9. We owe to him, further, an elaborate discussion of the chemical 
 action of light, supported in a great measure by his own experiments, 
 and proving conclusively, and, as we believe, for the first time, that rays 
 of all wave-lengths are capable of producing chemical changes, and that 
 too little account has hitherto been taken of the nature of the substance 
 in which the decomposition is produced. 
 
 " 10. Finally, Dr. Draper has recently published researches on the dis- 
 tribution of heat in the spectrum, which are of the highest interest, and 
 which have largely contributed to the advancement of our knowledge 
 of the subject of radiant energy. 
 
 G G 
 
46(3 APPENDIX. 
 
 " And now, in the absence of Dr. Draper, unable at this inclement 
 season to execute a fatiguing journey, it gives me pleasure to recognize 
 you, Mr. Quincy, as his worthy and competent representative. 
 
 " I pray you, in receiving these two medals on his behalf, in accordance 
 with the terms of the original trust, to assure him, on the part of the 
 Academy, of the high satisfaction taken by all its Fellows in doing 
 honor to those who, like him, take a prominent rank in the advance of 
 science throughout the world." 
 
 Mr. Quincy, on receiving the medals, said : 
 
 " MR. PRESIDENT, In the name and on the behalf of Dr. Draper, I 
 have the honor to receive the Rumford medals in gold and silver which 
 the Academy has been pleased to award to him, and I will have them safe- 
 ly conveyed to him to-morrow, together with the assurances of the sat- 
 isfaction of the Academy in this action which you wish me to com- 
 municate to him. In common with yourself, sir, and all the Fellows 
 present, I regret that that eminent person is unable to attend this meet- 
 ing and receive the medals himself. And, personally, I regret the ab- 
 sence of Dr. Wolcott Gibbs, who had promised to perform this grate- 
 ful service for his friend, and who would have been able to make a more 
 suitable reply to the able discourse with which you have accompanied the 
 presentation of the medals, and to have done more justice to the claims 
 of Dr. Draper to this distinction, than I can pretend to do. Dr. Gibbs 
 having also been unavoidably prevented from being present this evening, 
 I have now the honor to read a communication from Dr. Draper to the 
 Academy, in acknowledgment of this testimony to his services to science." 
 
 Mr. Quincy then read the following letter : 
 
 " To THE AMERICAN ACADEMY OF ARTS AND SCIENCES : Your favor- 
 able appreciation of my researches on radiations, expressed to-day by the 
 award of the Rumford medals the highest testimonial of approbation 
 that American science has to bestow on those who have devoted them- 
 selves to the enlargement of knowledge is to me a most acceptable re- 
 turn for the attention I have given to that subject through a period of 
 more than forty years, and I deeply regret that through ill-health I am 
 unable to receive it in person. 
 
 " Sir David Bre water, to whom science is under so many obligations 
 for the discoveries he made, once said to me that the solar spectrum is a 
 world in itself, and that the study of it will never be completed. His 
 remark is perfectly just. 
 
 " But the spectrum is only a single manifestation of that infinite ether 
 which makes known to us the presence of the universe, and in which 
 whatever exists if I may be permitted to say so lives and moves and 
 has its being. 
 
APPENDIX. 
 
 467 
 
 " What object, then, can be offered to us more worthy of contempla- 
 tion than the attributes of this intermedium between ourselves and the 
 outer world ? 
 
 " Its existence, the modes of motion through it, its transverse vibra- 
 tions, their creation of the ideas of light and colors in the mind, the in- 
 terferences of its waves, polarization, the conception of radiations and 
 their physical and chemical effects these have occupied the thoughts of 
 men of the highest order. The observational powers of science have 
 been greatly extended through the consequent invention of those grand 
 instruments, the telescope, the microscope, the spectrometer. Through 
 these we have obtained more majestic views of the nature of the uni- 
 verse. Through these we are able to contemplate the structure and 
 genesis of other systems of worlds, and are gathering information as to 
 the chemical constitution and history of the stars. 
 
