ATMOSPHERIC ELECTRICITY. 319 



are raised or lowered, liow we may explain these indications of negative 

 electricity in clear weather. 



It appears to ibllow, from what precedes, that the plienomena which 

 accompany evaporation and vegetation are not sufficient, in tlie present 

 state of science, to explain atmospheric electricity. M. A. de la Rive* 

 made in respect to this the following observations : " Vegetation and 

 chemical action which take place on the surface of the globe produce 

 so little electricity, and especially afford signs of electricity the nature 

 of which is so variable, that we can hardl}^ attribute to them the con- 

 stantly positive electricitj'- with which the atmosphere is charged. 

 Moreover, these are phenomena which almost wlioUy cease during 

 winter, and yet observation teaches us that the atmosphere is very 

 frequently charged and sometimes with more electricity at this period 

 of the year than in any other. What is then the cause which pro- 

 duces this agent? May it not be sought for in a more general phe- 

 nomenon than any to which it has been hitherto attributed, to wit : 

 the unequal distribution of the temperature of the atmosphere?" 



The idea of regarding atmospheric electricity as a thermo-electric 

 phenomenon had already been propounded by the philosophers who 

 were first engaged in researches of this kind. After having ascer- 

 tained the laws of electric polarity which the tourmalin acquires 

 when its temperature is raised or lowered, Cantonf advanced the 

 opinion that heat might probably exercise an influence in the elec- 

 tric phenomena of the atmosphere. ''If we supposed/' says he, 

 " that the air possesses qualities similar to tliose of the tourmalin, 

 that is to say, the capacity of becoming electric by the increase or 

 decrease of its heat, we can easily explain thunder-clouds, positive or 

 negative, as well as the peals of thunder.*' A similar sup[)osition 

 was also made by Th. Pionayne,| in the course of his observations on 

 atmospheric electricity. 



The 0})inion which consists in regarding heat as the cause of atmos- 

 pheric electricity has received some degree of probability from the 

 labors of M. Becquerel. It is known that this philosopher succeeded in 

 demonstrating that when caloric is propagated in a metallic body 

 it produces in its whole extent such a series of decompositions and re- 

 combinations of the natural electricity that the positive electricity, 

 successively imparted by one molecule to the next, goes from the warm 

 to the cold part, and that the negative electricity follows the contrary 

 course. Hence a particle which becomes warm in receiving its heat 

 from a neighboring one, takes from the latter positive electricity and 

 gives it negative electricity. Ought not the same effect to be pro- 

 duced, though in a less degree, in bodies which are bad conductors of 

 electricity ? This ap})ears the more probable, when we recollect that 

 the conducting power of these bodies for electricity increases with the 

 temperature. § M. NobiliU succeeded in producing tliermo-electric 



° Essai Historique sur rElectiicitd, page 140. 



f Philosophical Transactions for 1750, page 403. 



j Philosophical Transactions for 1772, page 139. 



§ M. Becquerel verified by M. llousseau's apparatus that the glass which had been 

 beatcil from 90 to 80°, and even below, becomes a conductor of tlie elcctricit)' even for 

 very weak tensions. — Annalcs de Chiraie et de Physique, torn. XLI, page 35G, 1829. 



U Bibliotheque Uuiverselle, torn. XXXVJI, page 125, 1828. 



