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.LIBRARY 
 
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 UNIVERSITY OF CALIFORNIA 
 
 Accessions No. \3tf & J Shelf No. 
 
$ 
 
 ENERGY IN NATURE 
 
ENERGY IN NATURE, 
 
 15E1XG, WITH SOME ADDITIONS, THE SUBSTANCE OF A COURSE 
 OF SIX LECTURES UPON THE 
 
 FORCES OF NATURE AND THEIR MUTUAL RELATIONS, 
 
 Delivered tinder the auspices of the Gilchrist Educational Trust, 
 in the Autumn of 1881, 
 
 WM. LAKT CAKPENTEK, B.A., B.Sc., 
 
 KKI.LOW OF THE CHEMICAL AND PHYSICAL SOCIETIES, AM) OF THE 
 SOCIETY OF CHEMICAL INDUSTBY. 
 
 CASSELL & COMPANY, LIMITED: 
 
 LONDON, PARIS $ NEW YORK. 
 
 [ALL RIGHTS BESF.BVED.] 
 1883. 
 
to* m|i 
 WILLIAM B. CARPENTER, C.B., M.D., LL.D , 
 
 F.R.S., F.G.S., F.L.S., &c. &c., 
 WHO HAS DEVOTED MUCH OF THE ENERGY OF A LIFETIME 
 
 TO THE SCIENTIFIC EDUCATION OF THE PEOPLE, 
 
 31 DcMtate ti)is f ittlc gook. 
 
PREFACE. 
 
 THE object of the following pages is to give to 
 those who have had little or no opportunity of 
 studying the subject, some idea of the mutual 
 relations existing between the various so-called 
 " Forces of Nature," expressed in the simplest 
 language, but at the same time, it is hoped, with 
 sufficient scientific accuracy. Whenever it is im- 
 possible to avoid the use of technical terms r they 
 will be carefully defined and explained, and the 
 illustrations used will, as far as possible, be 
 matters of common experience. An attempt is 
 made in the earlier pages to lay down clearly 
 the distinction between Force and Eneryy, and 
 to explain how the language of the older books 
 on the so-called Forces of Nature, the Correla- 
 tion of Forces, &c., has been of late modified 
 
viii PREFACE. 
 
 by the development of the doctrine of Energy 
 and its Conservation. The book may be shortly 
 described as an endeavour to expound in popu- 
 lar, yet accurate language, the meaning and 
 consequences of that important principle known 
 as the Conservation of Energy. Considerable 
 pains, however, have been taken, especially in 
 dealing with Electricity, to illustrate and ex- 
 plain the very latest developments of the 
 subjects treated in the text, since the trans- 
 formation of Mechanical into Electrical Energy 
 by the dynamo-machine is a remarkably good 
 example of the general principle. The sub- 
 stance of what is here written was delivered 
 extenrpore by the writer to large audiences 
 of artisans and others, under the auspices 
 of the Gilchrist Educational Trust, in five 
 Lancashire towns during the autumn of 1881, 
 the lectures were abundantly illustrated by ex- 
 periments, and by the projection of photographs 
 upon the screen. It is believed that this was 
 the first occasion on which the attempt was made 
 to bring home to those who were not, in the 
 
PREFACE. ix 
 
 ordinary sense of the term, either educated 
 persons or students of science, the important 
 practical consequences arising out of, as well 
 as some of the glorious thoughts suggested by 
 the consideration of, that grand doctrine of the 
 Conservation of Energy, probably the most 
 sublime generalisation of modern times, since its 
 effects are not confined to our own planet, but 
 pervade the Universe. 
 
 So much interest was excited on the subject 
 among the audiences to whom the lectures were 
 addressed, and so many enquiries have been 
 made for a book in which the subject is treated 
 in a popular manner, as to create a belief that 
 the publication of the substance of the lectures 
 in book-form will meet a real want; although 
 no one, probably, is more conscious than the 
 writer, of the different impressions produced by 
 essays on a subject, and by the same matter 
 treated orally, with experimental demonstrations 
 and other illustrative aids to the comprehension 
 of what may be to many readers comparatively 
 unfamiliar scientific truths. 
 
x PREFACE. 
 
 I wish to express my obligation to Pro- 
 fessor Balfour Stewart, and to one or two other 
 friends, for valuable suggestions made to me 
 while the book was passing through the press ; 
 and to Messrs. Macmillan, Mr. Stanford, Messrs. 
 Longman, and the publishers of Engineering, 
 for permission to reproduce certain illustra- 
 tions. 
 
 3d, Craven Park, 
 
 Harlesden, London, N.IV. 
 September 1st, 1883. 
 
C N T E N T[8 
 
 CHAPTER I. PAGE 
 
 MATTKR AND MOTION FORCE AND ENERGY 1 31 
 
 CHAPTER II. 
 HEAT, A FOKM OF ENERGY 32 60 
 
 CHAPTER III. 
 
 CHEMICAL ATTRACTION, ESPECIALLY COMBUSTION ... ... 61 86 
 
 CHAPTER IV. 
 ELECTRICITY AND CHEMICAL* ACTION 87126 
 
 CHAPTER V. 
 MAGNETISM AND ELECTRICITY ,. 127 172 
 
xii CONTENTS. 
 
 CHAPTER VI. 
 ENERGY IN ORGANIC NATURE .. 173206 
 
 NOTE. The following- outline of the Course of Six Lectures, as 
 arranged with the tlilchrist Trustees, will convey a fair idea of the 
 contents of the six chapters, whose headings are given above. The 
 delivery of the Course was prefaced with an Introductory Lecture by 
 Dr. W. B. CARPENTER (the Secretary of the Trust), on "The Philosophy 
 of a Lucifer Match," illustrating the connection between Mechanical 
 Force, Heat, Light, Chemical Action, Electricity, and Vital Energy, 
 all of them concerned in the simple act of "striking- a light." 
 
 LECTURE I. 
 
 MATTER AND MOTION FORCE AND ENERGY. 
 
 Different states of Matter (solid, liquid, and gaseous), all having weight, 
 and offering Resistance Indestructibility of Matter Changes in 
 its state. 
 
 Different kinds of Motion Visible Motion of Masses Invisible Motion 
 of Particles, or Molecular Motion. 
 
 Different Modes of Force Attraction of Gravitation giving Motion to 
 Masses Attraction of Cohesion holding together the particles of 
 Masses. 
 
 Energy, or the Power of doing work Energy of Motion, and Energy 
 of Position, or Potential Energy Relation of Energy to Mass and 
 Motion Its Disappearance really a change in its mode of mani- 
 festation. 
 
CONTENTS. xiii 
 
 LECTURE II. 
 
 HEAT, A FORM OF ENKKf;Y. 
 
 Source of Heat in check to visible Motion, as in Friction and Percussion 
 Mechanical Equivalent of Heat Communication of Heat by Con- 
 duction, Convection, and Radiation Action of Heat in the Change 
 of state of Matter Relation of Heat to Light and to Chemical 
 Action. 
 
 LECTURE TIL 
 
 CHEMICAL ATTRACTION, ESPECIALLY COMBUSTION. 
 
 Chemical Action generally Elementary and Compound bodies Changes 
 of Composition due to Chemical Attraction Ordinary Combustion 
 the union of combustible substances with Oxygen, producing Light 
 and Heat, thereby generating Mechanical Force Influence of Heat 
 in exciting Combustion or other kinds of Chemical action Influence 
 of Light in producing Chemical Change Photography. 
 
 LECTURE IV. 
 
 ELECTRICITY AND CHEMICAL ACTION. 
 
 Production of Electricity : (1) by Friction of dissimilar Substances ; 
 (2) by Heat affecting two metals unequally ; (3) by Chemical Action, 
 as in the galvanic battery Difference between Electricity at rest, 
 
xiv CONTENTS. 
 
 and Electricity in motion Transmission of Electricity by Conduction 
 Electric Currents Heat and Light produced by opposing resist- 
 ance to their passage Mechanical effects of their interruption 
 Induced Currents Ruhmkorff's Coil Production of Chemical 
 Action by Electricity Law of Equivalence Electrotyping and 
 Electro-plating Electrical Storage of Energy in Secondary Bat- 
 teries. 
 
 LECTURE V. 
 
 MAGNETISM AND ELECTRICITY. 
 
 Magnetism originally derived from Lodestone ; now induced in iron 
 and steel bars by Electric Currents Electro-Magnets Mariner's 
 Compass ; its direction due to the Magnetism of the Earth 
 Deflection of Magnetic Needle by Electric Current Induction 
 of Electric Currents by giving Motion to Magnets Magneto - 
 Electricity. 
 
 Application of these facts in the Electric Telegraph, the Telephone, 
 Microphone, and Photophone, and in the Dynamo-machine for 
 Electric Lighting, and for the Transmission of Power. 
 
 LECTURE VI. 
 
 ENERGY IN ORGANIC NATURE. 
 
 Growth of Plants, and formation of their material, dependent on the 
 Light and Heat of the Sun Reproduction of this Light and Heat 
 
CONTENTS. xv 
 
 in the combustion of Wood, Kf.-sin, &c. Storage of Solar Energy 
 in Coal. 
 
 Growth of Animals also dependent upon Heat ; but their material derived 
 from plants Animal Energy dependent on Combustion of Food 
 Material Power directed, but not generated, by Human Will 
 Balance of Organic Nature. 
 
 These mutual relations of the Powers of Nature, and the Uniformities 
 expressed in its Laws, prove a Common Source, and suggest a 
 Designing Mind as their Originator. 
 
[UNIVERSITY; 
 
 ENERGY IN NATURE. 
 
 CHAPTER I. 
 
 MATTER AND MOTION; FORCE AND ENERGY. 
 
 BEFORE studying any of the so-called Forces 
 of Nature, some of which are known under the 
 names of Gravity, Chemical Attraction or Affinity, 
 Electricity, Magnetism, Heat, and so on, it is 
 desirable to say clearly and emphatically that 
 these so-called Forces are not " things" in them- 
 selves, in the sense in which sand, wood, &c., are 
 " things " in themselves, and that they are only 
 known, and can only be investigated, by their 
 effects upon substances, or, to use a more pre- 
 cise phrase, upon what the scientific man calls 
 Matter. We cannot lay hold of some thing and 
 handle it, or deal with it in any way, and say, 
 " this is electricity," or "that is heat," in the 
 sense in which we can say, " this is iron," or " that 
 is clay." The so-called forces of nature have 
 been well and truly spoken of as the moods, or 
 affections, of matter. The relations of an indi- 
 vidual to the objects which surround him, vary 
 B 
 
2 BNEUGY IN NATURE. 
 
 with the mood in which ho is ; in a similar way, 
 the relations of any object in nature to other 
 objects in its immediate neighbourhood vary 
 with the mood of these objects, or with the way 
 in which they affect each other. Thus, for 
 example, a leaden rifle-bullet weighing exactly 
 one ounce may be held in the hand and will 
 produce the sensation of weight and of metallic 
 coldness, but in other respects it appears to be in 
 a very impassive mood ; if it be melted in an 
 iron spoon its weight will be unchanged, but 
 it can no longer be handled with impunity, and 
 is in a very heated mood ; and again, if it be 
 fired off from a rifle it will still be the same 
 leaden bullet, but in a very destructive mood. 
 Lastly, if it be brought under the influence of a 
 current of electricity it will be found to behave to- 
 wards iron and steel very much as a magnet does, 
 and it may be said to be in an attractive mood. 
 
 Hitherto we have spoken of Force, and of the 
 forces of nature; and so long as heat, chemical 
 attraction, electricity, magnetism, &c. &c., were 
 regarded as so many distinct and separate forces 
 (called in the older books on natural philosophy 
 the " imponderables," i.e., the un-weigh-ables), 
 this phraseology served its purpose very well. 
 As, however, the idea gradually became developed 
 that these so-called various forces were all diffe- 
 rent manifestations of a power of doing work (i.e., 
 causing change), residing in or acting through 
 
FORCE AND ENERGY. 3 
 
 matter, the need was felt for some general phrase 
 which should include them all ; accordingly the 
 word Energy was adopted, as expressing more 
 accurately this fundamental idea. Energy, then, 
 is the power of doing work, in the strict scientific 
 sense of the term. The phrase is an expressive 
 one, and the consideration of what is implied in 
 the phrase " an energetic man" will assist the 
 reader in grasping the idea implied in it. We 
 do not exactly know what energy is, but we 
 recognise it, just as we recognise life (about 
 the nature of which we are equally ignorant) in 
 various forms, and, as will presently be seen, we 
 can measure it very exactly. On this view then 
 all the so-called forces of nature, or the various 
 moods that affect matter, are so many kinds of 
 energy, which is capable of assuming various 
 forms, and of being changed from one form to 
 another by apparatus arranged for the purpose 
 by man, but is never created afresh or destroyed en- 
 tirely, by any contrivance of his. This is the 
 idea intended to be conveyed by the modern 
 phrase, " The Conservation of Energy " (in place 
 of that of the " Correlation of Forces"), which is 
 the subject of the following pages, an idea which 
 the diagram, Fig. 2 (p. 13) is designed also to 
 convey to the eye. 
 
 Let us consider for a moment what is implied 
 exactly by the term Force. A little thought givei? 
 to the subject will show that whenever it is de- 
 B 2 
 
4 ENERGY TN NATURE. 
 
 sired to set anything in motion, or to bring to rest 
 anything already in motion, the use of force is 
 necessary. The exertion required to bowl, and 
 to catch, a ball at cricket, and the action of the 
 locomotive and the brake in the railway train, 
 are familiar examples of this. Hence, Force may 
 be defined as any cause which alters or tends to 
 alter a body^s natural state of rest, or of uniform 
 motion in a straight line, and it should be borne 
 in mind that the states of rest and of motion here 
 referred to are not merely molar, but also molecular, 
 i.e., not merely motion of the body as a whole, 
 but of the motion among themselves of the very 
 minute particles or molecules (vide pp. 17-20) of 
 which the body is made up. There is very good 
 reason to believe that all the various energies of 
 nature, light, heat, electricity, magnetism, &c., 
 are derived from different kinds of molecular 
 motion. Prof. Tyiidall wrote a book several 
 years ago entitled u Heat considered as a Mode 
 of Motion." Prof. Hughes has quite lately (Feb- 
 ruary, 1883) exhibited to the Royal Society some 
 very curious experiments, which seem to confirm 
 the idea long entertained by Sir W. Thomson 
 and others that magnetism is another form of 
 molecular motion. A wire along which an 
 electric current is made to pass undergoes no 
 change in weight thereby, nor, if it is large 
 enough (Chap. IV.) in any outward appearance, 
 but under certain conditions (in a dynamo- 
 
FORCE AND ENERGY. 5 
 
 machine, for example, Chap. V.), it can produce 
 not only molecular motion, but even that kind 
 of motion familiar to us as mechanical. 
 
 It is evident, therefore, that Force also can 
 only be known by its effect upon Matter, and that 
 it is not a " thing' 7 (in the ordinary sense of that 
 word) any more than the bank-rate of interest is 
 a sum of money, or a birth-rate of children is 
 the actual group of children who are born in a 
 year. Force is simply the expression of the 
 rate or speed at which any change takes place 
 in matter ; what its essence, or primordial cause 
 is, is a problem that science does not attempt to 
 solve. 
 
 The terms Energy and Force do not, therefore, 
 mean exactly the same thing, and, indeed, it is 
 most important to grasp, and to bear in mind, 
 the distinction between them. Energy involves 
 two distinct ideas combined, whereas Force in- 
 volves only one. Energy has been defined as 
 "the power of doing work," and work is force 
 exerted through space, i.e., the idea of motion 
 of some kind is connected with it.* The dis- 
 tinction again between energy and what the 
 
 * When a weight rests on the ground, the weight pushes the 
 ground down with a certain force, and the ground pushes the weight 
 up with the same force. It is obvious, however, that a weight in 
 this condition is incapable of doing any work, i.e., of producing 
 motion of any kind ; indeed, the very expression " dead- weight " 
 applied to it is a practical admission of its inefficiency in this re- 
 spect. The pressure that this inert weight produces is force and 
 not energy, but the operation of raising tho weight is doing work, 
 
6 ENERGY IN NATURE. 
 
 artizan and engineer recognise as " power " is 
 essentially difficult, and lies probably in the idea 
 of direction any form of directed energy is 
 power. In some lectures on the Transmission 
 of Energy, recently delivered before the Society 
 of Arts, Prof. Osborne Reynolds compared the 
 difference between undirected and directed energy, 
 to the difference between a mob and a trained 
 army, the individuals in whom the energy re- 
 sided being, in both cases, the molecules (p. 20) 
 or ultimate particles, of matter. 
 
 Reference has been frequently made to the 
 term " Matter." What is this Matter, in the 
 different moods or affections of which we re- 
 cognise what is now known as Energy? It 
 may be stated in general terms to be " what- 
 ever can affect one or more of our senses;" but 
 there are certain kinds or conditions of it for 
 which we need to increase the sensitiveness of 
 what have been called our u five gateways of 
 knowledge" by suitable apparatus, in order to 
 become aware of this affection of our senses. 
 Thus, for example, the sense of sight may be 
 assisted by magnifying glasses, and those of 
 touch and hearing by certain electrical contri- 
 vances, such as the telephone and the micro- 
 
 and involves the expenditure of energy. In every case in which 
 force is said to act, what is really observed is a transference (or a 
 tendency to transference) of energy from one portion of matter to 
 anothr:- and the so-called force in any direction is simply the rate 
 of that transference. 
 
MATTElt. 7 
 
 plionc (Chap. V.), with which it is possible to 
 hear the footsteps of a fly crawling upon a piece 
 of wood, and to converse in spoken language 
 with a friend a thousand miles off.* 
 
 A little attentive consideration will show that 
 we usually meet with matter in one of three 
 states, known as solid, liquid, and gaseous, f It 
 will presently be shown that, in the majority 
 of instances, the occurrence of any substance in 
 one form or the other depends upon the tem- 
 perature and the pressure to which the matter 
 is subjected. A very familiar example of the 
 three states of matter is to be found in ice, 
 water, and steam, each of which forms of water 
 can be changed into either of the others by 
 regulating the amount of heat to which it is ex- 
 posed, while it still remains through all these 
 changes the same compound of two measures 
 of hydrogen with one measure of oxygen (vide 
 page 68), which the chemist recognises as 
 
 water bv its behaviour towards other kinds of 
 
 * 
 
 matter. 
 
 It will be desirable now briefly to consider 
 one or two properties which are possessed, with- 
 out any exception, by matter in all its states. 
 A stone left to itself in the air falls until it 
 touches the ground; a round stone or ball rolls 
 
 * Vide Nature for March 22nd, 1883. 
 
 f For the purposes of the present enquiry the question of the 
 existence of a fourth state of matter need not be discussed. 
 
ENEliQY IN NATVUE. 
 
 down a sloping surface ; a mass of water, such 
 as a stream or river, flows from a higher to a 
 lower level in the channel in which it is con- 
 fined ; clouds appear to rise in the air, and 
 descend again as rain ; all these occurrences 
 
 are due to the fact 
 that all matter pos- 
 sesses weight. That 
 this is the case with 
 solids and liquids is 
 a matter of such com- 
 mon experience as to 
 need no more than a 
 reference to it ; but 
 it is not so apparent, 
 although equally and universally 
 true, in the case of gases. It may, 
 however, be readily proved by 
 very simple apparatus (a balloon 
 for example), and may be shown 
 more exactly by weighing differ- 
 ent kinds of gas in a glass-bottle 
 provided with a stopcock, attached 
 to one end of a delicate balance 
 or pair of scales, as in Fig. 1 . If 
 all the air be removed from the bottle by an air- 
 pump, and the bottle be then carefully weighed, it 
 will be found that when the stopcock is opened, 
 and air is admitted into the bottle, weights must 
 be put in the other scale-pan to restore the balance. 
 
 FIG. 1. 
 
FORCE AND ENERGY. 9 
 
 In this way it can be shown that five gallons of 
 common air weigh very nearly an ounce, or, in 
 the so-called metrical system in use on the con- 
 tinent, not only by scientific men but also in 
 all the dealings of daily life, that one litre of air 
 weighs 1*29 gramme. Instead of common air 
 other gases may be allowed to enter the bottle, 
 being conveyed there by a pipe, and it can 
 thus be shown that they differ greatly in 
 weight. Thus : 
 
 100 cubic inches of air weigh . . .31 grains. 
 100 ,, of carbonic acid weigh . 47 ,, 
 100 ,, ,, of hydrogen weigh only . 2 
 
 Now here we have an illustration of the 
 fact which is of considerable importance in 
 connection with this property of weight, that 
 substances differ largely in the mass or quantity 
 of matter contained in the same measure thereof. 
 Thus, for example, a gallon of water weighs 
 exactly 10 Ibs. (and in the metrical system a 
 cubic centimetre of water weighs exactly one 
 gramme), while a gallon of oil, or of spirit, 
 weighs only 9 Ibs., or less. If the weight of 
 one-tenth of a gallon of water (or 27*27 cubic 
 inches) which is 1 lb., be taken as a standard, 
 and compared with the weight of the same mea- 
 sure of other substances, we shall find that the 
 weights of equal measures, expressed in Ibs., 
 give us figures like the following : 
 
10 
 
 ENERGY IN NATURE. 
 
 Hammered Copper . 8*9 
 Gold . . . . 19-6 
 Cast Iron . . . 7'0 
 Wrought Iron . .7*7 
 Lead .... 114 
 Silver . . . . 10'5 
 Platinum . . . 21 '5 
 Tin .... 74 
 Zinc . . . .7-0 
 Mercury . . .13-6 
 
 Brick . 
 Clay . 
 Chalk . 
 Dry Sand 
 Glass . 
 Spruce Fir 
 Oak . 
 Proof Spirit 
 Ether . 
 Oil 
 
 2-1 
 1-9 
 2-3 
 1-4 
 2-7 
 0-6 
 0-8 
 0-9 
 0-7 
 0-9 
 
 The annexed drawings of spheres (Fig. 2), and 
 figures, also present the same relationship between 
 other substances : 
 
 Hydrogen 
 Air . 
 
 Water . 
 Platinum 
 
 FIG. 2. 
 
 1 
 
 14-4 
 11,943 
 256,764 
 
 1 
 
 829 
 17,831 
 
 1 
 21-5 
 
 Thus, for equal bulks, the metal platinum is 21 J times 
 as heavy as water, 17,831 times as heavy as air, 256,764 
 times as heavy as hydrogen, and so on. 
 
 This relation between the measure (or volume) 
 and the weight of any substance is known as its 
 specific gravity, and is recognised in common life 
 by such phrases as u Oil is lighter than water," 
 " Lead is heavier than iron," &c., the words 
 
MATT Eli. 11 
 
 " heavier" and "lighter" really implying the 
 addition to them of the phrase " bulk for bulk." 
 
 Another characteristic of all kinds of matter 
 is that it offers resistance. This, again, so far 
 as our sense of touch is concerned, conies within 
 our common experience, even in the case of 
 gases, for who has not experienced the resis- 
 tance offered to his progress by air in violent 
 motion ? Another mode of expressing the same 
 thing is to say that matter possesses impenetra- 
 bility, which means that no two portions of 
 matter can occupy the same place at the same 
 time. An amusing story illustrating this is told 
 of King Charles II., in whose reign the Royal 
 Society was founded. The king propounded to 
 the philosophers composing it the following 
 query: "Why is it that when two gold-fish 
 are put into a bowl of water full up to the 
 brim, the water does not overflow?" The ques- 
 tion was gravely debated at considerable length, 
 until some one suggested that the question should 
 be put to Nature, or in other words, that the ex- 
 periment should be tried. The result, of course^ 
 showed that the " Merry Monarch " had been 
 amusing himself at their expense. 
 
 It may be pointed out here that by no process 
 at the command of man can matter be either 
 created or destroyed. It is a most important 
 truth that the utmost man can do in this direction 
 is to change the combinations of matter. For 
 
12 ENEliGY IN NATURE. 
 
 example, when coal is burnt in the fire it is not 
 destroyed, but in the act of burning (Chap. III.) 
 its carbon and hydrogen unite with the oxygen 
 of the air, forming gases which pass away into 
 the atmosphere. The main object of this little 
 book is to show that the same thing is true of the 
 so-called Forces of Nature, that is to say, of the 
 forms of Energy, which can neither be created 
 nor destroyed by man, whose power over them 
 is limited to changing their form, or mode of 
 manifestation. This doctrine may be looked upon 
 as the outcome of the scientific work done during 
 the last thirty years by such men as Meyer, Helm- 
 holtz, Clausius, and others, in Germany, and as 
 Grove, Joule, Thomson, Balfour Stewart, Tait, 
 Tyndall, and W. B. Carpenter in England. Pro- 
 perly regarded in its various aspects, it is one of 
 the most sublime generalisations of modern times ; 
 the most stupendous phenomena of the universe 
 are seen to happen in accordance with it, while 
 the life and movements of the minutest living 
 being, and the most insignificant natural phe- 
 nomenon occurring in human experience, alike 
 bear witness to its truth. 
 
 That when Energy seems to be destroyed, or 
 to disappear, it does not really do so, but, like the 
 magician in the " Arabian Nights," only vanishes 
 to reappear under some other form (or mode of 
 manifestation) it will be the object of the follow- 
 ing pages to prove. A careful inspection of the 
 
FORCE AND ENERGY^ 1:' 
 
 accompanying engraving (Fig. 3) im^ $\ 
 reader in grasping this grand idea, to which 
 name " Correlation of the Physical Forces " was 
 
 
 first given, but the progress of research has shown 
 (Chap. VI.) that it may be extended also to the 
 so-called Vital Forces, i.e., to the Energy dis- 
 played in the phenomena of Vitality. The circles 
 are intended to represent movable discs, upon 
 which are written the names of some of the forms 
 
14 ENERGY IN NATURE. 
 
 of energy existing in Nature. It is obvious that 
 any two of these discs may be made to inter- 
 change places, and thus to express graphically the 
 change or transformation of one form of energy 
 into another. A grand thought, which follows 
 from the recognition of the indestructibility of 
 energy, is that the total quantity of it in the 
 universe is unchangeable, and can neither be 
 increased nor diminished ; this is strictly what 
 is meant by the phrase " Conservation of 
 Energy," and it is expressed in the diagram by 
 the ring which surrounds all the smaller discs. 
 
 One other property of matter remains to be 
 considered that called Inertia, in virtue of which 
 matter can neither start into motion of itself, 
 nor, when it is once in motion, can it stop of 
 itself. It may be objected that although the 
 first part of that sentence is obviously true, 
 daily experience tends to contradict the second. 
 A ball rolled along the ground, for example, 
 speedily comes to rest. True, but if that same 
 ball be rolled along a level surface of ice it 
 will travel very much farther, and the more 
 the resistance of friction is removed from any 
 body in motion, the longer will that motion 
 continue. If a strong mental effort be made to 
 put aside the impressions caused by the familiar 
 facts of the case, it will then be evident to the 
 accurate thinker that a state of motion is just 
 as natural to a body as a state of rest, provided 
 
MATTER AND MOTION. 15 
 
 that no external cause acts upon it, or, in otlier 
 words, that there is nothing in the inherent nature 
 of a moving body to cause it to cease moving. 
 
 Here it is desirable that we should pause 
 and reflect upon what is implied by the term 
 Motion, and on the meaning of the phrases 
 used to describe it. Motion means change of 
 place, and it is obvious that in order to have 
 a clear idea of the movement of any body, 
 we must know the direction or line in which 
 it is moving, and also the rate or velocity at 
 which it moves. This velocity is often mea- 
 sured in ordinary life in miles per hour ; thus 
 an express train is said to go at the rate of 
 forty miles an hour, meaning that if it continued 
 moving for a whole hour at an unchanged speed 
 it would rush over forty miles of railroad in 
 that time ; but that if it only travelled at that 
 rate for a fraction of an hour it would move 
 over a proportionate distance two miles in three 
 minutes, for example. It is more usual, in scien- 
 tific language, to speak of velocity as measured in 
 feet per second ; for instance, sound travels through 
 the air at the rate of about 1,100 feet per second. 
 
 The Laws of Motion were very carefully 
 studied by that great philosopher, Sir Isaac 
 Newton, who reduced them to three. The first 
 one states that " a body at rest will continue 
 at rest, and a body in motion will continue in 
 motion, unless acted on by some external force;" 
 
16 ENERGY IN NATURE. 
 
 and it has already been discussed in connection 
 with Inertia. The second one states that "when 
 two or more forces act upon a body each pro- 
 duces its full effect, whether the body be at 
 rest or in motion." Simple illustrations of this 
 are seen in the fact that if a stone be dropped 
 from the window of a moving train it falls on, 
 and not behind, the carriage step, or, when let 
 fall from the top of the mast of a ship in motion, 
 it falls at the foot of the mast ; in the feats 
 of circus performers, who, in jumping through 
 hoops, &c., simply jump upwards into the air, 
 and are carried forwards by the motion derived 
 from the horse ; and in a boat which, propelled 
 straight forward by oars, while carried sideways 
 by the tide, moves in a line which is really the 
 result of the action of both these forces. The 
 third law states that "Action and Reaction are 
 equal and opposite ; " but for present purposes 
 it need not be here discussed. 
 
 It may now be noted, however, with advan- 
 tage, that although a body may be apparently 
 at rest, it may have a very decided tendency to 
 move, actual movement being prevented by some 
 counteracting tendency, a very slight diminution 
 in which may cause a vast movement to take 
 place. The case of heavy weights carefully 
 balanced, and then moved with a very slight 
 addition to them, illustrates this. 
 
 Before leaving the subject of Motion, it should 
 
MATTUli .l\l> MOTION. 
 
 17 
 
 not be forgotten tl at a body may be affected by 
 motion in two ways ; the first of these, viz., the 
 change of place of the whole mass of the body, 
 has been already considered; the other way is 
 by the movement among themselves of the small 
 particles of which the body is made up. This 
 kind of movement is known as Molecular Motion. 
 There arc the strongest possible grounds for 
 believing that all matter, whether solid, liquid, 
 or gaseous, is made 
 up of exceedingly 
 small particles, in visi- 
 ble even with the aid 
 of the highest powers 
 of the microscope, 
 Which are in a more 
 or less continual state 
 
 FIG. 4. 
 
 of movement among each other. When a blow, 
 or impulse of any kind, is sent through a solid or 
 liquid substance, it is passed on, as it were, from 
 particle to particle, somewhat in the manner re- 
 presented on a comparatively gigantic scale in the 
 annexed figures (Fig. 4). 
 
 A good instance of this molecular motion, 
 or movement of the particles of an apparently 
 
18 ENERGY IN NATURE. 
 
 solid body amongst themselves, is afforded by 
 that curious alteration in the internal structure 
 of the iron in girders of bridges, &c., in con- 
 sequence of which the iron becomes more brittle, 
 and the bridge therefore unsafe, after the lapse 
 of some years. Illustrations of molecular motion 
 in liquids are afforded by the beautiful forms 
 assumed when they solidify as for example, 
 the crystalline appearance of melted metals that 
 
 are allowed to cool slowly (as seen in ornamental 
 tin ware, &c.), or of different salts crystallising 
 from their solution in water; a very familiar 
 and a very beautiful instance is seen in the 
 exquisite crystals of snow, a few of the varieties 
 of which are seen in Fig. 5, and the same general 
 plan of structure exists also in ice, and is re- 
 
FORCE AND ENERGY. 
 
 vealed therein by passing a. beam of light through 
 it, and throwing the image on a screen by a lens, 
 as shown in Fig. 6. 
 
 A very pretty ex- 
 periment illustrating the 
 molecular movement in 
 gases is as follows : A 
 round porous pot (such 
 as is used in voltaic 
 batteries) is closed at 
 the open end with a 
 perforated cork, through 
 which a glass tube about 
 two feet long is inserted, 
 and the lower end of 
 this tube dips into co- 
 loured fluid (Fig. 7). 
 A bell-jar full of hydro- 
 gen gas (p. 70) is in- 
 verted over the porous 
 pot, when bubbles of air 
 will at once issue from 
 the mouth of the tube ; 
 the reason of this is that 
 the molecules of hydro- 
 gen pass into the pot, 
 through its pores, faster 
 than the molecules of 
 air can pass out. If the bell- jar be now re- 
 moved, the hydrogen will pass out faster than 
 c 2 
 
20 
 
 ENERGY IN NATURE. 
 
 the air can pass in again, a partial vacuum will 
 be formed, and the fluid will rise in the tube. 
 
 Although no one has ever yet seen a single 
 molecule, it is a remarkable fact that four distinct 
 lines of mathematical reasoning, 
 based upon as many different sets 
 of experiments, have led emi- 
 nent men like Sir W. Thomson, 
 Clausius, and others, to the 
 conclusion that in any ordi- 
 nary solid or liquid the mean 
 distance between the centres of 
 contiguous molecules is less than 
 the 1-5, 000, 000th and greater than 
 the 1-1, 000, 000, 000th of a centi- 
 metre (2*5 centimetres = one inch).* 
 To form some idea of what this 
 implies, let the reader imagine a 
 solid sphere the size of a football 
 magnified to the size of the globe 
 of our earth, which is about 8,000 miles in dia- 
 meter ; if the molecules were magnified in the 
 same proportion, the structure would be more 
 coarse-grained than a globe of small shot, but less 
 coarse-grained than a globe of footballs. 
 
 Since we have been considering the two kinds 
 of motion, molar and molecular, which affect 
 
 FIG. 7. 
 
 * For fuller information 011 this subject, the reader may consult 
 with advantage Sir W. Thomson's Jeeture, reported in three num- 
 bers of Nature for July, 1883. 
 
