RELATION OF VAPOUR AND AIR. 563 



rents of the atmosphere. Wallerius conceives that 

 the vapour will rise till it gets into air of the same 

 density as itself, and being then in equilibrium, 

 will drift to and fro. 



The two rival theories of evaporation, that of 

 chemical solution and that of independent vapour, 

 were, in various forms, advocated by the next gene- 

 ration of philosophers. De Saussure may be consi- 

 dered as the leader on one side, and De Luc on the 

 other. The former maintained the solution theory, 

 with some modifications of his own. De Luc denied 

 all solution, and held vapour to be a combination 

 of the particles of water with fire, by which they be- 

 came lighter than air. According to him, there is 

 always fire enough present to produce this combina- 

 tion, so that evaporation goes on at all temperatures. 



This mode of considering independent vapour as 

 a combination of fire with water, led the attention 

 of those who adopted the opinion to the thermo- 

 metrical changes which take place when vapour 

 is formed and condensed. These changes are im- 

 portant, and their laws curious. The laws belong 

 to the induction of latent heat, of which we have 

 just spoken ; but a knowledge of them is not abso- 

 lutely necessary in order to enable us to under- 

 stand the manner in which steam exists in air. 



De Luc's views led him 6 also to the considera- 

 tion of the effect of pressure on vapour. He ex- 



8 Fischer, vol. vii. p. 453. Nauvelles Idees sur le Meteoro- 

 logie, 1787. 



O02 