 " In this noble advancement of science you, through some of your 
 members, have taken no inconspicuous part. It adds impressively to 
 the honor you have this day conferred on me that your action is the 
 deliberate determination of competent, severe, impartial judges. I can- 
 not adequately express my feelings of gratitude in such a presence, pub- 
 licly pronouncing its approval on what I have done. 
 " I am, gentlemen, very truly yours, 
 
 " JOHN W. DRAPER." 
 
 The investigations and memoirs referred to by the committee of tin- 
 Academy are contained in this volume. 
 
INDEX. 
 
 A. 
 
 Absorbent media, action of 197 
 
 Absorption of chemical rays 231 
 
 of radiant heat 200 
 
 " of radiations, necessity of. 186 
 
 Achromaticity of camera 210 
 
 Actinic rays 130-132 
 
 Adhesion and capillary attraction. . . 346 
 
 Air in interior of flame 68 
 
 Albertus Magnus on phosphores- 
 cence 135,159 
 
 Alchemy 313-315 
 
 Alcohol flame spectrum, analysis of. 55 
 
 Allotropism in living beings 375 
 
 its successive stages 310 
 
 of chlorine 284 
 
 Analogies of heat and chemical rays. 230 
 
 Angstrom's discovery 186 
 
 law 428 
 
 Animal and flame, analogy of. 189 
 
 Animals, circulation in 370 
 
 Antagonism in spectra 98 
 
 Anthracite, burning, analysis of 57 
 
 Arseniuretted hydrogen 55 
 
 Arterialization of blood 378 
 
 Atmospheric lines 75 
 
 Attraction, capillary 342 
 
 B. 
 
 Baldwin's phosphorus 134 
 
 Bancalari's experiment 456 
 
 Becquerel on rayons continuateurs. . . 278 
 
 k ' on ultra red lines 79 
 
 Bending of stems of plants 413 
 
 Berzelius on allotropism 376 
 
 " on carbon compounds 303 
 
 Bitumens and resins 408 
 
 Blood, arterialization of 378 
 
 Blow-pipe flame 68-69 
 
 Bolognian stone 1 34, 166 
 
 Boracic acid, spectrum of 55 
 
 Bouguer's photometric method.. . 39, 156 
 " " " criticised 
 
 byMel- 
 loni.... 49 
 Boyle, experiments of, on diamond. . 135 
 
 " on movements in solids 343 
 
 Brewster's experiments, criticism on. 36 
 
 Bromide of silver as test 202 
 
 Bromine cannot decompose water. . . 445 
 
 Bubbles, soap, endosmosis through. . 356 
 
 Budde on chlorine 427 
 
 Bunsen and Roscoe, experiments of. 429 
 
 " quoted 263 
 
 Burning glasses and mirrors . . . 436-441 
 
 " driven by clock 443 
 
 C. 
 
 Calcium light, radiations of 320 
 
 Calorific hypothesis of vision 1 02 
 
 Camphor, experiments with 203 
 
 Canton's phosphorus 136 
 
 Capillary attraction 342 
 
 " " an electric phe- 
 nomenon 346 
 
 Carbon, allotropism of 285 
 
 " nucleus in vision 105 
 
 " solid, disengaged in flame .. 60 
 Carbonic acid decomposed by color- 
 ed solutions 172 
 
 " decomposed by light.. 167 
 " decomposed in spec- 
 trum 169, 410 
 
 Carbonic oxide, spectrum analysis 
 
 of * 54,66 
 
 Carbuncle, legend of. 133 
 
 Chemical force, distribution of, in 
 
 spectrum 404 
 
 " hypothesis of vision 105 
 
 " radiations, absorption of. . 201 
 " rays, action of silver iodide 
 
 on 236 
 
 Chlor-hydrogen photometer 245 
 
 Chlorine and hydrogen, relation to 
 
 light 248, 282, 425, 431 
 
 Chlorine, active and inactive 271 
 
 " duration of its modifica- 
 tion 273,283,313 
 
 indigo rays absorbed by. . . 276 
 
 its allotropism 2S4 
 
 modified 271 
 
 substitutes hydrogen 306 
 
 two stages in its modifica- 
 tion 274 
 
 " water decomposed by 286 
 
 Chlorophane, its phosphorescent prop- 
 erty 141 
 
470 
 
 INDEX. 
 