FORCE AND ENERGY. 21 
 
 matter, it may be convenient here to discuss 
 briefly the two forms of ordinary attraction 
 which affect matter, simply (so far as is at pre- 
 sent known) as matter, i.e., irrespective of such 
 attractions as those due to electricity, magnetism, 
 and chemical agency. The first, or the attrac- 
 tion between whole masses of bodies, is known 
 as Gravitation, while the second, which expresses 
 the attraction of molecules for each other, is 
 called Cohesion. 
 
 It is a universal physical law that every par- 
 ticle of the universe attracts every other particle 
 with a certain force, the amount of which de- 
 pends on conditions to be presently explained. 
 If a body (such as a stone or a rifle bullet) be 
 projected into the air, experience tells us that 
 it will almost immediately return to the surface 
 of the earth, in consequence of the attractive 
 force exerted by the earth upon it. It is equally 
 true, however, that, in an infinitesimal degree, 
 the earth falls towards the stone. Very delicate 
 experiments have been made in a laboratory with 
 two large masses of lead, accurately balanced 
 and free to move, which proved the fact of the 
 attractive force that they exerted on each other. 
 A good illustration of the attraction of a large 
 for a small mass of matter is seen in the fact 
 that a plumb-line in the neighbourhood of large 
 mountain masses does not hang perfectly vertical, 
 the " bob " being attracted towards the moun- 
 
2-2 ENERGY IN NATURE. 
 
 tain, as was clearly shown by experiments made 
 near Schehallion, an isolated mountain in Scot- 
 land. Astronomers tell us how gravitation ex- 
 tends its power to the most remote parts of the 
 heavens; how the moon and the earth gravitate 
 towards each other, and both together towards 
 the sun ; how the whole solar system is gravi- 
 tating in a given direction among the stars ; 
 how double stars are systems of suns, revolving 
 round each other under the action of the same 
 force, each of which has probably its own system 
 of planets, all moving in accordance with the 
 same law. If we confine our attention to this 
 earth and its satellite, we shall see that to gravi- 
 tation in great part are due, in one sense the 
 rise and fall of the tides, all the phenomena of 
 wind and rain, all the mechanical energy derivable 
 from falling water (in the form of torrents, so 
 apparently irresistible), and, in fact, the regu- 
 lation of the whole of the existing fabric of the 
 earth's surface. Were it not for the attraction 
 of gravitation, the speed at which the earth is 
 rotating on its axis would send every loose thing 
 upon its surface flying into space ; if we jumped 
 into the air, or went up in a balloon, we should 
 have 110 means of returning to the earth's sur- 
 face. Thoughts like these illustrate the truth of 
 the remark that " gravitation is the force which 
 keeps the universe going." It is by virtue of 
 gravitation that matter possesses weight ; for 
 
PORGE AND ENERGY. 23 
 
 the weight of any thing is the expression of 
 the force with which it tends towards the earth, 
 and, as we have seen, depends upon the mas,;, 
 or quantity of matter, in the body. Hence it 
 is evident, first, that weight is a particular case 
 of the universal law of gravitation, and, secondly, 
 that the force of gravitation between two bodies 
 depends upon their mass. It remains to enquire 
 what effect distance lias upon the force also. 
 Experiment shows that if the distance between 
 two bodies be doubled, the force of attrac 
 tion between them will only be one-fourth as 
 great ; if the distance be trebled, one-ninth as 
 great ; if quadrupled, one-sixteenth as great, and 
 so on; this is expressed in general terms by 
 saying that the force of gravitation varies in- 
 versely as the square of the distance. Other 
 physical phenomena follow the same rule, e.g., 
 the distribution of light from any light-source. 
 Another illustration of the law of squares is seen 
 in the phenomena of falling bodies ; a stone falls 
 towards the earth 16 feet in the first second, 64 
 feet (or 16 multiplied by the square of 2) in 
 2 seconds, 144 feet (or 16 x 3 2 ) in 3 seconds, 
 and so on. If the resistance of the air were 
 removed all bodies would fall towards the earth 
 at the same rate, irrespective of their compara- 
 tive weight. This may be roughly shown by 
 placing upon a penny a flat disc of paper the 
 same size as the coin, and letting both fall to- 
 
24 
 
 ENERGY IN NATURE, 
 
 gether, when they will be found to reach the 
 ground at the same instant. 
 
 In all bodies there is some point at which 
 its whole weight may be considered as concen- 
 trated, and about which it will balance in all 
 positions ; it is called the centre of gravity. 
 When a body is free to move it takes up such 
 
 a position that its centre of 
 gravity occupies the lowest 
 possible point, hence by 
 hanging up an irregularly 
 shaped body in two posi- 
 tions (Fig. 8), and noticing 
 where vertical lines from 
 the points of support in- 
 tersect each other, its 
 centre of gravity may bo 
 found. Many remarkable 
 feats of balancing, of re- 
 covery of equilibrium, &c., 
 depend upon the position of the centre of gravity 
 of the performer or of the thing balanced. The 
 general rule governing these cases is that the 
 equilibrium will be maintained as long as a ver- 
 tical line drawn downwards from the centre of 
 gravity of the body, falls within the base-line 
 of the support (Figs. 9 and 10). The celebrated 
 leaning tower of Pisa is a standing instance of this. 
 The attraction of cohesion differs from that 
 of gravitation, inasmuch as it is only exerted 
 
 FIG. 8. 
 
VUUi'K AND ENERGY. 
 
 25 
 
 FIG. 9. 
 
 between very minute particles of matter, and at 
 
 exceedingly small distances ; it is, however, that 
 
 which binds 
 
 together the 
 
 various par- 
 ticles of a 
 
 body, and 
 
 resists our 
 
 attemptsto 
 
 break it; 
 
 on the other 
 
 hand, when 
 
 the body is 
 
 once bro- 
 ken, and the particles separated by a measurable 
 
 distance, it is 
 very difficult 
 to get them 
 to adhere to- 
 gether again.* 
 It is obvious, 
 therefore, that 
 this molecu- 
 lar attraction 
 acts very 
 powerfully 
 
 through a certain small distance, but disappears 
 
 * A notable instance to the contrary is afforded by lead ; u 
 bullet may be cut in half, and the two clean surfaces -will adhere 
 strongly if pressed together. >4dhes:on is the name sometimes 
 gi\en to this form of cohesion. 
 
26 ENERGY IN NATURE. 
 
 entirely when the distance becomes percep- 
 tible. Cohesion is strongest in solids, in liquids 
 it is much diminished, and in gases it may 
 be said to vanish altogether. Reference was 
 made to the conditions of material life with- 
 out gravitation ; a similar line of thought with 
 regard to cohesion shows that if that attraction 
 suddenly ceased to be exerted, our houses, our 
 own bodies, and, in fact, the whole surface of the 
 earth, would be instantly resolved into a heap 
 of dust ! This is well illustrated by the so- 
 called Prince Rupert's drops, formed by allowing 
 drops of melted glass to fall into water, which 
 assume, on cooling, the form of a pear-shaped 
 crescent. If the conditions of equilibrium (i.e., 
 the balancing of the forces) of cohesion are dis- 
 turbed by breaking off the minutest fragment 
 from the small end, the whole mass (the thick 
 part of which will resist severe blows from a 
 hammer) will suddenly fall into fine powder. 
 
 Having thus disposed of some of the more 
 important properties of Matter, simply as matter, 
 let us proceed to inquire a little more in detail 
 into what we have ventured to term its moods, i.e., 
 to discuss the states and forms of Energy, which 
 forms the special subject of this book. 
 
 Energy may exist in two states, that of 
 motion, and that of repose. It is obvious that 
 any moving body possesses energy, or the power 
 of doing work, but it is not quite so clear at first 
 
FORCE AND ENERGY. 27 
 
 sight how there can be energy in repose. In 
 order to understand it, let us recur again to the 
 analogy of the energetic man ; lie may be very 
 quiet, and yet be able to do a great deal of 
 work when he chooses to se!: about it. Energy 
 in repose is in many case j due to its position, 
 as may be seen from tliij example : Imagine a 
 mill or factory driven by a water-wheel, and near 
 this mill two ponds of water of equal capacity, 
 one below the level of the wheel, the other at 
 a considerable elevalion above it. It is obvious 
 that, as far as the mill is concerned, there is no 
 work at all to bj got out of the lower pond, 
 while the fall of water from the upper pond 
 will drive t:ie wheel. It should be carefully 
 noted tluit the work is done by the passage of 
 the water from a higher to a lower level, and 
 this will help us to understand later on, how, in 
 dor.ig work, energy passes from a higher to a 
 lov.-er grade. Examples, then, of energy due to 
 position, or, as it is called, potential energy, are 
 to be found (mechanically) in a raised weight, 
 a stretched spring, and a "head" of water; 
 chemically, in coal, gunpowder, corn, and other 
 foods, &c. (Chaps. III. and VI.) ; electrically, in 
 voltaic batteries, and in accumulators or storage 
 batteries (Chap. IV.). 
 
 A little consideration will show that all poten- 
 tial energy must ultimately be converted into 
 energy of motion (called scientifically kinetic 
 
28 ENERGY IN NATURE. 
 
 energy), whether that motion be molar (i.e., of 
 the mass), as in the case of mechanical motion, 
 or one of the various forms of molecular motion. 
 
 The following table gives the various supplies 
 of natural energy and their sources, it being 
 borne in mind that the words are used in a wide 
 sense fuel and chemical affinity, for example, 
 being intended to include the burning of zinc 
 in a voltaic battery (pp. 102, 119) as well as of 
 fuel in the air. 
 
 Supplies of Natural Energy. 
 Potential. Kinetic. 
 
 (1) Fuel. (1) Winds. 
 
 (2) Food of animals. (2) Water and ocean currents. 
 
 (3) Ordinary water-power. (3) Volcanoes and hot-springs. 
 
 (4) Tidal water-power. 
 
 The sources of these supplies are : 
 
 (1) Chemical affinity. (2) Solar Energy. (3) Energy 
 of the earth's rotation on its axis. (4) Internal heat of 
 the earth. 
 
 These energies may be conveniently distri- 
 buted into three groups, as follows : (1) Visible 
 energies, including all cases of visible motion, 
 such as those produced by gravitation, &c ; (2) 
 heat energies, in which the motion is molecular 
 (Chap. II.) ; and (3) chemical and electrical 
 energies, in which the motion is rather that of 
 atoms than of molecules (see Chaps. III. and IV.) 
 It must not be imagined, however, that each of 
 these groups or systems is complete in itself and 
 
29 
 
 lias no sort of connection with its neighbours, for 
 the various forms of energy are inseparably inter- 
 mingled with each other. 
 
 Energy, then, being the power of doing work, 
 let us next enquire how that work is measured, 
 a point of the utmost importance, as to which 
 clear ideas are essential. When a man raises a 
 weight from the ground, against the action of 
 gravity, he does a certain amount of work ; if 
 the number of pounds in the weight raised be 
 multiplied by the number of feet through which 
 it is raised, a number will be obtained which 
 expresses the measure of that work ; these numbers 
 are called foot-pounds, and foot-pounds are the 
 standard in which all work is measured scientifically. 
 One foot-pound, then, is 1 Ib. raised 1 foot ; 
 100 foot-pounds is either 1 Ib. raised 100 feet, 
 or 100 Ibs. raised 1 foot, or 10 Ibs. raised 10 
 feet, and so on. 
 
 The " horse-power" is the measure of the rate 
 of work most commonly used, but it is obviously 
 an unscientific one, because the strength of horses 
 varies so much. What is now recognised as 
 one horse-power is 550 foot-pounds of work per 
 second, or 33,000 foot-pounds per minute. 
 
 In the metrical system, to which reference 
 has been made, the standards taken are the 
 kilogramme (or 1,000 grammes, about 1-1 Ib.) 
 and the metre (39*37 inches), and hence the 
 unit-measure of work on the continent, and among 
 
30 ENERGY IN NATURE. 
 
 many English scientists, is the kilogram-metre, 
 which is equal to about 7^ foot-pounds. 
 
 Since the various forms of energy are inter- 
 changeable, this work-measure can be applied to 
 any one of them, whether it be heat, or electricity, 
 or any other form, and so it comes to pass, for 
 example, that when electrical energy is applied 
 to produce mechanical motion (Chap. V.), and 
 is distributed over our towns for this purpose^ 
 the amount supplied will be measured, and can 
 be expressed in foot-pounds, by meters con- 
 structed for the purpose. 
 
 It is a matter of common experience that the 
 energy of a moving body depends upon, or varies 
 with, its mass. The relation that the velocity 
 of a body bears ta its energy is expressed by the 
 law of squares (p. 23) ; i.e., if the velocity be 
 doubled, the energy is quadrupled, three times 
 the velocity gives nine times the energy, and 
 so on. 
 
 Having now cleared the ground somewhat, 
 and gained some exact ideas on the subject 
 of matter and its moods, let us enquire into the 
 truth of the statement so often insisted on, that 
 energy is never destroyed, but only transmuted, 
 or changed in form ; and, to begin with, let us 
 try to find an answer to the enquiry, What 
 becomes of the energy of mechanical motion 
 when it is retarded, or altogether arrested ? It 
 is a matter within the common experience of 
 
FORCE AND ENERGY. 31 
 
 almost every one, that any attempt to stop such 
 motion, to "put the brake on," results in the 
 production of heat, and it will be the object of 
 the next chapter to point out the relations between 
 mechanical energy and heat energy. 
 
CHAPTER II. 
 
 HEAT, A FORM OF ENERGY. 
 
 LET us now consider some instances of the state- 
 ment that whenever mechanical energy is re- 
 tarded or destroyed, heat makes its appearance. 
 All who have anything to do with machinery 
 are familiar with the fact in the form of hot 
 bearings, which need to be lubricated with oil 
 (or water) in order to diminish the friction ; in 
 the absence of that appliance of civilised life, 
 the lucifer match (which is itself ignited by the 
 heat produced when it is forcibly drawn over a 
 rough surface) ; the untutored savage spends a 
 great deal of energy in producing fire by rubbing 
 together two pieces of wood ; a clever blacksmith 
 can heat a large nail red-hot by simply hammering 
 it upon his anvil ; when a cannon-shot strikes a 
 target, its energy of motion is at once destroyed, 
 but both shot and target become very hot, and 
 fragments of the metal are often heated sufficiently 
 to glow perceptibly in diffused daylight. The 
 annexed engraving (Fig. 11) represents a Shoe- 
 buryness target struck with an amount of energy 
 expressed by about 2,200 foot-tons. 
 
 The first person to whom science was in- 
 
HEAT, A FORM OF ENERGY. 
 
 33 
 
 debted for proving clearly the relationship be- 
 tween mechanical and heat energy, was Count 
 Rumford, who, on January 25th, 1798, read an 
 elaborate paper to the Royal Society, entitled 
 " An Enquiry concerning the Source of Heat 
 which is" ex cited by Friction." In it lie detailed 
 
 FIG. 11. 
 
 experiments showing the large amount of heat 
 produced in boring brass guns by horse-power 
 in the military arsenal at Munich, during which 
 the gun itself became hot enough to boil large 
 quantities of water, and the metallic chips were 
 intensely heated. These experiments, taken in 
 connection with the very obvious fact that a 
 hot body does not weigh more (or less) than the 
 same body when cold, effectually disposed of 
 D 
 
34 
 
 ENERGY IN NATURE. 
 
 the notion that heat was a material substance. 
 " What is heat ? " he asked. " Is there any such 
 thing as an igneous fluid? Is there anything 
 that, with propriety, can be called caloric ? " 
 and, after reviewing the whole series of experi- 
 ments, he concluded, "It appears to me ex- 
 tremely difficult, if not impossible, to form any 
 distinct idea of anything capable of being ex- 
 cited and communicated in these experiments, 
 
 except it be MOTION." 
 There is, as will be seen in 
 the sequel, good reason to 
 
 FIG. 12. 
 
 believe that when motion disappears in this way, it 
 is changed from the motion of the whole mass of 
 the body into the motion of its particles, or, in other 
 words, that the molar motion becomes molecular. 
 Count Rumford's experiment may be illustrated on 
 a small scale by the apparatus represented in Fig. 
 12, where a tube four inches long, and three- 
 quarters of an inch in diameter, containing water, 
 and corked, is rapidly rotated, while it is gently 
 squeezed with a broad pair of wooden tongs. 
 The heat thus developed speedily boils the water 
 and blows the cork out. 
 
HEAT, A FORM OF ENERGY. 
 
 35 
 
 To Dr. Joule, of Manchester, belongs the 
 credit of first expressing in numbers this relation- 
 ship between heat and work, by a long series 
 of laborious researches, extending from 1843 to 
 1849, and his experiments were of this kind. 
 He took a closed vessel B, Fig. 13, contain- 
 ing water, in which a paddle fixed on an axis 
 was caused to rotate by gearing connected 
 
 FIG. 13. 
 
 with falling weights E, F; and as the amount 
 of this weight was known, and also the dis- 
 tance through which it fell, the work done 
 could be calculated in foot-pounds; the fric- 
 tion of the paddles against the water heated 
 the whole contents of the box, and thus Dr. 
 Joule established the fact that the mechanical 
 work represented by 772 foot-pounds would, 
 when converted into heat, raise the temperature 
 of one pound of water by one degree of Fahreii- 
 D 2 
 
36 ENERGY IN NATURE. 
 
 Kelt's thermometer (the ordinary scale in use in 
 England). This number, then, is known as the 
 mechanical equivalent of heat. Conversely, the 
 amount of heat necessary to raise 1 Ib. of water 
 1 Fahr. would, if all applied mechanically, raise 
 772 Ibs. 1 foot high, nearly equal to 1J horse- 
 power for 1 second. 
 
 The heat generated by the collision of a fall- 
 ing body with the earth depends on the height 
 from which it falls, and as that height is pro- 
 portional to the square of the velocity (p. 23), 
 it follows that the heat generated increases as 
 the square of the velocity; hence, if we double 
 the velocity of a projectile, we increase four- 
 fold the heat generated when that motion is 
 destroyed, and so on. A velocity of about 1,400 
 feet per second in a rifle bullet would, when 
 it struck the target, raise its own temperature 
 nearly 1,100 Fahr. (if no heat were absorbed 
 by the target), which would melt a portion 
 of the lead. Calculating on this principle 
 it has been shown that if the earth were 
 suddenly stopped in her motion through space, 
 as much heat would be generated as would 
 be developed by the combustion of fourteen 
 globes of solid carbon, each as large as the 
 earth; and that if the earth fell into the sun 
 the heat thus produced would be equal to that 
 of the combustion of 5,600 such worlds. Specula- 
 tions upon how the vast amount of the sun's 
 
HEAT, A FORM OF ENERGY. 87 
 
 energy* is maintained have led to the suggestion 
 that it may be kept up, in part at any rate, by 
 the constant showering down of solid meteoric 
 
 o 
 
 matter upon its surface ! 
 
 It is a very curious fact, and one that has 
 an important bearing upon the economy of na- 
 ture, that equal weights of various bodies require 
 different quantities of heat to bring them to the 
 same temperature, or, in other words, that bodies 
 vary very much in their capacity for heat. If 
 1 Ib. of water at 60 Fahr. be mixed with 1 Ib. 
 of water at 212 Fahr. (its boiling-point) the 
 temperature of the mixture will be found to be 
 half-way between the two, or 136 ; but if 1 Ib. 
 of mercury at 212 be used instead, the tempera- 
 ture of the mixture will only be about 65. 
 Water has a very great capacity for heat; the 
 same amount of heat that will raise the tem- 
 perature of 1 Ib. of water 1 Fahr. will raise 
 9 Ibs. of iron, 11 Ibs. of zinc, and 30 Ibs. of 
 mercury by the same amount. This difference 
 in capacity for heat may be also thus shown 
 experimentally ; balls of different metals, iron, 
 lead, bismuth, tin, and copper, are heated in oil 
 
 * It is difficult to convey any idea of this. The amount of heat 
 received by the earth from the sun in one year would liquefy a 
 layer of ice 100 feet thick all over it, and yet this is only the 
 l-2,138,000,000th part of the total heat emitted. Sir W. Thom- 
 son has lately put it in another form (Nature for January 18, 1883), 
 and expresses the radiant energy of the sun as equal to 7,000 horse- 
 power per square foot, or 50 horse-power per square inch, every 
 second of time, from the whole of his vast surface. 
 
38 
 
 ENERGY IN NATURE. 
 
 to about 350 Fahr. and 
 laid upon a cake of wax 
 (Fig. 14). The iron and 
 copper balls will work 
 themselves through first, 
 the tin will follow, while 
 the lead and bismuth 
 scarcely sink more than 
 half the depth of the 
 wax. 
 
 The following table in- 
 dicates in figures the rela- 
 tive capacity for heat, or specific heat, of various 
 substances : 
 
 FIG, 14. 
 
 Substances. 
 
 Water . 
 Turpentine 
 Air 
 
 Glass . 
 Iron 
 Copper . 
 Tin -'. 
 Mercury 
 Lead 
 
 Specific Heat. 
 
 , i-ooo 
 
 , 0426 
 , 0-237 
 
 0-198 
 
 0-114 
 
 , 0-095 
 
 , 0-056 
 
 . 0-033 
 
 0-031 
 
 The specific heat of water is nearly the 
 greatest of all known substances, hydrogen, 
 and a certain mixture of alcohol and water, 
 being the only exceptions. Comparing equal 
 weights the relation of the specific heat of water 
 to that of air is as 1 to 0'237, or 4-2 to 1; 
 but as water is 770 times as heavy as air, if 
 
HEAT, A FORM OF ENERGY. 39 
 
 equal measures are compared it will be seen that 
 the proportion is as 770 x 4*2, or 3,234 to 1. 
 Hence a cubic foot of water in cooling 1 will 
 warm by the same amount 3,234 cubic feet of 
 air. A little reflection will show the - bearing 
 of this fact upon the influences of the sea on 
 climate, and the reason of the comparative ab- 
 sence of extremes of heat and cold in an island 
 climate ; it will presently be shown also how the 
 system of circulation of the waters of the great 
 oceans, from the equator to the poles, and from 
 the lowest depths to the surface, demonstrated 
 by such researches as those of H.M.S. Challenger, 
 assists in moderating the severity of the land 
 climate in various parts of the earth, and notably 
 in the western shores of Europe. 
 
 We have now to inquire into the mode in 
 which heat passes through bodies. Experiment 
 shows us that substances vary very much in 
 their power of allowing this kind of molecular 
 motion to pass through them, or, in other words, 
 of conducting heat. A simple illustration will 
 show this : place two spoons, one of pure silver, 
 the other of German silver, in the same vessel of 
 hot water; in a few moments the free end of 
 the silver one will be much hotter than its neigh- 
 bour, and if the experiment be repeated, and 
 fragments of phosphorus placed on the ends, that 
 on the end of the silver spoon will fuse and catch 
 
 ^ while the other piece will be unaffected. 
 
40 
 
 ENERGY IN NATURE. 
 
 Of all solid bodies, metals (except bismuth) 
 conduct heat best; next in order come stone, 
 glass, and marble, wood-charcoal, and animal 
 and vegetable tissues. The following table 
 shows the relative conducting powers of metals 
 for heat and for electricity (pp. 106-7), both of 
 which we have good reason to believe are dif- 
 ferent kinds of molecular motion. It will be 
 noticed how alike they are : 
 
 Substance. 
 
 Silver 
 
 Copper . ".."'" 1- 
 
 Gold. | .-'.. .. 
 
 Brass . .. . 
 
 Tin . 
 
 Iron . . 
 
 Lead . 
 
 Platinum . 
 
 German Silver . 
 
 Bismuth . . . 
 
 Advantage is taken of the unequal conductivity 
 of solids in many ways. Tools and metal uten- 
 sils are furnished with non-conducting handles 
 of wood or ivory : a wooden house is cooler in 
 summer and warmer in winter than a stone one ; 
 ice is packed in sawdust or flannel to prevent its 
 melting ; steam boilers and pipes are (or should 
 be) covered with felt. The natural clothing of 
 the animal creation, and its application to the 
 needs of human life, depend upon the same 
 circumstance ; and the effect is heightened in this 
 case by the fact that the air entangled among the 
 
 Conductivity 
 for Heat, 
 
 100 ' 
 
 Conductivity for 
 Electricity. 
 
 100 
 
 74 
 
 ''": . 73 
 
 53 
 
 59 
 
 24 
 
 22 
 
 15 
 
 23 
 
 12 
 
 13 
 
 9 
 
 11 
 
 8 
 
 10 
 
 6 
 
 6 
 
 2 
 
 2 
 
HEAT, A FORM OF ENERGY. 41 
 
 fibres, feathers, &c., is an almost perfect non- 
 conductor, so long as it is at rest. 
 
 It should be noticed in this connection that 
 the sensation of cold is really due to an abstrac- 
 tion of heat from our own bodies. The tem- 
 perature of the blood is about 98 Fahr., and 
 when the hand is laid upon any substance at the 
 air-temperature, it feels more or less cold, ac- 
 cording to the rate at which heat passes from the 
 hand to it, i.e., according to the conductivity 
 of the substance. In the severe winter of the 
 Northern United States it is absolutely necessary 
 for all who have to handle metals in the open air 
 to wear gloves ; the metal is not colder than the 
 surrounding air, but, being a good conductor, it 
 robs the human body of heat very rapidly, and 
 will produce blisters upon the naked skin. 
 
 The experiments which have been made to 
 measure the conductivity of liquids 'and gases, 
 prove that it is very slight indeed. Heat is, 
 however, rapidly transmitted through these 
 media by actual transport of the heated parts, a 
 process to which the term convection (i.e., con- 
 veyance, carrying,) is given ; it depends 011 the 
 fact that (as will be immediately shown) the 
 heated portions of the liquid expand, and thus 
 become specifically lighter, when the balance is 
 therefore disturbed, and motion throughout the 
 mass ensues. The annexed cut (Fig. 15) shows 
 the convection currents set up in water when 
 
42 
 
 ENERGY IN NATURE. 
 
 heated from below. Numerous examples of con- 
 vection on a grand scale in nature will readily 
 occur to the thoughtful mind: the cooling of 
 large masses of fresh water in winter, the move- 
 ments of air in all systems of ventilation, the 
 trade- winds, produced by the sun heating the air 
 at the equator, and, indeed, all 
 winds. The investigations into 
 the temperature of the sea at 
 various depths, made on board 
 H.M.S. Porcupine, Challenger, &c., 
 show that in the great oceanic 
 basins there is a constant cir- 
 culation of their waters, both 
 vertical and horizontal ; that the 
 upper layers are moving from the 
 equator towards the poles, while 
 the lower layers are gradually 
 creeping along the floor of the 
 oceans from the poles towards the equator ; both 
 upper and lower currents, however, being in- 
 fluenced in their direction, just as the trade- 
 winds are, by the rotation of the earth upon its 
 axis, the northerly and upper current in the 
 northern hemisphere being turned in an easterly 
 direction, and the southerly and lower current 
 in the same hemisphere taking a westerly set. 
 There is good reason to believe that the maintain- 
 ing cause of this grand circulatory system is to be 
 sought rather in polar cold than in equatorial heat. 
 
 FIG. 15. 
 
HEAT, A FORM OF ENERGY. 
 
 43 
 
 The interchange of heat between bodies not 
 in contact, takes place by the process known as 
 radiation. It is in this way that any heated body 
 cools in the air that we are sensible of heat 
 when we approach a fire that the heat of the 
 sun reaches the earth and the various planets. 
 Eadiant heat passes through space in straight 
 lines at the same speed as light, and can be re- 
 flected by mirrors and refracted by lenses in pre- 
 cisely the same manner, a familiar illustration of 
 which is seen in the so-called burning-glass. 
 
 The nature of the surface of a body has a 
 very great influence upon the rate at which it 
 loses heat by radiation, and since those surfaces 
 which radiate heat best, also absorb it most 
 readily, these differences have very important 
 bearings upon natural phenomena, and, there- 
 fore, indirectly upon human life. Moreover, the 
 power of a surface to reflect heat is the com- 
 plement of its power to radiate or absorb it, 
 as is seen by the following table, the totals of 
 the two numbers opposite any substance being 
 100 in all cases : 
 
 Eadiating 
 powers. 
 
 . 100 
 90 
 17 
 17 
 
 7 
 
 7 
 
 5 
 
 3 
 
 Lamp black 
 Glass . 
 Steel . 
 Platinum 
 Polished brass 
 Red copper 
 Gold . 
 Polished silver 
 
 
 Reflecting 
 powers. 
 
 
 
 
 10 
 
 
 83 
 
 
 -83 
 
 
 93 
 
 
 93 
 
 
 95 
 
 
 97 
 
44 ENERGY IN NATURE. 
 
 The properties of radiant heat have been made 
 a special subject of experimental study by Prof. 
 Tyndall, who has shown how closely allied they 
 are to those of the other form of energy which 
 we recognise as light. The term radiant energy 
 may, perhaps, be used to include them both, 
 and a very pretty illustration of 
 its effects is seen in the Kadio- 
 meter of Mr. Crookes, in which 
 four small mica vanes, blackened 
 on one side, are mounted upon a 
 pivot, and the whole arrangement 
 is placed in a globe from which 
 air is then almost completely ex- 
 hausted (Fig. 16). When radiant 
 energy, as the light of a candle, 
 is allowed to fall on the globe, 
 it is absorbed by the black sides 
 
 FIG 16 ^ ^ e vanes > an ^ the molecular 
 
 motion thus set up among the 
 particles of gas still left inside the globe is at 
 once transformed into visible motion, and the 
 vanes rotate rapidly, with a speed varying partly 
 with the intensity of the light. 
 
 It must not be hastily concluded, however, 
 that radiant heat and light are identical, although 
 they are propagated in the same way, viz., by 
 wave-motion in that ether, which, according' to 
 the " undulatory theory" now generally accepted, 
 pervades all space. The length of these waves 
 
HEAT, A FORM OF ENERGY. 45 
 
 can be measured, and those which affect the 
 sense of touch as heat are much longer than 
 those which affect the eye as light. Further, 
 many substances, glass for example, are quite 
 transparent to light, but opaque to radiant heat ; 
 while others, such as iodine in solution, are abso- 
 lutely opaque to light, but permit radiant heat 
 to pass with the greatest ease. The presence of 
 more or less moisture in the air has a most im- 
 portant influence on the passage through it of 
 radiant heat. Moreover, it must be borne in 
 mind that the passage of radiant heat, as such, 
 through any medium does not heat it at all; 
 radiant heat only becomes sensible heat when 
 the waves by which it is propagated are ab- 
 sorbed, instead of being either reflected or trans- 
 mitted. 
 
 We now pass to the consideration of the 
 effects of heat upon Matter, one of the first and 
 most obvious of which is that, with one or two 
 exceptions, all bodies expand when heated, and 
 contract when cooled. In the case of solids this 
 expansion takes place in the three directions of 
 length, breadth and thickness. The linear ex- 
 pansion of a rod by heat has been used in an 
 instrument called a pyrometer to measure very 
 high temperatures, in which an expanding rod 
 presses against the short end of a bent lever, 
 the long end of which moves a pointer upon 
 a scale. Solids vary much in the amount of 
 
46 ENERGY IN NATURE. 
 
 their expansion under the same rise of tempera- 
 ture, and the mechanical energy of this mole- 
 cular motion under changes of temperature, is 
 very great; an iron bar, for example, one inch 
 square, cooled through 80 Fahr., contracts with 
 a pull of fifty tons. Advantage has been some- 
 times taken of this fact to restore to the per- 
 pendicular the bulging walls of a building, iron 
 tie-rods being placed across, and while they 
 were red-hot the nuts on the ends of the rods 
 outside the walls were screwed up tight; on 
 cooling, the contraction of the rods drew the 
 walls together. The practice of " shrinking" 
 tyres on wheels is an example of the force of 
 contraction. 
 
 As a general rule liquids expand more than 
 solids, but also vary much among themselves. 
 The indications of the thermometer depend 
 upon the expansion of liquids by heat. A glass 
 bulb, from which projects a long fine tube, is 
 filled with either mercury or alcohol ; in order 
 to graduate it, it is plunged first into melting ice, 
 arid a mark is made on the stem where the column 
 of fluid becomes stationary ; the same process is 
 then repeated with boiling water. The interval 
 between these two points is variously divided ; 
 the scientific division, also very largely used on 
 the continent, is into 100 parts, the lowest mark 
 (melting ice) being called 0, and the highest 
 (boiling water) 100. This is called the centi- 
 
HEAT, A FORM OF ENERGY. 47 
 
 grade, or " hundred steps" scale, and ought to 
 be universally adopted. In Northern Europe 
 the interval is divided into 80, melting ice being 
 0, a scale known as Reaumur's. In English- 
 speaking countries the scale is that of Fahrenheit, 
 in which the interval is divided into 180 parts; 
 in this case, 
 
 however, the 
 0, or zero- 
 point, is not 
 the tempera- 
 ture of melt- 
 ing ice, but 
 a point as 
 much below 
 that as cor- 
 responds in 
 distance on 
 the scale to 
 32 of these 
 180 parts. 
 Hence, on 
 
 Water 
 
 4) 
 
 _M 
 
 "c 
 
 
 
 S Reaumur 
 
 rf o 
 
 boils 
 Water 
 
 90 
 80 
 70 
 60 
 50 
 40 
 30 
 20 
 
 io c 
 
 _^o 
 
 70 
 60 
 60 
 40 
 30 
 20 
 10 
 
 
 freezes 
 
 C 
 
 ; 
 
 FIG. I 7 . 
 
 1 
 
 I 
 
 d 
 
 " o 
 -212 
 
 32- 
 
 Fahrenheit's scale, the " freezing-point," which 
 is the zero of the two other scales, is marked 
 32, and the boiling-point of water is 32 + 
 180, or 212. Since, therefore, the same in- 
 terval, i.e., that between the temperatures of 
 melting ice and of boiling water, is variously 
 divided (Fig. 17) into 100, 80, and 180, the 
 proportions of degrees on the Centigrade, Reau- 
 
48 ENERGY IN NATURE. 
 
 mur, and Fahrenheit scales are as 100 to 80 to 180, 
 or as 5 to 4 to 9, or in other words, 5 C. = 
 4 R. = 9 Fahr. The temperatures of melting 
 ice and of boiling water being the two fixed 
 points, all degrees above and below those are 
 obtained by simply prolonging the scale in either 
 direction. 
 
