242 



SCIENCE. 



[N. S. Vol. IV. No. 87. 



seems to have adopted the contrary opinion, 

 and to have believed that matter is infin- 

 itely divisible. But Bernouilli made the con- 

 jecture, which has since been verified, that a 

 ■ given volume of gas consists of a very large 

 number of very small discrete particles, 

 which we now call molecules ; and Higgins, 

 an English chemist, a contemporary of Dal- 

 ton, was the first to apply the notion of atoms 

 to the explanation of chemical phenomena, 

 although he did not think clearly in regard 

 to the weight of atoms, and so formed no 

 useful hypothesis. Accordingly the net re- 

 sult of twenty-two centuries of thought on 

 this subject was to form a conception of a 

 possible structure of matter, without imag- 

 ining any way of establishing the truth or 

 error of this conception, or even of gaining 

 any evidence whatever in regard to it. 

 But, if any are inclined to visit this failure 

 with reproach, it is interesting to notice 

 that the first man who was aware of the 

 quantitative relations which are adapted to 

 throw light on the matter did not fail to 

 make the most full and complete use of this 

 knowledge. 



Dalton, and not the ancients, ought to be 

 regarded as the discoverer of the atomic 

 structure of matter, because he invented a 

 hypothesis, involving such a structure, 

 which was capable of being so compared 

 with facts as to be proved or contradicted ; 

 because he actually began such a compari- 

 son of the hypothesis with the facts ; and 

 because all the evidence from facts, varied 

 as it has since become, supports the hypoth- 

 esis substantially in the form which he 

 gave it. He who suggests that a certain 

 benefit is desirable, or who conjectures that 

 it is possible, shall not fail of due ci^edit ; 

 but he who confers the benefit will receive 

 the credit due the benefactor. 



Since Dalton's discovery, much has been 

 done to confirm and enlarge our knowledo-e 

 of the atomic structure of matter. New 

 evidence has been acquired in favor of it 



because the theory has been ready to extend 

 over whole realms of facts of a kind un- 

 known to Dalton, to explain them, to facili- 

 tate their study; and also ready to predict 

 facts, unknown till they were sought in con- 

 sequence of the prediction, but found when 

 they were sought. 



The history of the atomic theory for 

 ninety years would fall into several distinct 

 chapters. One of these chapters, not the 

 least interesting of them, would tell of a 

 very large amount of work, some of it of 

 consummate accuracy, of which the object 

 was to attain some knowledge of the nature 

 or construction of atoms. Since the last 

 meeting of our Association in this city, 

 work has been accomplished which, if I 

 rightly judge, has ended this particular 

 chapter. That the chapter may at some 

 future time be resumed is, of course, not 

 absolutely impossible ; but for the present 

 it has come to a definite close. My own 

 interest in the matter suggests, and the co- 

 incidence in time now mentioned perhaps 

 justifies, my selection of this completed 

 chapter in the history of the atomic theory 

 as the subject of the address which our 

 constitution requires of me this evening. 



This chapter naturally concerns more 

 intimately the members of the sections of 

 Physics and Chemistry. To these I can 

 hardly hope to say anything not already 

 well known to them ; but members of other 

 sections may, perhaps, not be entirely un- 

 interested in an account of the conclusions 

 reached. 



Dalton's theory was founded on three 

 facts. These facts are often called Dalton's 

 laws ; one of them, because he discovered it; 

 the others because he first recognized their 

 important relations to chemical theory. 

 One of these is the law of definite propor- 

 tions : in any chemical compound, the ratio 

 of the components is constant, is invariable, 

 is definite. This truth had been recognized 

 by others ; it was finally established as a 