 Chlorophyl 410 
 
 Choroid, function of 10G 
 
 Christie on magnetism of light 192 
 
 Circulation in animals 370 
 
 in plants 367 
 
 Claudet on red rays 422 
 
 Clayton's phosphorescing diamond. . 135 
 
 Coins, slow movement in 344 
 
 Colored flames, nature of 63 
 
 Congestion, explanation of. 381 
 
 Conical refraction 117 
 
 Conservation of force 186 
 
 Continuous spectra of ignited solids. 32 
 Cooling, law of 37 
 
 " of the earth f>0 
 
 Correlation of force 186 
 
 Cracks, movement of liquids in 344 
 
 Crevice, movement of liquids in 344 
 
 Croll on age of earth 50 
 
 Currents, electrical, in water 351 
 
 " in capillary tubes 345 
 
 Cyanogen flame spectrum, analysis 
 of 55, 65, 67 
 
 D. 
 
 Daguerreotype, condition of surface. 222 
 
 " engraving of. 229 
 
 fading of... 233 
 
 " plate, its properties. 89 
 
 " process of 206-208 
 
 Dark heat rays 131 
 
 " spaces of spectrum 124 
 
 Darkness, spectrum of 92 
 
 Daubeny's experiments 167 
 
 Davy on flame 53 
 
 Daylight in spectrum analysis 56, 91 
 
 Deoxidizing rays 183 
 
 Diamond, effects of heat in its phos- 
 phorescence 160 
 
 " phosphorescence of 140 
 
 Diffraction and dispersion spec- 
 tra. 386, 416 
 
 Diffraction spectrum, advantages of. . 95 
 how obtained. . 97 
 
 laws of 120 
 
 photograph- 
 ed.. 112,113,126 
 
 studies in 115 
 
 what it is 115 
 
 Distribution of colors in spectrum. . . 121 
 
 of heat in spectrum 99 
 
 Dove, experiments of. 81 
 
 Draper, H., on diffraction-spectrum 
 
 photographs 125 
 
 " " on fixed lines 80 
 
 Dufay on Bolognian stone 161 
 
 ** on diamonds 137 
 
 " on theory of phosphorescence. 137 
 
 Dulong and Petit, law of cooling 37 
 
 Dumas, experiment by 308 
 
 Dumas, theory of substitution of. 304 
 
 Dutrochet on* bending of roots 413 
 
 " on endosmosis 352 
 
 E. 
 Earth, age of. 50 
 
 Eisenlohr's experiments 97 
 
 Electric spark, interference of 454 
 
 " its phosphorescent 
 
 rays 321 
 
 Electromotive power of heat 324 
 
 Endosmosis 352 
 
 " decompositions by 355 
 
 " force of. ." 362 
 
 " force of through India- 
 rubber 359 
 
 Engraving of daguerreotypes 229 
 
 Etiolated plants turn green 410 
 
 Eye and ear, analogies of 123 
 
 F. 
 
