14 Meldrum, DevclopDient of the Atomic Theory. 



without calling in any other repulsive power than the 

 well known one oi heat" This, he says, occurred in 1805.°^ 

 In later life, Dalton gave up this amended hypothesis, 

 and reverted to his original one.™ But in 1808 he ex- 

 pounded them both in the " New System." This may seem 

 inconsistent of him, inasmuch as the two hypotheses are 

 different from one another. Yet they are both forms of 

 the physical atomic theory. Dalton's consistency lies in 

 his adherence to a mechanical hypothesis in contrast to a 

 chemical one. The question of the precise mechanism 

 was subsidiary, and the mixed gases controversy turned 

 entirely on the theory which Dalton advanced in 1801. 

 No one took any notice of his change of front. It 

 has, therefore, not been necessary to consider the 

 amended diffusion hypothesis till now. The hypothesis 

 is less important for its own sake than in its bearing, 

 or supposed bearing, on the development of Dalton's 

 chemical theory. 



The amended hypothesis and the chemical atomic theory. 



In the lecture already quoted, Dalton connects this 

 amended hypothesis with the genesis of his chemical 

 atomic theory. " The different sizes of the particles of 

 elastic fluids under like circumstances .... being once 

 established, it became an object to determine the relative 

 sizes and lueigJits together with the relative nnmber of 

 atoms in a given volume. This led the way to the 

 combination of gases .... Thus a train of investigations 

 was laid for determining the nnmber and zveight of all 

 chemical elementary principles which enter into any sort 

 of combination with one another." -' 



-' Roscoe and Harden, " New view of the origin of Dalton's Atomic Theory," 



pp. 16-17. 

 -- Phil. Trans., 1826, part 2, p. 174. 

 -" Roscoe and Harden, loc. cil. 