 The most notable exception to the law of 
 expansion of liquids by heat is the case of fresh 
 water, which is at its greatest density (i.e., is 
 heaviest in proportion to its bulk) at 4 C. 
 (39 Fahr.). Heated above, or cooled below, 
 this point, it expands, and when it is converted 
 into ice the expansion is sudden and consider- 
 able, the ice, as is well known, floating on the 
 surface. Were it not for this remarkable excep- 
 tion to the general law, fresh- water lakes would 
 become solid masses of ice in severe winters, all 
 animal life therein would probably perish, and 
 the climate in their neighbourhood would be- 
 come quite rigorous. 
 
 It is a somewhat remarkable fact that, in 
 the absence of experiments, physicists somewhat 
 hastily assumed that sea- water followed the same 
 rule, and hence predicted that "in all deep seas 
 a temperature of 4 C. (39 Fahr.) would be 
 found to prevail."* Experiment shows, however, 
 that sea- water continues to contract down to its 
 freezing-point, and the deep-sea temperature 
 
 * Sir J. Herschel's "Physical Geography," 1861. 
 

 HEAT, A FORM OF EXfl&C&.j 4i> 
 
 i^JY^ 
 
 observations before referred to confinf 
 
 and demonstrate the important consequences that 
 
 follow from it. 
 
 The linear expansion of metals heated be- 
 tween the freezing and boiling points of water, 
 varies from about one to three parts in 1,000. 
 Water similarly treated 
 undergoes a total in- 
 crease in volume of 
 43-15, i.e., 1,000 gallons 
 would become 1,043*. 
 Three cubic feet of air, 
 or of almost any gas, 
 heated under the same 
 circumstances, would 
 become four, or, more 
 exactly, 1,000 cubic feet 
 would become 1,807, 
 provided that the pres- 
 sure were unaltered. 
 
 The expansion of gases by heat may be 
 readily shown by heating some in a vessel (Fig. 
 1) provided with a tube, the open end of which 
 dips under a vessel previously closed at one end, 
 filled with water, and inverted. The ascent of 
 a fire-balloon, the ventilation of mines, the as- 
 cending' currents of air which produce winds 
 (and which under various conditions of moisture 
 (ause clouds also), are all illustrations of the 
 effects of alterations in the density or specific 
 E 
 
 FIG. 18. 
 
50 ENERGY IN NATURE. 
 
 gravity (p. 10) of air, produced by the expan- 
 sion caused by heat. 
 
 Having now considered the alterations in the 
 mass of a body produced by heat, let us consider 
 more closely its effect upon the molecules of 
 which that body is composed. It was pointed 
 out in the last chapter, water being taken as an 
 illustration, that whether any substance was in 
 the solid, liquid, or gaseous form depended in 
 great measure upon the temperature and pressure 
 to which it was exposed. By the use of intense 
 eold and severe pressure, even the so-called per- 
 manent gases such as oxygen and hydrogen 
 have recently been condensed to liquids. On the 
 other hand, the intense heat of the arc between 
 the carbon points of the electric light (Chap. V.) 
 is sufficient not only to melt, but to turn into 
 vapour, the most infusible metals. Heat-energy, 
 therefore, changes the molecular state of matter. 
 
 If, however, the change be more closely 
 examined, it will be found that in the passage 
 of any substance from the solid to the liquid, or 
 from the liquid to the gaseous state, an enormous 
 quantity of heat disappears ; and that any change 
 in the reverse direction is always accompanied by 
 the apparent production of heat. For example, 
 if 1 Ib. of water at C. (32 Fahr.), and 1 Ib. of 
 water at 77-8 C. (172 Fahr.) be mixed together, 
 the result will be 2 Ibs. of water at 39 C. (102 
 Fahr.). On the other hand, the temperature of a 
 
HEAT, A FORM OF ENERGY. 51 
 
 mixture of 1 Ib. of ice at C. and 1 Ib. of water 
 at 77-8 C., will be found to be only C. 
 What lias become of this heat ? The only dif- 
 ference between the two experiments is, that in 
 the first case liquid water was used, and in the 
 second ice, or solid water. In the older books 
 the phenomena was said to be explained by say- 
 ing that the heat which thus disappeared " became 
 latent," and the phrase is still in use to express 
 the fact, but it does not explain it. Why does the 
 heat become latent ? The true explanation, upon 
 the principle of the conservation of energy, is 
 that it is used up in overcoming the cohesive force 
 of the molecules of water, and is thus trans- 
 formed into a kind of energy of position. 
 
 Again, it can be shown experimentally that 
 1 Ib. of water at 100 C. in being turned into 
 steam, absorbs enough heat to raise 537 Ibs. one 
 degree in temperature, and yet the steam is no 
 hotter. In this case also the heat has " conferred 
 potential energy upon the atoms," as any attempt 
 to make the experiment in a confined space will 
 immediately render evident ! A cubic inch of 
 water produces nearly a cubic foot of steam. 
 
 It may be stated generally, then, that change 
 of state in the direction of solid to gas is accom- 
 panied by the absorption of heat, or, in other 
 words, the production of cold. Freezing mix- 
 tures, in which certain substances snow and 
 salt, for example rapidly liquefy when brought 
 E 2 
 
52 ENERGY IN NATURE. 
 
 into contact, depend upon this principle, as well as 
 all those freezing machines which owe their action 
 to the rapid vaporisation of some volatile liquid, 
 as ether, liquid sulphurous acid, or solution of 
 ammonia. In all these cases the heat which is 
 thus abstracted, reappears in that part of the 
 machine devoted to the condensation of the 
 vapour. 
 
 In a similar way, whenever work is spent 
 upon a gas, as it is when air (or gas) is com- 
 pressed by mechanical means, heat is evolved, 
 and when that gas is allowed to expand again by 
 the removal of the pressure, heat is absorbed, or, 
 in other words, cold is produced. The heat pro- 
 duced by the compression of air may be shown 
 experimentally by placing a piece of dry tinder 
 under the piston of an air-syringe, closing the 
 mouth of the cylinder, and smartly driving the 
 piston to the bottom of it ; the heat thus evolved 
 will ignite the tinder, as may be seen when the 
 piston is withdrawn. 
 
 During the last few years several very success- 
 ful attempts have been made by various practical 
 inventors, such as Bell-Col email, Hargreaves, and 
 others, to take advantage of the production of 
 cold when compressed air is allowed to expand, 
 and to construct cooling machines upon this prin- 
 ciple. Such machines (Fig. 19) are now exten- 
 sively used for freezing meat, and for maintain- 
 ing so low a temperature in the chambers in 
 
tttiAT, A FORM OF ENERGY. 53 
 
 which it is stored, that several cargoes of fresh 
 meat have lately been brought to England from 
 Australia and New Zealand in such good con- 
 dition as to be indistinguishable from home- 
 grown meat. The air is compressed by pumps, 
 operated by a small steam-engine, and confined 
 in strong reservoirs, round which sea-water is 
 
 allowed to flow, in order to cool the compressed 
 air to the surface-temperature of the ocean, after 
 which it is allowed to expand into the frozen 
 meat-chamber, which is of course kept closed 
 (except as to the air entrances and exits) and 
 surrounded with non-conducting material. The 
 temperature of this chamber is controlled by 
 regulating the volume of air passing into it, the 
 quantity required being naturally dependent 
 
54 ENERGY IN N 
 
 upon its temperature, and this again upon the 
 pressure from which it is allowed to expand. A 
 very obvious advantage of this process is that no 
 " chemicals " are employed in it by which the 
 flavour of the meat can possibly be affected. 
 
 Let us now consider that machine which 
 effects the transformation of heat into work for 
 practical purposes the Steam-engine. It is well 
 known that the work, or motion, is produced 
 by the expansive force of steam, which is ad- 
 mitted alternately on either side of a piston 
 fitted tightly in a closed cylinder in which it 
 moves to and fro, and that when it has done 
 its work the steam passes either into the air (as 
 in locomotives and other " high-pressure " en- 
 gines), or into a cold chamber or condenser (as 
 in the ordinary marine and stationary engine). 
 It will probably surprise many, however, to be 
 told how very imperfect a machine even the 
 most modern type of engine is, not more than 
 eighteen per cent., or less than one-fifth, of the 
 energy generated by the combustion of the fuel 
 being given back as mechanical work. In fact, 
 a no less eminent authority than Sir W. Arm- 
 strong has stated that, for practical purposes, 
 if the whole potential energy of the coat be 
 divided into ten parts we should find that two 
 went up the chimney, one was lost by radia- 
 tion and friction, only one was turned into work, 
 and the remaining six were wasted ! If it were 
 
HEAT, A FORM OF ENERGY. 55 
 
 not that coal is so cheap, and every other form 
 of potential energy that we can buy is so dear, 
 we should find the steam-engine very expensive 
 to use. An attempt will now be made to show 
 how this comes about, and why no very great 
 improvement in the steam-engine is to be ex- 
 pected. 
 
 There is one condition which must be rigidly 
 fulfilled in order to get mechanical work out of 
 heat there must be a difference of temperature, 
 and the heat must pass from a body of high 
 temperature to one of low. An analogy may 
 here be drawn with the case of water, out of which 
 no work can be got unless it flows from a higher 
 to a lower level (p. 27). What difference in 
 temperature, then, is it possible to maintain in 
 practice between the boiler and the condenser of 
 the steam-engine ? In giving an answer to this 
 question we must take into account not merely 
 the thermometric differences, but the absolute 
 quantity of heat, or of heat-units, in each, and 
 to do this we must enquire what is the absolute 
 zero of temperature. The law of expansion of 
 gases by heat tells us that gases expand 373 of 
 their volume for every increment of 1 C. in 
 temperature between and 100 C., so that 
 at 4- 273 C., the elastic force of a gas is double 
 what it is at Q C. Supposing the same law 
 to hold good in the other direction, at -273, 
 i.e., 273 below zero, the gas would have no 
 
53 ENERGY IN NATURE. 
 
 elastic force at all. This point, then, -273 Q C., 
 or -4()1 Falir., may be considered as probably 
 the absolute zero of temperature, although it has 
 never been actually readied. This number of 
 degrees, therefore, must be added to the thermo- 
 metric degrees in any such calculation. Let us 
 assume the case of an ordinary engine with 
 steam at three and a half atmospheres pressure, 
 or 53 Ibs. per square inch. The temperature of 
 this is 300 Fahr. The condenser cannot prac- 
 tically be kept below 110 Fahr. Hence we 
 have : 
 
 Heat in boiler . . 300 + 461 = 761 units of heat. 
 Heat in condenser . 110 +461 = 571 
 
 Difference, available for work . 190 ,, ,, 
 
 Or only one-fourth of the total energy (100 
 parts out of 761) is available for the production 
 of motion, even supposing that there were no 
 other sources of loss, such as friction, radiation, 
 &c. How much more perfect a machine in this 
 respect is nature's engine, i.e., the human body, 
 in which energy is derived from the combustion 
 of food, will be seen in Chap. VI. (p. 189). 
 
 It has been pointed out by Thomson, that 
 although work can be transformed into heat with 
 the greatest ease, there is no process known by 
 which all the heat can be changed back again into 
 work ; that, in fact, the process is not a reversible 
 one. The consequence is that the mechanical 
 
HEAT, A FORM OF ENERGY. 57 
 
 energy of the universe is daily becoming more 
 and more changed into heat, that heat being of 
 a low grade, and (as we have seen in the case 
 of the steam-engine) inconvertible. Hence it is 
 conceivable that a time may ultimately arrive 
 when " the universe will become an equally 
 heated mass, utterly worthless as far as the pro- 
 duction of work is concerned, since such produc- 
 tion depends upon difference of temperature. 
 Although therefore, in a strictly mechanical sense, 
 there is a conservation of energy, yet, as regards 
 usefulness or fitness for living beings, the energy 
 of the universe is in process of deterioration. 
 Universally diffused heat forms what we may call 
 the great waste-heap of the universe, and this is 
 growing larger year by year. . . . We are 
 led to look to a beginning in which the particles 
 of matter were in a diffuse chaotic state, 
 but endowed with the power of gravitation, 
 and we are led to look to an end in which 
 the whole universe will be one equally-heated 
 inert mass, and from which everything like life, 
 or motion, or beauty, will have utterly gone 
 away."* This is the doctrine known under the 
 name of the " Dissipation of Energy," and, 
 although very suggestive, its consideration should 
 bo entered upon with the recollection that it 
 applies solely to the physical universe, or rather 
 to such portions of it as our senses can appreciate. 
 
 * " Conservation of Energy," by Bulfour Stowurt, p. 153. 
 
58 ENERGY IN NATURE. 
 
 Reference has already been made to the in- 
 timate association of heat and light, and it may 
 be well here to point out that all bodies when 
 heated sufficiently give out light, and that the 
 colour of the light in the case of solids and 
 liquids (melted metals, for example) depends 
 upon the temperature to which the body is 
 heated. The phrases in common use, such as red- 
 hot, white- 
 hot, & c . , 
 applied to 
 metals, re- 
 cognise 
 this f act. 
 A very re- 
 fined me- 
 t h o d of 
 examining; 
 
 FIG. 20. -, 
 
 or analy- 
 sing the colour of light, is to pass a beam of 
 it through a triangular piece of glass, called a 
 prism. A coloured band is then seen, in which 
 the various tints are separated from each other 
 (Fig. 20), and to this coloured band the name 
 spectrum is given. In the case of the rainbow, 
 we see the spectrum of the sun's light apparently 
 projected in the air. The examination of vari- 
 ous kinds of light with an instrument called a 
 spectroscope (the essential parts of which are a 
 prism, a slit to narrow the beam of light, lenses, 
 
HEAT, A FORM OF ENERGY. 59 
 
 and an eye-piece) has shown (1) that all solid and 
 liquid bodies when heated sufficiently give out 
 all the various kinds of light, or, in other words, 
 that their spectra are all the same, whatever their 
 substance is, and are continuous bands of colour ; 
 and (2) that when gases or vapours are heated 
 sufficiently to give out light they only give out a 
 few kinds, and that no two elementary substances 
 give out the same kind. In other words, the 
 spectra of glowing gases (i.e., of the vapours of 
 metals, for example) are isolated bands of various 
 colours, in groups characteristic of each gas. 
 These facts are at the base of the whole science 
 of spectrum analysis, whether applied to the 
 detection of minute quantities of terrestrial sub- 
 stances on the earth, or to the recognition 
 of their presence in the atmospheres of the 
 sun, stars, comets, nebulae, &c., since the mere 
 examination of any light enables the trained 
 observer to say with certainty whether the light- 
 source is a glowing solid or liquid on the one 
 hand, or glowing gas on the other, and if it be a 
 gas, to form an accurate notion of its nature. He 
 is also able to watch the constant changes going 
 on in the atmosphere of that source of nearly all 
 terrestrial energy, the sun, to say approximately 
 the temperatures and pressures to which the 
 glowing gases are subjected, and, more wonderful 
 still, to estimate with tolerable accuracy the rate 
 at which some of the heavenly bodies are moving 
 
60 ENERGY IN NATURE. 
 
 towards and from the earth in the direct line of 
 sight ! 
 
 Hitherto we have only considered the phy- 
 sical effects of heat upon matter, in which no 
 change takes place in the nature of the substance 
 itself, but only in its mood or condition. One of 
 the most important of the effects of heat, how- 
 ever, is that of promoting in matter those changes 
 which are known as chemical, in which more 
 than one kind of matter, or one substance, takes 
 part, and which result in the production of a 
 third substance different from either. For ex- 
 ample, in every coal-cellar containing coal there 
 is in the fuel a large amount of potential energy 
 ultimately derived from the sun (Chap. VI.), and 
 the oxygen necessary for the combustion of that 
 fuel is present also ; the fire, however, does not 
 barn. In order to bring into play the energy of 
 chemical attraction, the application of heat to 
 a portion of the coal is necessary, and when this 
 has once been done the chemical action continues, 
 a large amount of heat is produced, and at the 
 same time the coal disappears, being converted 
 into carbonic acid gas. In this conversion of 
 chemical energy into heat energy, which will be 
 specially considered in the next chapter, the heat 
 may be regarded as the mechanical result of the 
 collision of the atoms of the carbon and oxygen. 
 
CHAPTER III. 
 
 CHEMICAL ATTRACTION, ESPECIALLY COMBUSTION. 
 
 THUS far we have been considering the effect of 
 heat- energy upon one kind of matter or one sub- 
 stance at a time, changes in which the body 
 undergoes no alteration in its kind, but only in its 
 mood or condition. To changes of this nature 
 the term physical is frequently given, in contrast 
 to the term chemical, which implies the fact that 
 two or more different kinds of matter are con- 
 cerned in producing the effect observed, and 
 result in the formation of a third substance dif- 
 fering in properties from either of the two with 
 which the experiment is made. The present 
 chapters deals with the relations between heat 
 and chemical attraction, while the next one will 
 deal, in part, with the relations between chemical 
 attraction and electrical energy. We shall pre- 
 sently see evidence of the broad fact that when- 
 ever different substances combine under the in- 
 fluence of chemical attraction, heat is produced 
 or evolved ; and it will also be shown that, when 
 it is desired to reverse this change, when it is 
 wished to undo that work and to separate the two 
 substances again, the application of heat to them 
 
62 ENERGY IN NATURE. 
 
 will in many instances effect that as well. In 
 short, we shall see how much "potential energy" is 
 stored up in substances between which there is a 
 strong chemical attraction , and how great an in- 
 fluence heat has upon the question whether these 
 different substances shall be attracted to, or re- 
 pelled from, each other. 
 
 Although a strongly-marked distinction has 
 been drawn between physical and chemical 
 changes, it must be taken in the same sense, for 
 example, as when the difference between plants 
 and animals is exemplified by the instances of an 
 oak-tree and a cow. The tendency of all scientific 
 research is to obliterate these strongly-marked 
 lines of demarcation and classification, just as 
 among the lower forms of life it is frequently 
 a matter of doubt whether a given organism is 
 an animal or a vegetable (and, indeed, there is 
 the best reason to believe, in one instance at least, 
 that the same organism may be both animal and 
 vegetable at different periods of its life history), 
 so also are there actions, partly physical, partly 
 chemical, which it is difficult to assign to either 
 one class or the other. 
 
 Let us consider now a couple of simple illus- 
 trations of chemical action between two different 
 bodies, and the influence of heat upon them. 
 If a mixture be made of iron filings and flowers 
 of sulphur, a grey powder is produced, in which 
 the particles of iron and of sulphur each have 
 
CHEMICAL ATTRACTION. 63 
 
 the properties that characterise larger masses of 
 iron and sulphur ; for example, a magnet applied 
 to the mass will draw out of it all the iron filings. 
 If, however, heat be applied to one part of it, 
 enough to melt a very small portion of the 
 sulphur, the whole mass will speedily glow with 
 a bright red heat, and when it is cold it will 
 be a dense compact mass, utterly unlike either 
 iron or sulphur, and unacted upon by a magnet. 
 If the experiment be made out of contact of 
 air, the mass will be found to weigh exactly 
 the same as the powder did before it was heated. 
 Again, powdered charcoal and sulphur mixed 
 together give another grey powder ; if, however, 
 the sulphur be (not melted this time but) turned 
 into vapour in a closed vessel, and bits of hot 
 charcoal be dropped into it, the charcoal and sul- 
 phur will unite chemically, and if the closed vessel 
 be connected with a condenser, the result of the 
 experiment will be found to be a clear bright 
 liquid, as white as water, but very much heavier, 
 known as carbon bisulphide, which is highly 
 inflammable, and will dissolve many things which 
 water will not. 
 
 In order, then, that chemical attraction may 
 take place between the (atoms or) molecules of 
 two substances, it is necessary to bring them into 
 very close and intimate contact. This is very 
 conveniently done by heat, which, as we have 
 seen, tends to separate molecules from each other, 
 
64 ENERGY IN NATURE. 
 
 and thus to make easier the passage of two sets 
 of molecules between each other. * Any very fine 
 extension of their surface, or severe pressure, 
 however, will often bring two bodies within the 
 sphere of chemical attraction without any heat 
 whatever, a subject which has formed the basis 
 of some curious recent investigations. Instances 
 of some of these points will come before us in 
 this chapter. 
 
 Since we have now to deal with different 
 Jdnds of matter, it may be well to state here that 
 all kinds of matter known to the chemist are 
 either simple, i.e., of one kind of substance only, 
 or compound, made up of two or more of the 
 simple ones. These simple substances, which 
 cannot by any known process be separated into 
 two others, are called elements. About seventy 
 elements are at present known, and of these 
 about sixty are the pure metals, iron, copper, 
 silver, &c. Of the remaining ten or twelve, 
 which are not metals, some are solid, such as 
 charcoal, sulphur, and iodine ; one is liquid (bro- 
 mine), while others are gases at the ordinary pres- 
 sure and temperature, such as oxygen, nitrogen, 
 hydrogen, and chlorine. As examples of chemical 
 compounds we may cite chalk, made up of carbon, 
 
 * A rough illustration of this effect of heat was afforded by 
 the speaker presenting his right and left hands to each other, with 
 the fingers closed, when they could not interlace. Supposing heat 
 to expand the interval between the molecules i.e., the fingers the 
 hands could then be interlocked. 
 
CHEMICAL ATTRACTION. 65 
 
 oxygen, and the metal calcium ; common salt, 
 made up of sodium and chlorine ; water, made 
 up of oxygen and hydrogen ; and bread, meat, 
 and most foods, made up of the four simple ele- 
 ments carbon, hydrogen, oxygen, and nitrogen. 
 (The amount of energy to be got out of different 
 kinds of food when we eat them, and the chemi- 
 cal changes which they undergo in our bodies, 
 will be considered in Chap. VI.) 
 
 Having now a clear idea as to the difference 
 between an element and a compound, let us 
 consider some instances of the general statement 
 that when two or more elements combine and 
 produce a compound, their atoms rush together 
 with great force, and the amount of heat 
 developed by their collision is proportional 
 to the mutual attraction of their respective 
 atoms. 
 
 The most familiar example is presented by 
 all cases of combustion, whether that be carried 
 on merely for the domestic uses of warming and 
 cooking, or for metallurgical purposes, as in the 
 smelting of metals, or for the production of 
 mechanical power by the aid of the steam-engine, 
 or for the projection of missiles of war and the 
 blasting of rocks, as in the combustion of gun 
 powder. In all these instances, then, the heat- 
 energy is developed from the chemical attraction 
 between the fuel that is burnt and the element 
 oxygen, which, in all cases except the last (to be 
 F 
 
66 ENERGY IN NATURE. 
 
 considered later), is supplied by the air around us, 
 of which it forms one-fifth, and as a result com- 
 pounds are produced which are neither fuel nor 
 oxygen, but which contain both, and from which, 
 by undoing the work done in combustion by 
 reversing the process, by unburning them, as it 
 were the oxygen and the constituents of the 
 fuel may be recovered again. 
 
 Speaking broadly, all the substances that are 
 used as fuel, or for the production of light (ex- 
 cept electric lighting, which will be dealt with in 
 Chap. V.) are made up of very little else than the 
 two elements carbon and hydrogen, in different 
 proportions. Charcoal, and that peculiar hard 
 shiny coal called anthracite, are nearly pure 
 carbon. Paraffin oil arid coal-gas are almost 
 entirely composed of carbon and hydrogen. 
 Wood and bituminous coal, i.e., the ordinary 
 caking coal that we use, contain small quanti- 
 ties of oxygen, in addition to the carbon and 
 hydrogen, as do also the animal and vegetable 
 oils, and that elegant but expensive form of fuel, 
 spirits of wine. The difference in properties of 
 these various substances, and the various ways 
 in which they behave when burnt, are largely 
 due to the great differences in the relative pro- 
 portions of hydrogen and carbon which they 
 contain. Paraffin oil and spirits of wine, when 
 burnt in similar lamps, behave very differently, 
 the former giving a dull and very smoky 
 
CHEMICAL ATTRACTION. 67 
 
 flame, the latter a clear non-luminous blue one; 
 and chemical analysis shows that the propor- 
 tion by weight of carbon to hydrogen is as 
 4 to 1 in the first, and as 6 to 1 in the 
 second. Smoke, as is well known, arises from 
 the imperfect combustion of the fuel, due to 
 a deficiency in the supply of air, or oxygen, 
 and it is almost entirely composed of a mix- 
 ture of other compounds of hydrogen and carbon, 
 and of almost pure carbon. The formation of 
 these products is due to the fact that in so many 
 of our fire-places the coal frequently undergoes 
 a sort of rough distillation (before it is actually 
 burnt) without any attempt being made to con- 
 dense the products. Before going farther into 
 this question, however, it will be well to be- 
 come acquainted with the chemical properties 
 of oxygen, hydrogen, &c., and with the laws 
 according to which these substances combine 
 together. 
 
 One of the most important laws in chemistry 
 is known as the law of combining proportions ; 
 it is the general expression of the fact that when 
 two (or more) substances combine together to 
 produce a third, they do so in certain definite 
 quantities, or multiples of those quantities, which 
 never vary. It is upon this invariability that 
 the whole science of chemistry depends ; but for 
 it exact chemical analysis would be impossible. 
 Each element has its own combining proportion, 
 F 2 
 
68 ENERGY IN NATURE. 
 
 and the numbers in the case of the three ele- 
 ments we are considering- are- 
 Carbon . . . * . ..12 
 Oxygen . , . . .16 
 
 Hydrogen . . . . . . 1 
 
 Hence 12 parts of carbon combine with 16 
 parts (or a multiple of 16) of oxygen, and so 
 on. In this particular case the compound of 
 12 parts of carbon and 16 of oxygen is a colour- 
 less, poisonous, inflammable gas, called carbonic 
 oxide, which we often see burning with a pale 
 blue or yellowish flame on the top of a clear 
 coal fire; while the compound of 12 carbon to 
 (twice 16 or) 32 oxygen is the gas usually known 
 as carbonic acid, Avith which we shall shortly be 
 concerned. No other compounds of carbon and 
 oxygen only, are known, and should such be dis- 
 covered they will be found to contain carbon and 
 oxygen, in the proportion of multiples of 12 and 16. 
 
 The application of this law in the present 
 instance is that when fuel is completely burnt 
 
 12 parts (by weight) of carbon take 32 parts oxygen, 
 
 forming 44 parts carbonic acid. 
 2 parts (by weight) of hydrogen take 16 parts oxygen, 
 
 forming 18 parts water. 
 
 Or, in this case by measure, oxygen being six- 
 teen times as heavy as hydrogen 
 
 2 volumes of hydrogen take 1 volume of oxygen, form- 
 ing 2 volumes of steam. 
 
CHEMICAL ATTRACTION. 69 
 
 It is a very practical point, and one which 
 cannot be too strongly or too clearly brought 
 home to those in charge of furnaces, steam-boilers, 
 &c., that (assuming for a moment that coal is 
 nearly pure carbon, as it actually is in some 
 instances) every 12 tons of coal require 32 tons 
 of oxygen for their complete combustion, and as 
 oxygen is only one-fifth of the air, they require 
 1GO tons of air, or practically a ton of coal re- , 
 quires 14 tons of air (or nearly 410,000 cubic feet] 
 to be passed over it in order to burn it completely. 
 To move this mass of air requires the expen- 
 diture of considerable energy, and here we 
 see one source of that loss of available energy 
 pointed out in connection with the steam-engine. 
 
 Oxygen may be 
 readily obtained in the 
 pure state by heating 
 some of its compounds 
 in a closed vessel provi- 
 ded with an exit tube, 
 the end of which dips 
 under water, and the gas 
 as it bubbles up (Fig. 21) 
 is collected in a jar pre- 
 viously filled with water 
 and inverted over the 
 end of the pipe. Chlo- 
 rate of potash, and oxide 
 of manganese are the two compounds generally 
 
 FIG. 21. 
 
70 ENERGY IN NATURE. 
 
 employed for this purpose. Oxygen is the most 
 abundant of all the elements, forming eight-ninths 
 
 of the water, nearly one-fourth the air. and about 
 j 
 
 one-half of sand, chalk or limestone, and clay, the 
 three most abundant minerals on the earth's sur- 
 face, as well as entering largely into the composi- 
 tion of most substances. Under ordinary conditions 
 it is a transparent colourless gas, but it has been 
 liquefied by cold and pressure. It is not inflam- 
 mable itself, but substances burn in it with much 
 greater brilliancy than in air, evolving a large 
 amount of heat energy. A steel watch-spring, 
 for example, one end of which is heated white 
 hot in air, if plunged into a jar of oxygen, begins 
 to burn with brilliant scintillations. 
 
 Hydrogen is usually obtained from water by 
 decomposing it (i.e., pulling it asunder), either by 
 the energy of the electric current (p. 116) or 
 by some metal which unites with the oxygen of 
 the water, and turns out the hydrogen. Zinc 
 
 */ 
 
 put into water, to which a little acid has been 
 added, effects this easily ; heat is not required 
 in this case, and the mode of doing it is shown 
 in Fig. 22. 
 
 Hydrogen is the lightest substance in nature, 
 being about one-fourteenth as heavy as air ; soap- 
 bubbles blown with it easily ascend in the atmos- 
 phere. It has also the greatest capacity for 
 heat, or specific heat (p. 38), of any known sub- 
 stance. It is usually a transparent colourless gas, 
 
CHEMICAL ATTRACTION. 
 
 but, like oxygen, it has been liquefied, and 
 probably also solidified, by cold and pressure. 
 Unlike oxygen, it will not allow substances to 
 burn in it, but it is itself inflammable, burning 
 in air with a pale blue flame, and producing 
 water by its combination with oxygen.* The 
 chemical attraction between hydrogen and oxygen 
 is greater than between any other two 
 known substances, and hence their 
 combination pro- 
 duces the most 
 powerful artifi- 
 cial heat which 
 can be produced 
 by purely chemi- 
 cal means. If 
 the two gases be 
 mixed together 
 in the propor- 
 tions by measure necessary to form water (p. 68), 
 and a flame, or red-hot wire, be brought in contact 
 with them, they unite with great explosive 
 violence. In the oxy -hydrogen blow-pipe, the 
 gases are brought to the jet by two separate 
 tubes which unite, like a A, each provided with 
 a cock for regulating the gas supply. The heat 
 
 * The formation of water from the hydrogen of fuel or of gas 
 when burnt, may be readily shown by holding a cold tumbler or 
 bell-glass momentarily over a flame, and is also seen in the water 
 which on a cold night trickles down the inside of a shop window 
 where many lamps are burning. 
 
 FIG. 22. 
 
72 ENERGY IN NATURE. 
 
 of this jet melts some of the most infusible 
 metals, and heats lime to intense whiteness, pro- 
 ducing the lime-light. For this latter purpose 
 coal-gas is frequently employed in place of pure 
 hydrogen, with but little diminution in the effect. 
 The properties of the other element in fuel, 
 carbon, in its usual form are familiar to most 
 persons a dense black solid, devoid of taste or 
 smell.* When it unites with oxygen, the pro- 
 duct is a heavy poisonous gas known as carbonic 
 acid. This gas is exhaled naturally from various 
 parts of the earth, as a result of the energies 
 that are at work in its interior. It may be 
 readily prepared in the gas-bottle (Fig. 22) by 
 acting on chalk or limestone, which contain 
 nearly half their weight of carbonic acid, by 
 another acid, such as hydrochloric acid (spirits 
 of salt). When limestone is " burnt" in a kiln, 
 the heat expels the carbonic acid and leaves 
 pure lime. Carbonic acid is also a transparent 
 colourless gas, much more soluble in water than 
 oxygen or hydrogen ; it will neither burn itself 
 nor allow anything else to burn in it, and it is 
 poisonous to animal life. It forms the chief part 
 of the fatal choke-damp or after-damp found in 
 coal mines after an explosion. It is given off 
 by all animals when breathing, and is one of the 
 
 * It occurs pure in nature, however, in a crystalline form, and 
 is then known as the diamond. Jet, lampblack, charcoal, &c., as well 
 as the diamond, are other forms of carbon, and they all produce 
 carbonic acid when burnt. 
 
CHEMICAL ATTRACTION. 73 
 
 results of fermentation and of the decay of ani- 
 mal and vegetable matter. A portion of Chap. 
 VI. will be devoted to explaining the source of 
 the physical energy of the human body, which is 
 closely associated with the production therein of 
 carbonic acid resulting from the union of the 
 carbon in our food with the oxygen of the air. 
 
 The frequent formation and occurrence of this 
 gas render it very desirable that a knowledge of 
 its properties, and of some simple tests for its 
 presence, should be widely spread. It is an ex- 
 ceedingly heavy gas, being about half as heavy 
 again as air, with which it does not readily mix ; 
 this can be shown by actually pouring the gas 
 from a vessel full of it into a vessel full of air, 
 when the test of a burning taper will show that it 
 has actually displaced the air, although there was 
 no visible passage of matter from the first vessel 
 to the second.* It cannot be too carefully re- 
 membered that air containing so much car- 
 bonic acid gas that a candle will not burn 
 therein, is unfit also to support human life. A 
 very simple test for it is afforded by clear lime- 
 water (lime stirred up with water, and allowed 
 to settle clear), which becomes milky directly it 
 is shaken up in a bottle with air containing 
 carbonic acid. The production of it in the 
 
 * One practical consequence of this is that it has a great ten- 
 dency to remain at the bottom of old wells, mines, brewers' 
 vats,' &c. 
 
