PRIMARY CONCEPTS OF MODERN SCIENCE. 707 



terminate specific weights, corresponding to their equivalents of com- 

 bination? 



Confessedly the atomic theory is but an hypothesis. This in itself 

 is not decisive against its value ; all physical theories properly so 

 called are hypotheses whose eventual recognition as truths depends 

 upon their consistency with themselves, upon their agreement with 

 the canons of logic, upon their congruence with the facts which they 

 serve to connect and explain, upon their conformity with the ascer- 

 tained order of Nature, upon the extent to which they approve them- 

 selves as reliable anticipations or previsions of facts verified by subse- 

 quent observation or experiment, and finally upon their simplicity, or 

 rather their reducing power. The merits of the atomic theory, too, 

 are to be determined by seeing whether or not it satisfactorily and 

 simply accounts for the phenomena as the explanation of which it is 

 propounded, and whether or not it is in harmony with itself and with 

 the known laws of Reason and of Nature. 



For what facts, then, is the atomic hypothesis meant to account, 

 and to what degree is the account it offers satisfactory? 



It is claimed that the first of the three propositions above enu- 

 merated (the proposition which asserts the persistent integrity of 

 atoms, or their unchangeability both in weight and volume) accounts 

 for the indestructibility and impenetrability of matter; that the sec- 

 ond of these propositions (relating to the discontinuity of matter) is 

 an indispensable postulate for the explanation of certain physical phe- 

 nomena, such as the dispersion and polarization of light ; and that the 

 third proposition (according to which the atoms composing the chem- 

 ical elements are of determinate specific gravities) is the necessary 

 general expression of the laws of definite constitution, equivalent pro- 

 portion, and multiple combination, in chemistry. 



In discussing these claims, it is important, first, to verify the facts 

 and to reduce the statements of these facts to exact expression, and 

 then to see how far they are fused by the theory : 



1. The indestructibility of matter is an unquestionable truth. But 

 in what sense, and upon what grounds, is this indestructibility predi- 

 cated of matter ? The unanimous answer of the atomists is : Expe- 

 rience teaches that all the changes to which matter is subject are but 

 variations of form, and that amid these variations there is an unvary- 

 ing constant the mass or quantity of matter. The constancy of the 

 mass is attested by the balance, which shows that neither fusion nor 

 sublimation, neither generation nor corruption, can add to or detract 

 from the weight of a body subjected to experiment. When a pound 

 of carbon is burned, the balance demonstrates the continuing exist- 



1 To avoid confusion, I purposely ignore the distinction between molecules as the ulti- 

 mate products of the physical division of matter, and atoms as the ultimate products of 

 its chemical decomposition, preferring to use the word atoms in the sense of the least 

 particles into which bodies are divisible or reducible by any means. 



