Page:Popular Science Monthly Volume 8.djvu/631

 Rh The experiment of the Florentine academicians, whereby they proved the electric attraction of a liquid, is pretty, and worthy of repetition. Fill a very small watch-glass with oil, until the liquid forms a round curved surface, rising a little over the rim of the glass. A strongly excited glass tube, held over the oil, raises not one eminence only, but several, each of which finally discharges a shower of drops against the attracting glass.

Cause the excited glass tube to pass close by your face, without touching it. You feel, like Hauksbee, as if a cobweb were drawn over the face. You also sometimes smell a peculiar odor, due to a substance developed by the electricity, and called ozone.

Long ere this, while rubbing your tubes, you will have heard the "hissing" and "crackling" so often referred to by the earlier electricians; and, if you have rubbed your glass tube briskly in the dark, you will have seen what they called the "electric fire." Using, instead of a tube, a tall glass jar, rendered hot, a good warm rubber, and vigorous friction, the streams of electric fire are very surprising in the dark.

6. Discovery of Conduction and Insulation.—Here I must again refer to that most meritorious philosopher, Stephen Gray. In 1729, he experimented with a glass tube stopped by a cork. When the tube was rubbed, the cork attracted light bodies. Gray states that he was "much surprised" at this, and he "concluded that there was certainly an attractive virtue communicated to the cork." This was the starting-point of our knowledge of electric conduction.

A fir-stick four inches long, stuck into the cork, was also found by Gray to attract light bodies. He made his sticks longer, but still found a power of attraction at their ends. He then passed on to packthread and wire. Hanging a thread from the top window of a house, so that the lower end nearly touched the ground, and twisting the upper end of the thread round his glass tube, on briskly rubbing the tube, light bodies were attracted by the lower end of the thread.

But Gray's most remarkable experiment was this: He suspended a long hempen line horizontally by loops of packthread, but failed to transmit through it the electric power. He then suspended it by loops of silk and succeeded in sending the "attractive virtue" through 765 feet of thread. He at first thought the silk was effectual because it was thin; but, on replacing a broken loop by a still thinner wire, he obtained no action. Finally, he came to the conclusion that his loops were effectual, not because they were thin, but because they were silk. This was the starting-point of our knowledge of insulation.

It is interesting to notice the devotion of some men of science to their work. Dr. Wells finished his beautiful essay on "Dew" when he was on the brink of the grave. Stephen Gray was so near dying,