30 THE AIM AND ACHIEVEMENTS OF SCIENTIFIC METHOD. 



gas " fly each other " could be supplied by surrounding them 

 with a relatively bulky elastic atmosphere of heat.* But 

 Dalton's problem was to understand how uniform diffusion 

 could occur in a mixture of atoms of different densities ; and it 

 proved to be impossible to bring about this result by adjusting 

 their atmospheres of heat, without contradicting known facts 

 .about the specific gravities of the gases involved.! Then it 

 occurred to Dal ton to make the particles ' of his oxygen, 

 ^azote, and water vapour of different sizes (meaning by size 

 "the hard particles at the centre and the atmosphere of heat 

 taken together "). If this were done, he supposed that (as in 

 a mixture of shot of two or three different sizes) the particles 

 -of one size would not be able to form a system in equilibrium 

 with particles of another size, but would ignore their presence ; 

 -equilibrium being reached only when the different types of 

 particles had become equally diffused. But there was an 

 apparent obstacle to this view. " At the time " when Dalton 

 "formed the theory of mixed gases" he "had a confused idea, 

 as many have, that the particles of elastic fluids are all of the 

 same size ; that a given volume of oxygenous gas contains just 

 -as many particles as the same volume of hydrogenous."! 



Fortunately, perhaps, for the history of chemistry, Dalton 

 .saw reason to reject a doctrine which later became equally 

 with his own ideas part of the fundamental basis of chemical 

 theory. This obstacle removed, he believed that his theory of 

 gaseous interdiffusion held the field, and at once " it became an 

 object to determine the relative sizes and weights, together with the 

 relative number of atoms in a given volume. . . . Other bodies 

 besides elastic fluids, namely, liquids and solids, were subject, to 

 investigation, in consequence of their combining with elastic 

 fluids. Thus a train of investigation was laid for determining 



* Roscoe and Harden, op. cit., p. 19. 



t Pp. 14, 15. 



% Dalton, A New System of Chemical Philosophy, 1808, p. 188. 



Dalton, op. cit., p. 71. 



