RELATION OF VAPOUR AND AIR. 571 



quences. 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 con- 

 sequence 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 the Laws of the Elastic 

 Force of Steam. 



THE expansion of aqueous vapour at different tem- 

 peratures is governed, like that of all other vapours, 

 by the law of Dalton and Gay-Lussac, already men- 

 tioned ; and from this, its elasticity, when its expan- 

 sion 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 supposition that the steam is cut off from con- 

 tact 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 

 vapour, under these latter circumstances, must be 

 briefly noticed. 



During the period of which we have been speak- 

 ing, the progress of the investigation of the laws of 

 aqueous vapour was much accelerated by the grow- 

 ing importance of the steam-engine, in which those 



