172 HISTORY OF THEEMOTICS. 



gated, all assertions respecting the properties of the particles of bodies 

 their sizes, distances, attractions, and the like, are insecure and super- 

 fluous. Passing over, then, such hypotheses, the inductions which 

 remain are these ; that two gases which are in communication will, 

 by the elasticity of each, diffuse themselves in one another, quickly or 

 slowly ; and that the quantity of steam contained in a certain 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 

 together 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 certainly the touchstone of the mechanical and chemical 

 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 ; 1S and Ave may, therefore, accept it as 

 well established. 



This doctrine, along with the principle of the constituent tempera- 

 ture of steam, is applicable to a large scries of meteorological and other 

 consequences. But before considering the applications of theory to 

 natural phenomena, which have been made, it will be proper to speak 

 of researches which were carried on, in a great measure, in consequence 

 of the use of steam in the arts : I mean the laws which connect its 

 elastic force with its constituent temperature. 



Sect. 4. Determination of (he Laws of (he Elastic force of Steam. 



THE expansion of aqueous vapor at different temperatures is governed, 

 like that of all other vapors, by the law of Dalton and Gay-Lussac, 

 already mentioned ; and from this, its elasticity, when its expansion is 

 resisted, will be known by the law of Boyle and Mariotte'; namely, by 

 the rule that the pressure of airy fluids is as the condensation. But it 

 is to be observed, that this process of calculation goes on the sup- 

 position that the steam is cut off from contact with water, -so that no 

 more steam can be generated ; a case quite different from the common 

 one, in which the steam is more abundant as the heat is greater. The 

 examination of the force of vapor, when it is in contact with water, 

 must be briefly noticed. 



During the period of which we have been speaking, the progress 01 

 the investigation of the laws of aqueous vapor was much accelerated 



15 New System, vol. i. p! 160, <fcc. 