 Fading of daguerreotypes 233 
 
 Ferric oxaiate for photographs 266 
 
 ' 4 its properties 265 
 
 Fishes, circulation in 373 
 
 Fixed lines, atmospheric 75 
 
 " " identification of 77 
 
 " " nature of. 82 
 
 " " ultra red 77 
 
 " " ultraviolet 74 
 
 Fixing of daguerreotype 212 
 
 Flame and animals, analogy of. 189 
 
 " Heumann on 60 
 
 " passes into nonentity 190 
 
 Flames composed of shells 59, 6 
 
 " spectrum analyses of 52, 64 
 
 Flowers, their colors 411 
 
 Fluorescence, spectrum examined by. 124 
 Fluor-spar, its phosphorescent quali- 
 ties 141 
 
 Force derived from the sun 187 
 
 " included in plants 177 
 
 Foucault and Fizeau, their experi- 
 ments 421 
 
 " on fixed lines 78 
 " " " on wave the- 
 ory 117 
 
 Frankenheim on allotropism 377 
 
 Frankland on flame 60 
 
 Fraunhofer lines not in ignited solids. 31 
 Frequency of undulation, effect 
 
 of 439, 452 
 
 Fresnel on transverse vibration 116 
 
 G. 
 
 Gardner on bending of stems 413 
 
 Germination of seeds 184 
 
 Grating 35 
 
 " Rutherfurd's 117 
 
 Grotthus, his law 412 
 
INDEX. 
 
 471 
 
 Growth of plants in darkness 177 
 
 " of plants in light 178 
 
 Gnm on daguerreotype 225 
 
 G un-barrel ignition experiment 29 
 
 H. 
 
 Heat distribution in spectrum... 100, 383 
 " effect of, on phosphores- 
 cence 159, 162, 165 
 
 " invisible action of, on carbonic 
 
 acid 183 
 
 ' measurement of, in spectrum. . 99 
 " radiation of, at different tem- 
 peratures 43 
 
 " rays and chemical rays, their 
 
 analogies 230 
 
 Henry on phosphorescence 318 
 
 Herschel on heat radiations 79 
 
 " on photographic spec- 
 trum 418, 420 
 
 " on spectrum scale 127 
 
 Heumann on flame 60 
 
 Horizontal element of flames ana- 
 lyzed 54, 61 
 
 Huggins on nebula in Draco 32 
 
 Hydrogen flame, spectrum analysis of 55 
 
 " its relation to chlorine 294 
 
 " substituted by chlorine... 306 
 
 " union of, with chlorine 298 
 
 Hypothesis of vision 101 
 
 I. 
 
 Ignited solids, their spectra 31 
 
 Ignition of platinum, apparatus for.. 26 
 
 " " '' on Laplace's co- 
 efficient 30 
 
 Ignition, temperature of, by Davy 23 
 
 " " by Newton. 23 
 
 Incandescence, temperature of 23 
 
 India-rubber, endosrnosis through... 357 
 
 Inflammation, explanation of 380 
 
 Ingenhousz, experiments of 180 
 
 Intensity of light, effect of. 439, 448 
 
 Interference of radiations. . . 92, 240, 454 
 
 Inverse vision Ill 
 
 Iodide of silver, its decomposition. . . 406 
 
 Iodine cannot decompose water 445 
 
 " corrodes daguerreotype plate. 228 
 Isinglass on daguerreotype plate 227 
 
 J. 
 
 Jamin, quotation from 85 
 
 K. 
 
 Kirchoff on fixed lines 84 
 
 " on spectrum analysis 86 
 
 Kunckel, the alchemist 134 
 
 L. 
 
 Lamansky on fixed lines 384 
 
 Lavoisier, chemical theory of 52 
 
 Le'rnery, theory of phosphores- 
 cence of. 137, 1 60 
 
 Leyden spark 247 
 
 Lime, incandescence of, compared 
 
 with electric spark 320 
 
 Light, action of, on daguerreotype. . . 235 
 decomposes chlorine water . . . 287 
 " intensity of, at different tem- 
 peratures 41 
 
 " magnetism of 191 
 
 " transference of energy 117 
 
 Lunar photography 214 
 
 M. 
 
