Sect. V.] Meteorology, t47 



quantity of electricity is as much increased when 

 water is decomposed as when water is dropped on 

 red-hot iron. On the other hand, when steam is 

 condensed into vesicular vapour, or into water, the 

 air becomes negatively electric. Mr. Canton has 

 ascertained that dry air, when heated, becomes ne- 

 gatively electric, and positively when cooled, even 

 when it is not permitted to expand or contract ; 

 and the contraction and expansion of air also oc- 

 casion changes in its electric state. It is discover- 

 ed, therefore, by these experiments, that there are 

 four sources of atmospheric electricity known ; vix. 

 1. Friction; 2. Evaporation; 3. Heat and Cold; 

 4. Expansion and Contraction ; not to mention the 

 electricity evolved by the melting, freezing, solu- 

 tion, &c., of various bodies in contact with the air*. 

 Closely connected with the doctrines which have 

 been taught on the subject of evaporation, are the 

 several theories of Bain to which modern times 

 have given birth. The phenomenon of vapour 

 becoming condensed, or of air in any manner pro- 

 ducing water, and falling in the form of rain, hall, 

 and snoiv, has long been considered a point of dif- 

 ficult solution among meteorologists. All the sup- 

 positions to account for this fact were for a con- 

 siderable time insufficient and unsatisfactory ; and 

 even now the subject is far from being fully unfold- 

 ed. At one time, the condensation and fall of va- 

 pour in different forms has been accounted for by 

 referring to the influence of electricity ; at another, 

 by considering water as held in solution in air, 

 ^nd precipitated by streams of air of different tem- 



* Thomson's Chemistn/, 