74 ENERGY IN NATURE. 
 
 human body may be shown by drawing air into 
 the lungs through lime-water, which remains 
 clear, and then breathing the same air out again 
 into the lime-water, which at once becomes 
 milky. The amount of carbonic acid in the 
 breath is about 5 per cent (five parts in 100), 
 and since pure air in the open country contains 
 only about three parts in 10,000, and in towns 
 seldom more than four or five in 10,000, while 
 air with only six parts of carbonic acid in 10,000 
 is felt at once to be close and disagreeable, the 
 need of the ventilation of confined spaces is very 
 evident. It has been shown by very careful ex- 
 periments and calculations that in ordinary dwel- 
 ling-rooms of moderate size, the amount of fresh 
 air necessary to be passed through the room in 
 order to keep the proportion of carbonic acid 
 below 6 in 10,000 is about 3,000 cubic feet per 
 hour for each human being, for each lamp or 
 gas-burner, and for each pair of candles.* 
 
 It will now be desirable to consider a little 
 more in detail the conditions under which fuel is 
 burnt for the production of light, or, in other 
 words, the structure of flame. It may be stated 
 broadly at the outset that, under the ordinary 
 practical conditions of daily life, no flame is 
 luminous in which there are not some solid par- 
 
 * It is beyond the scope of this little be ok to go more fully iiito 
 this important question of respiration and ventilation, but the 
 reader will find it fully and yet popularly treated in Professor 
 Hartley's " Air and its Relations to Life" (Longman's). 
 
CHEMICAL ATTRACTION. 75 
 
 tides. The flame of burning hydrogen, for ex- 
 ample, or of the mixture of coal-gas and air in a 
 gauze burner or Bunsen lamp (Fig. 23), gives out 
 scarcely any light ; but if some iron-filings are 
 sprinkled into it, or a metallic wire be held in it, 
 these solid particles will be intensely heated, and 
 will glow, or become incandescent, enough to give 
 out light. Solids may be thus made to glow by 
 the energy of the electric current, as will be seen 
 when the " incandescence electric 
 lamps " are explained (Chaps. IV. 
 and V.). It has been shown above 
 that all our fuel is essentially some 
 compound or other of carbon and 
 hydrogen, and also that the energy 
 of chemical attraction between hy- 
 drogen and oxygen is much greater FIG. 23. 
 than that between carbon and oxy- 
 gen. When, therefore, the combustion is so ar- 
 ranged that there is not enough oxygen present 
 to burn both completely, the hydrogen is burnt 
 first, and either pure carbon, or, as is more 
 probable, certain other compounds of carbon 
 and hydrogen containing a greater proportion 
 of carbon than the original fuel, is left in the 
 flame, to be raised to glowing point by the heat 
 resulting from the collision of the atoms of 
 hydrogen and oxygen. A careful study of a 
 candle-flame (Fig. 24) will show that it consists 
 in the main of three parts : (1) the exterior 
 
ENERGY IN NATURE. 
 
 shell, very faintly luminous, where the combustion 
 of both carbon and hydrogen is complete ; (2) the 
 luminous part of the flame, containing the glow- 
 ing and unburnt carbon compounds ; and (3) the 
 inner, non-luminous portion, where no oxygen 
 penetrates, which consists chiefly of the gaseous 
 fuel. If a cold plate, or even a piece of thick 
 white paper, be suddenly depressed 
 upon a flame, so as 
 to cut it across the 
 middle, a ring of 
 black carbon (or hy- 
 drocarbon) will be 
 deposited on it from the unburnt por- 
 tions in the flame; and if the broken 
 stem of a tobacco-pipe be inserted 
 into the inner non-luminous part of 
 the flame and steadily held in it (Fig. 
 24), inflammable vapours will issue 
 from the other end, and can be igni- 
 ted there. When a candle is blown 
 out these vapours rise from the wick, 
 and the candle can, with care, be rekindled by 
 holding a light in these vapours several inches 
 from it.* 
 
 It appears, then, that the production of arti- 
 ficial light from fuel depends upon a proper 
 
 * These and many other interesting points are fully dealt [with 
 in Faraday's "Chemical History of a Candle" (Chatto and 
 Wmdus). 
 
 FIG. 24. 
 
CHEMICAL ATTRACTION. 77 
 
 adjustment of the supply of oxygen, i.e., of air, 
 which varies with the nature of the fuel em- 
 ployed, and must be so arranged as to prevent 
 the escape into the air of unburnt particles of 
 fuel (i.e., smoke), and yet to leave enough uncon- 
 sumed in the flame to give out light by their incan- 
 descence. The various arrangements of chim- 
 neys, globes, &c., around our lamps all have this 
 object in view. It is, moreover, a curious fact 
 that if the gaseous fuel be heated before it is 
 burnt, it produces much more light ; this has 
 recently been taken advantage of by Sir W. 
 Siemens in the construction of large gas-lamps 
 of enormous power, rivalling the electric light. 
 
 We have seen that if hydrogen and oxygen 
 are mixed and heated to a given point, they 
 combine with explosive violence ; the same thing 
 happens with a proportionately less development 
 of energy, when coal-gas or other inflammable 
 vapours are mixed with air, and more or less 
 heated. To this cause are due the lamentable 
 explosions in our coal mines ; fire- damp, a mix- 
 ture of gases, chiefly hydrocarbons, is pent up in 
 the coal, and is sometimes released, either by the 
 mechanical process of mining, or by a diminution 
 in the pressure of the air, owing to atmospheric 
 changes indicated by the barometer, a sudden 
 fall of which at certain periods of the year is 
 frequently followed in a few hours by a colliery 
 explosion. The fire-damp thus set free mixes 
 
78 ENERGY IN NATURE. 
 
 with the air, and when it comes in contact with a 
 naked flame, or even with red -hot iron (heat, not 
 actual combustion, being all that is necessary to 
 kindle it), a, very rapid explosive combustion takes 
 place. Moreover, recent experiments have con- 
 clusively shown that the presence of finely- divided 
 coal-dust in the air of the mine will communicate 
 this rapid combustion, either alone or assisted by 
 gas ; and further, that any inflammable substance, 
 if sufficiently finely-divided and suspended in the 
 air, will communicate an explosive flame over a 
 large area. Many fires in flour-mills, &c., have 
 been traced to this cause. In fact, the rate at 
 which a flame spreads is largely determined by 
 the extent of surface available for contact be- 
 tween the combustible substance and the oxygen, 
 and also by the heat-conducting power of the 
 substance. Lead-foil, for ^example, cannot be 
 made to burn in the air, but it is possible chemi- 
 cally to obtain lead in such a very fine state of 
 division, that when air has access to it, it begins 
 to burn of itself. The lead pyrophorus, or fire- 
 bearer, is made by heating tartrate of lead in a 
 glass tube till vapours cease to come off, and 
 then sealing the tube. This action, due to in- 
 crease of surface, is the explanation of most 
 cases of spontaneous combustion. Oily rags and 
 greasy sawdust, for example, present large sur- 
 faces of oil to the action of the air they begin 
 to absorb oxygen therefrom, the energy of chemi- 
 
CHEMICAL ATTRACTION. 
 
 79 
 
 cal attraction is exerted, and presently the whole 
 mass bursts into flame. The same process goes 
 on in hay-ricks, and it is not the least of the re- 
 commendations of the new process of preserving 
 green fodder, called ensilage, that 
 the exclusion of oxygen is an 
 essential feature in it, fire-risks 
 being thus avoided.* 
 
 Allusion has been made to the 
 important influence of the heat- 
 conducting power of metals on the 
 spread of flame, and it is upon this 
 that the principle of the safety- 
 lamp (Fig. 25) used in coal mines 
 is based. Such lamps consist es- 
 sentially of an ordinary oil lamp, 
 the flame of which is completely 
 surrounded with fine wire gauze, 
 through the meshes of which only 
 has air any access to the burning 
 oil. When the lighted lamp is 
 placed in an explosive mixture of 
 gas and air, the mixture is kindled and con- 
 tinues to burn inside the gauze cylinder, but 
 the metal wire conducts away the heat so rapidly 
 from the flame thus produced, that the gas on 
 the outside of the cylinder is not kindled, since 
 
 * The introduction of this process from America into this 
 country was recently advocated at the Society of Arts by Prof. 
 Thorold Rogers, M.P.,in a paper which has since been extended and 
 published. 
 
80 ENERGY IN NATURE. 
 
 it is not heated to the necessary point of igni- 
 tion. 
 
 From what has been said about the theory 
 of combustion, it is evident that when it is 
 desired to obtain the greatest possible amount of 
 heat-energy out of fuel, that fuel must be com- 
 pletely burnt, so as to get the energy developed 
 by the burning of the carbon as well as of the 
 hydrogen. On a large scale this is far more prac- 
 ticable when the fuel is gaseous than when it is 
 either solid or liquid, since the supply both of fuel 
 and of air can be regulated to. a nicety by valves, 
 and hence it is that many furnaces now con- 
 structed for large metallurgical and other manu- 
 facturing operations are built in two or more 
 parts, one of which is known as the gas-producer, 
 the object of which is to roughly distil the coal, 
 i.e., turn it into gas before it is burnt. Great 
 economy and many advantages result from this 
 method. The cleanliness and convenience of 
 coal-gas as a fuel for domestic use are now 
 coming to be more generally recognised, o wing- 
 to the perfection of the combustion causing no 
 smoke, and to the absence of dust, &c., arising 
 from the ash, or unburnt (and unburnable) mineral 
 constituents of the coal ; nor must the difference 
 in the mode of delivery into houses of solid and 
 gaseous fuel be overlooked. In the increased 
 use of gas - stoves, gas - fires, and gas - cooking 
 ranges, is unquestionably to be found the remedy 
 
CHEMICAL ATTRACTION. 81 
 
 for the smoke and smoke-fogs of our large towns, 
 and proposals have been made by Sir W. Siemens 
 and others to separate the gaseous products of 
 the distillation of coal into two portions, collecting 
 the first and last portions in one gasometer, and 
 the middle portions in another, in order thus to 
 supply lighting gas of higher illuminating power 
 than at present (the middle portions), and, by a 
 second set of pipes, gas of low illuminating, but 
 great heating, power, for use in stoves, fires, 
 &c. 
 
 In addition to the energy obtained indirectly 
 through heat from chemical attraction, mechanical 
 energy is often obtained directly, in the shape of 
 an explosion. A simple case of this is the gas- 
 engine (Fig. 26), which has been developed 
 during the last few years, and marks an entirely 
 new departure in the artificial development of 
 mechanical energy. In these engines the piston 
 is driven backwards and forwards in the cylinder 
 by the explosion, either in the cylinder itself or 
 in an adjoining chamber, of a mixture of coal- 
 gas and air. The explosion in the early forms 
 was very sudden and rapid, but latterly, by means 
 of automatic valves regulating the supply of air 
 and gas, it has been made more gradual, so as 
 to produce a tolerably uniform motion. The 
 chemical changes that occur in the action of this 
 engine are somewhat complicated, and there is 
 no doubt that the heat produced by the com- 
 G 
 
82 
 
 ENERGY IN NATURE. 
 
 bination ) which expands the gases, largely in- 
 creases the mechanical effect. 
 
 Directly connected with this subject is the 
 transmission of energy from one place to another 
 by the flow of gas. The use of the gas-engine 
 
 Fio. 26. 
 
 is probably at present the largest example of 
 transmitting power, and according to recent cal- 
 culations power can be transmitted by gas at 
 one-twenty-fifth the cost of its transmission by 
 compressed air. 
 
 In most cases where explosive energy is exerted 
 for mechanical purposes, however, matters are so 
 
CHEMICAL ATTRACTION. 83 
 
 arranged by the admixture of various solid or 
 liquid chemical substances, that under the influ- 
 ence of heat and of the chemical action induced 
 by heat, these various substances suddenly re- 
 arrange themselves in different combinations, 
 some of which in all cases are gaseous at the 
 temperature of their production, and it is to this 
 sudden production of gas, whose expansive force 
 is enormous, in a very confined space, that the 
 destructive action of explosives is due. Grun- 
 cotton, for example, cannot be distinguished by 
 the eye from ordinary cotton, but the influence 
 of a comparatively low degree of heat (whether 
 applied directly or derived from percussion) will 
 cause the atoms of which it is composed to change 
 their grouping instantly, and to form fresh com- 
 pounds, all of which in this case are gaseous. 
 This is owing to the fact that, in making this 
 explosive, ordinary cotton (which contains carbon, 
 hydrogen, and oxygen) is soaked in nitric acid, 
 whereby it loses some of its hydrogen, and takes 
 up some nitrogen and a great deal of oxygen, 
 and this oxygen eventually helps to burn the 
 carbon and hydrogen. A similar change is made 
 in glycerine, during the preparation of nitro-gly- 
 cerine, from which dynamite, litho-fracteur, &c., 
 are manufactured by taking up the liquid explo- 
 sive with some absorbent powder. In Ms annual 
 address as President of the Society of Chemical 
 Industry, Sir F. Abel stated that the manufacture 
 G 2 
 
84 ENERGY IN NATURE. 
 
 of dynamite had now (1883) reached the astonish- 
 ing total of 9,000 tons per annum. Blasting 
 gelatine, the " king of explosives," as it has been 
 termed by Sir F. Abel, is a solution of gun-cotton, 
 or nitro-cellulose, in nitre-glycerine. In the case 
 of gunpowder, the charcoal and sulphur which it 
 contains are burnt at the expense of the oxygen 
 in the nitre, a salt which contains nearly half its 
 weight of that gas. When powder explodes, the 
 gases produced by it, when measured at the stan- 
 dard temperature and pressure of C. and 760 
 mm. (i.e., 32 Fahr. and nearly 3 Gin. barometer) 
 are 270 times the original volume of the powder, 
 but so much heat is produced by the chemical 
 action that if the explosion takes place in a space 
 confined to the original volume of the powder, a 
 pressure of 42 tons per square inch is exerted on 
 the walls of that space. The energy imparted to 
 a shot depends upon the rapidity with which the 
 powder disengages its gases, and slow burning 
 powder is, for artillery purposes, much better than 
 quick. If the energy were developed too rapidly, 
 it would be spent upon the powder-chamber, 
 instead of upon the propulsion of the shot, and 
 the gun would burst. The larger the " grain" 
 of the powder the more slowly does it burn, other 
 things being equal, and for very heavy guns the 
 ." grain" is so large as to cause the term " pebble- 
 powder " to be employed. In the case of some 
 experiments in 1881 with the 100-ton gun, a 
 
CHEMICAL ATTRACTION, 85 
 
 projectile of 2,000 Ibs. weight was fired with a 
 charge of 448 Ibs. of pebble-powder, each pebble 
 being about one inch in diameter. The striking 
 energy of this shot as it left the muzzle was 33,500 
 toot- tons. 
 
 Hitherto we have been concerned chiefly with 
 the production of heat when atoms collide, and 
 chemical combination take place, although, in 
 passing, instances have been mentioned where 
 the decomposition or separation of substances 
 was also effected by heat, as, for example, the 
 "burning" of lime, when 50 parts of limestone 
 lose 22 of carbonic acid, leaving 28 of lime. 
 When a compound of two (or more) elements 
 is resolved by great heat into its constituent 
 elements, the process is called dissociation. Thus 
 water may, if the steam be heated hot enough, 
 be dissociated into hydrogen and oxygen; car- 
 bonic acid into carbonic oxide and oxygen, and 
 so on. It is a remarkable fact that the other 
 form of radiant energy which excites in our eyes 
 the sensation of light, also effects both these 
 kinds of chemical change. If a mixture of 
 hydrogen gas and chlorine gas (which is best 
 prepared by decomposing hydrochloric acid by 
 electric energy, Chap. IV.) be exposed to light 
 in a glass vessel, they will unite with explosive 
 violence. On the other hand, a familiar example 
 of the decomposition of compounds by light is 
 found in the beautiful art of photography, where 
 
86 ENERGY IN NATURE. 
 
 colourless substances containing silver are so 
 affected by light that the image or picture pro- 
 duced by a lens is painted by the silver thus 
 set free from its compounds. Another instance, 
 which will be developed at some length in Chap. 
 VI., is the decomposition, or dissociation, of car- 
 bonic acid by growing plants, under the influ- 
 ence of light, when the oxygen is set free, and 
 the carbon enters into the substance of the plant, 
 forming therein a store of potential energy. 
 
CHAPTER IV. 
 
 ELECTRICITY. 
 
 THE first observation recorded with regard to 
 this form of Energy in Nature was made by 
 Thales of Miletus about 2,400 years ago, who 
 observed that the curious 
 substance amber, when 
 rubbed, became temporarily 
 endowed with the property 
 
 FIG. 27. 
 
 (which it had not previously) of attracting 
 to itself light particles in its neighbourhood 
 (Fig. 27). It is from the Greek name for 
 amber yXetcrpov, Electron, that the term Electricity 
 takes its name. The next step was made by 
 
88 ENERGY IN NATURE. 
 
 Dr. Gilbert , in the reign of Queen Elizabeth, 
 who, repeating the experiment of Thales, ex- 
 tended it to other bodies, such as diamond, rock- 
 crystal, glass, sulphur, resin, &c., and showed 
 that the same energy of attraction could be 
 similarly excited in them, and he termed such 
 bodies electrics. Otto Gaiericke, Sir Isaac Newton, 
 and Dr. Franklin made further experiments ; but 
 many years elapsed before the various isolated 
 observations were systematised, and brought into 
 that condition of exact knowledge which we re- 
 cognise as scientific. The science of electricity 
 may be said to have been established by the 
 labours of Franklin, Volta, Galvani, Davy, and 
 above all, of Faraday, during the latter half of 
 the last, and the earlier part of the present cen- 
 tury. 
 
 It may be stated broadly that in every 
 case of friction between (and probably even of 
 contact of) two different bodies, there is a de- 
 velopment of electricity. This is sometimes ex- 
 pressed in another way ; it is said that ' i different 
 bodies are at different potentials with regard to 
 electricity;" the word "potential," in an electric 
 sense, being used merely to express the degree 
 in which a body is electrified. In the majority 
 of instances, however, the effect is not percep- 
 tible, since the electricity passes away instan- 
 taneously by conduction, a process analogous 
 to the conduction of heat (p. 40). Further 
 
ELECTRICITY. 
 
 89 
 
 researches have shown that the phenomena ob- 
 served are best explained (or, at any rate, best 
 described) by assuming the existence of two 
 kinds of electricity, of opposite properties, to 
 which the terms positive and negative, or vitreous 
 
 and resinous, are given ; 
 the production of elec- 
 tricity by friction ap- 
 pears to be due to the 
 separation of the two 
 kinds of electricity, one 
 
 Yin. 28. 
 
 accumulating on the rubber, the other upon the 
 thing rubbed. If a pith-ball suspended by a 
 silk thread (which being a non-conductor pre- 
 vents the electric influence from passing away 
 from the ball) be touched with a piece of glass 
 that has been rubbed with silk, and the glass 
 be then withdrawn, the ball will no longer be 
 attracted by it, but repelled (Fig. 28). If then 
 
90 ENERGY IN NATURE. 
 
 a piece of shell-lac, resin, or sealing-wax, be 
 rubbed with flannel and presented to the pith-ball, 
 the ball will be attracted by it. From this we 
 learn that bodies charged with the same kind of 
 electricity repel one another, and that if charged 
 with opposite kinds of electricity they attract 
 one another. This repulsion is felt by many 
 persons in electrical states of the air, their hair 
 having a tendency to stand out from the head 
 owing to the mutual repulsion between the fibres 
 charged with the same kind of electricity. There 
 are many mechanical operations, especially in 
 the textile industries, in which these phenomena 
 of electrical attraction and repulsion play an im- 
 portant part. When one excited body is brought 
 near an unexcited one, which is a conductor, the 
 first attracts towards itself the opposite kind of 
 electricity existing in the second, and as these 
 two opposite kinds have a tendency to rush to- 
 gether, they do so if the distance between them 
 be not too great, and an electric spark passes 
 across the interval ; the duration of this spark 
 is not longer than the twenty-four-thousandth 
 part of a second, and yet considerable heat, light, 
 and noise are developed, and hence the energetic 
 nature of electrified bodies is apparent. This 
 apparent production of an opposite electrical state 
 in bodies brought near to a substance previously 
 electrified is known as induction. 
 
 When it is desired to obtain larger supplies of 
 
ELECTRICITY. 
 
 91 
 
 electric energy than are produced by rubbing rods 
 of glass, shell-lac, &c., electrical machines are em- 
 ployed, all of which consist of two parts, one for 
 
 producing, the other for col- 
 lecting, the electricity (Fig. 
 29). The producing part con- 
 sists usually of one or more 
 
 FIG. 29. 
 
 plates of glass, to which a rapid movement of rota- 
 tion can be given, during which it is rubbed ; the 
 collecting part comprises metal points and conduc- 
 tors, placed near the rotating plate, and mounted 
 upon some non-conductor, as a stem of glass or ebo- 
 
92 ENERGY TN NATURE. 
 
 nite. A description of the details of their construc- 
 tion and mode of action would occupy more space 
 than is here available, but it may be found in any 
 elementary treatise on electricity.* They may be 
 regarded generally as contrivances to develop elec- 
 trical energy of a peculiar kind at the expense of 
 mechanical energy. The most recent types of 
 these machines depend very largely for their 
 effects upon the influence of induction above 
 alluded to, and are known as " induction" and 
 also as " influence" machines. In consequence 
 of the fact that damp air is a very much better 
 conductor of electricity than dry air, or, in other 
 words, is a much worse insulator than dry air, all 
 experiments with such machines succeed best in 
 a warm dry room. The electrical effect of these 
 machines may be stored or accumulated in a Ley- 
 den jar (Fig. 30), which is simply a wide-mouthed 
 glass jar coated with tinfoil inside and outside for 
 about three-fourths of its height from the bottom, 
 and provided with a wooden cover, through 
 which a metal rod passes, having a knob on its 
 outer end, and a chain on the lower end which 
 lies on the inner tinfoil coating. When a charge 
 of one kind of electricity is driven into the jar, 
 by connecting the rod with an electrical machine, 
 
 * "Elementary Lessons in Electricity and Magnetism," by 
 Silvanus P. Thomson (Macmillan, 18S2), Ferguson's "Electricity" 
 (Chambers, 1882), Balfour Stewart's " Primer of Physics " (Mac- 
 millan), and Worinell's "Electricity and Magnetism" (Murray, 
 1882), may be consulted with advantage for further information 
 on the subjects of this and the following chapter, 
 
ELECTRICITY. 93 
 
 it spreads over the inner surface, and drives the 
 whole of the other kind away on to the outer 
 surface, whence it passes into the earth in contact 
 with it. The jar may be retained in this con- 
 dition for some hours, or even days ; but when- 
 ever a path is provided by which the two kinds 
 
 of electricity can re- 
 unite, they will flow 
 to one another, and 
 the jar will be dis- 
 
 J FIG. 30. 
 
 charged. If this 
 
 path is through the human body a shock will 
 be felt ; if it is through a metal rod and across 
 a space of air (Fig. 30) the discharge will 
 be sudden, and accompanied by sound, light, 
 and heat (from the transformation of electrical 
 energy) ; if it is through a fine wire or a wet 
 strino-, the discharge will be slower and more 
 
94 ENERGY IN NATURE. 
 
 quiet. It is important to remember that, as was 
 iirst shown by Dr. Franklin, who constructed a 
 jar with movable coatings, the charges of the 
 jar really reside in the glass itself, and not in 
 the metallic coatings. These effects may be 
 largely increased by employing a number of 
 Leyden jars in a " battery," when energetic 
 mechanical and heating effects may be accom- 
 plished by the discharge, such as the perfora- 
 tion of cards, and of plates of glass, the fusion 
 of metallic-foil and wire, the combination of 
 mixed gases, the ignition of combustibles, explo- 
 sives, &c. By far the grandest exhibition of 
 these, however, is seen in the lightning-flash, 
 the destructive energy of which is well known, 
 and with good reason feared. This flash, which 
 is a discharge either between a cloud and the 
 earth, or between two clouds, at the point where 
 the air offers the least resistance, may some- 
 times be a mile in length, and its duration is not 
 really more than the l-100,000th part of a second, 
 although the impression on the retina of the 
 eye lasts much longer. Its electrical energy is 
 sometimes changed into mechanical, as when 
 buildings are destroyed ; and sometimes into 
 heat, as when metallic wires and rods in its path 
 are melted. Dr. Franklin, who first established 
 the identity of atmospheric electricity and that 
 of the machine, suggested in 1749 the use of 
 pointed metallic rods to protect property from 
 
ELECTRICITY. 95 
 
 destruction by lightning thus affording a path 
 by which the discharge should take place quietly. 
 Care should be taken in fixing these rods that 
 their tips are of some metal which does not tar- 
 nish in air, that the points project well above 
 the highest part of the building, that metal work 
 about the roofs be connected with them by stout 
 wires, and, most of all, that their lower ends 
 lead into damp ground ; the neglect of this last 
 precaution, which is frequently unattended to, 
 will make the best-laid conductor practically use- 
 less , while the owner sleeps in fancied security. 
 A rough practical rule for the height of a con- 
 ductor is, that it will protect a circular area at 
 its base whose radius is equal to its own height, 
 so that a rod 50 feet high will protect a circle on 
 the ground round it whose diameter is 100 feet. 
 
 Electrical energy may be developed in many 
 other ways than by friction ; a violent blow, and 
 even steady pressure, produces opposite electrical 
 states on the two opposing surfaces the tearing 
 of paper or linen, the crushing of sugar, the 
 cleaving of a sheet of mica, all produce it. Many 
 bodies in passing from the liquid to the solid 
 state become electrical, the phenomena of com- 
 bustion and evaporation are attended by it, and 
 in the evaporation of water over the surface 
 of the oceans is seen one source of atmo- 
 spheric electricity. Certain crystals (e.g., tour- 
 maline) when heated are found to develop 
 
96 ENERGY IN NATURE. 
 
 opposite electrical charges at opposite poles. 
 Many animals (notably the electric eel), and 
 some plants, produce electrification, and Volta 
 showed that the mere contact of certain metals 
 caused them to assume opposite electrical states. 
 Hence, as has been pointed out by Fleeming 
 Jenkin,* " a sense enabling us to perceive elec- 
 tricity would frequently disclose a scene as varied 
 as a gorgeous sunset . . . Every movement 
 of our body, each touch of our hand, and the 
 very friction of our clothes would cause a play of 
 effects analogous to those of light and shadow on 
 the eye. . . . Without eyes we might never 
 have discovered the existence of light. By direct 
 perception we have become aware of the vast 
 importance of light, and it is probably owing to 
 the absence of direct perception that we do not 
 yet know the part which electricity plays in the 
 economy of nature." 
 
 Thus far we have been considering chiefly 
 the production of opposite electrical states in 
 bodies, of static electricity, i.e., of electricity at 
 rest ; the only instance of electricity in motion 
 being afforded by the spark, or discharge, passing 
 between two oppositely electrified substances, as 
 in the Ley den jar or the lightning-flash. It will 
 be desirable now to study a little more in detail 
 some of the other modes of producing electricity 
 
 * S.P.C.K. Manuals of Elementary Science : " Electricity " 
 (pp. 51-53). 
 
ELECTRICITY. 97 
 
 111 motion, i.e., electric currents, since they afford 
 remarkable instances of the general doctrine of 
 the conservation of energy/ The transformation 
 of mechanical into electrical energy will form the 
 chief subject of the next chapter, the remainder 
 of the present one being devoted to the connec- 
 tion between heat and moving electricity, and 
 between chemical attraction and moving elec- 
 tricity. 
 
 It has been already pointed out that the mere 
 contact of two different metals gives rise to 
 opposite electrical states in them, but so long as 
 there is no difference in temperature between 
 various parts of their junction there is no dis- 
 charge, or movement of electricity no current is 
 produced. If, however, heat be applied to the 
 point of contact of any two dissimilar metals, and 
 their free ends be united by a wire, a current of 
 electricity will be found to flow through the wire 
 and through the point of junction, in a direction 
 varying with the pair of metals employed. This 
 phenomenon is known as thermo-electricity, and 
 it was first observed in 1822 by Seebeck. The 
 best mode of detecting the existence of the cur- 
 rent is by its action upon a magnetic needle (to 
 be fully explained in Chap. V.), which is turned 
 to one side or the other of its normal* position 
 when an electric current circulates near to it. 
 An instrument for subjecting a magnetic needle 
 to the influence of a current is called a galvano- 
 H 
 
98 
 
 ENERGY IN NATURE. 
 
 meter, and such an arrangement is shown in 
 Fig. 31. The intensity of the effect depends 
 upon the metals employed, and upon the tem- 
 perature ; the pair that produce the greatest 
 
 effect are bis- 
 muth and anti- 
 mony. If a 
 series of these 
 bars be solder- 
 ed together in 
 such a way 
 that all the 
 odd-numbered 
 joints are on 
 one side and 
 all the even- 
 numbered Oil 
 the other, a 
 thermo-elec- 
 tric battery is 
 formed, and 
 the electrical 
 energy of the arrangement depends upon the dif- 
 ference in temperature which can be maintained 
 between the two sets of joints. Such batteries 
 have been constructed in a form powerful enough 
 to produce the electric light, and other familiar 
 effects of strong currents. When made on a very 
 small scale the arrangement is known as a thermo- 
 electric pile (Fig. 32), and in combination with a 
 
 FIG. 31. 
 
ELECTRICITY. 
 
 99 
 
 delicate galvanometer is an exceedingly sensitive 
 instrument for detecting minute changes in tem- 
 perature, being largely used in researches upon 
 radiant heat. It should be noted here, as another 
 
 FIG. 32. 
 
 illustration of the conservation of energy, that 
 when an electric current is passed through a 
 junction of dissimilar metals, the junction is 
 cither heated or cooled, according to the direction 
 of the current. 
 
 We have now to consider the connection 
 between chemical attraction and electricity, or, 
 as it is often called, the production of electricity 
 H 2 
 
100 ENERGY IN NATURE. 
 
 by chemical action. This was due, in the first 
 instance, to two Italian men of science, Galvani 
 and Volta, who early in the present century in- 
 vestigated this subject, which we now recognise 
 as one of the transmutations of energy, and whose 
 names are perpetuated in the terms Voltaic and 
 Galvanic batteries. Volta showed that when any 
 two dissimilar metals are brought into contact, 
 each of them is found to be in an opposite elec- 
 trical state to the other, one becoming positively 
 (+), and the other negatively ( ), electrified. 
 The amount of difference between these states, 
 and whether any given metal was in a + or 
 condition depended upon the pairs of metals em- 
 ployed iron, for example, being positive towards 
 copper, silver, and gold, and negative towards tin, 
 lead, and zinc. The reality of the existence of this 
 "contact force," as it is often called, was doubted 
 for a long time, but such doubts have recently 
 been set at rest by some very delicate experiments 
 of Sir W. Thomson's. It will be noticed, however, 
 that this effect is a mere state, or condition ; in 
 order to produce a flow, continuous discharge, or 
 current of electricity, something more is necessary, 
 and this is found in the arrangement known as 
 the voltaic cell (Fig. 33). To construct it take 
 a vessel of water, to which a few drops of sul- 
 phuric acid, or a few crystals of common salt or of 
 sal-ammoniac, may be added with advantage, and 
 place therein two plates, z, c, of dissimilar metals, 
 
ELECTRICITY. 
 
 101 
 
 taking care to keep them apart. A very usual 
 and effective pair is copper and zinc, with sul- 
 phuric acid and water. As long as the two plates 
 are kept apart scarcely any effect is observable, 
 and none if the zinc be quite pure or be previously 
 rubbed over with mercury, i.e., amalgamated. If, 
 however, a wire, M, be attached to each plate, 
 and examined as to its electrical 
 condition, the two wires are 
 found to be in opposite electrical 
 states, and as soon as they are 
 joined the two electricities rush 
 together, and continue to flow or 
 circulate along the wire. In the 
 case of zinc and copper, the 
 wire attached to the copper is 
 called the positive pole, but, 
 as in Volta's contact experi- 
 ments, the intensity of the effect 
 produced, and whether any given metal is + or , 
 depends on the particular pair of metals employed. 
 It will presently be seen that a wire joining 
 two plates under these conditions, is in a very 
 different state or mood from an ordinary wire. 
 It does not weigh any more while in this state, 
 but it has many curious properties chemical, 
 magnetic, and physiological and these are ex- 
 pressed by saying that a current of electricity 
 circulates or flows in the wire. It is, however, 
 important to remember that this is merely a 
 
 FIG. 33. 
 
102 ENERGY IN NATURE. 
 
 convenient phrase to express a set of facts. We 
 do not Jcnoiv that anytJiing actually flows along 
 the wire, although there are some reasons for 
 believing that these observed effects are due to 
 a peculiar condition of vibration, or motion, set 
 up in the wire, different from those accompanying 
 the manifestations of heat-energy. 
 
 One remarkable fact, however, in connection 
 with this production of electrical energy by a 
 voltaic cell, is invariably noticed, viz., that it is 
 always accompanied by chemical action going on in 
 the cell. One of the two metals must have a 
 considerable chemical attraction for oxygen, and 
 the liquid must be one capable of acting on the 
 metal ; there is, however, no proof that their 
 electrical behaviour is due to their chemical be- 
 haviour, nor vice versa ; but the two sets of 
 phenomena invariably occur together. When 
 zinc foil is burnt in the air, or a mass of zinc 
 in a crucible is heated by fire, the zinc is oxidised 
 at the expense of the oxygen of the air, and heat 
 energy is produced. When, however, zinc is 
 oxidised at the expense of the oxygen of water 
 (and hydrogen is given off), which is the chemical 
 change that occurs in the voltaic cell, electrical 
 energy is produced. 
 
