September 12, 1901] 



NA TURE 



473 



are mere fragments of ordinary matter, they cannot be used as 

 aids in explaining those qualities of matter which they them- 

 selves share. 



We cannot explain things by the things themselves. If it be 

 true that the properties of matter are the product of an under- 

 lying machinery, that machinery cannot itself have the proper- 

 ties which it produces, and must, to that extent at all events, 

 differ from matter in bulk as it is directly presented to the 

 senses. 



If, however, we can succeed in showing that if the separate 

 parts have a limited number of properties (different, it may be, 

 from those of matter in bulk), the many and complicated proper- 

 ties of matter can, to a considerable extent, be explained as 

 consequences of the constitution of these separate parts ; we 

 shall have succeeded in establishing, with regard to quantitative 

 properties, a simplification similar to that which the chemist 

 has established with regard to varieties of matter. The many 

 will have been reduced to the few. 



The proofs of the physical reality of the entities discovered by 

 means of the two analyses must necessarily be different. The 

 chemist can actually produce the elementary constituents into 

 which he has resolved a compound mass. No physicist or 

 chemist can produce a single atom separated from all its fellows, 

 and show that it possesses the elementary qualities he assigns to 

 it. The cogency of the evidence for any suggested constitution 

 of atoms must vary wi'th the number of facts which the hypo- 

 thesis that they possess that constitution explains. 



Let us take, then, two steps in their proper order, and inquire, 

 first, whether there is valid ground for believing that all matter 

 is made up of discrete parts ; and secondly, whether we can 

 have any knowledge of the constitution or properties which those 

 parts possess. 



The Coarse-grainedness of Matter. 



Matter in bulk appears to be continuous. Such substances 

 as water or air appear to the ordinary observer to be perfectly 

 uniform in all their properties and qualities, in all their parts. 



The hasty conclusion that these bodies are really uniform is, 

 nevertheless, unthinkable. 



In the first place, the phenomena of diffusion afford conclusive 

 proof that matter when apparently quiescent is in fact in a state 

 of internal commotion. I need not recapitulate the familiar 

 evidence to prove that gases and many liquids when placed in 

 communication interpenetrate or diffuse into each other ; or that 

 air, in contact with a surface of water, gradually becomes laden 

 with water vapour, while the atmospheric gases in turn mingle 

 with the water. Such phenomena are not exhibited by liquids 

 and gases alone, or by solids at high temperatures only. Sir 

 W. Roberts-Austen has placed pieces of gold and lead in 

 contact at a temperature of i8' C. After four years the gold 

 had travelled into the lead to such an extent that not only were 

 the two metals united, but, on analysis, appreciable quantities of 

 the gold were detected even at a distance of more than 5 milli- 

 metres from the common surface, while within a distance of 

 three quarters of a millimetre from the surface gold had pene- 

 trated into the lead to the extent of I oz. 6 dwts. per ton, an 

 amount which could have been profitably extracted. 



Whether it is or is not possible to devise any other intelligible 

 account of the cause of such phenomena, it is certain that a 

 simple and adequate explanation is found in the hypothesis that 

 matter consists of discrete parts in a state of motion, which can 

 penetrate into the spaces between the corresponding parts of 

 surrounding bodies. 



The hypothesis thus framed is also the only one which affords 

 a rational explanation of other simple and well-known facts. If 

 matter is regarded as a continuous medium, the phenomena of 

 expansion are unintelligible. There is, apparently, no limit to 

 the expansion of matter, or, to fix our attention on one kind of 

 matter, let us say to the expansion of a gas ; but it is incon- 

 ceivable that a continuous material which fills or is present in 

 every part of a given space could also be present in every part 

 of a space a million times as great. .Such a statement might be 

 made of a mathematical abstraction ; it cannot be true of any 

 real substance or thing. If, however, matter consists of discrete 

 particles, separated from each other either by empty space or 

 by something different from themselves, we can at once under- 

 stand that expansion and contraction may be nothing more than 

 the mutual separation or approach of these particles. 



Again, no clear mental picture can be formed of the pheno- 

 mena of heat unless we suppose that heat is a mode of motion. 

