RELATION OF VAPOR AXli AIR. 16 1 



uiight be saturated ; and that when the water was beyond the amount 

 required for saturation, it appeared in a visible form. The saturating 

 quantity was held to depend mainly on warmth and wind. 



This theory w r as by no means devoid of merit; for it brought 

 together many of the phenomena, and explained a number of the expe- 

 riments which Le Eoi made. It explained the facts of the transparency 

 of vapor, (for perfect solutions are transparent,) the precipitation of 

 water by cooling, the disappearance of the visible moisture by warming 

 it again, the increased evaporation by rain and wind ; and other 

 observed phenomena. So far, therefore, the introduction of the notion 

 of the chemical solution of water in air was apparently very successful. 

 But its defects are of a very fatal kind ; for it does not at all apply to 

 the facts which take place Avhen air is excluded. 



In Sweden, in the mean time, 7 the subject had been pursued in ;i 

 different, and in a more correct manner. TVallerius Ericsen had, by 

 various experiments, established the important fact, that Avater evapo- 

 rates in a vacuum. His experiments are clear and satisfactory; and 

 he inferred from them the falsity of the common explanation of evapo- 

 ration by the solution of water in air. His conclusions are drawn in a 

 very intelligent manner. He considers the question whether water can 

 be changed into air, and whether the atmosphere is, in consequence, a 

 mere collection of vapors ; and on good reasons, decides in the nega- 

 tive, and concludes the existence of permanently-elastic air different 

 from vapor. He judges, also, that there are two causes concerned, one 

 acting to produce the first ascent of vapors, the other to support them 

 afterwards. The first, which acts in a vacuum, he conceives to be the 

 mutual repulsion of the particles ; and since this force is independent 

 of the presence of other substances, this seems to be a sound induction. 

 When the vapors have once ascended into the air, it may readily be 

 granted that they are carried higher, and driven from side to side by 

 the currents of the atmosphere. "Wallerius conceives that the vapor 

 will rise till it gets into air of the same density as itself, and being then 

 in equilibrium, \vill drift to and fro. 



The two rival theories of evaporation, that of chemical solution and 

 that of independent vapor, were, in various forms, advocated by the 

 next generation of philosophers. De Saussure may be considered as 

 the leader on one side, and De Luc on the other. The former main- 

 tained the solution theory, with some modifications of his own, De 



7 Fischer, Gcsch. Phys. vol. v. p. 63. 