 Volta increased these effects by arranging 
 a number of the cells in a series, in the manner 
 indicated in Fig. 34, to which the name of 
 " crown of cups " was first given, and this is the 
 
ELECTRIC 
 
 principle of the arrangement of 
 the same kind in a Voltaic battery. It was soon 
 found that the 
 bubbles of hy- 
 drogen gas 
 evolved stuck 
 to the plates 
 (especially to 
 the copper one, 
 from the sur- 
 face of which 
 they chiefly 
 rose, although 
 produced at 
 that of the zinc 
 plate), and that 
 the energy of 
 the cells rapid- 
 ly became less. 
 Numerous plans 
 have been de- 
 vised to over- 
 come this diffi- 
 culty, usually 
 by the adoption 
 of two fluids, 
 separated by a 
 diaphragm of porous earthenware, as well as two 
 metals. DanielPs constant battery (Fig. 35) has 
 the copper-plate immersed in a solution of copper 
 
104 
 
 ENERGY IN NATURE. 
 
 coming 
 
 as 
 
 gas, 
 
 sulphate (blue vitriol), the zinc plate and dilute 
 
 sulphuric 
 acid being 
 contained in 
 a porous 
 vessel, 
 t h r o u g h 
 which the 
 hydrogen 
 passes, and 
 instead of 
 off 
 , de- 
 posits cop- 
 per on the 
 copper- 
 
 P late ; kec P- 
 iiig its sur- 
 face always clean and bright. Nearly 88,000 
 cells of this type are employed in 
 the British postal telegraph service. 
 In the Leclanche battery (Fig. 36) 
 the only exciting liquid is a solu- 
 tion of sal-ammoniac, and the por- 
 ous vessel, M M, contains a carbon 
 rod, c, surrounded by a mixture 
 of carbon and oxide of manganese : 
 56,000 of these are used in the 
 same service. In the so-called 
 bichromate cell, the plates are zinc and 
 
 FIG. 
 
 gas- 
 
ELECTRICITY. 
 
 105 
 
 carbon, surrounded by a mixture of bichromate of 
 potash and sulphuric acid (Fig. 37). More than 
 20,000 of a modification of these are used in the 
 post-office telegraphs. Many other forms have 
 been devised, the most energetic for a short period 
 being Grove's nitric acid battery, in which the 
 two metals are zinc and platinum, and the two 
 liquids are weak sulphuric acid 
 and strong nitric acid. Each 
 arrangement has advantages of 
 its own, and is best suited for 
 particular kinds of work. 
 
 It will be convenient here to 
 define certain terms that are 
 frequently employed in con- 
 nection with electrical energy. 
 In a single cell or battery, the 
 path of the positive current 
 from the zinc through the liquid, 
 copper-plate, and wire back to 
 the zinc again, is spoken of as 
 the electric circuit, consisting of the liquid part and 
 the metallic part. When the metallic circuit is not 
 continuous, it is said to be broken, and unless the 
 current has enormous energy it will not leap over 
 the smallest break of continuity ; when two surfaces 
 touch so closely that the current passes from one to 
 the other, they are said to be in contact, A con- 
 ductor is a substance along which the current flows 
 more or less freely, such as most metals ; and it 
 
106 ENERGY IN NATURE. 
 
 is a curious fact that those metals which conduct 
 heat best (p. 40) also conduct electricity best : 
 the order of conducting power is the same for 
 both. An insulator is a substance through which 
 the current will not pass, such as silk, glass, 
 earthenware, gutta-percha, &c., and insulators are 
 used for preventing the electric energy from leak- 
 ing out of the conductors. The phrase electro- 
 motive force (for brevity often written E.M.F.) 
 denotes that which moves, or tends to move 
 (p. 4) electricity from one place to another, 
 just as the pressure in a system of water-pipes 
 sets, or tends to set, the water in motion along it ; 
 and the phrase " potential" is used to express 
 the degree to which a body is electrified, a great 
 difference of potential between any two bodies 
 corresponding to considerable E.M.F. between 
 them. For further information on these subjects, 
 the reader should consult the various text-books 
 on Electricity before referred to. 
 
 It will be well now to consider some of the 
 effects of electricity in motion, or current elec- 
 tricity, and to observe how they differ from those 
 of electricity at rest, or the static electricity of 
 bodies in opposite electrical conditions. In the 
 discharge of the Leyden jar, we noticed some of 
 the work that was done by moving electricity, 
 but the discharge and its immediate conse- 
 quences were of excessively short duration. In 
 the case of current electricity, however, the 
 
ELECTRICITY. 107 
 
 effects produced by it last as long as the current 
 continues to flow, or, in the case now being con- 
 sidered, as long as the chemical changes go on 
 in the cells of the battery, provided also that the 
 circuit is maintained untouched, and that contact 
 is nowhere broken. 
 
 It should be carefully borne in mind, however, 
 that the phrase " current of electricity" is purely 
 a conventional one, for there is no proof that any- 
 thing " flows " along the wire. Our actual know- 
 ledge is confined to the fact that a wire under 
 these conditions possesses certain remarkable pro- 
 perties, and that this change, whatever it be, is 
 communicated along the wire at a speed closely 
 approaching the velocity at which light travels, con- 
 siderably exceeding 150,000 miles in one second. 
 
 We have seen what occurred when resistance 
 was offered to the energy of mechanical motion 
 viz., that it was converted into heat-energy. 
 The same thing happens when resistance is 
 opposed to electricity in motion. This resistance 
 is offered by bad and small conductors, and 
 accordingly we find that when a current of elec- 
 tricity meets a high resistance in its path, the 
 place where that occurs is more or less heated. 
 The experiment may be effectively made by in- 
 troducing into a circuit along which a strong 
 current is flowing, a short fine wire, too small 
 to convey the whole of the current, when it 
 will be seen that the wire will get intensely 
 
108 ENERGY IN NATURE. 
 
 hot, and if the energy of the current is sufficient 
 it will melt, and the circuit will be broken. This 
 power of exciting heat- energy at will by means 
 of electricity is, as is well known, extensively 
 used for firing mines at a safe distance, thus 
 avoiding the possible accidents from time-fuses ; 
 for discharging heavy artillery, either singly or 
 simultaneously, in a broadside on a man-of-war, 
 and, as will shortly be seen, it is at the basis of 
 all applications of electricity to lighting purposes, 
 and more especially to that form known as incan- 
 descence lighting. The resistance offered by a 
 conductor depends upon its size, and also upon 
 the material of which it is made. A very pretty 
 experiment illustrative of this latter point is to 
 construct a chain of alternate links of the same 
 gauge and lengths of silver and of platinum wire. 
 When a strong current is sent through this, the 
 platinum links become red-hot, while the silver 
 links, being better conductors i.e., offering less 
 resistance remain comparatively cool. 
 
 The principles of electric-lighting will be 
 explained in the next chapter, since, as practi- 
 cally carried out at present, they involve those 
 questions of the relations between magnetism 
 and electricity which will there be specially con- 
 sidered. It will be sufficient to remark here 
 that there are, broadly, two systems [1], the 
 Arc system, in which the light is produced when 
 the resistance opposed by two pieces of carbon 
 
ELECTRICITY. 109 
 
 (with a thin stratum of air between them) is 
 introduced into the circuit; and [2], the Incan- 
 descence, or glow, system, in which the necessary 
 resistance is given by a continuous thin filament 
 of carbon interposed in the circuit. It may be 
 remarked here that neither system is as new as 
 is generally supposed, the arc-light having been 
 produced by Sir H. Davy three-quarters of a 
 century ago,* and an incandescent carbon lamp 
 having been publicly exhibited in Birmingham by 
 Mr. W. Mattieu Williams more than thirty years 
 ago ; in all such cases, however, the electrical 
 energy was developed by chemical means, which 
 were so costly as to prohibit the use of the light 
 except for scientific experiments, and on occa- 
 sions when expense was no object. The reason 
 why so much has been heard during the last 
 few years of the application of electricity to 
 lighting, and to various other purposes of prac- 
 tical life (such as motive power, &c.) is, that 
 only recently have the means been discovered 
 of transforming the cheapest source of energy 
 known to us, viz., mechanical, into electrical, 
 and also of effecting the reverse change. The 
 apparatus which effects this is called a dynamo- 
 machine, and will be fully explained in the next 
 chapter. It may be interesting here to note the 
 
 * The first production of an arc-light is probably due to Etienne 
 Gaspard Robertson, and is recorded in the Journal de Paris, for the 
 date 22 Yentose, An X (March 12, 1802). Yide Nature, June 7, 1883 
 
110 ENERGY IN NATURE. 
 
 comparative cost of electrical energy developed 
 chemically and mechanically ; the following table 
 gives the number of foot-pounds of energy which 
 can be got out of one ounce weight of different 
 substances : 
 
 Gunpowder 100,000 
 
 Coal 695,000 
 
 Zinc 113,000 
 
 Copper 69,000 
 
 Hydrogen 2,925,000 
 
 From this it appears that coal is capable of 
 giving out six times as much energy as zinc, 
 so that even if coal and zinc were the same 
 price per ton, the (electrical) energy produced 
 by zinc would be six times as costly as the 
 (mechanical) energy produced by coal. As is 
 well known, the price of a ton of zir.c is many 
 times that of a ton of coal, and hence, even 
 under the most favourable circumstances, the 
 cost of electrical energy developed chemically 
 compares most unfavourably with the cost of 
 that developed mechanically, in the manner to 
 be described in the next chapter. 
 
 We have seen that the mere approach of an 
 electrified body towards a conductor in a neutral 
 condition promotes, or induces, an opposite elec- 
 trical state in that other body ; a phenomenon of 
 a similar kind in current electricity was observed 
 by Faraday in 1831, and the fundamental fact 
 of current induction may be thus stated. If two 
 
ELECTRICITY. Ill 
 
 wires are laid parallel to, but insulated from, 
 each other, and a current be sent along one of 
 them, it is found that at the very instant when 
 the current commences to flow in it, a momen- 
 tary current passes along, or is induced in, the 
 second wire. This induced current is in the 
 opposite direction to the current in the first wire, 
 and it ceases immediately, no change occurring 
 in the second wire until the current in the first 
 wire ceases to flow; at this instant another in- 
 duced current makes its appearance in it, but 
 in the reverse direction to the first induced cur- 
 rent, and therefore in the same direction as that 
 of the continuous current in the first wire. The 
 same effects are observed when a wire along 
 which a continuous current is flowing is brought 
 near to, and removed away from, another sepa- 
 rate wire. The experiment is best shown by 
 insulating both wires with silk or gutta percha, 
 and winding them in coils on thin wooden bob- 
 bins (Fig. 38); the ends of the first, or " pri- 
 mary " wire, are connected to a battery, and 
 the ends of the second to a testing instrument 
 called a galvanometer (Chap. V.). When the 
 primary coil is put inside the secondary, and 
 when it is removed from it, evidence is obtained 
 of the development of the induced currents above 
 described. The same effect may be produced 
 in a more perfect way by allowing the primary 
 coil to remain inside the secondary, and fitting 
 to the former a little mechanical arrangement 
 
112 
 
 ENERGY IN NATURE. 
 
 which makes and breaks contact rapidly with 
 the battery, and consequently produces a very 
 rapid succession of these induced currents. The 
 primary coil is a short length of comparatively 
 thick wire, of low resistance, and the secondary 
 coil is composed of many turns of very fine 
 
 and well-insulated wire. 
 
 Such an arrangement 
 
 FIG. 38. 
 
 is known as an Induction coil, and sometimes 
 as a Ruhmkorffs coil, from the name of a very 
 celebrated maker of them (Fig. 39). Induced 
 currents always possess great electro -motive force, 
 and their sparks will strike across intervals that 
 no battery will reach ; their physiological effects 
 are very strong, even comparatively small medi- 
 cal coils giving most unpleasant shocks, while 
 the discharge from some large coils is sufficient 
 to kill a man. The largest coil yet constructed 
 
ELECTRICITY. 
 
 113 
 
 belonged to the late Mr. Spottiswoode, President 
 of the Royal Society (1883) ; it is 4 feet long 
 and 20 inches in diameter, weighing altogether 
 15 cwt. ; its primary coil is 660 yards of wire 
 nearly yVth inch in diameter, and the secondary 
 coil contains 280 miles of wire, nearly yiroth inch 
 
 FIG. 39. 
 
 in diameter ; when excited by a Grove's battery 
 of 30 cells it gives a spark 42 J inches long, a 
 veritable miniature flash of lightning! 
 
 Many of the effects of the ordinary " electrical 
 machines" (p. 91) can be produced by these coils, 
 but their chief use is for the study of the electric 
 discharge under various conditions either in air 
 or in a partial vacuum, some of the experiments 
 upon which last are probably the most beautiful 
 in the whole range of experimental physics. The 
 
114 ENERGY IN NATURE. 
 
 character of the luminous discharge is subject to 
 almost innumerable variations, depending upon 
 the degree of exhaustion of the glass tube or 
 vessel (Figs. 40, 41), and 
 upon the kind of gas or 
 f gases contained in it at these 
 exceedingly minute pres- 
 sures, but it is a remarkable 
 fact that when the exhaus- 
 tion has reached the stage 
 of a nearly perfect vacuum, 
 the discharge will not take 
 place at all. The study of 
 these discharges has thrown 
 considerable light upon that 
 wonderful natural phenome- 
 non, the . Aurora Borealis, 
 and also upon the molecular 
 theory of matter referred to 
 in Chap. I. The very re- 
 cently published investiga- 
 tions of Prof. Selim Lein- 
 strom (vide Nature , Xos. 
 709, 710, &c.) into auroral 
 
 phenomena, have thrown considerable light upon 
 that subject. 
 
 In the last chapter we saw several instances 
 of the heat-energy developed by the force of 
 chemical attraction, and it was also pointed out 
 that under certain conditions the process could 
 
ELECTRICITY. 115 
 
 be reversed, i.e., that if heat-energy were allowed 
 to act on the compounds thus produced, the work 
 could be undone, and the compounds separated 
 or decomposed. Now it is found that electrical 
 energy is one of the most powerful agents with 
 which we are acquainted for decomposing 
 chemical compounds, especially when they are 
 
 Fro. 41. 
 
 in the liquid state. No liquid (except melted 
 metal) which conducts electricity at all, conducts 
 it without being thus decomposed, and when 
 this occurs the electrical energy is spent in 
 overcoming the resistance of the chemical attrac- 
 tion for each other of the two substances 
 forming the compound. The most familiar ex- 
 ample of this is the electrical analysis, or the 
 electrolysis, as it was called by Faraday, of 
 water. Pure water belongs, like turpentine, 
 petroleum, and many oils, to the class of non- 
 conductors, but if a few drops of sulphuric acid 
 be added, it may readily be decomposed by the 
 i 2 
 
116 
 
 ENERGY IN NATURE. 
 
 current from a few cells of a battery. The 
 arrangement is shown in Fig. 42 ; the wires from 
 the battery are attached to two platinum plates, 
 over each of which is inverted a tube closed at 
 one end, and filled with water. When the con- 
 nections are made, bubbles of gas rise from each 
 plate to the top of the tube, displacing the water, 
 
 and in a short time it 
 will be evident that 
 twice as much gas 
 
 is 
 
 Fia. 42. 
 
 collected in the tube which is connected with 
 the zinc end of the battery, as in that con- 
 nected with the copper end. If the tubes be 
 then removed and their contents examined by 
 chemical tests, the larger volume of gas will be 
 found to be hydrogen, and the smaller, oxygen, 
 thus demonstrating the statement of the compo- 
 sition of water given on p. 68. If, instead of 
 water alone, a solution of any metallic salt be 
 similarly treated, as, for example, sulphate of 
 
ELECTRICITY. 117 
 
 copper (blue vitriol), sulphate of nickel, acetate 
 of lead (sugar of lead), &c., oxygen will still be 
 given off in one tube, but in the place where hy- 
 drogen before appeared no gas will be given off ; 
 instead of it the platinum plate will be covered 
 with a deposit of the metal itself, copper, nickel, 
 or lead, according to the salt employed. These 
 facts form the foundation upon which the whole 
 art of electrotyping and electro-plating has been 
 reared, to which industry the phrase electro-metal- 
 lurgy is often given. The articles .to be plated 
 are connected with the zinc end of the battery, 
 or with the corresponding end of some other 
 source of electricity, and are suspended in a tank 
 containing a solution of the metal to be deposited 
 on them. In the case of silver-plating, a double 
 cyanide of silver and potassium is used, and for 
 gilding, a similar salt of gold in a hot solution. 
 The other end of 'the battery (or dynamo-machine, 
 Chap. V.) is attached, not to a platinum plate, 
 but to a sheet of the metal whose salt is in 
 solution, which is also hung in the tank. When 
 the circuit is completed, the electrical energy 
 tears asunder the metal from the acid in the salt, 
 depositing it upon the substance to be plated, 
 and the acid and oxygen thus set free at once 
 exercise their attraction for the metallic plate, 
 which is gradually dissolved away, and thus the 
 strength of the solution is kept up. Electrotyping 
 is the name given to the art of copying seals, 
 
118 
 
 ENERGY IN NATURE. 
 
 medals, engraved plates, &c., in copper, and may 
 be readily practised by the amateur. A sharp 
 mould must first be taken of the object to be 
 copied, in fusible metal, sealing-wax, gutta-percha, 
 or plaster of paris, and if it be made of any 
 except the first named, it must be rubbed over 
 with plumbago (black-lead) to make the surface 
 conduct electricity. The mould is then hung in 
 
 a tank as above de- 
 scribed, containing a 
 solution of sulphate 
 of copper and a sheet 
 of copper, connected 
 with a battery, and 
 left until a sufficient 
 thickness of metal 
 has been deposited, 
 when the electrotype 
 and the mould are 
 pulled asunder. A 
 large number of the cuts in this book have been 
 printed from electrotypes thus produced. For 
 small objects a simpler arrangement suffices, known 
 as the single cell apparatus (Fig. 43). It consists 
 of an earthenware vessel half filled with a strong 
 solution of sulphate of copper, in which also stands 
 a porous earthenware pot containing dilute sul- 
 phuric acid and a plate or rod of zinc, which is 
 connected by a wire to the mould hung in the 
 solution, and the strength of this is kept up by 
 
 FIG. 43. 
 
ELECTRICITY, 119 
 
 suspending some crystals of sulphate of copper 
 in it. 
 
 We have seen that whenever chemical com- 
 bination takes place some form of energy is 
 developed, and also that energy is absorbed in 
 undoing that work. Now, since the amount of 
 energy produced in the first of these processes 
 varies with the quantity and with the kind of 
 the materials employed, but is always the same 
 when the same quantity of similar materials are 
 used, we should naturally expect to find that the 
 amount of electric energy necessary to undo this 
 work varied in the same way, and this is actually 
 found to be the case. In other words, the ivork 
 done is proportional to the electricity generated, 
 and to the amount of zinc " burnt" in the battery. 
 When water is thus decomposed, the amount 
 of gas evolved depends upon the strength of the 
 current employed, and upon the time for which 
 it acts, and Faraday constructed an instrument 
 called a Voltameter, in which the mixed gases are 
 measured in a graduated tube, and from the quan- 
 tity collected in a given time, the strength of the 
 current can be calculated. Mr. Edison has re- 
 cently proposed to apply the same principle to a 
 meter intended to measure the amount of elec- 
 tricity drawn from the town -supply by any pri- 
 vate house, whether for lighting or as motive- 
 power (Chap. V.). In this ingenious instrument 
 the weight of copper deposited by a known 
 
120 ENERGY IN NATURE. 
 
 fraction of the current upon a plate of metal 
 gives the necessary data for the calculation, and 
 by a curious mechanical contrivance the instru- 
 ment is provided with a set of dials, to be read 
 off just like a gas-meter. But further, if the 
 same strength of current, i.e., the same quantity 
 of electric energy produced by the consumption 
 of the same amount of zinc, is used to decompose 
 salts of various metals, it is found that the amount 
 of metal deposited differs in each case, and, in 
 fact, that these different weights stand to each 
 other in the same relation as the numbers which 
 express (pp. 67, 68) the combining proportion or 
 equivalent of each element. Thus, for every 32 J 
 parts of zinc consumed in the battery (or de- 
 posited by a given strength of current) 31^ parts 
 of copper, 108 parts of silver, 29 J parts of nickel, 
 and 65 J parts of gold would be deposited. More- 
 over, as hydrogen develops the greatest amount 
 of energy in its combinations, it is not surprising 
 to find that the greatest amount of energy is 
 necessary to decompose its compounds; accord- 
 ingly only one part (by weight) of hydrogen is 
 produced by the consumption of 32 J parts of 
 zinc. 
 
 Recently, the electro-chemical decomposition 
 of water, or rather of weak sulphuric acid, has 
 assumed exceptional importance in connection 
 with the so-called " storage of electricity," or 
 " storage of force/' erroneous expressions, as will 
 
ELECTRICITY. 
 
 121 
 
 be seen in the sequel, for which should be substi- 
 tuted the phrase u electrical storage of energy." 
 The first observation on the subject is due to 
 Sir W. Grove, who, in 1842, noticed that if he 
 decomposed water in a voltameter, and then, 
 having disconnected the bat- 
 tery, joined the two plati- 
 num plates by a wire (Fig. 
 44), a current of electricity 
 passed along the wire, and 
 at the same time some of 
 the gas disappeared. More- 
 over, the current was from 
 the oxygen to the hydrogen, 
 or in the reverse direction to 
 that which separated them. 
 A battery made up of fifty 
 of these cells possessed suffi- 
 cient energy to decompose 
 water, and even to produce 
 the electric light, and their 
 energy appears to be due to, at any rate it is 
 always accompanied ly, chemical action between 
 the hydrogen and oxygen, just as in the simple 
 voltaic cell, chemical action between the zinc and 
 oxygen (p. 102) always accompanies the produc- 
 tion of current electricity there. 
 
 The first person to apply this fact upon an 
 industrial scale was M. Gaston Plante*, who in 
 I860 constructed his storage -battery, or accu- 
 
 FIG. 44. 
 
122 
 
 ENERGY IN NATURE. 
 
 mulator, in the following simple manner : Two 
 long- wide slips of lead are laid upon each other, 
 separated by narrow strips of gutta-percha ; the 
 whole is then rolled up in a spiral form and im- 
 mersed in a (glass) jar containing weak sulphuric 
 acid ; a connecting strip is at- 
 tached to each plate, and'car- 
 ried outside the cell (Fig. 45). 
 When the current from a bat- 
 tery (or a dynamo-machine) is 
 sent through the cell, water is 
 decomposed, and the plate by 
 which the current enters (i.e., 
 that attached to the " copper," 
 or positive, end of the battery) 
 is covered with a thin film of 
 oxide of lead, while the hydro- 
 gen is absorbed by the other 
 lead plate. When the battery - 
 is disconnected the cell may 
 be kept some time in the same 
 condition, but as soon as the 
 two lead-plates are joined by a wire a current 
 passes between them, and at the same time some 
 of the hydrogen combines with the oxygen of the 
 oxide of lead on the other plate. If the cell be 
 now charged again, but in the reverse direction, 
 more gas will be absorbed by eacli .plate, and if 
 this process of charging and discharging alter- 
 nately in opposite directions be repeated several 
 
 FIG. 45. 
 
ELECTRICITY. 
 
 123 
 
 times, the surface layers of both lead-plates will 
 get into such a porous condition that they will 
 absorb very large quantities of gas. This pro- 
 cess is known as " forming " the cell, and is 
 somewhat tedious. To obviate this M. Camille 
 Faure in 1880 coated both plates with red-lead, 
 and thus obtained the neces- 
 sary porous condition of me- 
 tallic lead with a much less 
 number of charges and dis- 
 charges of the cell than was 
 necessary in Planters form. A 
 plan of the arrangement of 
 the plates in a Faure cell is 
 given in Fig. 46. In the 
 Faure-Sellon-Volckmar accu- 
 mulators probably the most 
 efficient yet (June, 1883) 
 produced on a commercial 
 scale, thick leaden gratings 
 are cast, the perforations in 
 which are then filled with oxides of lead; 
 and, indeed, nearly all the modifications pro- 
 posed have for their object the increase of 
 surface of the lead employed, upon a support 
 which is not acted on by the liquid contents of 
 the cell. 
 
 Opinions are somewhat divided as to the 
 exact nature of the chemical changes that occur 
 in these cells, but there can be no question that 
 
 J 
 
 
 
 ~N 
 
 
 L 
 
 ^ 
 
 r 
 
 -N 
 
 1 
 
 ,j 
 
 ( 
 
 -\ 
 
 -i 
 j 
 
 
 
 ] 
 
 \ 
 
 3. 
 
 46 
 
 
 
 
 
124 ENERGY IN NATURE. 
 
 sulphuric acid, sulphate of lead, peroxide of lead 
 and water play an important part therein, and that 
 they are continually undergoing decomposition 
 and ^-composition. The main points, however, 
 to be remembered in connection with them 
 are that when the cell is being charged the 
 electric energy does chemical work in the cell, 
 and in so doing ceases to be electricity ; that 
 the work which is done is the overcoming of 
 the chemical attraction of certain substances 
 for each other ; that the cell in the charged 
 condition resembles an ordinary voltaic cell, 
 from which electric energy can be drawn at 
 pleasure by completing the circuit outside it ; 
 and that when the cell is discharged, and the 
 electric energy is drawn off, the electricity is 
 produced at the time, and is always accompanied 
 by, and probably caused by, the re-union of 
 the substances previously separated. Hence 
 these accumulators do not contain electricity as 
 such, but only the means of producing it when 
 wanted energy, not electricity, is stored in 
 them. A precisely parallel case to this will be 
 found in Chap. VI., where the storage of solar 
 energy in wood and coal (through the medium 
 of growing plants), and its reproduction as heat 
 and light when the carbon and oxygen thus 
 separated are brought together again, are de- 
 scribed at some length. In fact, in almost all 
 cases a store of energy arises from the separa- 
 
ELECTRICITY. 125 
 
 tion of two bodies which desire to come to- 
 gether. 
 
 Nearly all the practical uses of these accu- 
 mulators, or secondary batteries, are dependent 
 upon the employment of dynamo-machines for 
 charging them, the electric energy being thus 
 produced cheaply. Their use in this connec- 
 tion will be alluded to in the next chapter, but 
 it may be convenient to state here the amount 
 of energy that can be thus stored electro-chemi- 
 cally. The numerical relations between mecha- 
 nical and electrical energy will be explained in 
 the next chapter, and hence, without giving the 
 intermediate steps, it may suffice to say that a 
 fully-charged Plante cell can store about 11,000 
 foot-pounds of energy per pound of lead in the 
 cell, and that a Faure cell can store nearly 
 13,000 foot-pounds in the same weight of lead. 
 According to Sir W. Thomson a single Faure 
 cell of the spiral form, weighing 165 Ibs., can store 
 2,000,000 foot-pounds of energy. Prof. Henry 
 Morton, of the Stevens Institute of Technology, 
 New York, in a report (dated February 17, 
 1883) upon the Faure-Selloii-Volckmar accumu- 
 lators, states that this same quantity of 2,000,000 
 foot-pounds of energy, or one horse-power work- 
 ing for one hour, can be stored in a cell of 
 that kind which weighs only 80 Ibs., and that 
 three cells, while standing fully charged for six- 
 teen days, only lost by leakage 7 per cent, of 
 
126 ENERGY TN NATURE. 
 
 the energy contained in them. The latter part 
 of this result the author can corroborate from 
 his own experience of the working of these cells, 
 under very trying conditions. 
 
CHAPTER V. 
 
 MAGNETISM AND ELECTRICITY. 
 
 BEFORE proceeding to the study of the very 
 intimate relations existing between these two 
 forms of Energy in Nature relations upon which 
 all the applications of electricity to the purposes 
 of practical life depend it will be well to con- 
 sider briefly some of the elementary phenomena 
 of, and some of the terms used in connection 
 with, Magnetism. The force of attraction in this 
 form of energy was first noticed many hundred 
 years ago in certain hard black stones found in 
 Magnesia in Asia Minor (whence the name mag- 
 net), but afterwards in other parts of the world. 
 This lodestone (i.e., leading-stone) as it was 
 called, was afterwards shown to be a peculiar 
 iron ore, a combination of iron and oxygen, and 
 although it was at first thought that its attrac- 
 tive influence was confined to iron and steel, 
 further experiments (as in the case of electric 
 attraction) showed that a large number of other 
 substances were thus affected by it, though in 
 a less degree. In the tenth or twelfth century 
 it was noticed that such stones when hung up 
 
128 ENERGY IN NATURE. 
 
 by a thread pointed in definite directions, North 
 and South, rapidly taking up that position again 
 when disturbed from it, and shortly afterwards 
 it was found that iron and steel, when rubbed 
 in certain directions with a piece of lodestone, 
 acquired these same properties. In 1600 Dr. 
 Gilbert published the results of very careful ex- 
 periments with magnets, adding greatly to the 
 knowledge then existing. It is, however, per- 
 haps scarcely necessary to say that even at the 
 present day we are as ignorant of the nature 
 of magnetism as we are of electricity none of 
 these forms of energy are recognisable apart 
 from matter, as has been already pointed out. 
 There are strong reasons for believing that the 
 phenomena of magnetism are in some way con- 
 nected with the motion of the particles of those 
 bodies, which, like iron, become magnetic ; that, 
 in fact, it is another form of the molecular motion 
 spoken of in Chaps. I. and II. On this view of 
 the case, which has quite recently received strong 
 support from experiments exhibited to the Royal 
 Society by Prof. D. E. Hughes, the difference 
 between the arrangement of the particles in a 
 magnet and in an ordinary piece of steel or 
 iron, might be likened to the difference in the 
 packing arrangements of two boxes of eggs 
 in the first (corresponding to the magnet) the 
 eggs are carefully packed, lying side by side 
 parallel to each other and to the sides of the 
 
MAGNETISM AND ELECTRICITY. 120 
 
 box, with their small ends all turned in the same 
 direction, and therefore touching the larger end 
 of the adjoining egg ; while in the second (ordi- 
 nary iron or steel), badly packed, the separate 
 eggs lie in all sorts of positions with regard to 
 each other, and at all angles of inclination to 
 the sides of the box. Another and quite different 
 physical theory of magnetism will be alluded 
 to later; but it may be noted with advantage 
 in this connection that the magnetism of iron 
 and steel is always materially lessened, and some- 
 times entirely destroyed, by changing the mole- 
 cular condition of the iron ; this may be done 
 by subjecting a magnetic rod to a mechanical 
 twist, or strain of any kind, or by heating it, 
 all magnetism disappearing at a cherry-red heat. 
 The influence of temperature upon magnetism 
 is well seen in the case of manganese, which 
 only becomes magnetic when cooled to below 
 zero Fahr. 
 
 If a bar-magnet, i.e., a straight piece of steel 
 which has been magnetised, be carefully ex- 
 amined, by holding the different parts of it near 
 to a small nail suspended by a long thread (Fig. 
 47), it will be found that its attractive force is 
 most strongly exerted by the two ends, to which 
 the term poles is given. Moreover, if one pole 
 be thrust into a heap of nails, and withdrawn 
 again, it will be observed that not only do many 
 nails adhere to the magnet itself, but that other 
 j 
 
130 
 
 ENERGY IN NATURE. 
 
 nails adhere to them, and that quite a string of 
 them may be thus drawn up ; if, however, the 
 nails actually in contact with the magnet be 
 removed from it, the attraction of the other nails 
 for each other at once ceases. This shows that a 
 
 FIG. 47. 
 
 magnet is capable of inducing magnetism tem- 
 porarily in other pieces of iron somewhat in the 
 same manner as is observed in the case of elec- 
 trified bodies (p. 90). Further, this attractive 
 force between a magnet and iron is mutual, for if 
 a magnet which is free to move be brought near 
 to a fixed piece of iron, the latter will attract the 
 magnet (Fig. 48). 
 
MAGNETISM AND ELECTRICITY. 
 
 131 
 
 It has been already stated that a bar-magnet, 
 freely suspended by the middle of its length 
 (either by a loop of string or on a point of support), 
 will take up a definite position. If, now, another 
 bar-magnet be brought near to one end (say that 
 
 FIG. 48. 
 
 pointing northwards) of it, the two ends of the 
 second magnet will behave very differently to- 
 wards it, and careful observation will show that 
 when two poles of the same kind (/.<?., two N- 
 pointing or two S -pointing) are brought near to 
 each other, they tend to repel each other, while 
 between two poles of opposite kinds there is 
 j 2 
 
132 
 
 ENERGY IN NATURE. 
 
 attraction. These phenomena are of the utmost 
 possible consequence in the theory of the dynamo 
 machine, and in the transformation of mechanical 
 work into electricity, and vice versa, and when 
 the attractions and repulsions are strong the 
 
 energy brought into 
 play is very great in- 
 deed. On the other 
 hand, no matter how 
 small the pieces into 
 which a magnet may 
 be broken, each piece 
 has opposite proper- 
 ties at opposite ends, 
 or is said to be en- 
 dowed with polar- 
 ity. In these phe- 
 nomena of attrac- 
 tion and repulsion, 
 again, the careful 
 reader will note the 
 corresponding electrical condi- 
 
 FIG. 49. 
 
 with 
 
 analogy 
 tions. 
 
 The north-pointing property of a magnetic 
 needle (Fig. 49) has been known and used for 
 the purposes of navigation for more than 2,000 
 years. The mariner's compass consists essentially 
 of a magnetic needle balanced on a pivot and 
 supporting a card on which are marked the 
 "points," and the whole is enclosed in a box 
 
MAGNETISM AND ELECTRICITY. 133 
 
 which is mounted on two rings with pivots called 
 gimbals, in such a way as to be independent of 
 the motion of the ship. The use of the compass, 
 however, is not nearly as simple as might be 
 supposed. In the first place, it is affected by the 
 iron of which most ships are built, and by that fre- 
 quently contained in the cargo, and in the second 
 place, careful and exact observations show that 
 the direction in which the needle points is itself 
 subject to constant change. It varies slightly 
 with the time of day ; it varies a good deal with 
 the particular place on the earth's surface where 
 it is observed, and it varies from year to year in 
 the same place. These different changes, as well 
 as the fundamental phenomena of N. and S. 
 pointing, are best explained by regarding the 
 whole earth as a huge magnet, whose magnet- 
 ism is constantly varying in different parts 
 of it. 
 