 In the words of Rumford, it is "extremely difficult, if not 



NO. 1663, VOL. 64] 



quite impossible, to form any distinct idea of anything capable 

 of being excited and communicated in the manner the heat 

 was excited and communicated in [his] experiment [on friction] 

 except it be motion" (Phil. Trans., 1798, p. 99). And if heat 

 be motion there can be no doubt that it is the fundamental 

 particles of matter which are moving. For the motion is 

 not visible, is not motion of the body as a whole, while 

 diffusion, which is a movement of matter, goes on more quickly 

 as the temperature rises, thereby proving that the internal 

 motions have become more rapid, which is exactly the result 

 which would follow if these were the movements which 

 constitute sensible heat. 



Combining, then, the phenomena of diffusion, expansion, and 

 heat, it is not too much to say that no hypotheses which make 

 them intelligible have ever been framed other than those which 

 are at the basis of the atomic theory. 



Many other considerations also point to the same conclusion. 

 Many years ago Lord Kelvin gave independent arguments, 

 based on the properties of gases, on the constitution of the sur- 

 faces of liquids, and on the electric properties of metals, all of 

 which indicate that matter is, to use his own phrase, coarse- 

 grained — that it is not identical in constitution throughout, but 

 that adjacent minute parts are distinguishable from each other 

 by being either of different natures or in different states. 



And here it is necessary to insist that all these fundamental 

 proofs are independent of the nature of the particles or 

 granules into which matter must be divided. 



The particles, for instance, need not be different in kind 

 from the medium which surrounds and separates them. It 

 would suffice if they were what may be called singular parts of 

 the medium itself, differing from the rest only in some peculiar 

 state of internal motion or of distortion, or by being in some 

 other way earmarked as distinct individuals. The view that the 

 constitution of matter is atomic may and does receive support 

 from theories in which definite assumptions are made as to the 

 constitution of the atoms ; but when, as is often the case, these 

 assumptions introduce new and more recondite difficulties, it 

 must be remembered that the fundamental hypothesis — that 

 matter consists of discrete parts, capable of independent 

 motions — is forced upon us by facts and arguments which are 

 altogether independent of what the nature and properties of 

 these separate parts may be. 



As a matter of history, the two theories, which are not by 

 any means mutually exclusive, that atoms are particles which 

 can be treated as distinct in kind from the medium which sur- 

 rounds them, and that they are parts of that medium existing 

 in a special state, have both played a large part in the theoreti- 

 cal development of the atomic hypothesis. The atoms of 

 Waterston, Clausius, and Maxwell were particles. The vortex- 

 atoms of Lord Kelvin, and the strain-atoms (if I may call them 

 so) suggested by Mr. Larmor, are states of a primary medium 

 which constitutes a physical connection between them, and 

 through which their mutual actions arise and are transmitted. 



Properties of the Basis of Matter. 



It is easy to show that, whichever alternative be adopted, we 

 are dealing with something, whether we consider it under the 

 guise of separate particles or of differentiated portions of the 

 medium, which has properties different from those of matter in 

 bulk. 



For if the basis of matter had the same constitution as 

 matter, the irregular heat movements could hardly be maintained 

 either against the viscosity of the medium or the frittering away 

 of energy of motion which would occur during the collisions 

 between the particles. Thus, even in the case in which a hot 

 body is prevented from losing heat to surrounding objects, its 

 sensible heat should spontaneously decay by a process of self- 

 cooling. No such phenomenon is known, and though on this, as 

 on all other points, the limits of our knowledge are fixed 

 by the uncertainty of experiment, we are compelled to 

 admit that, to all appearance, the fundamental medium, if it 

 exists, is unlike a material medium, in that it is non-viscous ; 

 and that the particles, if they exist, are so constituted that 

 energy is not frittered away when they collide. In either case, 

 we are dealing with something different from matter itself in 

 the sense that, though it is the basis of matter, it is not identical 

 in all its properties with matter. 



The idea therefore that entities exist possessing properties 

 different from those of matter in bulk is not introduced at the 

 end of a long and recondite investigation to explain facts with 



