570 HISTORY OF THERMOTICS. 



the particles, which would, he thinks 12 , produce the 

 same effect. 



In selecting, as of permanent importance, the 

 really valuable part of this theory, we must endea- 

 vour to leave out all that is doubtful or unproved. I 

 believe it will be found that in all theories hitherto 

 promulgated, all assertions respecting the properties 

 of the particles of bodies, their sizes, distances, at- 

 tractions, and the like, are insecure and superfluous. 

 Passing over, then, such hypotheses, the inductions 

 which remain are these ; that two gases which are 

 in communication will, by the elasticity of each, dif- 

 fuse themselves in one another, quickly or slowly : 

 and that the quantity of steam contained in a cer- 

 tain space of air is the same, whatever be the air, 

 whatever be its density, and even if there be a 

 vacuum. These propositions may be included toge- 

 ther by saying, that one gas is mechanically mixed 

 with another; and we cannot but assent to what 

 Mr. Dalton says of the latter fact, "this is cer- 

 tainly the touchstone of the mechanical and che- 

 mical theories." This doctrine of the mechanical 

 mixture of gases appears to supply answers to all 

 the difficulties opposed to it by Berthollet and 

 others, as Mr. Dalton has shown 13 ; and we may, 

 therefore, accept it as well established. 



This doctrine, along with the principle of the 

 constituent temperature of steam, are applicable to 

 a large series of meteorological and other conse- 



12 New System, p. 188. 13 Ib. vol. i. p. 160, &c. 



