668 POPULAR SCIENCE MONTHLY. 



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 photo- 

 graphs of the human hand were taken, one through the rarefied 

 cylinder and the other through an equivalent thickness of air, no 

 diil'erence in clearness or depth of definition could be perceived. 

 The amount of absorption by a colunm 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 

 Tenard; 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 beau- 

 tiful 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 emit- 

 ting the X rays. If the finger was brought near the glass walls 

 of the cylinder a stream of light apparently erhanated 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 in- 

 visible. 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 clonds or the drifting of moist- 

 ure-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 sim. 

 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. 