 Magnetism of light 191 
 
 Mascart on cadmium 125 
 
 Measuring lens 265 
 
 Mechanical hypothesis of vision 108 
 
 Melloni on heat radiations 129 
 
 Melloni's critique 46 
 
 Mercurialization of daguerreotype. . . 211 
 
 Mercury, tides on 349 
 
 Metals not phosphorescent 140 
 
 Microscopic photography 338 
 
 Millon on bleaching compounds 293 
 
 " on carbon compounds 302 
 
 Mirrors, action of light on 220 
 
 Modified chlorine 271 
 
 Moon, phosphorescent picture of. ... 162 
 " photographic picture of. . 213,214 
 
 " rays of heat of. 131 
 
 Mosotti on wave-lengths 112 
 
 Muller on heat distribution 383 
 
 N. 
 
 Nebula3, spectra of 32 
 
 Negative rays 87 
 
 Newton on decomposition of light. . . 128 
 
 " on temperature of ignition. . 23 
 
 Nitrogenized bodies, allotropism of. . 379 
 
 Nonentity., flame passes into 190 
 
 O. 
 
 Ocelli, their use in vision 102 
 
 Octaves, effect of, in decomposition. . 124 
 
 " several seen by eye 123 
 
 Oil-flame, spectrum analysis of 55 
 
 Optical qualities, control of. 238 
 
 Organic matter, production of. 366 
 
 Oxygen, combustion in 58 
 
 P. 
 
 Passive and active states of chlorine. 301 
 
 " condition, effect of 4~>7 
 
 Phantom images 207 
 
 " subjective 107 
 
 Phosphorescence 133 
 
 Phosphorescent bodies, ignition of. . . 31 
 
 " " their volume. 142 
 
472 
 
 INDEX. 
 
 Phosphorescent bodies, effect of heat 
 
 on.... 159-161 
 " " effect of mag- 
 netism on.. 151 
 " " effect of press- 
 ure on 144 
 
 " " effect of press- 
 
 ure on, by 
 
 forceps 145 
 
 " " electricity of. 151 
 " emit heat. 148-150 
 " " examined by 
 Bouguer's 
 method.... 156 
 " " examined by 
 photogra- 
 phy 155 
 
 *' examined by 
 the polari- 
 
 scope 146 
 
 ** " examined by 
 
 vapors 147 
 
 " " quantity of 
 
 light of 
 
 153, 155, 157 
 " structural 
 
 change of.. 146 
 
 Phosphorescent images 322 
 
 Phosphorogenic experiments by Hen- 
 ry 318 
 
 105 
 313 
 281 
 457 
 215 
 338 
 
 Phosphorus, action of light on 
 
 modified by light.. 281, 
 
 Phosphuretted hydrogen 
 
 Photo-decomposition, cause of. 
 
 Photographic portraits 
 
 Photography, microscopic 
 
 " specimens 
 
 of. 341 
 
 Photometer, chlor-hydrogen.. . . 245, 253 
 its sensi- 
 tive- 
 ness.. 247 
 
 ferric-oxalate 267 
 
 selective 269 
 
 Photometric methods 39 
 
 Pigment, dark, its use in vision 103 
 
 Plants and animals, balance of 181 
 
 " circulation in 367 
 
 1 ' decomposition of carbonic acid 
 
 by 169 
 
 ' ' force included in 177 
 
 Platinum, emission of light by 25 
 
 Polarization bv magnetism 456 
 
 effect of 455 
 
 Portal circulation 371 
 
 Portraits from life by photogra- 
 phy 215 
 
 Priestley's experiments 181 
 
 Prismatic spectrum, its defects 112 
 
 Prisms, distribution of heat by 400 
 
 Protecting rays 87 
 
 Pulmonary circulation 371 
 
 Q- 
 
 Quartz and glass in phosphorescence. 316 
 
 R. 
 