 The subject of terrestrial magnetism is a 
 large one, and is being daily investigated at ob- 
 servatories, but space does not admit of its further 
 discussion here at present. It is, however, in- 
 teresting to note here that the regular changes hi 
 the earth's magnetism are associated in some way 
 with those periodic changes in the atmosphere of 
 the sun which we recognise as sun-spots, and also 
 with displays of the aurora borealis, and other 
 phenomena. 
 
 It will now be convenient to examine the 
 
134 ENERGY IN NATURE. 
 
 directions in which the lines of force emanate 
 from a magnet, and these are best seen in the 
 study of what Faraday called magnetic fields. 
 A magnetic field is simply that portion of space 
 which is within the sphere of action of a magnet. 
 
 FlG. 50. MAGNETIC FIELD OF A BAR-MAGNET. 
 
 (By permission from "Engineering.") 
 
 If a card or piece of stiff paper be laid upon a 
 magnet, and very fine iron filings be sifted there- 
 upon, the iron filings will (especially if the card 
 be gently tapped) take up certain definite positions 
 (Figs. 50, 51) indicating the lines of magnetic 
 force in the field. 
 
 We now approach the connection between 
 magnetism and electricity, which may be thus 
 
MAGNETISM AND ELECTRICITY. 135 
 
 stated broadly : .1 wire conveying electricity acts 
 Wee a macjnct. In Fig. 52 we see the u magnetic 
 whirls " surrounding a conducting wire the 
 magnetic field of that wire, in fact. One 
 direct consequence of this, that a copper wire, 
 
 FlG. 51. MAGNETIC FIELD OF ONE POLE OF A BAll-MAOXET PLACED 
 
 " END-OX." 
 
 (By permission f rom "Engineering.") 
 
 along which an electric current is flowing, will 
 attract iron filings to itself, is very easily 
 shown by making the experiment in the same 
 way as with a magnet. Moreover, if a con- 
 ducting wire be delicately balanced on a pivot 
 (Fig. 53) like a magnetic needle, it will be found 
 to be attracted and repelled by the poles of a 
 magnet, according to the direction of tlie current 
 
136 ENERGY IN NATURE. 
 
 ill it ; and further, if two such wires be presented 
 to each other, all the experiments of attraction 
 and repulsion performed with two magnets, can 
 be repeated with them. Fig. 54 shows the in- 
 
 FiG. 52. MAGNETIC FIELD OF A CONDUCTING WIRE PLACED AT WIGHT 
 ANGLES TO THE PLANE OF THE PAPER. 
 
 (By permission from " Engineering.") 
 
 fluence which the direction of the current has 
 on the experiment ; parallel currents passing in 
 the same direction attract, and passing in the 
 contrary direction repel, each other. Lastly, if a 
 long hollow spiral coil of insulated wire (technically 
 called a solenoid) be suspended so that it is free 
 to move, when a current is sent along it, it will 
 behave like a magnetic needle, and take up a 
 
MAGNETISM AND ELECTRICITY. 
 
 137 
 
 N. and S. position. On these facts Ampere founded 
 a theory of magnetism, according to which electric 
 currents were perpetually circulating in the same 
 direction round 
 the particles of 
 the magnetised 
 body (Fig. 55), 
 and to these cur- 
 rents the mag- 
 netism was due. 
 The disco- 
 very of the in- 
 fluence of a con- 
 
 ducting wire 
 
 upon a magnetic 
 
 needle was made 
 
 by Oerstedt in 
 
 1820, who found that if a stout wire were laid 
 
 parallel to such 
 
 ^ _ a needle moun- 
 
 ted on a pivot, 
 as soon as a cur- 
 rent passed 
 along the wire 
 the needle ten- 
 ded to place 
 itself across it 
 
 FIG. 54. (Fig. 56). It 
 
 should be noted 
 that this was the first instance of mechanical 
 
 FIG. 53. 
 
138 
 
 ENERGY IN NATURE. 
 
 motion being produced by current electri- 
 city. The side to which the needle turns, or 
 is deflected, depends upon the direction in 
 
 which the current flows 
 in the wire, and upon its 
 position with regard to 
 the N. and S. pointing 
 poles of the magnet. It 
 may be determined by 
 Ampere's rule, which is 
 
 fully explained in the text -books referred to 
 on p. 92. 
 
 The effect of a weak current may be largely 
 
 increased by winding insulated wire several times 
 round the needle, thus causing the current to 
 pass over it in one direction, and underneath it in 
 the opposite (Fig. 57), and still greater sensitive- 
 ness is given by mounting a pair of parallel 
 needles on one axis, with their poles, a, b, and 
 
MAGNETISM AND ELECTRICITY. 
 
 139 
 
 a,' b' (Fig. 58), in opposite directions. If the 
 needles are equal in. magnetic strength, they are 
 said to be astatic, and the combination being 
 
 uninfluenced by the 
 earth's magnetism, has 
 110 tendency to point 
 in any particular direc- 
 tion, and is therefore 
 much more sensitive to 
 electric currents. 
 
 On this principle 
 the action of a cur- 
 rent upon a magnetic 
 needle are constructed galvanometers (Fig. 59), 
 instruments to detect the presence of, and in 
 some modifications to measure the force of, 
 electrical currents. On the 
 same principle also depends 
 one form of the electric 
 telegraph, that known as 
 the single-needle instru- 
 ment, in which letters are 
 indicated by beats to the 
 right and left of the needle 
 on the dial underneath which are the coils of wire. 
 These beats are produced by sending the current 
 from the distant station in two different directions, 
 by a mechanical arrangement of springs operated 
 with a handle. This was the earliest practical in- 
 strument devised, and it is still largely employed, 
 
 58. 
 
140 
 
 ENERGY IN NATURE. 
 
 both by the British Post-office and on rail- 
 way lines, for which it is peculiarly adapted, 
 about 16,000 being in use for this latter purpose 
 alone.* 
 
 No single letter requires more than four beats 
 
 of the needle, and an expert operator can transmit 
 from thirty to thirty-five words per minute in 
 this way. 
 
 * For a popular account of Electric Telegraphy, consult " Tele- 
 graphy," by Messrs. Preece and Sivewright, in Longmans and Co.'s 
 series of Text-books of Science. 
 
MAGNETISM AND ELECTRICITY. 141 
 
 Having seen the influence of the magnetic 
 field of a conducting wire upon a magnet, let us 
 now examine its influence upon soft iron. The 
 fact that a magnet induces magnetism in soft 
 iron in contact with it, leads us to expect that 
 current electricity will do the same thing, and 
 experiment justifies that expectation. Soon after 
 Oerstedt's discovery described above, Sturgeon 
 showed that soft iron placed in a coil of wire 
 through which a current was passing, induced 
 magnetism in itself from that '/field," but that 
 this was only temporary, and that as soon as 
 the current ceased, the iron lost its magnetic 
 properties. 
 
 Steel thus treated, however, is permanently 
 magnetised, though in a less degree ; in fact, 
 electro-magnets, as they are called, are weight 
 for weight very much more powerful than per- 
 manent steel magnets (Fig. 60). Few things are 
 more remarkable in this connection than the 
 rapidity with which soft iron may be thus 
 magnetised and de-magnetised. The ordinary 
 trembling electric bell, so largely used now, 
 supplies audible evidence of the fact; every 
 time the vibrating hammer strikes the bell, 
 an electro-magnet is excited, and loses its 
 magnetism. In some dynamo-machines also 
 these changes take place many hundred times 
 in a minute. 
 
 The practical applications of this fact are 
 
142 
 
 ENERGY IN NATURE. 
 
 almost innumerable, and are daily increasing in 
 number. The elaborate systems of fire-alarms, 
 which have been in operation in American cities 
 for the last quarter of a century, but are only now 
 
 FIG. 60. 
 
 beginning to be adopted in England, consist 
 almost entirely of electro-magnetic signalling 
 appliances. In Boston it is in the power of any 
 person to give an alarm of fire by simply pulling 
 down a handle in one of the very numerous street- 
 boxes, when, without any further human inter- 
 
MAGNETISM AND ELECTRICITY. 143 
 
 vention, automatic machinery causes the church 
 bells all over the city to ring out the number of 
 the district whence the alarm came. So perfect 
 also is the fire-drill, not only in Boston, but in 
 nearly every large town in the Union, and so much 
 assistance is given by electro-magnetic arrange- 
 ments, that the author has seen naked horses 
 loosened from their stalls, harnessed, arid started 
 off with a steam fire-engine, in less than ten 
 seconds from the receipt of the signal. By an 
 ingenious combination of a thermometer with 
 this system, fires are made to signal themselves 
 automatically, an arrangement found sufficiently 
 effective to justify a considerable reduction in 
 insurance premiums. Similar arrangements are 
 employed for the protection of banks, safes, and 
 private dwellings from burglars, for the regu- 
 lation of clocks, the automatic registration of 
 certain occurrences, and a variety of somewhat 
 similar purposes.* 
 
 The system of telegraphic signalling most 
 largely employed upon land lines, depends upon 
 the attraction of an iron bar, or keeper, by an 
 electro-magnet, magnetised by a current sent from 
 the distant station. In the older instrument 
 known as the Morse recorder (Fig. 61), of which 
 
 * Among the various practical applications alluded to in the 
 lectures in Lancashire, was a very ingenious electric stop-motion 
 applied to " drawing-frames " in a cotton spinning-mill, the inven- 
 tion of Mr. John Bullough of Accrington, the action of which 
 depended on the insulating properties of cotton. 
 
144 
 
 ENERGY IN NATURE. 
 
 there are still about 40,000 in use on the European 
 continent, this keeper, c, c, is placed at one end 
 of a lever, d, d, to the other end of which is 
 attached a style or ink-pen, under which a con- 
 tinuous strip of paper is made to pass by clock- 
 work, (j. The attraction downwards of the keeper, 
 
 FIG. 61. 
 
 by the electro-magnet, &, , presses the pen up 
 against the moving paper, and a mark is made, 
 the length of which depends on the time for which 
 the current is allowed to pass. In the Morse 
 alphabet, two signals, only a (.) and a ( ) are 
 employed, by combinations of which all letters 
 are signalled, no single letter requiring more than 
 four signals, and the most frequently used let- 
 
MAGNETISM AND ELECTRICITY. 145 
 
 ters being represented by the simplest signals ; 
 thus 
 
 MAGNET 
 
 The Morse recorder thus gives visible signals, 
 of which a record is kept. The English and 
 American systems, however, have for the last 
 fourteen years largely employed an instrument 
 known as the Sounder, which is simply a re- 
 corder deprived of its recording apparatus. The 
 interval between two attractions of the keeper, 
 each of which makes a sharp " click," may 
 be either long or short, like a minim and 
 crotchet in music, corresponding to the dash 
 and the dot of the Morse alphabet. The signals 
 made in this way are audible and non-recording; 
 but the alphabet, once learnt, is not readily 
 forgotten. 
 
 Submarine cables consist essentially of a 
 copper conducting wire, closely and carefully 
 surrounded with gutta-percha as an insulator, 
 and the whole protected by an outer coating of 
 hemp and steel wire. The signalling through 
 them is carried on either by a very delicate 
 galvanometer, to the needle of which is attached 
 a delicate mirror on which a beam of light falls, 
 the movements of the needle being indicated 
 by the oscillations of the reflected spot of light, 
 or by a complicated instrument known as Sir W. 
 K 
 
146 ENERGY IN NATURE. 
 
 Thomson's Siphon recorder. There are now 
 80,000 miles of submarine cable at work, costing 
 30,000,000. A fleet of twenty-nine ships is 
 employed in laying, watching, and repairing 
 these cables, of which there are nine across the 
 Atlantic alone, the last one having been laid in 
 twelve days without hitch or stoppage. Since 
 the purchase of inland telegraph lines by 
 the British Government (1870), the average of 
 126,000 messages per week has grown to 603,000, 
 and the ic Press messages " have increased 
 from about 5,000 words per day to very nearly 
 1,000,000.* According to the Postmaster- General 
 the introduction of sixpenny telegrams involves 
 the erection of 15,000 additional miles of wire 
 in the British Islands. 
 
 Having seen how magnetism may be excited 
 by electricity, we will now turn our attention 
 to the reverse change the development of elec- 
 tricity from magnetism which is the founda- 
 tion of all the modes of converting mechanical 
 into electrical energy. This discovery, which 
 is of superlative importance, was made by 
 Faraday in October, 1831, quietly working ex- 
 perimentally in his laboratory in the Royal 
 Institution in London, solely in the pursuit of 
 truth, without any thought of the possible 
 
 * Vide lecture on the " Progress of Telegraphy," at the Insti- 
 tution of Civil Engineers, by Mr. W. H. Preece, F.R.S., reported 
 in Nature, Feb. 22, 1883, 
 
MAGNETISM AND ELECTRICITY. 
 
 147 
 
 consequences that might ultimately arise from 
 his results. It is thus described in his own 
 words : * 
 
 " A cylindrical bar-magnet, three-quarters of 
 an inch in diameter, and eight and a half inches 
 in length, had one end just inserted into the end 
 of the helix cylinder (220 
 feet long) ; then it was 
 quickly thrust in the whole 
 
 FIG. 62. 
 
 length, and the galvanometer needle moved, then 
 pulled out, and again the needle moved, but in the 
 opposite direction. This effect was repeated 
 every time the magnet was put in or out, and 
 therefore a wave of electricity was so produced 
 from mere approximation of a magnet, and not from 
 its formation in situ." 
 
 * Dr. Bence Jones's " Life and Letters of Faraday," Vol. II. 
 p. 5. 
 
 jc 3 
 
118 
 
 ENERGY IN NATURE. 
 
 Fig. 62 shows this fundamental experiment, 
 which is of superlative importance. The ends 
 of a hollow coil of insulated wire are con- 
 
 FIG. 63. 
 
 nected with a galvanometer, and the insertion 
 of the magnet, into the coil, or its withdrawal 
 
 therefrom, i.e., the motion of a 
 
 magnetic 
 
 field in the neighbourhood of a conducting wire 
 (or vice versa), produces a momentary current 
 
MAGNETfSM AND ELECTRICITY. 149 
 
 of electricity. Evidently, then, the rapid oscil- 
 lation of this bar-magnet would give rise to a 
 rapid succession of electric currents alternately in 
 opposite directions. Similar effects, but of greater 
 intensity, are produced by the rapid rotation of 
 coils of wire in front of the poles of a permanent 
 horse-shoe magnet. Such an arrangement is often 
 known as a magneto-electric machine (Fig. 63), 
 in which two coils of wire, t and t ', revolve on a 
 horizontal ' axis, /, in front of the poles of a 
 magnet, A, B. Small machines of this kind are 
 commonly sold by opticians, and are used for 
 giving shocks for medical purposes. The first 
 machine built for thus converting mechanical 
 into electrical energy on a large scale, was simply 
 a repetition of these parts, and one of them, 
 driven by a steam-engine, was used for some 
 years to produce the electric light in a lighthouse 
 on the coast of Kent. The term " armature" is 
 given to the rotating coils of wire, and in 1857 
 Sir W. Siemens devised one of peculiar form, in 
 which the wire is wound lengthways on a spindle, 
 for the production of the u field'' in which the 
 armature revolved. The next step was made 
 by Mr. Wilde, who employed electro-magnets 
 (excited by the current produced by a small 
 auxiliary magneto-electric machine), which, as 
 we have seen, are, weight for weight, very 
 much more powerful than permanent steel- 
 magnets. 
 
150 
 
 ENERGY IN NATURE. 
 
 The dynamo-machine in its present form, 
 however, owes its origin to a suggestion made 
 independently, and at the same time (L867), by 
 Sir W. Siemens and Sir Charles Wheatstone. In 
 it no extraneous source of electricity is needed to 
 
 FIG. 64. 
 
 keep up the current in the electro-magnet, which 
 is excited either by the whole or by a portion of 
 the current produced when the armature begins 
 to rotate, since matters are so arranged that the 
 coils of wire in the armature and those round the 
 large electro or " field" magnets, all form part 
 of one continuous circuit (Fig. 64). It will be 
 
MAGNETISM AND ELECTRICITY. 151 
 
 remembered that all iron is more or less mag- 
 netic, owing, in great part, to the influence 
 of the earth, and when the armature begins to 
 revolve, this residual magnetism in the cores of 
 the electro-magnets induces feeble currents in 
 the armature ; these currents then circulate 
 round the field -magnets, making them much 
 stronger, and they in their turn re -act upon the 
 armature, " inducing stronger currents in its 
 coils. After a few moments' revolution, there- 
 fore, during which the machine becomes in- 
 creasingly difficult to drive, its full power is 
 exerted, and the mechanical energy put in at 
 one end can be drawn off as electrical energy 
 at the other. The speed of rotation most suit- 
 able for the armature depends upon the par- 
 ticular construction of each machine, and 
 ordinarily varies from 600 per minute to four 
 times as much. 
 
 Such being the principles of the dynamo- 
 machine, it is almost needless to add that they 
 are capable of being carried out in practice in a 
 great variety of w r ays of greater or less efficiency, 
 according to the particular kind of work which 
 the machine is intended to do. In the present 
 transitional state of our knowledge, it may 
 safely be said that there is no such thing yet 
 as a " best dynamo." It may, however, be 
 well to point out in this connection what a 
 very efficient machine it is in comparison with 
 
152 
 
 ENERGY IN NATURE. 
 
 the steam-engine. It was shown in Chap. II. 
 that a steam-engine only yielded about one- 
 fourth of the total quantity of heat in it as 
 available for work. In the conversion of me- 
 
 FIG. Go. 
 
 chanical into electrical energy by the dynamo, 
 not three-fourths but only one-seventh of the 
 energy put into it is lost, or, in other words, 
 a good dynamo-machine will give out as elec- 
 trical energy 85 per cent, of the energy put into 
 it in the mechanical form. 
 
 The various types of dynamo -machines are 
 
MAGNETISM AND ELECTRICITY. 153 
 
 usually known by their inventor's names. The 
 first practical one, still very largely used, is 
 called the "Gramme" (Fig. 64), in which the 
 wire of the armature is wound upon a ring, the 
 principle of which was invented some years pre- 
 viously by Paccinotti. In the "Siemens" 1 
 machine (Fig. 65) the field-magnets are flat, 
 and connected by pole pieces, and the wire is 
 wound lengthways on the armature. In the 
 "Brush" the wire is wound in sections on a 
 ring, with intervening spaces, intended partly to 
 check the tendency to waste of energy as heat. 
 In the "Edison" the field-magnets are of 
 enormous length, and the armature resembles 
 that of the " Gramme ; " and in the " Ferranti" 
 the armature is of copper ribbon, rotated in 
 a very powerful field. The largest machine 
 yet built is the " Gordon," weighing twenty- 
 two tons, and capable of supplying energy 
 enough for 7,000 or 8,000 incandescence lamps. 
 It was designed in order to do the work of a 
 small gas works, and to show how much more 
 cheaply electricity could be produced when the 
 process was carried out on a comparatively large 
 scale.* 
 
 A very important division of dynamo- 
 machines is into those that supply alternating 
 currents i.e., currents alternately in oppo- 
 
 * Vide Mr. Gordon's lecture, printed in the "Journal of the 
 Society of Arts," upon " Electric Lighting," 
 
154 ENERGY IN NATURE. 
 
 site directions, as by the oscillation of the bar- 
 magnet in Faraday's original experiment, and 
 those that supply continuous currents in one 
 direction, this being effected by suitable com- 
 mutators for reversing the direction of half 
 the currents. For many systems of electric 
 lighting the former are preferable ; but for the 
 economical electric transmission of power, a 
 continuous current machine is almost indis- 
 pensable. 
 
 It was stated in the last chapter that the 
 production of light from electric energy was 
 due to some resistance interposed in the pas- 
 sage of the current. In the lt arc system " 
 this resistance is given by two carbon-points 
 and a thin stratum of air between them, the 
 thickness of which stratum varies with the 
 intensity of the current, but must be kept 
 constant for the same current if the light 
 is to be steady. The discharge between the 
 points (Fig. 66) is accompanied by an actual 
 transport of particles of carbon from the posi- 
 tive to the negative pole, and as the carbon 
 points are intensely heated they oxidise, and 
 bum away with the production of carbonic 
 acid (Chap. III.). 
 
 The proportion of light produced by this 
 combustion is insignificant (and not essential, 
 since the luminous arc can be maintained in 
 a vacuum or under water) ; but the fact of 
 
FIG. 66. 
 
156 ENERGY IN NATURE. 
 
 the combustion necessitates some contrivance 
 for keeping the points at the proper distance 
 apart, since they gradually waste away. This 
 is the object sought to be attained by the 
 innumerable arc-lamps, or regulators, before 
 the public, the general principle of the ma- 
 jority of which is, the mutually-opposing action 
 of a spring (or clockwork) which tends to 
 bring the points together, and of an electro- 
 magnet operated by the current that has passed 
 through the points, which tends to draw them 
 asunder. When their distance increases the 
 resistance is higher, less electricity circulates in 
 the magnet, and as its withdrawing influence is 
 lessened, the spring causes the points to approach, 
 and vice versa. 
 
 In other lamps gravity tends to bring the 
 points together, while a similar application of 
 electro -magnetism keeps them apart. In the 
 so-called electric candles, such as .Jablochkoff's, 
 the two pencils of carbon arc placed parallel to 
 each other, and separated by some china-clay 
 or other infusible insulating material. Since the 
 pencils are always at the same distance apart they 
 require no machinery to regulate their distance, 
 but they can only be used with alternating current 
 dynamos. 
 
 In the incandescence, or glow, system of 
 lighting, with which the names of Swan and 
 Edison are honourably associated as the practical 
 
MAGNETISM AND ELECTRICITY. 
 
 157 
 
 pioneers, who were succeeded later by Maxim 
 
 and Lane-Fox, the source of light is a con- 
 
 tinuous filament or narrow ribbon of carbon, 
 
 F, placed in a glass globe, A, from which all 
 
 air has been exhausted (Fig. 67). The ends 
 
 of the carbon filament are 
 
 attached, G, G, to platinum 
 
 wires, E E,' which are fused 
 
 into the substance of the 
 
 glass in their passage 
 
 through it, and serve to 
 
 convey the current to the 
 
 carbon. Should the globe 
 
 be accidentally broken, ilie 
 
 lamp is instantly extin- 
 
 guished without setting 
 
 alight to anything, because 
 
 the access of air to the 
 
 white hot carbon consumes 
 
 it almost instantaneously. 
 
 Swan's filaments are pro- 
 
 pared by heating to red- 
 
 ness, out of contact of air, slips of parchment 
 
 paper; Edison's by similarly treating bamboo 
 
 fibre. There is 110 difference in principle, but 
 
 only in detail, between the lamps of the 
 
 various makers named above ; the heat and 
 
 light are in all cases due to the resistance 
 
 offered to the passage of the current by the 
 
 carbon filament. In the manufacture of some 
 
 FIG. 67. 
 
158 ENERGY IN NATURE. 
 
 of these lamps, great pains are taken to adjust 
 the resistance of each filament to an exact 
 quantity. 
 
 Although arc-lighting is well adapted for 
 large public rooms, railway stations, factories, 
 and large open spaces, it is not suitable for 
 dwelling-houses, owing to its great intensity, 
 which cannot be diminished below a certain 
 point, and at the same time kept steady. For 
 such purposes, the incandescence system is 
 eminently suited. The best example of this 
 at present (1883) is seen in the Savoy Theatre 
 in London, where 1,200 lamps are arranged 
 in six circuits of 200 each, 824 of them being 
 used to light the stage. * Several of the 
 highest class of ocean passenger steam-ships are 
 now lighted in this way, the freedom from risk 
 of fire, and the absence of any noxious products 
 of combustion, such as carbonic acid, &c., ren- 
 dering the system peculiarly advantageous for 
 this purpose, as it is also for libraries, picture- 
 galleries, silver smiths' -shops, &c., the contents 
 of which are seriously injured by the sulphuric 
 
 The following statistics of the Electric Lighting at the Fisheries 
 Exhibition (first shown on June 26th) may be interesting. As 
 many as 4,000 incandescence lamps and nearly 300 arc lamps are 
 employed, the latter varying from 280 to 500 candle-power. The 
 amount of real, not nominal, candle-power thus distributed through 
 twenty-five miles of wire is estimated at more than half-a-million of 
 candles, and to produce it, 1,100 indicated horse-power is used up in 
 the steam-engines, of which there are more than fifty, each driving 
 its own dynamo-machine. 
 
MAGNETISM AND ELECTRICITY. 159 
 
 and other acids arising from the combustion of 
 gas. 
 
 It may be interesting to note here the differ- 
 ence in the amount of mechanical energy necessary 
 to produce the same amount of light by these 
 two systems of electric lighting. The expenditure 
 of one horse-power upon a good dynamo and arc- 
 lamp will give in the latter light equal to from 
 1,000 to 1,200 candles. The same energy ex- 
 pended upon incandescence lamps, will only yield 
 about 180-candle-power light, i.e., will only keep 
 going from 8 to 9 of Swan's standard 20-candle- 
 power lamps. 
 
 Into the comparative cost of lighting by gas 
 and by electricity, it is beyond the scope of this 
 volume to enter, since it involves questions of 
 the cost of obtaining power, which is so very 
 variable, especially when the supplies of natural 
 energy running to waste in waterfalls, wind, and 
 tide, are taken into consideration ; supplies which, 
 through the agency of the dynamo-machine and 
 the storage batteries (Chap. III.), may now be 
 rendered available for the production of electrical 
 energy. 
 
 It may, however, be worth notice, as an 
 interesting illustration of various transformations 
 of energy, that if a given volume of gas be burnt 
 in the ordinary way, and the amount and duration 
 of the light be noticed, and if an equal volume 
 of the same gas be burnt in an Otto gas-engine 
 
160 ENERGY IN NATURE. 
 
 (p. 81), which drives a dynamo-machine sup- 
 plying an arc-lamp, the amount of light in the 
 second case will be about seven times as much 
 as in the first, owing in great measure to the 
 fact that so little energy is wasted in produ- 
 cing the heat which always accompanies gas- 
 lighting.* 
 
 It may be stated broadly that when equal inten- 
 sities of light (i.e., the same number of " candle- 
 power") are produced, arc-lighting is, under almost 
 any practical set of conditions, cheaper than gas- 
 lighting. The same, however, cannot be said 
 of the only form of electric light available for 
 domestic use, viz., the incandescence system, 
 since it requires at least five times as much 
 mechanical energy as an arc-lamp for the 
 same amount of light. The comparison of 
 cost of electric and gas-lighting varies with 
 the local conditions in each town, such as the. 
 price paid there for gas, the price of steam 
 coal, and the presence or otherwise of waste 
 water-power ; hence each case can only be 
 dealt with on its own merits ; in making the 
 comparison, however, the advantages of electric 
 lighting alluded to on p. 158 should not be over- 
 looked. 
 
 Having now a clear conception of the way in 
 which mechanical energy is changed into electrical, 
 
 * Vide the Modern Applications of Electricity, by E. Hospitalier, 
 translated by Julius Maier, pp. 264, 265. (Kegan Paul & Co.) 
 
MAGNETISM AND ELECTRICITY. 161 
 
 let us consider the reverse change, viz., how 
 to produce mechanical energy from electrical, 
 or, in colloquial language, the use of electri- 
 city as a motive power. As long as electri- 
 city could only be produced by chemical, and 
 therefore costly, means (the relative costs of 
 the chemical and mechanical methods being as 
 120 to l; vide p. 110), this subject attracted 
 comparatively very little attention from the 
 so - called practical men, although scientific 
 workers had paid much attention to it, and 
 had devised various pieces of apparatus for 
 effecting it. 
 
 As already pointed out, Oerstedt's discovery 
 (p. 137) of the deflection of a magnetic needle 
 when an electric current passed near it, was 
 the first instance of the electrical production 
 of mechanical motion. The discovery of the 
 electro-magnet, and of the great attractive force 
 it exerted on its keeper, gave a great im- 
 petus to the construction of electro-magnetic 
 engines of various kinds, in which mechanical 
 motion was usually produced by the attraction 
 for a piece of iron of magnets alternately ex- 
 cited. In 1839 Jacobi propelled a boat along 
 the river Neva with such an engine of about 
 one horse-power, worked by a battery of sixty- 
 four large Daniell cells (p. 104). The electric 
 launch which was recently propelled on the 
 Thames had this very great advantage over it 
 
162 ENERGY IN NATURE. 
 
 in point of cost, viz., that the electricity had 
 been produced mechanically by a dynamo- 
 machine, and the energy had been stored electri- 
 cally in some secondary batteries (pp. 122-126) 
 until required for use. 
 
 In the present day, however, the transforma- 
 tion of energy necessary for the employment of 
 electricity as a motive power, is effected by the 
 agency of the dynamo-machine. One of the 
 keenest intellects of the age, the late Prof. Clerk 
 Maxwell, of Cambridge, replying. to an enquiry 
 as to what he considered the greatest discovery 
 of the last twenty-five years, said, " That the 
 Gramme machine is reversible." In other words, 
 a dynamo-machine (for the remark applies to 
 them all, though Gramme's was the best known 
 at the time) may be worked in both directions, 
 forwards and backwards. If supplied with 
 mechanical energy, it will give out electrical,, 
 but if supplied with electrical, it will give out 
 mechanical. 
 
 This is the principle upon which all the uses of 
 electricity as a motive power, and the electrical 
 transmission of power to a distance, depend. To 
 make an electric current set a machine in motion, 
 then, it is necessary to interpose between the ends 
 of the conducting wire and the driving pulley of 
 the machine, a small dynamo-machine worked 
 backwards. To such small machines the term 
 electric motor is usually given, but it must be 
 
MAGNETISM AND ELECTRICITY. 163 
 
 distinctly understood that the same machines are 
 capable of being used both as electric generators 
 and as motors. A large central generator may 
 (and at the present time in New York does) supply 
 electrical energy to a large number of small 
 motors (Fig. 68), all connected with it by wires, 
 each of which may work a separate machine. This 
 is practically carried out by Sir. W. Siemens on 
 
 FIG. 68. 
 
 his own farm, a saw-mill, a set of pumps, a chaff- 
 cutter and other machines, in different parts 
 of the farm, all being driven and controlled from 
 the dynamo and central steam-engine. When the 
 comparative cost of working large and small steam- 
 engines is taken into account, this mode of " dis- 
 tributing power" electrically will be found to be 
 very economical. It was in this sense that the late 
 Dr. Spottiswoode, President of the Royal Society, 
 recently expressed his belief that " before long 
 electricity would prove the poor man's friend," 
 enabling small amounts of motive power to be 
 brought to his own door for industrial purposes, 
 L 2 
 
164 ENERGY IN NATURE. 
 
 instead of his being obliged to go to the factory 
 for it. By these means electric railroads and 
 tram-roads are worked, the energy being in some 
 cases drawn from storage batteries (previously 
 charged by a dynamo-machine) carried on the 
 vehicle, and in others transmitted along a wire 
 or cable, between which and the car there is 
 maintained a " rolling contact."* In the Paris 
 electrical exhibition of 1881 there were examples 
 of almost every kind of mechanical operation 
 (even to rock-boring) carried on by electrical 
 energy. At the electrical exhibition at Munich 
 in 1882, twelve horse-power of energy was 
 daily transmitted to the building from some 
 waterfalls five miles off, the dynamo-machines 
 there being turned by large turbines, which 
 are peculiarly fitted for such work, from their 
 rapid speed. In all cases, however, where 
 machines are apparently " driven by electricity," 
 it is important to remember that there is (or 
 has been) mechanical energy at their back, in 
 some shape. 
 
 This electrical transmission of power, again. 
 
 * The special advantage, speaking in general terms, of electric 
 railroads, is the very small weight of the motor, as compared witli 
 that of a steam locomotive. At present, all bridges, embankments, 
 &c., have to be made two or three times as strong as they need be, 
 if the weight of the train only, without the locomotive, had to be 
 considered. In particular cases, such as badly ventilated under- 
 ground railways, long tunnels, &c., electric motors possess other 
 obvious advantages, since no noxious products of combustion are 
 given off by them. 
 
MAGNETISM AND ELECTRICITY. 165 
 
 affords the means, either with or without the in- 
 tervention of secondary batteries, of utilising 
 sources of natural energy that hitherto have 
 been running to waste, and it is only in this 
 limited sense that electricity may be considered 
 as a new force, or " the coming force," as it is so 
 often erroneously called, and only thus can it be 
 regarded as an addition to the means already 
 possessed by man of doing work. 
 
 One instance of the amount of this "wasted 
 energy" may be cited. At the site of the 
 Severn Tunnel, ten miles from Bristol, the 
 river is two and a half miles wide, and the 
 average rise of tide is fifty feet. If the 
 average rate of flow across this section were 
 only one mile per hour (which is certainly a 
 very low estimate), 100,000 horse-power could 
 be utilised ; the market value of this at present 
 is something like 1,000,000 per year, which 
 is now allowed to be wasted. The neces- 
 sary engineering works would be very simple, 
 though on a large scale, and the action could 
 be made continuous, and independent of the 
 state of the tide, without using electric accumu- 
 lators. 
 
