Manchester Memoirs, Vol. Iv. (191 1), No.*it%. 3 



different kinds combine in the proportion i : i rather than 

 in any other. It was this rule, and no other, which led each 

 chemist to precisely the same conclusions regarding water, 

 and the oxides of sulphur, respectivel}'. 



How the rule was arrived at is a matter of the 

 historical origin of the theories. As I have already 

 shown, they arose from the same central capital idea : 

 Newton's postulate of "particles mutually repulsive" was 

 the starting point in each case. The thoughts of each 

 chemist ran in the same groove. Similar particles repel 

 one another, consequently particles of different kinds 

 tend to unite in pairs. 



Bryan Higgins was the first to reach this stage of 

 thought, and he would not depart from it in any way. He 

 supposed that the combination of one atom of alkali and 

 two atoms of acid (or two of alkali and one of acid) must 

 be prevented by the mutual repulsion of the two similar 

 atoms, so that combination could not proceed further than 

 I : I. 



Better acquainted than he with the facts of chemical 

 combination, William Higgins imagined the combination 

 of atoms in multiple proportion. But he laid it down 

 that the combination in the proportion i : i was the most 

 stable, thus adhering to the original idea of mutually 

 repulsive particles. 



The train of thought which Dalton followed had 

 features of its own. His physical atomic theory was 

 plainly an extension of Newton's, and was called for by 

 the discovery of the existence of different gases, of their 

 property of diffusing into one another, and of the properties 

 of the resulting mixture. As I have shown in the fifth 

 paper of this series, he held the physical theory for two 

 years before he formed the chemical one. He was able 



