436 



SCIENCE. 



[N. S. Vol. XIV. No. 351. 



to extend the line of reasoning that has 

 been followed. 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 difBculties 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 recognize that, 

 though the fundamental assumptions of the 

 atomic theory receive overwhelming sup- 

 port from a number of more detailed argu- 

 ments, they are themselves almost of the 

 nature of axioms, in that the simplest phe- 

 nomena are unintelligible if they are aban- 

 doned. 



THE RANGE OP 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, gen- 

 eral 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 con- 

 stitution can as yet be given ; that perfec- 

 tion has not yet been attained here or in 

 any other branch of science ; but the num- 

 ber 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 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 

 [Dal ton'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 difliculties, 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-book on 

 physics. It must suffice 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 in- 

 stances'of the wonderful success which has 

 been achieved in the explanation of physical 

 phenomena by the theory we are consider- 

 ing, and I select them because they are in 

 harmony with the line of argument I have 

 been pursuing. 



When a piece of iron is magnetized its 

 behavior is different according as the mag- 

 netic force applied to it is weak, moderate 

 or strong. When a certain limit is passed 

 the iron behaves as a non-magnetic sub- 

 stance to all further additions on magnetic 

 force. With strong forces it does and with 

 very weak forces it does not remain mag- 



* Thorpe, 'Essays on Historical Chemistry,' 1849, 

 p. 368. 



