566 HISTORY OF THERMOTICS. 



peratures, is produced in this way, (by elasticity 

 alone.) But we cannot be of this opinion; and 

 must still think that this kind of evaporation is pro- 

 duced by the dissolving power of the air." He then 

 gives some reasons for his opinion. "When moist 

 air is suddenly rarefied, there is always a precipita- 

 tion of water. But by this new doctrine the very 

 contrary should happen, because the tendency of 

 water to appear in the elastic form is promoted by 

 removing the external pressure." Another main 

 difficulty in the way of the doctrine of the mere 

 mixture of vapour and air was supposed to be this ; 

 that if they were so mixed, the heavier fluid would 

 take the lower part, and the lighter the higher 

 part, of the space which they occupied. 



The former of these arguments was repelled by 

 the consideration that in the rarefaction of air, its 

 specific heat is changed, and thus its temperature 

 reduced below the constituent temperature of the 

 vapour which it contains. The latter argument is 

 answered by a reference to Dalton's law of the 

 mixture of gases. We must consider the esta- 

 blishment of this doctrine in a new section, as 

 the most material step to the true notion of evapo- 

 ration. 



Sect. 3. Dalton's Doctrine of Evaporation. 



A PORTION of that which appears to be the true 

 notion of evaporation was known, with greater or 



