6 Meldrum, Development of the Atomic Theory. 



succeeded in transforming hygrometry, and " raising it to. 

 the rank of an exact science." " 



Dalton's tlieory and Henrfs laiv. 



Dalton's theory had been only a short time before the 

 world, when it was reinforced in a remarkable way. It 

 was found to have an important bearing on the solubility 

 in water of a gas under various pressures. The study of 

 this subject had been undertaken by William Henry^ 

 already mentioned in the second paper of this series as 

 a friend of Dalton. 



Henry had discovered the law, which is now called 

 after him, that at a given temperature, " water takes up 

 the same volume of condensed gas as of gas under 

 ordinary pressure." "^ The amount dissolved is propor- 

 tional to the pressure. This, as Dalton pointed out to 

 Henry, is a strong argument in favour of the view that 

 solution is " purely a mechanical effect." If gas, in a state 

 of absorption by water, is retained entirely by the incum- 

 bent pressure, there is no need to call in the notion of 

 chemical affinity. 



Not only so, but in the matter of the solubility of a 

 mixture of gases, Dalton's theory proved able to sustain 

 a severe enough test. Henry found that each gas dissolved 

 in water as if the others were absent, " Each gas," he 

 concluded, " when dissolved in water, is retained in its 

 place by an atmosphere of no other gas but its own kind." " 

 This is precisely what was to be expected from Dalton's 

 theory. 



Henry had opposed the theory when it was first made 

 known. He now wrote Dalton a letter, which was read 



° See W. C. Henry, " Memoirs of Dalton," p. 226. 

 I'' JVu'I. Trans., p. 41, 1803. 

 ^^ Nicholson s Joiirn. , [2], vol. 9, p. 126, 1804. 



