576 HISTORY OF THERMOTICS. 



to the question, According to what rules does trans- 

 parent aqueous vapour resume its form of visible 

 water? This question includes, not only the pro- 

 blems of rain and dew, but also of clouds; for 

 clouds are not vapour, but water, vapour being 

 always invisible. An opinion which attracted much 

 notice in its time, was that of Hutton, who, in 1784, 

 endeavoured to prove that if two masses of air 

 saturated with transparent vapour at different tem- 

 peratures are mixed together, the precipitation of 

 water in the form either of cloud or of drops 

 will take place. The reason he assigned for the 

 opinion was this ; that the temperature of the mix- 

 ture is a mean between the two temperatures, but 

 that the force of the vapour in the mixture, which 

 is the mean of the forces of the two component 

 vapours, will be greater than that which corresponds 

 to the mean temperature, since the force increases 

 faster than the temperature 18 ; and hence some 

 part of the vapour will be precipitated. This doc- 

 trine, it will be seen, speaks of vapour as " saturat- 

 ing" air, and is therefore in this form, inconsistent 

 with Dalton's principle ; but it is not difficult to 

 modify the expression so as to retain the essential 

 part of the explanation. 



Dew. The principle of a " constituent tempe- 

 rature" of steam, and the explanation of the "dew- 

 point," were known, as we have said, to the mete- 

 orologists of the last century ; but we perceive 

 18 Edin. Tram. vol. i. p. 42. 



