MAGNETISM AND ELECTRICITY 205 



ment to test his theory. He told no one about it but his son, 

 who was to be a witness. In a thunder shower he sent up a silk- 

 covered kite. At first nothing happened, but as soon as kite 

 and string were sufficiently wet to serve as good conductors, the 

 current came down the string and jumped, in a succession of 

 sparks, from a key that Franklin had tied to it, to any good con- 

 ductor presented to it. Franklin was holding the kite string 

 with a piece of dry silk which is not a good conductor so that 

 the current would not pass into his body, for that might have 

 been dangerous. 



When the tiny particles of water are carried up, as warm air 

 rises from the earth, they rub against the surrounding air, and so 

 by friction generate electricity. Such electricity is carried on 

 the surface of the object that is charged with it. These tiny 

 electrified particles merge to form larger and larger drops that 

 make a visible group of them, which we call a cloud. Finally, 

 they may become so large and heavy that they can no longer 

 float in the air and they fall as rain. As two of these particles 

 fuse, the surface of the resultant droplet is not as great as their 

 combined surfaces, for surfaces increase only as the square of 

 the radius, while volumes increase as its cube. Surface does 

 not increase as rapidly, therefore, as volume. So the electricity 

 on the drops, growing constantly larger, becomes crowded. The 

 cloud becomes overcharged and finally much of its electricity 

 leaps toward another part of the cloud, to another cloud that 

 happens to have a charge of the opposite kind, or toward some 

 portion of the earth so charged. This discharge heats the air 

 and the dust particles through which it passes, the latter to 

 brilliant incandescence as the electric current heats the filament 

 in an incandescent light, so we see the flash of lightning. 

 Furthermore, the great heat expands the air suddenly, and the 

 thunderclap is produced just as a gun makes a loud noise when 

 it goes off because the confined gases suddenly expand. 



In 1789 what was supposed to be another sort of electricity 

 was discovered. Galvani, an Italian, found quite by accident 



