Page:Popular Science Monthly Volume 9.djvu/49

 Rh You will find it electric; and with it you can charge your electroscope, or attract from a distance your balanced lath.

The human body was ranked among the non-electrics. Make plain to yourself the reason. Stand upon the floor and permit a friend to strike you briskly with the fox's brush. Present your knuckle to the balanced lath, you will find no attraction. Here, however, you stand upon the earth, so that even if electricity had been developed, there is nothing to hinder it from passing away.

But, place upon the ground four warm glass tumblers, and upon the tumblers a board. Stand upon the board, and present your knuckle to the lath. A single stroke of the fox's fur, if skillfully given, will produce attraction. If you stand upon a cake of resin, of ebonite, or upon a sheet of good India-rubber, the effect will be the same.

Throw a mackintosh over your shoulders, and let a friend strike it with the fox's brush, the attractive force is greatly augmented.

After brisk striking, present your knuckle to the knuckle of your friend. A spark will pass between you.

This experiment with the mackintosh further illustrates what you have already frequently observed, namely, that it is not friction alone, but the friction of special substances against each other, that produces electricity.

Thus we prove that non-electrics, like electrics, can be excited, the condition of success being, that an insulator shall be interposed between the non-electric and the earth. It is obvious that the old division into electrics and non-electrics really meant a division into insulators and conductors.

9. Discovery of Two Electricities.—We have hitherto dealt almost exclusively with electric attractions, but, in an experiment already referred to, Otto von Guericke observed the repulsion of a feather by his sulphur globe. I also anticipated matters in the use of our Dutch gold electroscope, where the repulsion of the leaves informed us of the arrival of the electricity.

Du Fay, who was the real discoverer here, found a gold-leaf floating in the air to be at first attracted and then repelled by the same excited body. He proved that when it was repelled by rubbed glass, it was attracted by rubbed resin—and that when it was repelled by rubbed resin, it was attracted by rubbed glass. Hence the important announcement, by Du Fay, that there are two kinds of electricity.

The electricity excited on the glass was for a time called vitreous electricity—while that excited on the sealing-wax was called resinous electricity. These terms are, however, improper; because, by changing the rubber, we can obtain the electricity of sealing-wax upon glass, and the electricity of glass upon sealing-wax.

Roughen, for example, the surface of your glass tube, and rub it with flannel, the electricity of sealing-wax will be found upon the