e methods of
investigation used by Gilbert the ones which he had found so fruitful,
and wrote of him, "I extremely praise, admire, and envy this author."

Gilbert made many interesting contributions to magnetism, which we
shall notice in another lecture, and he also found that sulphur,
glass, wax, and other bodies share with amber the property of being
electrified by friction. He concluded that many bodies could not be
thus electrified. Gray, however, found in 1729 that these bodies were
conductors of electricity, and his discoveries and experiments were
explained and described to the president of the Royal Society while on
his death bed, and only a few hours before his death. If precautions
are taken to properly insulate conductors, all bodies which differ
in any way, either in structure, in smoothness of surface, or even in
temperature, are apparently electrified by friction. In all cases
the friction also produces heat, and if the bodies rubbed are exactly
alike, heat only is produced.

An electrified body will attract all light bodies. This gutta percha
when rubbed with a cat's skin attracts these bits of paper, and this
pith ball, and this copper ball; it moves this long lath balanced on
its center, and deflects this vertical jet of water into a beautiful
curve.

If a conductor is to be electrified, it must be supported by bad
conductors. This brass cylinder standing on a glass column has become
electrified by friction with cat's-skin. My assistant will stand upon
this insulating stool, and by stroking his hand you will observe that
with his other hand he can attract this suspended rod of wood, and you
will hear a feeble spark when I apply my knuckle to his.

Du Fay, of Paris, discovered what he called two kinds of electricity.
He found that a glass rod rubbed with silk will repel another glass
rod similarly rubbed, but that the silk would attract a rubbed
glass rod. We express the facts in the well-known law that like
electricities repel each other, and unlike attract. For a long time
the nature of the distinctions between the two electricities was not
understood. It was found later that when the two bodies are rubbed
together they become oppositely electrified, and that the two
electricities are always generated in equal quantity; so that if the
two bodies are held in contact after the rubbing has ceased the
two electricities come together again and the electrical phenomena
disappear. They have been added to