HISTORY OF CHEMISTRY. [LECTURE IV. 



these opinions into the science. In this connection he says, 

 that the observation of the existence of different states of 

 aggregation, has " led to the conclusion which seems univer- 

 sally adopted, that all bodies of sensible magnitude, whether 

 liquid or solid, are constituted of a vast number of extremely 

 small particles, or atoms of matter bound together by a force 

 of attraction, which is more or less powerful according to cir- 

 cumstances, and which as it endeavours to prevent their 

 separation, is very properly called in that view, attraction of 

 cohesion"^ And with regard to the "agency of heat" he 

 says: "An atmosphere of this subtile fluid constantly sur- 

 rounds the atoms of all bodies, and prevents them from being 

 drawn into actual contact." 17 



Dalton shows, however, in the course of his most interest- 

 ing work how the relative weights of these particles can be 

 ascertained ; and it will remain as his imperishable merit to 

 have shown the possibility of determining the atomic weights. 

 Higgins, it is true, tried to prove that he also participated in 

 this important discovery ; 18 but even if it must be admitted 

 that Higgins employed the atomic theory as early as lySg, 19 

 still his way of expressing himself is not, by a long way, so clear 

 and definite as Dalton's, and, so far as I know, he does not 

 mention atomic weights. 



Dalton turned the atomic hypothesis to account as the 

 basis of chemical considerations, after he had found that when 

 two substances unite in several proportions, these proportions 

 are always expressible in simple multiples by whole numbers. 

 He examined the two hydrocarbons, marsh gas and ethylene, 

 and found that, for the same weight of hydrogen, there was 

 twice as much carbon combined with it in ethylene as in marsh 

 gas. He then examined to see whether any such regularity 

 was to be found in other compounds, employing in particular 

 for this purpose the oxides of nitrogen, and he thereby ob- 



16 Dalton, A new System of Chemical Philosophy, I, 141-142 ; Alembic 

 Club Reprints. 2, 27. 17 New System. I, 143-144. 18 Experiments 

 and Observations on the Atomic Theory, by W. Higgins (1814). 19 A 

 Comparative View of the Phlogistic and Antiphlogistic Theories. 



