33fi On sovie Phcenomena which take place 



quenllv precedetl and accompanitd by ]oud explosions oc- 

 casioned by thunder, while those wliich fall in the nortbtra 

 regions do not present the same phajnonKtiun. But we 

 know very well that thunder is caustd bv the approximation 

 of clouds, sonie of them being affected with positive or 

 vitreous eleq^tricity, and others with negative or resinous 

 ekctricity*. Now we also know that when we pass a cer- 

 tain quantity of electrical sparks through a mixture of 

 azotic and oxygen gas, nitric acid is formed : therefore 

 when the same phaenomenon takes place in the air and in 

 our laboratories, the same results ought to take place; and 

 this is what actually happens : for it has been clearly proved 

 by Margraf, and subsequently by several other chemists, 

 that all rain water contams nitric acid. The stronger the 

 detonali(m, the greater quantity of this acid is formed : iu 

 Spain and India, therefore, where the thunder is more fre- 

 quent in rainy weather, and where tlie detonations are also 

 stronger than in northern regions, of course mpre nitric 

 acid is produced in the former than in the latter regions. 



By this formation of nitric acid in the air, may we not 

 explain a part of the phsnomena which we observe iu 

 storms? 1 si'.all examine therefore the theory which some 

 naturalists have given on the subject. M. de Saussure 

 thinks that the electric fluid exists in the higher regions ; 

 that it every where tends to find an equilibrium ; and that 

 our globe, being frcquentlv deprived of it by some given 

 causes of destruction, recovers it in the hit;her regions, 

 where the rarefaction of the air admits of its existing more 

 easily than in our atmosphere. From this he conceives 

 a part of the earth to be sufficiently heated to reduce into 

 vapour -a part of the water on its surface: this vapour, com- 

 municating caloric to the air which surrounds it, renders It 

 thinner and forces it to ascend. In this way there is a ver- 

 tical wind establislied which carries the heat into the upper 

 strata of the air, and renders them susceptible of dissolving 

 the vapours which it carries along with it. The air being 

 in no respect cold enough for condensing them, they are 

 diffused nearly unifornily into the mass of a very high ver- 

 tical colunni. But the small inequalities which exist in 

 this mass, and the agitation given to the air by the vertical 

 wind which carries it along with it, diminishes the trans- 

 parency of the column, which thereby becomes susceptible 

 of being more strongly healed by the rays of the sun; this 



♦ I have indiscriminately used the expressions vitremis or positive, because 

 both hypotJieses explain equally well these kinds of phxnomcna. 



: ' heat 



