RELATION OF VAPOR .4XD AIR. 169 



Girtanner, 9 in his Grounds of the Ant'qildogistlc Theory, may be con- 

 sidered as one of the principal expounders of this view of the matter. 

 Hube, of Warsaw, was, however, the strongest of the defenders of the 

 theory of solution, and published upon it repeatedly about 1790. Yet 

 he appears to have been somewhat embarrassed with the increase of 

 the air's elasticity by vapor. Parrot, in 1801, proposed another 

 theory, maintaining that De Luc had by no means successfully attacked 

 that of solution, but only De Saussure's superfluous additions to it. 



It is difficult to see what prevented the general reception of the 

 doctrine of independent vapor ; since it explained all the facts very 

 simply, and the agency of air was shown over and over again to be 

 unnecessary. Yet, even now, the solution of water in air is hardly 

 exploded. M. Gay Lussac, 10 in 1800, talks of the quantity of water 

 " held in solution" by the air ; which, he says, varies according to its 

 temperature and density by a law which has not yet been discovered. 

 And Professor Robison, in the article " Steam," in the Encyclopedia 

 Brilannica (published about 1800), says, 11 "Many philosophers ima- 

 gine that spontaneous evaporation, at low temperatures, 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 produced by the dis- 

 solving power of the air." He then gives some reasons for his opinion. 

 " When moist air is suddenly rarefied, there is always a precipitation 

 of water. But by this new doctrine the very contrary should happen, 

 because the tendency of water to appear in the elastic form is promot- 

 ed by removing the external pressure." Another main difficulty in 

 the way of the doctrine of the mere mixture of vapor and air was sup- 

 posed 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 vapoi 

 which it contains. The latter argument is answered by a reference to 

 Dalton's law of the mixture of gases. AYe must consider the esta- 

 blishment of this doctrine in a new section, as the most material step 

 to the true notion of evaporation. 



Fischer, vol. vii. 473. 10 Ann. Cfiiin. torn, xliii 



11 Robison's Works, ii. 37. 



