ADDRESS. 13 



heat unless we suppose that heat is a mode of motion. In the words of 

 Rumford, it is ' extremely dilhcult, if not quite impossible, to form any 

 distinct idea of anythuig capable of being excited and connnunicated in 

 the manner the heat was excited and communicated in [his] experiment 

 [on friction] except it be motion.' ^ And if heat be motion there can be 

 no doubt that it is tlie fundamental particles of matter which are moving. 

 For the motion is not visible, is not motion of the body as a whole, wliilc 

 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 beat, 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. 



Other considerations also point to the same conclusion. Many years 

 ago Lord Kelvin gave independent arguments, based on the proper- 

 ties of gases, on the constitution of the surfaces 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 distor- 

 tion, 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 assump- 

 tions introduce new and more recondite difficulties, it must be remem- 

 bered 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 dis- 

 tinct in kind from the medium which surrounds them, and that they are 

 parts of that medium existing in a special state, have both played a largo 

 part in the theoretical development of the atomic hypothesis. The atoms 

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



' Phil. Trans., 17DS, p. 99. 



