30 OF CLOUDS. CHAP. 1. 10. 



two processes may co-operate to the production 

 of clouds, so that, if this hypothesis be advanced 

 against M. Howard's theory, it cannot be re- 

 garded as constituting a valid objection.* 



* The following extract will, perhaps, furnish to the reader a 

 more perfect idea of M. Howard's theory of the origin of clouds: 



" On the remote and universal origin of clouds there can 

 be but one opinion that the water of which they consist has 

 been carried into the atmosphere by evaporation. It is on the 

 nature of this process, the state in which the vapour subsists 

 for a time, and the means by which the water becomes again 

 visible, that the greatest diversity of opinion has prevailed. 



fc The chemical philosopher, seduced by analogy, and 

 accustomed more to the action of liquids on solids, naturally 

 regards evaporation as a solution of water in the atmosphere, 

 and the appearance of cloud as the first indication of its 

 precipitation; which becoming afterwards (under favourable 

 circumstances) more abundant, produces rain. The theory of 

 Dr. Hutton goes a step further, assumes a certain rate of 

 solution differing from that of the advance of temperature by 

 which it is effected, and deduces a general explanation of 

 clouds and rain from the precipitation which, according to his 

 rule, should result from every mixture of different portions of 

 saturated air. The fundamental principle of this theory has 

 been disproved in an essay heretofore presented to the soci- 

 ety,* and which was written under the opinion, at present 

 generally adopted by chemists, that evaporation depends on a 

 solvent power in the atmosphere, and follows the general rules 

 of chemical solution. 



See Phil. Mag. vol. xiv. p. 55. 



