rays is allowed to play upon the air between these two plates, it is found that some of these neutral air molecules are split up by the X-rays into electrically charged parts, which fly instantly, one part to plate M and the other part to plate N. This shows conclusively that the ordinary neutral molecules of the air possess electrical constituents, that is, that they contain equal quantities of positive and negative electricity.
Fig. 2. A, arc light for illuminating droplet. H, chronograph for measuring speeds of droplet. R, lens for making the beam from the arc parallel or slightly convergent. I, shutter for intercepting altogether the light from the arc save when a reading of a transit of an "oil star" over a cross-hair was to be taken. This was used to insure entire stagnancy of the air between the plates M and N (Fig. 1). E, high potential static voltmeter from measuring PD produced by B. P, mercury pressure gauge or manometer for measuring the pressure of the air within C. C, airtight brass chamber containing the plates M and N of Fig. 1. W, pressure pump for forcing a puff of air through the atomizer inside of C and above the plates M and N. The same pump is also used for exhausting the cylinder C. T, telescope for observing illuminated droplet. S, switch for throwing on or off the electric field between the plates. B, ten-thousand-volt storage battery. T, spot of light produced by the beam after passing through the two windows of the chamber C. Q, opening in lead box through which X-ray beam emerges on its way to chamber C, where it ionizes the gas between plates M and N of Fig. 1. X, X-ray bulb. O, cylindrical glass trough 80 cm. long filled with water for absorbing the heat rays from the arc.
Both ultra-violet light and the rays from radium possess, like the X-rays, the power of thus ionizing a gas, and even when no external ionizing agent whatever is at hand, it is found that out of the 27 billion billion molecules which are present in each cubic centimeter of ordinary air, from two tO twenty split up per second into ions. As will presently be shown, this process of ionization consists in the detaching from a neutral molecule of an exceedingly minute fraction of its con-
