398 



SCIENTIFIC THOUGHT. 



1^. exactly what gave to Dalton's view its great plausibility/ 



propo?uons. ^^^^ ^^ ^^^ elementary atom of each substance had a definite 



weight, it might be that not one atom only combined with 



one other, but that one combined with two, or two with 



three, and so on. Indeed it was soon found that this was 



^ The different factors of thought 

 which combined to give the atomic 

 theory that definiteness and useful- 

 ness which it attained through and 

 since Dalton lay ready-made before 

 him ; but no one had seen so clearly 

 as he did how to combine them. 

 Proust had taught how to distin- 

 guish between chemical compounds 

 and mixtures. W^hen he prepared 

 carbonate of copper artiiicialij', he 

 found that it had the same com- 

 position as the mineral which he 

 found in nature. Richter had 

 shown that definite proportions de- 

 scribe the quantities in which acids 

 and bases exist in neutral salts. 

 Fischer had attached to his transla- 

 tion of Berthollet's work the first 

 table of equivalent quantities of 

 bases and acids which combine to 

 neutralise each other. Richter, and 

 after him Gay - Lussac, had also 

 found that the quantities of dif- 

 ferent metals which dissolve in the 

 same quantity of acid to form 

 saturated solutions combine also 

 with the same weights of oxygen 

 to form oxides. Richter, and after 

 him Proust, had found that certain 

 metals, like iron and mercury, form 

 more than one fixed compound with 

 oxygen, but without perceiving that 

 the different quantities of oxygen 

 in these fixed compounds stand in 

 simple proportions to each other. 

 So far as the theoretical side is 

 concerned, the idea that bodies 

 are formed of distinct particles 

 the notion of the ultimate hetero- 

 geneousness or discontinuity of mat- 

 ter was not only familiar to the 

 ancients, but was adopted by many 

 physicists before Dalton; though the 



chemical specialists who prepared 

 the way for Dalton do not seem to 

 have made use of this idea. Boer- 

 haave, and before him Boyle, had 

 spoken of atoms and of the massulce 

 or particles. Theories were not 

 wanting that these ultimate par- 

 ticles differed in size and form, nor 

 the opposite view, that the par- 

 ticles which combined had the same 

 weight. The latter was the view 

 of Higgins, in the exposition of 

 which (1790) he entangled himself 

 in contradictions, losing his chance 

 of being one of the founders of the 

 atomic theory. As Wurtz and Kopp 

 and others who have carefully in- 

 vestigated the rival claims have 

 said : This honour of founding the 

 atomic theory belongs undividedly 

 to Dalton. It seems important to 

 notice that his experiments with 

 mixtures of gases, which must have 

 begun about 1790, impressed upon 

 him the idea that different gases 

 could exist independently of each 

 other in the same sjjace, suggesting 

 the conception that neither of them 

 filled the whole space, but that 

 they consisted of discontinuous par- 

 ticles. He himself refers to these 

 first investigations as containing 

 the germ of his later opinions. It 

 must, however, be borne in mind 

 that Dalton was only imperfectly 

 acquainted with the writings of 

 contemporary especially Conti- 

 nental writers, and that he had 

 a wholesome distrust for state- 

 ments of facts which he had not 

 verified or observed himself. All 

 this is very clearly stated in Kopp's 

 ' Entwickelung der Chemie,' p. 285, 

 &c. 



