LETTER XVI. 



THE ELECTRICITY OF THE AIR, AND LIGHTNING. 



I 3irsT not conclude my account of the causes 

 of the fall of wet from the air, and of the con- 

 ditions which bring it on, without calling your 

 attention to the phenomena of thunder and light- 

 ning which so often forerun and accompany heavy 

 floods of rain. 



Lightning is, as every one now knows, an 

 electrical action on a vast scale, but the same in 

 kind as that which we obtain artificially by the 

 electrical machine and the Ley den-jar, and which 

 we consider as the work of a peculiar power of 

 nature, of Electricity. You are, of course, so far 

 generally acquainted with electricity, as to be 

 aware that it is a very subtle force, endowed with 

 peculiar powers ; — that it is sometimes considered 

 as a fluid which, like the air, strives for boundless 

 expansion, and which therefore has very different 

 degrees of density according to the conditio v 

 which it is placed; — that it is ^.j or less 

 checked, in its endeavours to spread itself, by 

 many substances, which are called non-conductors 

 — for instance, by the air ; while there are others, 



