by the rays. It was speedily seen that the absorption of the layer of air three feet thick could not be detected either by photographs or the fluorescent screen. The glass cylinder was then filled with rarefied hydrogen, but no advantage was apparent. If the photographs of the human hand were taken, one through the rarefied cylinder and the other through an equivalent thickness of air, no difference in clearness or depth of definition could be perceived. The amount of absorption by a column of air three feet in length is less than ten per cent. This result interested me greatly, for it shows the remarkable difference between the X rays and the cathode rays, which had been investigated by Crooke, Hittorf, and Lenard; for the cathode rays are greatly absorbed by atmospheric air, being reduced in passing through five or six inches of air to one four-hundredth part of their value.
The small amount of absorption of the X rays lifts them into the realm of very short wave lengths of light, for their behavior in regard to the absorption by air is very analogous to that of ultra-violet rays. Although the vacuum chamber, by which I looked, showed no absorption of the X rays, it disclosed a beautiful phenomenon. In a dark room this large tube, three feet long and eight inches wide, was filled with a roseate light, which wavered like the northern lights when the Crooke's tube was emitting the X rays. If the finger was brought near the glass walls of the cylinder a stream of light apparently emanated from a point on the inside wall of the cylinder. The hand thus had ghostly streamers giving an image of it, although the hand itself was invisible. These banners of light could be diverted in any direction by the hand or by any conducting body brought near, and gave a vivid conception of how the streaming of the aurora can be brought about by this flitting of conducting clouds or the drifting of moisture-laden strata of air below the rarefied space in which the beams of the northern light dart back and forth. Both in the case of the Crooke's tube and the aurora these streamers are produced by electrical discharges through rarefied air. The experiments show that outside the Crooke's tube there is a strong electrical attraction and repulsion, which is only revealed in darkness and in a cold, lifeless, airless space, such as exists between us and the sun. Can we not extend our thoughts from the contemplation of this laboratory experiment to that of the immensely greater play of electrical forces between the earth and the sun across the immense vacant space ninety millions of miles in distance?
The mysterious effects of the X rays on the molecules in the air form a great subject of inquiry, and the investigation of it promises to extend our knowledge of electricity and light and heat.
