176 HISTORY OF THERMOTICS. 



elasticity proceeds in a geometric series when the temperature pro- 

 ceeds in an arithmetical series nearly ; the differences of temperature 

 for equal augmentations of the ratio of elasticity being somewhat 

 greater for the higher temperatures. 



The forces of the vapors of other liquids in contact Avith theii 

 liquids, determined by Dr. Faraday, as mentioned in Chap. ii. Sect. 1, 

 are analogous to the elasticity of steam here spoken of.] 



Sect. 5. Consequences of the Doctrine of Evaporation. Explanation 

 of Rain, Dew, and Clouds. 



THE discoveries concerning the relations of heat and moisture which 



o 



were made during the last century, were principally suggested by 

 meteorological inquiries, and were applied to meteorology as fast as 

 they rose. Still there remains, on many points of this subject, so 

 much doubt and obscurity, that we cannot suppose the doctrines to 

 have assumed their final form ; and therefore we are not here called 

 upon to trace their progress and connexion. The principles of atmo- 

 logy are pretty well understood ; but the difficulty of observing the 

 conditions under which they produce their effects in the atmosphere 

 is so great, that the precise theory of most meteorological phenomena 

 is still to be determined. 



AVe have already considered the answers given to the question : 

 According to what rules does transparent aqueous vapor resume its 

 form of visible water ? This question includes, not only the problems 

 of Rain and Dew, but also of Clouds ; for clouds are not vapor, but 

 water, vapor being always invisible. An opinion which attracted 

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

 to prove that if two masses of air saturated with transparent vapor at 

 different temperatures 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 

 mixture is a mean between the two temperatures, but that the force of 

 the vapor in the mixture, which is the mean of the forces of the two 

 component vapors, will be greater than that which corresponds to the 

 mean temperature, since the force increases faster than the tempera- 

 ture; 81 and hence some part of the 'vapor will be precipitated. This 

 doctrine, it will be seen, speaks of vapor as " saturating " air, and is 



21 Edin. Trans, vol. i. p. 42. 