 Probably the best example of this use of waste- 
 energy is the case of waterfalls before alluded to, 
 whose power can now be transmitted electrically 
 to distant cities, and there used either for light- 
 ing or for mechanical work. In his address to 
 
166 ENERGY IN NATURE. 
 
 " Section A " of the British Association for the 
 Advancement of Science at its meeting in 1881, 
 Sir W. Thomson showed that a copper wire 
 of half an inch diameter charged at Niagara 
 with energy equal to 26,250 horse - power, 
 would give out at a distance of 300 miles 
 (a greater distance than New York, Boston, 
 Montreal, or Philadelphia) energy equal to 
 21,000 horse-power, and that the cost of this 
 length of wire at 8d. per pound is 37,000, 
 the interest on which at 5 per cent is 1,850 
 per year. 
 
 The numerical relation between heat-energy 
 and work was pointed out in Chap. II., 772 foot- 
 pounds being the number that expresses the 
 mechanical equivalent of heat. The same kind 
 of exact relation exists between electrical energy 
 and work. The mode of measuring electrical 
 energy by galvanometers and voltameters of 
 various kinds has been indicated generally in 
 the preceding pages. In measuring the energy 
 that can be got out of a given volume of water 
 used in hydraulic machinery it is necessary 
 to know not only the quantity of water 
 used, but also the pressure or "head" at which 
 it is supplied. The same thing is true of 
 electricity ; both the pressure, i.e., the electro- 
 motive force (p. 106), and the quantity, must be 
 known. 
 
 Now, the first of these (the E.M.F.) is measured 
 
MAGNETISM AND ELECTRICITY. 167 
 
 in Volts, and the quantity in Amperes, and hence 
 a given quantity of electrical energy is measured 
 in volt-amperes. One horse-power is equal to 746 
 volt-amperes, or, to use the single word proposed 
 by Sir W. Siemens, to 746 watts. So practical 
 a question have these measures become, that in 
 many of the Provisional Orders for electric light- 
 ing purposes now before Parliament, the proposal 
 is made* to call by the name " one unit" the 
 energy contained in a current of 1,000 amperes 
 flowing under an electro-motive force of one volt 
 for one hour, and for this " unit " of electrical 
 energy it is proposed to charge sevenpence. 
 Now, as "one unit" of energy is 1,000 volt- 
 amperes (or 1,000 watts), and as 746 watts are 
 equal to one horse-power, it follows that the price 
 of energy delivered electrically is about 5Jd. per 
 horse-power per hour.t 
 
 Another illustration on a very much smaller 
 scale, though perhaps still more interesting, of 
 the transformation of mechanical into electrical 
 energy and vice versa, is afforded by that wonder- 
 ful little instrument for transmitting articulate 
 speech to long distances, the Bell Telephone 
 (Fig. 69). Like the dynamo-machine, it is a 
 
 * Vide " Nature," No. 695, Yol. XXVII., p. 385. 
 
 f In connection with the general subject of the Transmission of 
 Energy, it may ho interesting to note that Professor Osborne Rey- 
 nolds, F.R.S., recently expressed his opinion before the Society of 
 Arts that it would be a profitable speculation to supply stored 
 energy in the shape of air compressed in steel bottles, at the rate 
 of 3d. per million foot-pounds. 
 
168 
 
 ENERGY IN NATURE. 
 
 reversible engine, and may be worked in either 
 direction, i.e., it may be used both to receive 
 and to transmit messages. It consists of a steel 
 bar-magnet (d), round one pole of which is a 
 coil of insulated wire (a), the ends of which 
 (e, (?) are connected (/, /) with the line running 
 to the other station and with the earth- circuit. 
 Immediately in front of the same pole, 
 but not actually touching it, is a circular 
 
 FIG. 69. 
 
 diaphragm, or disc, of thin sheet-iron (b), 
 supported only at its edge, close behind a 
 mouthpiece (c, c). 
 
 As is well known, sound travels through the 
 air in waves ; the number of these waves that 
 strike the ear in a second determines the pitch 
 of the sound (i.e., the musical note), and their 
 size or height influences the timbre or quality 
 of the sound. When these waves in the air 
 strike upon the drum of the ear they cause the 
 sensation of sound ; when they strike on the dia- 
 phragm of the telephone they cause it to vibrate. 
 
MAGNETISM AND ELECTRICITY. 169 
 
 The vibrations of this iron disc close to the pole of 
 the magnet alter its " field " continually, and these 
 alterations excite very feeble currents of electricity 
 in the coil surrounding that pole; in this manner 
 the atmospheric waves are converted into elec- 
 trical waves, or undulatory currents, as Graham 
 Bell calls them. On their arrival at the distant 
 station these currents enter a precisely similar 
 instrument; circulating round its coil they pro- 
 duce a succession of feeble changes in its mag- 
 netic field, each one corresponding to a similar 
 change in the transmitting instrument. These 
 feeble changes in its field cause the magnet 
 to attract and repel its disc, i.e., to~ set it 
 vibrating ; these mechanical vibrations are at 
 once communicated to the air, and, falling on 
 the ear of the listener, produce the sensation 
 of sound. With a pair of such instruments 
 only it is possible not merely to converse, but 
 to recognise a friend's voice, at a distance of 
 several miles. 
 
 A telephone, then, converts atmospheric into 
 electrical waves, and vice versa. Its currents are 
 too feeble to overcome the resistance of a great 
 length of wire, but if in any way an undulatory 
 movement could be set up in a stronger current, 
 capable of travelling a great distance, the tele- 
 phone would convert these undulations into 
 sound, and thus the distance-limits to conversa- 
 tion could be enormously increased. This is 
 
170 
 
 ENERGY IN NATURE. 
 
 actually effected by the microphone of Professor 
 D. E. Hughes (Fig. 70), in which two or three 
 pieces of carbon lying together in " loose con- 
 tact " are put into the circuit of a few cells of 
 a battery, in which circuit also a telephone is 
 included. As long as these pieces of carbon are 
 perfectly still, a constant current passes, and no 
 sound is heard in the telephone. The smallest 
 vibration (even that produced by a fly walking) 
 
 FIG. 70. 
 
 in the neighbourhood of the carbons, however, 
 whether produced by speech or in any other way, 
 shakes them and alters the contact, causing a 
 variation, or undulation, in the current, which 
 the telephone renders audible. All telephonic 
 transmitters are simply combinations of micro- 
 phones, and with their aid conversation is pos- 
 sible up to distances exceeding 200 miles, and 
 the sound of music can be transmitted above 
 500 miles. Conversation has taken place also 
 recently between New York and Chicago, a 
 distance exceeding 1,000 miles ; in this case, 
 
MAGNETISM AND ELECTRICITY. 171 
 
 however, an ordinary telegraph wire was not 
 employed, but a steel wire coated electrically 
 with copper. 
 
 A more remarkable feat still, perhaps, also 
 achieved by Graham Bell, and also illustrating 
 the transformations of energy, is the trans- 
 mission of articulate speech electrically, without 
 the intervention of any conducting wire, by an 
 instrument called the Pliotophone. It depends 
 for its action upon the very curious fact that the 
 electric resistance, or power of conducting elec- 
 tricity, of the rare element selenium, is dimin- 
 ished by the radiant energy of light. If there- 
 fore a surface of selenium be included in an electric 
 circuit with a battery and a telephone, no sound 
 will be audible in the telephone as long as there 
 is no change in the intensity of the light falling 
 upon the selenium, but any such change alters the 
 resistance of the selenium, and causes the current 
 to vary in strength, or to become undulatory in 
 character, producing sounds in the telephone. 
 The necessary changes in the light are produced 
 at the transmitting station, by speaking to the 
 back of a thin mirror, so adjusted as to reflect a 
 beam of light upon a parabolic (curved) mirror, 
 at the receiving station, which concentrates the 
 beams on the selenium cell ; the words spoken 
 at the transmitting station cause the mirror to 
 vibrate, hence the beam falling on the selenium 
 vibrates also, undulations are thus set up in 
 
172 ENERGY IN NATURE. 
 
 the electric current, and these are translated by 
 the telephone into articulate speech ! There are 
 probably few, if any, more curious and interest- 
 ing examples of the transformation of energy 
 than this. 
 
CHAPTER VI. 
 
 ENERGY IN ORGANIC NATURE. 
 
 AT the end of Chap. III. a brief reference was 
 made to the power of that form of radiant energy 
 which we call Light, to do chemical work in 
 the shape of effecting the separation of certain 
 chemical substances from each other ; and the 
 decomposition in that way of the salts of silver, 
 upon which the art of photography is based, was 
 adduced as an instance. It was also stated in 
 passing, that under the influence of the radiant 
 energy of the sun, growing plants decomposed 
 the carbonic acid existing in the air (pp. 74 and 
 86) setting free oxygen, and storing up carbon 
 in their own substance. This process we are 
 now to consider a little more in detail, and 
 for this purpose it will be well to glance 
 first, very briefly, at the life-history of some 
 familiar plant, that of the oak from the acorn, 
 for example. 
 
 If an acorn be slit lengthways down the 
 middle, the greater part of its contents will be 
 seen to be a yellowish white substance, but at one 
 
174 
 
 ENERGY IN NATURE. 
 
 end there is a little thing about the size of a pin's 
 head, which contains the germ of the future plant, 
 a rootlet, and a rudimentary stem. The rest of 
 the acorn, the yellowish white substance, is 
 nothing more nor less than a store of nourishment 
 ready prepared for the plantlet, until it is able, 
 as it were, to work for its 
 own living. When the acorn 
 falls into the ground, so long 
 as the earth is cold and dry, 
 it lies as though dead, or like 
 a person in a trance ; but as 
 soon as the sun waves pierce 
 down into the earth, their 
 energy wakes up the plantlet, 
 agitates the particles of mat- 
 ter in it, and causes them 
 to combine with the par- 
 ticles of ready-made food- 
 starch, oils, sugar, and albu- 
 minoids which are laid up for the plantlet in 
 the thick seed-leaves in which it is buried 
 (Fig. 71). 
 
 In this way, then, does the baby -plant spend 
 the first stage of its existence, and the root and 
 stem are developed at the expense of the pre- 
 viously prepared food carefully elaborated for 
 it by the full-grown oak-tree. By the time, 
 however, that this store is exhausted, portions 
 of the plant have appeared above the ground 
 
 FIG. 71. 
 
ENERGY IN ORGANIC NATURE. 175 
 
 and are exposed to light, and as soon as this 
 occurs, those portions have become capable of 
 preparing for themselves their own food, while 
 the radiant solar energy works through them, 
 as it were. 
 
 Careful experiments have clearly shown that 
 the green .surfaces of growing plants absorb 
 carbonic acid from the air, and give out oxygen, 
 at the same time that they increase in weight 
 from the deposition of carbon in their tissues, 
 which are chiefly formed by the combination of 
 this carbon with hydrogen and oxygen derived 
 from the water absorbed by the roots. Woody 
 fibre consists chemically of thirty -six parts by 
 weight of carbon, five of hydrogen, and forty 
 of oxygen. 
 
 The evolution of oxygen by growing plants 
 may be readily shown by putting some freshly 
 gathered green leaves (laurel leaves answer well) 
 into a tumbler full of, arid inverted over, 
 water; when the sun shines on this, bubbles 
 of oxygen are given off by the leaves in abund- 
 ance. In an aquarium also, a stream of such 
 bubbles may often be noticed rising from a 
 broken stem of the water-plants Anacharis or 
 Valisneria. 
 
 At the same time that this process goes on, 
 the root fibres of the plant absorb water, and, 
 dissolved therein, various mineral substances, as 
 potash, &c., and especially substances containing 
 
176 ENERGY IN NATURE. 
 
 nitrogen, all necessary for its proper nourishment. 
 Much of the nitrogen reaches the plant in the 
 shape of ammonia (composed of fourteen parts 
 by weight of nitrogen and three of hydrogen), 
 from which the plant prepares those elaborate 
 nitrogenous compounds, which, as we shall 
 presently see, form an essential part of the 
 food of man. 
 
 It is very interesting to note that the radiant 
 energy of the electric light is nearly, if not 
 quite, as effective in promoting these changes in 
 the plant, and this elaboration of its food, as 
 that of sunlight. This subject has been carefully 
 investigated by Sir W. Siemens, who has been 
 able, by a judicious combination of hot-house 
 warmth and electric light, to control at will the 
 time of the flowering of plants and the ripening 
 of fruits, and such flowers and fruit have 
 been pronounced by competent judges to be 
 equal in all respects to those grown in the 
 ordinary way. 
 
 In November, 1881, two plots of precisely 
 similar ground on Sir W. Siemens's farm were 
 sown with oats, wheat, and barley, the same seed 
 being used in each case. One plot was not inter- 
 fered with in any way ; the other was lighted by 
 the electric light from sunset to sunrise, until May, 
 1882, when ears were cut from each plot. Fig. 72, 
 from a photograph kindly permitted by Sir W. 
 Siemens, shows the result in each case. The 
 
ENERGY IN ORGANIC NATURE. 177 
 
 three long ears were grown in the portion elec- 
 trically lighted at night, the three short ears in 
 the other ; the difference is greatest in the case 
 of the oats, and least in that of the wheat. A 
 very remarkable feature in these experiments, 
 first pointed out by Sir W. Thomson, and 
 fully confirmed by Sir W. Siemens, is, that 
 no artificial light will effect this decompo- 
 
 FIG. 72. 
 
 sition of carbonic acid in growing plants, 
 unless the temperature of the source from 
 which it comes is above that necessary for 
 the dissociation of carbonic acid, viz., 4,000 
 Fahr. (p. 85). The electric light is, as yet, 
 the only artificial light that fulfils these con- 
 ditions. Another inference from the experiment 
 is, that the period of rest, hitherto considered 
 necessary for plants during their growth, is 
 not nearly so essential to them as was generally 
 supposed. 
 
 The question now arises, what part of the 
 plant is it that effects this decomposition ? The 
 M 
 
178 
 
 ENERGY IN NATURE. 
 
 flesh of all plants and animals is made up of 
 little (transparent) bags of fluid growing one 
 against another, called cells (Fig. 73). In pith 
 they are round, large, and easily seen ; in stalks 
 of plants they are usually long, and overlap each 
 other ; in other parts of the plant they vary much, 
 
 both in shape and 
 size. The fluid which 
 these cells contain is 
 a sticky liquid with 
 minute grains con- 
 stantly moving about 
 in it, to which liquid 
 the name u proto- 
 plasm," or first form of 
 life, is given. In the 
 growth of the plant 
 these cells multiply 
 by being sub-divid- 
 ed, the protoplasm of 
 
 one cell dividing into two parts and building 
 up a wall between them, one cell thus becoming 
 two. In the simpler plants and animals, com- 
 posed of single cells or of one kind of cell, one 
 cell has to do a great many kinds of work, per- 
 forming all the different functions of the organ- 
 ism ; but the more highly organised a plant (or 
 animal) is, the more are these different duties or 
 functions divided among various kinds of cells, 
 
 FIG. 73. 
 
ENERGY IN ORGANIC NATURE. 
 
 179 
 
 only one duty being- allotted to each. Thus (Fig. 
 
 74) one set of cells will secrete starch (st), another 
 
 oil (), another nitrogenous 
 
 compounds, a fourth will 
 
 decompose carbonic acid, a 
 
 fifth has to do with respira- 
 tion, and so on. Why one 
 
 cell is able to do many 
 
 kinds of work, while ano- 
 ther, apparently precisely 
 
 similar, is only able to do 
 
 its own special kind, is a 
 
 question about which we are as ignorant (or 
 perhaps more so) as we are of 
 the nature of the various forms 
 of energy around us, which are 
 unknown apart from matter. 
 
 The protoplasm in the cells 
 that decompose carbonic acid 
 (Fig. 75) is of a beautiful green 
 tint; to it the name chlorophyll is 
 given. It absorbs all the solar 
 rays except the green ones, which 
 it throws back again, producing 
 in our eyes the sensation of a 
 green colour. 
 
 Under the influence of solar 
 energy, then, carbon and oxygen 
 are separated from each other. 
 
 In Chap. III. it was shown that when chemical 
 
 M2 
 
 FIG. 75. 
 
180 ENERGY IN NATURE. 
 
 attraction exerted itself between these two ele- 
 ments, heat and light were the result. Hence, 
 when wood is burned (and, as we shall pre- 
 sently see, when vegetable food is " burned, 
 i.e., oxidised, in the animal body), the energy 
 thus generated is simply a reproduction of the 
 solar energy which separated the carbon and 
 oxygen during the growth of the plant, just 
 as in the secondary battery (pp. 122-126) the 
 electrical energy drawn off when the cell is 
 discharged, is due to the re-union of the 
 hydrogen and oxygen which had been pre- 
 viously separated from each other by the 
 decomposing action of the current employed 
 to charge the cell. As was remarked in the 
 detailed explanation of that, there is a complete 
 analogy between these two cases of the storage 
 of energy. 
 
 Now, this line of argument may be carried 
 back into those distant geological ages when 
 the mighty forests flourished which now form 
 coal, that great store of our national wealth, and, 
 regarding our coalfields in the light of a vast 
 store of potential energy, it may be worth while 
 briefly to trace their history. 
 
 A careful examination of many kinds of coal 
 shows that it will split into layers, and if the coal 
 be examined in the mine it will be found to lie 
 between beds of a hard mineral called shale, 
 which splits into layers much more readily than 
 
ENERGY IN ORGANIC NATURE. 181 
 
 the coal itself does. In this shale are frequently 
 found the impressions of various portions of 
 plants, such as those seen in Fig. 76. Moreover, 
 
 FIG. 76. 
 
 in the underclay, or floor, of the coal-bed, are 
 found curious fossils (Fig. 77), which investiga- 
 
 FIG. 77. 
 
 tion has shown to be the underground root-stems 
 of some of the trees, the remains of whose trunks 
 were found in the shale and coal. Speaking 
 in technical language, the Stigmaria has been 
 shown to be the root-stem of the Siyillaria. 
 There are found also occasionally imbedded in 
 
182 
 
 ENERGY IN NATURE. 
 
 the coal, round nodules of stone like cannon-balls, 
 which, when cut across (Fig. 78) in transparent 
 
 F IG .. 78. CONTENTS OF A COAL-BALL. (Carntthers.}* 
 
 S, Stem of Sigillaria cut across. _, Stem of Lepidodciidron cut across. 
 L', Stem of Lepidoderidron cut lengthways. /, cone of Lepidodcndron 
 (Lepidostrobus) cut across. C, Stem of Calamite cut across, c, c, c, 
 Fruit of Calamite lengthways and across. /, Stem of a fern with 
 fragments, of fern-leaves scattered round it. The small round dots scat- 
 tered here and there are the larger spores which have fallen out of the 
 fruit-cones. 
 
 slices, show the leaves, stems, &c., of the coal- 
 plants. 
 
 In the coal-forests, then, an ideal drawing of 
 a scene in which is given in Fig. 79, there were, 
 
 * I am much indebted to Miss Arabella Buckley for allowing me 
 to copy this figure from the original diagram of a coal-ball by 
 Mr. Carruthers, of the British Museum. 
 
FIG. 79. 
 
184 
 
 ENERGY IN NATURE. 
 
 broadly, three kinds of trees or plants a tree 
 with a scaly trunk, called Lepidodcndron ; a tree 
 with very curious marking's on its stem, called 
 Slgillaria ; and a plant with a reed-like stem, 
 called Catamites; and associated with these grew 
 large tree-ferns, such as are now seen in the 
 forests of New Zealand and Australia, as well as 
 many smaller ferns. None of these coal- forest 
 plants, however, bore any flowers, and when in pro- 
 cess of time flowering plants were developed, they 
 began to crowd out the giants of the coal-forest 
 in the struggle for 
 existence, and the lat- 
 ter gradually dwindled 
 down, so that their re- 
 presentatives living in 
 the present day are 
 very small indeed. 
 The little club - moss, 
 Lycopoditim (Fig. 80), 
 which grows best on 
 the moors in the North 
 of England, is the de- 
 scendant of the gi- 
 gantic Lepidodendron, FIG. so. 
 and the " horse-tail," 
 
 or Equisetum (Fig. 81), is all that remains of the 
 Galamites. 
 
 In connection with the history of the forma- 
 tion of coal it is interesting to note, that the same 
 
ENERGY IN ORGANIC NATURE. 185 
 
 processes which went on during the period of 
 the coal-formation are now going on in a marshy 
 district of North Carolina, in the United States ; 
 the plants are of course different, but the general 
 state of things as regards the atmosphere and 
 the ground is the same. In this great dismal 
 swamp trees and plants go on growing and 
 shedding their leaves year by year, and as 
 they fall into the water they are protected 
 from the decomposing 
 action of the air, and 
 accumulate layer upon 
 layer : the streams that 
 feed this swamp bring 
 perfectly clear water, 
 since all mud is fil- 
 tered from them by 
 the tangled mass of 
 roots, stems, &c., at 
 the outside edges of 
 the swamp ; thus a 
 great thickness of 
 vegetable remains is 
 being formed free 
 from any earthy mat- 
 ter, producing what we 
 now recognise as peat. 
 
 The transformation of the beds of peat into 
 coal was due to the various changes of level to 
 which they were subjected in the course of geo- 
 
186 ENERGY IN NATURE. 
 
 logical ages, owing partly to the " energies within 
 the earth" (such as volcanoes), and to the contrac- 
 tion of the crust of the earth as it gradually cooled 
 (causing cracks in its surface, which produced 
 mountain ranges), and partly to the wearing- 
 down action on the earth's surface of water, ice, 
 and atmospheric changes. Owing to combina- 
 tions of these various agencies, the peat-beds 
 gradually became covered with hundreds of feet 
 thickness of other material, usually rock, the 
 weight of which consolidated the peat, and ulti- 
 mately produced coal.* In some cases, too, the 
 outburst of volcanic rock near a partly-formed 
 coal-bed distilled portions of it, and produced 
 the natural mineral oils so abundant in some 
 parts of the world, and also gave rise to the gas 
 which, as is well known, often issues from cracks 
 in the coal, and in ill-ventilated mines produces 
 the disastrous results of an explosion (p. 77). 
 
 The unquestionably vegetable origin of coal 
 having now been shown, and the chemical changes 
 that occur during the growth of plants having 
 been pointed out, the truth of the statement that 
 coal is really a reservoir or store of the sun's 
 energy which poured down upon the earth in 
 past ages, will be apparent. Great and power- 
 ful minds often see things by instinct, as it 
 were, or intuitively, without going through the 
 
 * This conversion of peat iiito coal can be effected in the labora- 
 tory on a small scale by very strong hydraulic pressure. 
 
ENERGY IN ORGANIC NATURE. 187 
 
 whole steps of a demonstration, and it was the 
 intellect mil perception of the mutual relation of 
 these facts that led George Stephen son to say, 
 in reply to a question as to what drove his 
 locomotive, "The sun;" and on other occasions 
 to refer to coal as " bottled sunshine." 
 
 We now come to the consideration of Energy 
 in the animal kingdom. An exact definition of an 
 animal as distinguished from a plant, is exceed- 
 ingly difficult to frame ; probably a thoroughly 
 satisfactory one never will be found, since the 
 tendency of all modern scientific results, not only 
 in biology, but in other branches, is to obliterate 
 all such strongly marked distinctions and systems 
 of definite and self-contained classifications. In- 
 deed there is little doubt that organisms exist 
 which are, to all intents and purposes, plants at 
 one period of their life-history, and animals at 
 another. A broad distinction, however, between 
 these two great classes of living beings, is to be 
 found in the nature of their food, a distinction 
 which applies at any rate to the higher animals 
 and higher plants, and probably does to the lower 
 ones also, although it is difficult in some cases to 
 prove it. 
 
 Plants, as we have seen, derive their food 
 from simple substances, carbonic acid, ammonia, 
 and water from which they build up most 
 complicated compounds. Animals, on the other 
 hand, require that their food should be previously 
 
188 ENERGY IN NATURE. 
 
 prepared and elaborated for them, by plants or by 
 other animals. Plants, in fact, convert actual, 
 radiant, moving energy, into a store of potential 
 energy ; animals, on the other hand, may be re- 
 garded as machines which convert potential into 
 actual energy. The potential energy is supplied 
 to the animal in its food, and this is converted, 
 by the chemical changes within the body, into 
 the actual energy of heat and mechanical work. 
 These chemical changes are in the main, an 
 oxidation of the various constituents of the food, 
 a slow combustion of them in fact, a change of 
 precisely the same nature as that explained in 
 Chap. III. under the head of Combustion, although 
 occurring more slowly, and therefore its effects 
 are spread over a longer time, and are not so 
 immediately evident. If a given weight of 
 carbon be burnt in the air, or consumed in an 
 animal's food and slowly oxidised in his body, the 
 amount of heat (and therefore of work got out of 
 that heat) is the same in both cases. The income 
 of animal energy, therefore, consists in the oxi- 
 dation of food into its waste products, while the 
 expenditure takes the form of the heat necessary 
 to the maintenance of life, and of the mechanical 
 work done. 
 
 It has been pointed out by Helmholtz that 
 only one-fifth of the energy produced by the 
 combustion of food can be used outside the human 
 body; the remaining four-fifths being required 
 
ENERGY IN ORGANIC NATURE. 189 
 
 rt 
 
 for doing the vast amount of internal ivorJc neces- 
 sary in the body, such as the movements of 
 the heart and lungs, &c., and in the maintenance 
 of the animal heat. The normal temperature of 
 the human body is 98 Fahr., and it is obvious 
 that the combustion of a considerable amount of 
 material is .necessary to keep so large a surface 
 and so great a bulk at a temperature several 
 degrees above that of the surrounding air. It is 
 a well known fact that when people die of 
 starvation, the cause of death is really cold i.e., 
 inability to maintain the heat necessary for the 
 vital processes ; hence, when food is scarce, 
 much less is needed if the body be kept 
 warm ; a fact noted by a careful Lancashire 
 housewife, who, during the last cotton famine, 
 kept the men of her household in bed and well 
 covered up, for about twenty hours out of the 
 twenty-four. 
 
 Animal heat, then, is kept up by the slow , 
 oxidation of the carbon, hydrogen, and nitrogen V 
 in the food that we eat, and animal work, as we j 
 shall presently see, is derived from the same 
 source. " Observe," says Sir W. Armstrong in 
 his address on the steam-engine referred to on 
 page 54, " how superior the result is in Nature's 
 engine to what it is in ours. Nature only uses 
 heat of low grade, such as we find wholly un- 
 available. We reject our steam as useless at a 
 temperature which would cook the animal sub- 
 
190 ENERGY IN NATURE. 
 
 stance, while Nature works with a heat so mild 
 as not to hurt the most delicate tissue. And 
 yet, notwithstanding the greater availability of 
 high-grade temperature, the quantity of work 
 performed by the living engine, relative to the 
 fuel consumed, puts the steam-engine to shame." 
 It will be desirable here to point out the 
 three classes under which the various things 
 composing our food may be grouped. The first 
 group comprises all those articles of food in 
 which starch and sugar are the chief components. 
 rThese substances consist of carbon, hydrogen, 
 yand oxygen, but the two latter substances are 
 present in almost exactly the proportions neces- 
 sary to form water. Hence, the only energy 
 derivable from them is due to the oxidation 
 of the carbon which they contain. To the 
 jsecond group belong all oils, fat, butter, &c., 
 I substances which contain large quantities of 
 carbon and of hydrogen, united with only a 
 Ismail proportion of oxygen. Hence, for equal 
 weights, much more energy is obtained from 
 foods of this group than from those of the 
 previous one, and it will be remembered that 
 the amount of heat-energy developed by the 
 combustion of hydrogen (p. 71) is greater than 
 that from the same weight of any other known 
 substance. The third great group of foods con- 
 tains the element nitrogen, in addition to carbon, 
 ^hydrogen, and oxygen, and the purest example of 
 
ENERGY IN ORGANIC NATURE. 191 
 
 it is seen in the white of an egg, a substance called 
 albumen by the chemist ; other instances are seen 
 in the gluten of wheat (the stringy substance 
 that remains when all the starch has been washed 
 away from flour by a stream of water), in the 
 fibrin of meat, the casein of cheese, and also in cer- 
 tain vegetables, notably lentils, peas, and beans. 
 
 When these various substances are oxidised 
 in the animal body, by the oxygen drawn into 
 the lungs during breathing, carbonic acid is pro-N 
 duced from the carbon, and water from the hydrojy 
 gen, and both of these products pass off chiefly 
 in the breath. The oxidation products of the 
 nitrogen form a rather complex substance called 
 urea, in the getting rid of which from the body 
 the kidneys play a very important part. The 
 evil influence of carbonic acid when present even 
 in small quantities in the air, and the tests for it, 
 were pointed out in Chap. III. ; but it may be 
 interesting to note in this connection that, taking 
 the case of an adult with an average alternation 
 of activity and repose, about 360 cubic feet 
 of air pass through his lungs in twenty-four 
 hours, and as the air exhaled contains about 
 one -twenty -fourth part of carbonic acid, it 
 follows that about 15 cubic feet of that gas, 
 containing about 8 oz. of solid carbon, are 
 thrown off from the lungs of each adult in every 
 twenty -four hours. 
 
 It was formerly thought, on the authority of 
 
192 ENERGY IN NATURE. 
 
 Liebig who, although a very eminent chemist, 
 was not a physiologist that hard bodily work 
 was done at the expense of the muscular tissue 
 itself, which was actually destroyed in the pro- 
 cess. More careful experiments, however, showed 
 that this could not be the case, and one very 
 curious instance may be adduced the amount of 
 work done inside the human body by the heart 
 in maintaining the circulation of the blood is so 
 great, that if it were done at the expense of the 
 muscular tissue of the heart itself, a new heart 
 would be required every week ! Again, if 
 Liebig' s doctrine were true, whenever exces- 
 sively hard work is done, there ought to be 
 a corresponding increase in the production 
 of urea, the oxidation-product of nitrogen, and 
 this also is found not to be the case. It is, 
 however, the fact that during hard work a 
 larger supply than usual of albumenoid food 
 is necessary, but this is probably due rather to 
 the rate than to the extent of the chemical 
 change which violent exercise causes in the 
 human body. 
 
 Liebig's doctrine was completely disproved 
 by the careful experiments of two German 
 chemists, Fick and Wislicenus, who in their 
 own persons were the subjects of the ex- 
 periments. The work that they did was the 
 ascent of the Faulhorn, a Swiss mountain ; as its 
 height was known, and also their own weight, 
 
ENERGY IN ORGANIC NATURE. 193 
 
 the number of foot-pounds of work done by each 
 in making the ascent could be readily calculated. 
 For a few days previously they fed themselves 
 upon starch, sugar, &c., avoiding all nitrogenous 
 compounds, and they carefully measured and 
 weighed the products of the oxidation of this 
 food ; they then found not only that there was 
 no notable increase in the production of urea 
 (the result of the oxidation of the nitrogenous 
 constituents of the body), but that the amount 
 thus oxidised only corresponded to one-fifth 
 part of the total work done. It was clearly 
 shown also that the work ivas done at the 
 expense of the carl^^^i^lp^rogen contained 
 in the food. 
 
 Dr. Frankland has investigated the same 
 question from a different point of view, and 
 has shown the amount of heat that is pro- 
 duced by the complete combustion or oxida- 
 tion of different kinds of food, and this being- 
 known, the mechanical equivalent of heat in- 
 vestigated by Joule (pp. 35, 36) enables the same 
 thing to be expressed in terms of mechanical 
 work. 
 
 Here are some of Dr. Frankland's re- 
 sults : 
 
 Heat derived from the complete combustion of 1 Ib. of 
 each of the following substances, translated into foot-tons of 
 work (1 Ib. of water raised 1 Fah. 772 foot-pounds = Jrd 
 foot-ton nearly). 
 N 
 
194 ENERGY IN NATURE. 
 
 Wheat-flour . 
 
 , 2,283 foot-tons. 
 
 Oatmeal 
 
 . 2,439 
 
 Peas 
 
 . 2,341 
 
 Potatoes 
 
 618 
 
 Lean beef 
 
 885 
 
 Beef-fat . 
 
 , 5,626 
 
 Butter 
 
 . 4 507 
 
 Mackerel 
 
 . 1,000 
 
 Rice (Chinese) 
 
 . 2,330 
 
 Dr. Frankland put the same thing in another 
 way. He contrived a means for ascertaining the 
 amount and the cost of food necessary to be com- 
 pletely oxidised in order to do the same amount 
 of work as that done by Fick and Wislicerms, 
 viz. raising a man's weight (140 Ibs.) 10,000 feet 
 high, with the following results : 
 
 Ibs. Price per Ib. Cost. 
 
 Bread . .2-345 1|- 3J 
 
 Oatmeal. .1-281 2] 3J 
 
 Potatoes . 5-068 01 5| 
 
 Beef-fat. .. 0*555 10 5| 
 
 Cheese . . 1-156 10 11 J 
 
 Butter . . 0-693 16 1 
 
 Lean beef . 3 -532 1 3 6| 
 
 Pale ale . . 9 bottles 00 40 
 
 This table shows clearly the real value of 
 various kinds of food compared with each other, 
 provided that when taken they are completely 
 oxidised, and properly assimilated and digested ; 
 
ENERGY IN ORGANIC NATURE. 195 
 
 but in arriving at a sound conclusion as to the 
 practical value of any kind of food in any par- 
 ticular case, the two last points raised are of 
 great importance. Into the general question of 
 dietaries space forbids us to enter, but it may 
 be stated generally that the daily need of the 
 average adult requires as a minimum the assimi- 
 lation of 4,900 grains of carbon and 300 grains 
 of nitrogen, and that such quantities are found 
 in the following dietary : 
 
 Bread . . . . . .18 ozs. 
 
 Butter 1 
 
 Milk 4 
 
 Bacon . . . . 2 
 
 Potatoes . . . . 8 
 
 Cabbage . . . . . 6 
 
 Cheese 3J 
 
 Sugar 1 
 
 Salt . J 
 Water, alone, and in tea, coffee, 
 beer, etc. . 
 
