8 Meldrum, Deuclopuient of the Atomic Tlieoty. 



"a binary compound should always be specifically heavier 

 than the mere mixture of its two ingredients " [compounds 

 and ingredients being supposed to be gaseous]. This 

 rule is open to two objections : — (i) It is not true, as the 

 case of hydrochloric acid shows ; (2) it is of no use and 

 was not used for the problem that Dalton had to solve. 

 It cannot be used to ascertain whether the two gaseous 

 oxides of carbon ought to receive the formula CO and 

 CO2 respectively, or CoO and CO. In the 2nd part of the 

 " New System " he says : — " carbonic acid is of greater 

 specific gravity than carbonic oxide, and on that account 

 it may be presumed to be the ternary or more complex 

 element \^sic\. It must, however, be allowed that this 

 circumstance is rather an indication than a proof of the 

 fact." ' One can well believe that it was on this principle 

 Dalton arrived at the molecular constitution of these 

 gases, and of nitric and nitrous oxides as well, in the year 

 1803. 



The connection between the physical and the chemical 

 theories. 



The first rule has been called the rule of " greatest 

 simplicity," not only in allusion to its character, but as 

 meaning that it is based on the instinct for simplicity and 

 needs no other justification. As a matter of fact Dalton 

 deduced it from first principles. Dr. Bostock, in the 

 course of a criticism of the atomic theory, raised the 

 question, " When bodies unite only in one proportion, 

 whence do we learn that the combination must be 

 binary?" 



In answer Dalton gave an explanation, which shows 

 that Newton's postulate of similar particles, which are 

 *' mutually repulsive," was the fundamental idea of the 



"^ "New System ofCliemical Philosophy,"' iSiO, p. 369. 



