16 REPORT— 1901. 



purpose. The argument as to the constitution of matter could be de- 

 veloped further in the manner I have hitherto adopted, viz., by a series of 

 propositions, the proof of each of which is based upon a few crucial 

 phenomena. In particular, if matter is divided into moving granules or 

 particles, the phenomenon of cohesion proves that there must be mutual 

 actions between them analogous to those which take place between large 

 masses of matter, and which we ascribe to force, thereby indicating the 

 regular, unvarying operation of active machinery which we have not yet 

 the means of adequately understanding. For the moment, I do not wish 

 to extend the line of reasoning that has been folloAved. My main object 

 is to show that the notion of the existence of ultra-physical entities 

 and the leading outlines of the atomic theory are forced upon us at the 

 beginning of our study of Nature, not only by a priori considerations, 

 but in the attempt to comprehend the results of even the simplest 

 observation. These outlines cannot be effaced by the difficulties 

 which undoubtedly arise in filling up the picture. The cogency of 

 the proof that matter is coarse-grained is in no way affected by the 

 fact that we may have grave doubts as to the nature of the granules. 

 Nay, it is of the first importance to recognise that, though the funda- 

 mental assumptions of the atomic theory receive overwhelming support 

 from a number of more detailed arguments, they are themselves almost of 

 the nature of axioms, in that the simplest phenomena are unintelligible if 

 they are abandoned. 



The Range of the Atomic Theory. 



It would be most unfair, however, to the atomic theory to represent 

 it as depending on one line of reasoning only, or to treat its evidence 

 as bounded by the very general propositions I have discussed. 



It is true that as the range of the theory is extended the fundamental 

 conception that matter is granular must be expanded and filled in by 

 supplementary hypotheses as to the constitution of the granules. It may 

 also be admitted that no complete or wholly satisfactory description of 

 that constitution can as yet be given ; that perfection has not yet been 

 attained here or in any other branch of science ; but the number of facts 

 which can be accounted for by the theory is very large compared with the 

 number of additional hypotheses which are introduced ; and the cumula- 

 tive weight of the additional evidence obtained by the study of details 

 is such as to add greatly to the strength of the conviction that, in its 

 leading outlines, the theory is true. 



It was originally suggested by the facts of chemistry, and though, as 

 we have seen, a school of chemists now thrusts it into the background, it 

 is none the less true, in the words of Dr. Thorpe, that ' every great 

 advance in chemical knowledge during the last ninety years finds its 

 interpretation in [Dalton's] theory. ' ^ 



The principal mechanical and thermal properties of gases have been 



' Thorpe, Essays on Historical Chemistry, 1894, p. 368. 