 Radiant heat, absorption of 200 
 
 Radiations, interference of 89 
 
 " of red-hot bodies 23 
 
 Red heat 28 
 
 " rays 406,421 
 
 Reference spectrum 31, 57, 199 
 
 Reflections, repeated, effects of 449 
 
 " " in globe under 
 
 mercury 450 
 
 Refrangibility and chemical energy 
 
 connected 70 
 
 increases with temper- 
 ature 36 
 
 Resistance in thermo-electricity 330 
 
 Retina, function of. 103 
 
 Reversals in the spectrum 423 
 
 Hitter's experiments 202 
 
 Roscoe and Bunsen, quotation from. 264 
 
 Rumford's experiments on plants 1 80 
 
 photometric method 39 
 
 Rutherfurd's gratings . . .' 117 
 
 S. 
 
 Salt in flame 56 
 
 Sap, circulation of 367 
 
 Scheele on chemical radiations 130 
 
 Seeds, germination of 184 
 
 Selective photometer 269 
 
 Sensitiveness, cause of 233 
 
 control of 241 
 
 Shadows on phospliori 1 37 
 
 Shell construction of flame 59 
 
 " inner temperature of, of flame.. 70 
 
 Silk, evolution of gas by 1 82 
 
 Silver chloride, action of burning-lens 
 
 on 446 
 
 " salts, their decomposition. 406, 417 
 
 Soap-bubble, endosmosis through 356 
 
 Sodium line 62 
 
 Solids have same point of ignition.. . 30 
 " spectrum analvsis of radiations 
 
 of. 57 
 
 Somerville's experiments 191 
 
 Spectres evoked by breathing 137 
 
 Spectroscope of 1 848 54 
 
 Spectrum analysis of flames 52-54 
 
 dark spaces of 124 
 
 effect of daylight in 56 
 
 length increases with tem- 
 perature 33 
 
 length of. in ignited solids. 31 
 
 prismatic, its defects 112 
 
 structure of 402 
 
INDEX. 
 
 473 
 
 Stahl, chemical theory of 52 
 
 Stems bend to indigo rays 413 
 
 Stokes on electric light 125 
 
 Strontian, flame of 55 
 
 Substitution, theory of 104 
 
 Sun, force derived from 187 
 
 " surface of, its condition 81-83 
 
 Swan on yellow ray 63 
 
 Systemic circulation 370 
 
 T. 
 
 Thermo-electric couples 327 
 
 " currents 328 
 
 " pairs, effect of num- 
 ber of 334 
 
 " pairs, improved 
 
 forms of 336 
 
 " quantity 333 
 
 " thermometer 327 
 
 Thickness, effect of, in sensitiveness . 238 
 
 Tides in mercury 346 
 
 Towson on camera 211 
 
 Transmission from retina to brain. . . 109 
 
 Tyndall on distribution of heat 385 
 
 " on electric light 390 
 
 " on ignition of platinum 433 
 
 " on spectrum 384 
 
 U. 
 
 Undulations and colors 123 
 
 " interpretation of, by 
 
 brain ... 122 
 
 length of. 121 
 
 Undulatory theory, power of the 117 
 
 Unit lamp 45 
 
 V. 
 
 Vertical element of flame analyzed.. 55 
 Vibrations connected with tempera- 
 ture 36 
 
 effect of, on retina 108 
 
 " spectrum measure of 128 
 
 Virginia, experiments made in 197 
 
 ; ' spectrum 89,90 
 
 Vision and photographic impressions. 2 1 
 
 hypothesis of 101 
 
 " inverse Ill 
 
 Vital force, rejection of 131 
 
 Vogel, Professor, on increased sen- 
 sitiveness 451 
 
 W. 
 
 Water, decomposition of. 1 88 
 
 ' ' decomposition of, under press- 
 ure 363 
 
 " not decomposed by light 294 
 
 Wave-lengths as spectrum meas- 
 ures 113, 127 
 
 " " interpretation of, by 
 
 brain 122 
 
 " " measures of 12L 
 
 Wedgwood on ignition of gold and 
 
 earthenware 28 
 
 Wilson's experiments on phospho- 
 
 ri 318 
 
 Wollaston on spectrum 88 
 
 Y. 
 
 Yellow ray most luminous 105 
 
 Young discovers interference 116 
 
 THE END. 
 
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