Page:Popular Science Monthly Volume 9.djvu/362

340 the opening of the Academy of Sciences by Frederick the Great, at Berlin, on the 23d of January, 1744. With a spark from the sword of one of the court cavaliers present on the occasion, Ludolf ignited sulphuric ether.

Dr. Watson also made numerous experiments on the ignition of bodies by the electric spark. He fired gunpowder and discharged guns. Causing a spoon containing ether to be held by an electrified person, he ignited the ether by the finger of an unelectrified person. He also noticed that the spark varied in color when the substances between which it passed varied.

These, and numerous other experiments, may be made with a far simpler "machine" than any hitherto described. It was devised for your benefit by Mr. Cottrell. In the electric machine, as we have learned, the prime conductor is flooded with positive electricity through the discharge of the negative from the points against the excited glass. Your glass tube may be similarly turned to account. A strip of sheet-brass or copper, P, Fig. 30, about five inches long and one inch wide, is sewn on to the edge of the silk pad, R, employed as a rubber. Through apertures in the strip of metal about twenty pin-points are introduced and soldered to the metal. When the tube is clasped by the amalgam-covered rubber, the metal strip and points quite encircle the tube.

When a fine wire, w, connects the strip of metal with the knob of a Leyden-jar, by every downward stroke the glass tube is powerfully excited, and hotly following the exciting rubber is the circle of points. From these, against the rod, negative electricity is discharged, the free positive electricity escaping along the wire to the jar, which is rapidly charged.

Connecting the strip of metal with an insulated metallic knob, placed within a quarter or an eighth of an inch of an uninsulated argand burner, at every downward stroke of the rubber a stream of sparks passes between the knob and burner. If gas be turned on, it is immediately ignited by the stream, of sparks. Blowing out the flame and repeating the experiment, a single stroke of the tube infallibly ignites the gas. Sulphuric ether, in a spoon which has been previously warmed, is thus ignited: but the ether soon cools by evaporation; its vapor is diminished, and it is then less easy to ignite. Bisulphide of carbon may be substituted for the ether, with the certainty that every stroke of the rubber will set it ablaze. The spark thus obtained also fires an electric pistol charged with a mixture of oxygen and hydrogen. The two gases unite with explosion to form water, when an electric spark is passed through them.

Mr. Cottrell has mounted his glass tube so as to render friction in both directions available. The tube-machine is represented in Fig.