the jar, we know, attract each other, and if the insulating glass
is too weak to hold them asunder, the spark will pierce it.
Similarly, if the insulating air cannot resist the attraction
between the thundercloud and the earth, it will be ruptured by a
flash of lightning. The metal rod, however, tends to allow the two
charges of the cloud and earth to combine quietly or to shunt the
discharge past the house.





CHAPTER II.

THE ELECTRICITY OF CHEMISTRY.


A more tractable kind of electricity than that of friction was
discovered at the beginning of the present century. The story goes
that some edible frogs were skinned to make a soup for Madame
Galvani, wife of the professor of anatomy in the University of
Bologna, who was in delicate health. As the frogs were lying in
the laboratory of the professor they were observed to twitch each
time a spark was drawn from an electrical machine that stood by. A
similar twitching was also noticed when the limbs were hung by
copper skewers from an iron rail. Galvani thought the spasms were
due to electricity in the animal, and produced them at will by
touching the nerve of a limb with a rod of zinc, and the muscle
with a rod of copper in contact with the zinc. It was proved,
however, by Alessanjra Volta, professor of physics in the