CHAP, iv.] ELECTRICITY AND MAGNETISM. 255 



string sparks 9 feet long, and by Cavallo, who made 

 many important observations on atmospheric electricity. 

 In 1753 Richmann, of St. Petersburg, who was experi- 

 menting with an apparatus resembling that of Dalibard, 

 was struck by a sudden discharge and killed. 



3O3. Theory of Thunderstorms. Solids and 

 liquids cannot be charged throughout their substance ; 

 if charged at all the electricity is upon their surface (see 

 Art. 29). But gases and vapours, being composed of 

 myriads of separate particles, can receive a bodily charge. 

 The air in a room in which an electric machine is 

 worked is found afterwards to be charged. The clouds 

 are usually charged more or less with electricity, derived, 

 probably, from evaporation l going on at the earth's 

 surface. The minute particles of water floating in the 

 air being better conductors than the air itself become 

 more highly charged. As they fall by gravitation and 

 unite together, the strength of their charges increases. 

 Suppose eight small drops to join into one. That one 

 will have eight times the quantity of electricity dis- 

 tributed over the surface of a single sphere of twice the 

 radius (and, therefore, of twice the capacity, by Art. 247) 

 of the original drops ; and its electrical potential will 

 therefore be four times as great. Now a mass of cloud 

 may consist of such charged spheroids, and its potential 

 may gradually rise, therefore, by the coalescence of the 

 drops, and the electrification at the lower surface of the 

 cloud will become greater and greater, the surface of the 

 earth beneath acting as a condensing plate and becom- 

 ing inductively charged with the opposite kind of elec- 

 trification. Presently the difference of potential becomes 

 so great that the intervening strata of air give way under 

 the strain, and a disruptive discharge takes place at the 

 point where the air offers least resistance. This light- 

 ning spark, which may be more than a mile in length, 

 discharges only the electricity that has been accumulat- 



1 See Art. 63. 



