2 Meldrum, Development of tJie Atomic Theory. 



I. DaLTON'S theory of " MIXED GASES." 

 The question at issue. 



One of the burning questions in science, at the begin- 

 ning of the nineteenth century, was that of the constitution 

 of " mixed gases." The question could hardly have been 

 discussed much earlier, much less been settled, because 

 the existence of gases, different from atmospheric air and 

 from one another, had not been fully recognised till after 

 the discovery of oxygen in 1774. 



The properties of gases are accounted for now by the 

 Kinetic Theory, but this was not established till after the 

 middle of the century. Apart from this theory, men of 

 science explained matters as best they could. The 

 problem naturally arose in connection with the atmosphere, 

 the nitrogen and oxygen of which, although they have 

 different specific gravities, do not separate from one 

 another. Two opinions, says Dalton, arose on this matter : 

 the one supposed the two fluids were " merely mixed 

 together, but assigned no reason why they do not separate 

 . . . . The other supposes a true chemical union to 

 exist between the two, and thus obviates the difficulty 

 arising from the consideration of specific gravity.^ " The 

 first of these opinions was held by a few isolated indi- 

 viduals. Strange as it must seem now, the chemical 

 explanation of diffusion was not only widespread amongst 

 men of science, but was quite the predominant one. 



The germ of Dalton s theory. 



Dalton had early shown a tendency in the direction of 



a mechanical explanation of the state of the atmosphere. 



The " Meteorological Observations and Essays " published 



in i793CGntains, as he pointed out many years afterwards, 



"^Manchester Memoirs, [i], vol. 5, p. 538, 1802. 



