Munches ley Monoirs, Vol. Iv. (191OJ, No. %, 5 



Dalton's declarations in 1824 and 1830 to one and the 

 same effect regarding the influence of Richter must be set 

 aside.- 



2. The Conipositio}i of Marsh-gas and Olefiant Gas. 



Thomas Thomson says that the theory first occurred 

 to Dalton during his investigation of marsh-gas and 

 olefiant gas. The discovery of the composition of these 

 gases led to the discovery of the law of multiple propor- 

 tion, and the theory was then devised in order to explain 

 the law. His exact words are : — 



" Mr. Dalton informed me that the atomic theory first 

 occurred to him during his investigations of olefiant gas 

 and carburetted hydrogen gas, at that time imperfectly 

 understood, and the constitution of which was first fully 

 developed by Mr. Dalton himself It was obvious from 

 the experiments which he made upon them that the con- 

 stituents of both were carbon and hydrogen, and nothing 

 else. He found, further, that if we reckon the carbon in 

 each the same, then carburetted hydrogen contains exactly 

 twice as much hydrogen as olefiant gas does. This deter- 

 mined him to state the ratios of these constituents in 

 numbers, and to consider the olefiant gas a compound of 

 one atom of carbon and one atom of hydrogen ; and car- 

 buretted hydrogen of one atom of carbon and two atoms 

 of hydrogen. The idea thus conceived was applied to 

 carbonic oxide, water, ammonia, &c., and numbers were 

 given representing the atomic weights of oxygen, azote, 

 &c., deduced from the best analytical experiments which 

 chemistry then possessed.'"^ 



This narrative has passed muster for many years, and 

 is better known than any other. It was accepted with 



■" Roscoe and Harden, loc. <■//. 



" Tluimas Thomson, " History of Chemistry," vol. 2, p. 291. 



