THE X-RAYS. 145 



One may satisfy himself in various ways that this is the case. The 

 following-, though perhaps not the simi)iest method, may be mentioned: 



I employed a brass tube 3 centimeters wide and 45 centimeters long. 

 At L centimeter's distance from one end a portion of the tube was cut 

 away and replaced by a thin sheet of aluminum. At the other end 

 there was introduced a brass ball, which was supijorted by a metal 

 support, and this end was closed air-tight. BetAveen the brass ball 

 and the closed end of the tube a side tube was soldered in, which was 

 connected with an air-pump. By this means a current of air was 

 made to flow by the brass ball, after having* passed the aluminum 

 window. The distance from the ball to the window was 2b centimeters. 



I mounted this tube in the ziuc box in such a manner that the 

 X-rays entered the tube at right angles to its axis, and the insulated 

 ball lay outside the reach of these rays, in the shadow. The tube and 

 ziuc box were placed in contact and the ball was connected with a 

 Hankel electroscope. 



It was shown that a charge on the ball, whether positive or negative, 

 was not influenced by X-rays so long as the air remained quiet in the 

 tube, but that a marked diminution of the charge was produced by 

 sucking a strong current of air through. If the ball was kept at con- 

 stant potential by connecting it with accumulators, and a continuous 

 current of air was kept flowing in the tube, an electrical current was 

 set up j ust as if the ball was connected with the walls of the tube by 

 a conductor of high resistance. * * * 



20. In section 13 of my first article it was stated that the X-rays may 

 be generated not only in glass but in aluminum. In conducting experi- 

 ments in this direction no solid bodies were found which were not capa- 

 ble of producing X-rays when under the influence of cathode rays. I 

 know no reason to suppose that liquids and gases also do not act 

 similarly. 



Different substances, however, possess this property in different 

 degrees. For example, if cathode rays are caused to fall upon a plate 

 of which one-half is composed of platinum foil 0.3 millimeter thick and 

 the other half of aluminum 1 millimeter thick, one may observe iu the 

 photographic image taken with the pinhole camera that the platinum 

 foil sends out many more X-rays from the side bombarded by the 

 cathode rays than does the aluminum on the same side. But from the 

 back side of the plate there go out almost no X-rays from the platinum, 

 while the aluminum sends out a relatively large number. These latter 

 rays are generated at the front layers of the alumiuum and pass through 

 the x)late. 



It should be remarked that these observations have a practical sig- 

 nificance. For the generation of X-rays of the greatest possible inten- 

 sity my experience recommends the employment of platinum. I have 

 used for some weeks with advantage a discharge apparatus having a 

 concave mirror of aluminum as cathode, and as annode a platinum 

 SM 97 10 



