182 A MODEL OF NATURE. 



up the picture. The cogency of the proof that matter is coarse 

 grained is in no way affected by the fact that we have grave doubts as 

 to the nature of granules. Nay, it is of the first importance to recog- 

 nize that, though the fundamental assumptions of the atomic theory 

 receive overwhelming support from a number of more detailed argu- 

 ments, they are themselves almost of the nature of axioms, in that the 

 simples! phenomena are unintelligible if they are abandoned. 



THE RANGE OF THE ATOMIC THEORY. 



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

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

 as hounded 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 bt expanded and filled in by 

 supplementary hypotheses as to the constitution of granules. It may 

 also he admitted that no complete or wholly satisfactory description of 

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

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

 facts which can he accounted for by the theory is very large compared 

 with the number of additional hypotheses which are introduced; and 

 the cumulative 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 

 explained and in a large part discovered by the aid of the atomic 

 theory, and though there are outstanding difficulties, they are, for the 

 most part, related to the nature of the atoms and molecules, and do not 

 affect the question as to whether they exist. 



The fact that different kinds of light all travel at the same speed in 

 interplanetary space, while they move at different rates in matter, is 

 explained if matter is coarse grained. But to attempt to sum up all 

 this evidence would be to recite a text-hook on physics. It must suf- 

 fice to say that it is enormous in extent and varied in character, and 

 that the atomic theory imparts a unity to all the physical sciences 

 which has been attained in no other way. 



I must, however, give a couple of instances of the wonderful success 

 which has been achieved in the explanation of physical phenomena by 

 the theory we are considering, and 1 select them because they are in 

 harmony with the line of argument I have been pursuing. 



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