 Total 6 Ibs. 14J ozs. 
 
 The food that forms and repairs muscle, and 
 gives permanent capability of muscular power, 
 must not be confounded with that which sup- 
 plies the requisites for temporary activity. An 
 examination and analysis of various public 
 dietaries, containing the food found necessary 
 for men doing work of various degrees of 
 
 N2 
 
196 ENERGY IN NATURE. 
 
 hardness, shows clearly that while in very hard 
 /*work the amount of hydro-carbons (such as 
 ^fat, starch, sugar, &c.) has to be doubled as 
 / against the amounts necessary for a healthy 
 /adult in moderate exercise, the amount of nitro- 
 genous food need only be increased by one- 
 tenth. 
 
 In connection with the process known as 
 assimilation, by virtue of which food is digested, 
 and part of it is stored up in the body, as 
 a reserve in the shape of fat, or in renew- 
 in^ muscular tissue, it should be rioted that 
 
 . 
 there is good reason to believe that this is 
 
 by no means so simple a process as generally 
 
 /supposed ; that the assimilation is not as it 
 
 \ were direct, in the sense of the fat in food 
 
 / forming fat in the body, fibrin in the food 
 
 i becoming muscle in the body, and so on, 
 
 but that complicated processes of the re- 
 
 \ arrangement of compounds go on in the animal 
 
 Structure. 
 
 The question now arises as to how this oxi- 
 dation of the food, from which all animal energy 
 is derived, is carried on. It was first pointed out 
 /by the German physician Mayer, that " muscle 
 /is the instrument by which the transformation 
 pf force is accomplished, and not the material 
 iwhich is itself transformed." In fact, according 
 to the most recent researches on this question, 
 all the various chemical changes in food which 
 
ENERGY IN ORGANIC NATURE. 197 
 
 we have been considering, appear to be really 
 carried on in the muscular substance of the body, 
 through the medium of the blood. The general 
 facts of the circulation of the blood through 
 minute vessels in the body, and its maintenance 
 by the action of the heart, are well known. This 
 circulation serves a great many purposes, and 
 the liquid part of the blood carries to the mus- 
 cular tissues the results of the digestion of the 
 food in a state prepared for oxidation. The 
 minute red corpuscles or globules floating in the 
 blood carry with them the oxygen necessary for 
 the combustion of this food, and the blood is 
 distributed over the surface and through the sub- 
 stance of the muscular tissue in very fine vessels. 
 In some way, which is not yet perfectly understood, 
 owing chiefly to the difficulties of studying this 
 question in the living subject, the muscular fibres 
 absorb both the prepared food and the oxygen, 
 and in their substance these two re- act upon each 
 other. 
 
 But the function of the blood does not 
 end here ; it carries away the noxious products 
 of oxidation on its return journey, and takes 
 them to the lungs, where they are brought in 
 contact with the fresh air inspired in breathing. 
 Here an interchange again takes place, the blood 
 gives off these noxious products, which pass awav 
 in breathing outwards, and it also absorbs a fresh 
 portion of oxygen, which it again cantos to the 
 
198 ENERGY IN NATURE. 
 
 interior of the body. There is, therefore, always 
 a stream of oxygen from the blood to the muscles, 
 and of carbonic acid from the muscles to the 
 blood, and the oxygen is stored up in the tissues. 
 That this is really the case is proved by the fact 
 that certain animals, e.g., a frog, can produce or 
 give off carbonic acid in an atmosphere free from 
 oxygen. 
 
 It appears, therefore, that the oxidation in 
 the body is carried on by the tissues themselves ; 
 that the blood is merely a carrier, and the lungs 
 are the vehicle of discharge, and that all this 
 work is necessary for the proper maintenance of 
 life in the higher animals. Hence it has been 
 well remarked that "we live in a world of work, 
 of work from which we cannot possibly escape, 
 and that those of us who do not require to 
 work in order to eat, must at least in some sense 
 perform work in order to live." In connection 
 with the exertion of voluntary muscular power 
 in man, too, it should be noticed that although 
 it acts under the direction of the will, and is 
 more or less controlled by it, muscle does not 
 derive its power of acting from the will, any 
 more than a steamboat derives its power of 
 motion from the man at the helm who guides 
 it. The force that propels the boat is derived 
 from the chemical action in the furnace of its 
 steam-boiler (and that again from the sun, as 
 seen in the first part of this chapter), and the 
 
ENERGY IN ORGANIC NATURE. 199 
 
 motion thus produced is directed by the rudder 
 operated by the steersman, and is controlled 
 by the engineer in charge. So is it with the 
 will, in its relation to the energy of muscular 
 action. 
 
 A little attentive consideration of the fore- 
 going pages will show that animals throw off 
 from themselves as the result of the oxidation 
 processes going on in their bodies, precisely 
 those substances which are necessary for the 
 food of plants, viz., carbonic acid, water, and 
 the nitrogenous compound urea, which is 
 speedily changed into a simple salt of ammonia. 
 On the other hand, plants, under the influence 
 of solar energy, decompose these very substances, 
 and from them elaborate food for animals, 
 and whatever is necessary also for their 
 own growth. It is to this wonderful cycle 
 that the term " balance of organic nature " 
 is often given.* A very good example of it 
 
 * The following passage from Book II. (Population and Sub- 
 sistence) of Henry George's "Progress and Poverty," is very 
 suggestive in this connection : 
 
 " For that man cannot exhaust or lessen the powers of nature 
 follows from the indestructibility of matter and the persistence 
 of force. Production and consumption are only relative terms. 
 Speaking absolutely, man neither produces nor consumes. The 
 whole human race, were they to labour to infinity, could not make 
 this rolling sphere one atom heavier or one atom lighter, could not 
 add to or diminish by one iota the sum of the forces whose ever- 
 lasting circling produces all motion and sustains all life. As the 
 water that we take from the ocean must again return to the ocean, 
 so the food we take from the reservoirs of nature is, from the 
 moment we take it, on its way back to those reservoirs. What we 
 
200 ENERGY IN NATURE. 
 
 is seen in a well-managed aquarium, whether 
 of fresh or salt water. The animals living there 
 breathe the oxygen dissolved in the water, 
 and the resulting carbonic acid also remains in 
 solution, but is at once seized on by the grow- 
 ing plants that are present, and, as before 
 stated (p. 175), the liberation of oxygen by 
 them may often be noticed on a bright day in 
 the form of a stream of bubbles rising to the 
 surface ; in this way the water may be kept 
 constantly fresh. 
 
 We have now arrived at the conclusion of the 
 consideration of the special subject which it has 
 been the object of these pages to set forth the 
 mode in which the various forms of Energy are 
 interchangeable, without any diminution in their 
 total amount ; the manner in which the various 
 so-called Forces of Nature are related to each 
 other, or, to speak reverently, how the various 
 
 draw from a limited extent of land may temporarily reduce the 
 productiveness of that land, because the return may be to other 
 land, or may be divided between that land and other land, or 
 perhaps all land ; but this possibility lessens with increasing area, 
 and ceases when the whole globe is considered. That the earth 
 could maintain a thousand billions of people as easily as a thousand 
 millions is a necessary deduction from the manifest truth that, at 
 least so far as our agency is concerned, matter is eternal and force 
 must for ever continue to act. Life does not use up the forces that 
 maintain life. We come into the material universe bringing 
 nothing ; we take nothing away when we depart. The human 
 being, physically considered, is but a transient form of matter, 
 a changing mode of motion. The matter remains and the force 
 persists. Nothing is lessened, nothing is weakened. And from 
 this it follows that the limit to the population of the globe can 
 only be the limit of space." 
 
ENERGY IN ORGANIC NATURE. 201 
 
 ways in which the Deity works throughout Nature 
 are mutually interdependent. We have seen 
 reason also to believe that Energy, like Matter, 
 is never destroyed, and that its total amount in 
 the universe is "unchangeable, capable of neither 
 increase nor diminution. We know that there 
 are " other systems and other suns, each pouring 
 forth energy like their own, but still without in- 
 fringement of the law which reveals immutability 
 in the midst of change, which recognises incessant 
 transference or conversion, but neither final gain 
 nor loss. The law generalises the aphorism of 
 Solomon, that l there is nothing new under the 
 sun,' by teaching us to detect everywhere, under 
 its infinite variety of appearances, the same 
 primeval force. To Nature nothing can be 
 added, from Nature nothing can be taken away ; 
 the sum of her energies is constant, and the 
 utmost man can do in the pursuit of physical 
 truth, or in the applications of physical know- 
 ledge, is to shift the constituents of the never- 
 varying total. The law of conservation rigidly 
 excludes both creation and annihilation . . . 
 The flux of power is eternally the same. It rolls 
 in music through the ages, and all terrestrial 
 energy, the manifestations of life, as well as 
 the display of phenomena, are but modulations 
 of its rhythm." (Tyndall.) 
 
 The first scientific man in this country to draw 
 attention to the doctrine of the correlation of the 
 
202 ENERGY IN NATURE. 
 
 physical forces was Sir W. (Mr. Justice) Grove, 
 whose work under that title is still one of the best 
 treatises on the subject, but there is no doubt that 
 its truth had been intuitively apprehended by 
 others about the same time, and notably by Dr. 
 Joule and by Dr. Mayer, of Heilbronn, Germany. 
 To enumerate, however, the names of the scien- 
 tific workers to whose labours the establishment 
 of this doctrine on a firm and unassailable 
 foundation is due, would be an invidious task, 
 and out of place in these pages, which only 
 profess to be a resume, and not a history, of the 
 subject. The treatise on the " Conservation of 
 Energy," by Professor Balfour Stewart, and 
 Faraday's lectures on the various " Forces of 
 Nature" will give fuller information to those who 
 desire it. 
 
 Towards the end of his work upon " Cor- 
 relation and Continuity," Sir W. Grove 
 says : 
 
 " Reviewing the various relations between the 
 various forces we have been considering, it would 
 appear that where one of these is excited or 
 exists, all the others are also set in action ; thus, 
 when a substance such as sulphide of antimony is 
 electrified, at the instant of electrisation it becomes 
 magnetic in directions at right angles to the lines 
 of electric force ; at the same time it becomes 
 heated to an extent greater or less, according to 
 the intensity of the electric forces. If this in- 
 
ENERGY IN ORGANIC NATURE. 203 
 
 tensity becomes exalted to a certain point the 
 sulphide becomes luminous, or light is produced ; 
 it expands, consequently motion is produced, and 
 it is decomposed, therefore chemical action is pro- 
 duced . . . This is a strong argument in 
 favour of the theory which regards all these dif- 
 ferent forces as modes of motion" At another 
 place he says : " Tracing any force backwards 
 to its antecedents we are merged in an infinity 
 of changing forms of force ; at some point we 
 lose it, not because it has in fact been created 
 at any one point, but because it resolves itself 
 into so many contributing forces, that the 
 evidence of it is lost to our senses or powers of 
 perception." 
 
 Our consideration, then, of the various forms 
 of energy in nature has led us to see 
 
 1. That where one is excited or exists, many 
 others are also set in action ; hence, probably, all 
 are modes of motion. 
 
 2. That any one can be transformed, either 
 directly or by intermediate steps, into any 
 other. 
 
 3. That none of them can be produced but by 
 some other as an anterior. 
 
 4. That they act uniformly, i.e., according to 
 fixed laws. 
 
 Now the term " Law of Nature," in the scien- 
 tific sense, is simply the expression of the orderly 
 
204 ENERGY IN NATURE. 
 
 uniformity with which certain series of phenomena 
 invariably succeed each other, and is not the 
 immediate expression of a personal will. Let us 
 briefly consider the precise meaning that should 
 be attached to the term laiv, used in this con- 
 nection. To the question, " Why does a stone 
 fall to the ground?" the majority of educated 
 persons would reply, " Because of the law of 
 gravitation," or " Because of the earth's attrac- 
 tion," and would consider that they had given a 
 proper explanation of the phenomenon. A little 
 thought, however, will show that this is clearly 
 no explanation at all, but simply an assertion that 
 the stone falls in accordance with the law of gravi- 
 tation ; moreover, this law is simply the general 
 expression of the fact that all stones fall towards 
 the earth, or of the still more general fact that 
 all material bodies in the universe tend to fall 
 towards each other ; but the law has no coercive 
 power inherent in itself. A good analogy may 
 be drawn from the case of the laws of a state. 
 These are nothing more than the rules laid down 
 by the governing power of that state for the 
 conduct of its members the expression of their 
 (or its) will ; but they have no coercive action in 
 themselves. Mere laws are utterly unable to 
 compel the adoption of a certain line of conduct 
 by any individuals, as is clearly shown in that 
 condition of a country described as " lawless"; 
 the laws exist, but the inhabitants disregard them, 
 
ENERGY IN ORGANIC NATURE. 205 
 
 and do not regulate their conduct in accordance 
 with them. The expression, u a state governed 
 by law," simply means governed according to law, 
 i.e., that the controlling power acts within certain 
 fixed and determinate rules laid down by itself. 
 
 Similarly, when we speak of the universe 
 as governed by the laws of a Supreme Ruler, 
 we simply mean that it is governed according 
 to His laws that these laws are merely the 
 generalised expression of the way in which 
 the Supreme Ruler acts. It is related of the 
 great astronomer Kepler, who discovered the 
 laws of the motion of the planets in their 
 orbits, that when he had perceived their truth, 
 and given this grand generalisation an ex- 
 pression in a form of words, his devout mind 
 gave utterance to an ejaculation of thankfulness 
 at having been permitted " to think the thoughts 
 of God." He felt that he had discovered, as 
 it were, the way in which the Author of the Uni- 
 verse worked the mode in which he governed. 
 
 Hence, so far as pure science is concerned, 
 no law can be anything more than an expression 
 of the fact of the orderly uniformity of the 
 phenomena of the universe ; it expresses the re- 
 lationship of the forces, but gives no clue as to 
 their cause. On the other hand, however, there 
 is nothing in this fact to exclude, as is sometimes 
 maintained, the notion of an intelligent First 
 Cause. 
 
206 ENERGY IN NATURE. 
 
 The uniformities of Nature, so far from 
 suggesting blind force, have always seemed to 
 me and I trust that the perusal of these pages 
 may have led others to the same conclusion to 
 present in their wonderful combination of unity 
 and variety, of harmony and diversity, of gran- 
 deur and minuteness, the evidences of a design- 
 ing mind. As the distinguished man to whom 
 this book is dedicated wrote, more than forty 
 years ago: " Every step that we take in the pro- 
 gress of generalisation increases our admiration 
 of the beauty of the adaptation, and the har- 
 mony of the action, of the laws we discover ; 
 and it is in this beauty and this harmony, that 
 the contemplative mind delights to recognise the 
 wisdom and beneficence of the Divine Author of 
 the Universe." 
 
 There is, perhaps, no more fitting sentence 
 to close the consideration of this subject, than 
 those noble words in which Sir W. Grove con- 
 cludes his treatise : " In all phenomena, the 
 more closely they are investigated, the more are 
 we convinced that, humanly speaking, neither 
 matter nor force can be created or annihilated, 
 and that an essential cause is unattainable. 
 Causation is the will, creation the act, of God." 
 
INDEX. 
 
 Accumulator, Plante's, 122, 125. 
 Faure's, 123-126. 
 
 Faure-Sellon-Volckmar, 123 
 126. 
 
 Acorn, Life History of, 173 176. 
 
 Adhesion, 25. 
 
 Air, Amount of, necessary for Com- 
 bustion of Fuel, 69, 74. 
 
 Ditto for Respiration, 74, 191. 
 
 AMPERE'S Theory of Magnetism, 
 137. 
 
 Amperes, 167. 
 
 Analysis, Spectrum, 58 60. 
 
 Animal Energy, 187 199. 
 
 Heat, 189, 190. 
 
 Arc Lighting, 154156. 
 
 Armature, 149. 
 
 Artillery, 32, 33, 84 
 
 Assimilation, 196. 
 
 Astatic Needles, 139. 
 
 Attraction and Repulsion, Electric, 
 90. 
 
 - Molecular, 25, 26. 
 
 - of Cohesion, 25, 26. 
 of Gravitation, 21 24. 
 
 of Magnets, 129132. 
 Aurora Borealis, 114, 133. 
 
 B 
 
 Balance of Organic Nature, 199. 
 Batteries, Secondary, ^//"Accumu- 
 lator. 
 n.itteiy, Voltaic, 100105. 
 
 Bichromate, 105. 
 
 Daniell's, 104. 
 
 Grove's Nitric Acid, 105. 
 
 Batteiy, Grove's Gas, 121. 
 
 Leclanche, 104. 
 
 Simple Cell, 101. 
 Blasting Gelatine, 84. 
 
 Blood, Circulation of , 197, 198. 
 Blow-pipe, Oxyhydrogen, 71, 72. 
 Brush Dynamo-machine, 153. 
 
 Cables, Submarine, 145, 146. 
 Candle-flame, 76. 
 Candles, Electric, 156. 
 Capacity for Heat, 3739. 
 Carbon, 6669, 72, 76, 175. 
 Carbonic Acid, 7274, 175, 177, 
 
 191, 199. 
 Causation, 207. 
 Cells, Vegetable, 178, 179. 
 Centre of Gravity, 24. 
 Chemical Action and. Electricity, 
 
 100126. 
 
 Influence of Heat upon, 63. 
 
 Chemical Changes Distinguished 
 
 from Physical, 61. 
 Chemical Changes in Voltaic Cell 
 
 102. 
 
 Chemical Elements, 64. 
 Choke-damp, 72. 
 Circuit, Electric, defined, 105. 
 Circulation, Oceanic, 39, 42. 
 
 of Blood, 197, 198. 
 
 Climate, Influence of Sea on, 39, 
 
 Coal, 66, 69, 180187. 
 
 a Store of Energy, 180 
 187. 
 
208 
 
 INDEX. 
 
 Coal, Formation of, 180187. 
 
 Fossils, 181183. 
 
 Plants, 181185. 
 Cohesion, Attraction of, 2 5, 20. 
 Coil, Induction, 112, 118. 
 Cold, Sensation of, 41. 
 
 Production of, 5254. 
 Combining Proportions, Law of, 
 
 67, 68, 120. 
 
 Combustion, 60, 65, 81, 180 
 188. 
 
 Spontaneous, 78. 
 Comparison of Lighting by Gas 
 
 and by Electricity, 159, 160. 
 Conduction of Electricity, 40, 105, 
 106. 
 
 of Heat, 3941. 
 Conservation of Energy, 3. 
 Convection, 41, 42. 
 Cooling Machines, 52 54. 
 Correlation and Continuity, 202. 
 of Forces, 3. 
 
 Cost of Energy, 110, 167. 
 
 Cost of Potential Energy as Food, 
 
 194, 195. 
 Creation, 207. 
 Crown of Cups, 103. 
 Crystals of Snow, 18. 
 Currents, Electric, 97126, 135 
 
 172. 
 
 - Produced by Heat, 9799. 
 Induced, 110114, 147 
 
 149. 
 
 Oceanic, 42. 
 
 Telephonic, undulatory, 169 
 
 172. 
 
 D 
 
 Decomposition of Liquids by Elec- 
 tricity, 115126. 
 
 Definition of Energy, 3. 
 of Force, 4, 5. 
 
 Degradation of Energy, 57. 
 
 Density, Maximum, of Fresh and 
 of Sea Water, 48, 49. 
 
 Dietaries, 195, 1 DC-. 
 
 Discharge, Electric, in Vacuo, 114, 
 115 
 
 Dissipation of Energy, 57. 
 
 Dissociation, 85, 177. 
 
 Duration of Electric Spark, 90, 94 . 
 
 Dynamite, 83, 84. 
 Dynamo-machines, 1 50 1 54 . 
 
 E 
 
 EDISON Machine, 153. 
 
 Electric Attraction and Repulsion, 
 
 90. 
 -Bells, 141. 
 
 Candles, 156. 
 
 Circuit defined, 105. 
 Fire-alarms, 142, 143. 
 
 Lamps, 156 158. 
 Launch, 161. 
 
 Light, Influence of, upon 
 Vegetation, 176, 177. 
 
 Lighting, 1 54 - 1 60. 
 
 Lighting, Systems of, 108, 
 
 109. 
 
 .. Machines, 91, 92. 
 
 Measurements, 166, 167. 
 
 - Meter, 120. 
 Motors, 163. 
 
 - Railroads, 164. 
 
 Spark, Duration of, 90, 94. 
 
 Transmission of Power, 161 
 
 -167. 
 
 Electrical Sense, 96. 
 Electrical Energy, Various Modes 
 
 of Developing, 95, 96. 
 
 - Storage of, 120126. 
 Electricity and Chemical Action, 
 
 100126. 
 as Motive Power, 161167. 
 
 at Rest, 96. 
 
 Conduction of, 40, 105, 106. 
 Developed from Magnetism, 
 
 146149. 
 
 First Observations, 87, 88. 
 
 Flow of, 100, 101. 
 
 Origin of Name, 87. 
 
 - Storage of, 120126. 
 
 Two kinds of, 89, 90. 
 Electrics, 88. 
 Electrolysis, 115-126. 
 Electro -magnets, 141 146. 
 Electro -motive Force, 106, 166. 
 Electro typing and Plating, 117, 
 
 118. 
 
 Elements, Distinguished from 
 Compounds, 64, 65. 
 
INDEX. 
 
 209 
 
 Energies, Groups of, 28. 
 Energy, Conservation of, 3. 
 
 Cost of, 110, 167. 
 
 Definition of, 3. 
 
 Degradation of, 57., 
 
 Dissipation of, 57. 
 
 Distinguished from Force, 
 
 5, 6. 
 Electrical, Various Modes of 
 
 Developing, 95, 96. 
 Electrical Storage of, 120 
 
 126. 
 
 from Explosions, 81 85. 
 
 in Animals, 187 199. 
 
 in Repose, 27. 
 
 Indestructibility of, 1214. 
 
 Kinetic, 27. 
 
 Measure of, 29, 30. 
 
 of Motion, 27. 
 
 of Projectiles, 32, 33, 84, 
 
 85. 
 
 Potential, 27. 
 
 Price of Unit of, 167. 
 
 Relation of Mechanical, to 
 
 Heat, 3336. 
 
 Solar, 37. 
 
 Sources of, 28. 
 
 Stored in Coal, 180187. 
 Unit of, 167. 
 
 Utilisation of wasted, 165, 
 
 166. 
 Engine, G-as, 81, 82. 
 
 - Steam, 5456, 189, 190. 
 Ensilage, 79. 
 Equivalent, Mechanical, of Heat, 
 
 35, 36, 193. 
 
 Expansion by Heat, 45 49. 
 Explosions Caused by Light, 85. 
 Energy from, 81 80. 
 
 in Mines, &c., 7779. 
 
 FARADAY, 88, 110, 146. 
 
 FERRANTI Machine, 153. 
 
 FICK and WISLICENUS on Animal 
 
 Energy, 192, 193. 
 Fields, Magnetic, 134136. 
 Fire-alarms, Electric, 142, 143. 
 Fisheries Exhibition, Electric 
 
 Lighting at, 158, 
 
 O 
 
 Flow of Electricity, 100102. 
 Food, Classes of, 190, 191. 
 
 Cost of, 194. 
 
 of Plants, 174180. 
 
 Oxidation of, 196200. 
 
 Potential Energy of, 188 
 196. 
 
 Foot-pounds, 29. 
 
 Force, Definition of, 4, 5. 
 
 Distinguished from Energy, 
 
 5, 6. 
 
 Electro-motive, 106, 166. 
 
 Storage of, 120126. 
 
 Forces, Correlation of, 3. 
 
 of Nature, 1, 2. 
 
 Formation of Coal, 180187. 
 Fossils in Coal, 181183. 
 FRANKLAND on Food, 193196. 
 FRANKLIN, 88, 94. 
 Fresh Water, Maximum Density 
 
 of, 48. 
 
 Freezing Mixtures, 51. 
 Frozen Meat, Sea Transit of, 52 
 
 54. 
 
 Fuel (also vide Coal), 6669, 75. 
 Amount of Air necessary for, 
 
 69, 74. 
 Advantages of gaseous, 80. 
 
 GALVANI and VOLTA, 100103. 
 
 Galvanometers, 98, 139. 
 
 Gas and Electric Lighting Com- 
 pared, 159, 160. 
 
 Gas, as Fuel, Advantages of, 80. 
 
 Gas-battery, Grove's, 121. 
 
 Gas-engine, 81, 82. 
 
 Gas-lamps, Sir W. Siemens', 77. 
 
 Gelatine, Blasting, 84. 
 
 GORDON Machine, 153. 
 
 GRAMME Machine, 152. 
 
 Gravitation, Attraction of, 21 
 24. 
 
 Effects of, 22. 
 
 Gravity, Centre of, 24. 
 
 Specific, 9, 10. 
 
 Groups of Energies, 28. 
 
 GROVE'S Gas-battery, 121. 
 
 Gun-cotton, 83. 
 
 Gunpowder, 84. 
 
210 
 
 INDEX. 
 
 H 
 
 Heat, Animal, 189, 190. 
 
 - Appears when Mechanical 
 Motion is Destroyed, 30- 32. 
 
 Capacity for, 37 39. 
 
 Causes Expansion, 45 49. 
 
 Conduction of, 3941. 
 
 Effect of, in Changing State 
 
 of Matter, 50. 
 Influence of, upon Chemical 
 
 Action, 63. 
 
 Latent, 5052. 
 
 Mechanical Equivalent of, 35, 
 
 36, 193. 
 
 Radiation of, 4345. 
 
 Relation of, to Mechanical 
 
 Energy, 3236. 
 
 Source of, in Animals, 189. 
 
 Specific, 3739. 
 
 Horse-power, 29, 167. 
 Hydrogen, 70, 71. 
 
 Incandescence Lamps, 156 158. 
 
 of Solids, 75. 
 
 Indestructibility of Energy, 12 
 
 14. 
 of Matter, 11, 12. 
 
 Induced Currents, 110114, 147 
 149. 
 
 Induction Coil, 112113. 
 
 Electric, 90. 
 
 Machines, 92. 
 
 Inertia, 14. 
 
 Influence Machines, 92. 
 
 of Electric Light on Vegeta- 
 tion, 176, 177. 
 
 Insulators, 106. 
 
 JOULE'S Researches, 35, 36, 193. 
 
 Kilogram-metre, 29. 
 Kinetic Energy, 27. 
 
 Lamps, Electric, 156158. 
 Launch, Electric, 161. 
 Law of Squares, 23. 
 Laws of Motion, 15, 16. 
 
 of Nature, 204206. 
 
 Leyden Jar, 92 94. 
 LIEUIG'S Theory of Muscular Ex- 
 ertion, 192. 
 
 Life-history of Acorn, 173176. 
 Light, 44, 45, 85, 86. 
 
 Causes Explosions, 85. 
 
 from Fuel, 77. 
 
 Lighting, Electric, 109, 154160. 
 
 Systems of, 108, 109. 
 
 Lightning Conductor, 95. 
 
 Flash, 94. 
 Lime-light, 72. 
 
 Liquids Decomposed by Electricity, 
 
 115126. 
 Lodestone, 127, 128. 
 
 M 
 
 Machines, Dynamo, 149 154. 
 
 Electric, 91, 92. 
 
 Magneto-electric, 149. 
 
 Magnetic Fields, 134136. 
 Magnetism, Influence of Tempera- 
 ture on, 129. 
 
 Origin of Name, 127. 
 
 Terrestrial, 133. 
 
 - Theory of, 128, 129, 137. 
 Magneto -electric Machine, 149. 
 Magnets, Influence of, upon each 
 
 other, 131133. 
 Matter, 1, 68. 
 
 Indestructibility of , 11, 12. 
 
 Moods of, 2. 
 
 - Possesses Weight, 8, 9. 
 
 Resistance of, 11. 
 
 States of, 7, 50. 
 
 Measure of Energy, 29, 30. 
 Measurements, Electric, 166, 167. 
 Meat, Sea Transit of, 5254. 
 Mechanical Equivalent of Heat, 
 
 35, 36, 193. 
 
 Meter for Electricity, 120. 
 Microphone, 170. 
 Mines, Explosions in, 77 79. 
 
INDEX. 
 
 211 
 
 Molecular Attraction, 25, 26. 
 
 - Motion, 4, 1720. 
 Molecules, Size of, 20. 
 Moods of Matter, 2. 
 MORSE Telegraph, 143145. 
 MORTON, Prof. Henry, Report upon 
 
 Accumulators, 125. 
 Motion, 15. 
 
 Laws of, 15, 16. 
 
 Molecular, 4, 1720. 
 Motive Power, Electricity as, 161 
 
 167. 
 
 Motors, Electric, 163. 
 Muscle, 196198. 
 Muscular Action, LIEBIG'S Theory 
 
 of, 192. 
 
 N 
 
 Nature, Balance of Organic, 199. 
 
 Forces of, 1, 2. 
 
 Laws of, 204206. 
 
 Uniformities of, 206. 
 
 Needles, Astatic, 139. 
 Nitro- glycerine, 82. 
 
 
 
 Oceanic Circulation, 39, 42. 
 OERSTEDT'S Discovery, 137. 
 Organic Nature, Balance of, 199. 
 Oxidation of Food, 196200. 
 
 - of Fuel, 69, 74. 
 Oxygen, 69, 70. 
 
 Evolved by Plants, 175, 
 
 200. 
 Oxy hydrogen Blowpipe, 71'. 
 
 Peat, 185. 
 
 Photography, 86. 
 
 Photophone, 171. 
 
 Physical Changes distinguished 
 
 from Chemical, 61. 
 Plants, Food of, 174180. 
 
 of Coal-forests, 181185. 
 
 Poles, Magnetic, 129133. 
 Potential, Electric, 88, 106. 
 
 Potential Energy, 27. 
 
 Power, 6. 
 
 Electric Transmission of, 161 
 
 167. 
 
 Price of Unit of Energy, 167. 
 Prince Rupert's Drops, 26. 
 Projectiles, Energy of, 32, 33, 85. 
 Proportions, Combining, Law of, 
 
 67, 68, 120. 
 Pyrometer, 45. 
 Pyrophorus, Lead, 78. 
 
 R 
 
 Radiation of Heat, 43 45. 
 
 Radiometer, 44. 
 
 Relation of Mechanical Energy to 
 
 Heat, 3336. 
 
 of Velocity to Energy, 30. 
 Repulsion and Attraction, Electric, 
 
 90. 
 
 Resistance of Matter, 1 1 . 
 to Electricity in Motion, 107, 
 
 108. 
 Reversibility of Dynamo -machine, 
 
 162. 
 
 RUHMKORFF'S Coil, 112. 
 RULER, Supreme, His Mode of 
 
 Action, 205, 206. 
 RUMFORD, Count, 33. 
 
 S 
 
 Safety-lamp, 79. 
 
 Savoy Theatre, Electric Lighting 
 
 at, 158. 
 
 Scales, Thermometric, 4648. 
 Sea -Water, Temperature of, 48. 
 
 - Density of, 48, 49. 
 Secondary Batteries, vide Accumu- 
 lators. 
 
 Sense, Electrical, 96. 
 Shock, Electric, 93. 
 SIEMENS' Machine, 153. 
 
 Sir W., Gas-lamps, 77. 
 
 Experiments on Vegetation 
 with Electric Light, 176, 177. 
 
 Size of Molecules, 20. 
 Smoke, 67, 69, 81. 
 Snow Crystals, 18. 
 
212 
 
 INDEX. 
 
 Solar Energy, Amount of, 37. 
 
 Stored, 124. 
 Sources of Energy, 28. 
 Specific Gravity, 9, 10. 
 
 Heat, 3739. 
 
 Spectrum Analysis, 58 60. 
 Spontaneous Combustion, 78. 
 Squares, Law of, 23. 
 States of Matter, 7, 50. 
 Static Electricity, 96. 
 Steam-engine, 54 56, 189, 190. 
 Storage, Electrical, of Energy, 120 
 -126. 
 
 of Energy in Coal, 180187. 
 
 of Force, 120126. 
 Structure of Flame, 74 81. 
 Submarine Cables, 145, 146. 
 SUPEEME RULEE, Mode of Action 
 
 of, 205, 206. 
 
 Systems of Electric Lighting, 108, 
 109. 
 
 Telegraph, Electric, 139, 140, 143 
 
 145. 
 
 Telephone, 167169. 
 Temperature, Absolute Zero of , 56. 
 
 of Deep Sea, 42, 48. 
 Terrestrial Magnetism, 133. 
 Thermo-Electric Currents, 9799. 
 Thermometers, 46 48. 
 Transmission of Power, 161 167. 
 
 u 
 
 Uniformities of Nature, 206. 
 Unit of Energy, 167. 
 
 THB 
 
 HVERSITY 
 
 Urea, 191, 192. 
 
 Utilisation of Wasted Energy, 165, 
 166. 
 
 Vacuum-tubes, 114, 115. 
 Vegetable Cells, 178, 179. 
 Vegetation, Influence of Electric 
 
 Light upon, 176, 177. 
 Velocity, 15. 
 
 Relation of, to Energy, 30. 
 VOLTA and GALVANI, 100103. 
 Voltaic Cell, Chemical Changes in, 
 
 102. 
 
 Voltameter, 119. 
 Volt -amperes, 167. 
 Volts, 167. 
 
 W 
 
 Wasted Energy Utilised, 165, 
 
 166. 
 
 Water, Decomposition of, 116. 
 Watts, 167. 
 Weight, 23. 
 
 of Matter, 8, 9. 
 
 Will, Direction by, 198. 
 WISLICENUS and FICK on Animal 
 
 Energy, 192, 193. 
 Work (also vide Energy), 3, 5, 29, 
 
 198. 
 Measure of, 29, 30. 
 
 Zero, Absolute, of Temperature, 
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