THE X-RAYS. 141 



conceived wliich might have produced the increased darkening, and in 

 order to be sure I performed a second experiment, interposing a thin 

 sheet of aluminum foil (very transmissible to X-rays, but not to those 

 of the ultraviolet) between the metals and the sensitive film. Since in 

 this case again practically the same result was obtained, the fact of 

 reflection of X-rays by the metals above mentioned is established. 



Taking this result together with the observation that powder is as 

 transmissible as coherent substance, and further, that bodies with 

 rough surfaces behave in the transmission of X-rays and also in the 

 experiments just described exactly like polished bodies, the conclusion 

 is reached that there is, as before remarked, no regular reflection, but 

 that the bodies behave toward X-rays in the same manner as a turbid 

 medium with reference to light. 



As I have not been able to discover any refraction in the passage 

 from one medium to another, it appears as if the X-rays travel with 

 equal velocity in all bodies, and hence in a medium which is every- 

 where present and in which the particles of the bodies are embedded. 

 These latter act as a hindrance to the propagation of the X-rays, 

 which is in general greater the greater the density of the body in 

 question. 



9. In accordance with this supposition it might be possible that the 

 arrangement of the molecules of the body would exert an influence on 

 its transmissibility, and that, for example, a piece of calcspar would 

 be unequally transmissible for equal thicknesses wheu the rays i)assed 

 along or at right angles to the axis. Experiments with calcsi:)ar and 

 quartz gave, however, a negative result. 



10. It will be recalled that Lenard, in his beautiful experiments on 

 the transmission of the Hittorf cathode rays through thin aluminum 

 foil, obtained the result that these rays are disturbances in the ether, 

 and that they diftuse themselves in all bodies. We may make a similar 

 statement with regard to our rays. 



In his last research Lenard has determined the relative absorption of 

 diftereut substances for the cathode rays, and in determining the same 

 for air at atmospheric pressure has given the values 4.10, 3.40, 3.10 as 

 referred to 1 centimeter thickness according to the density of the gas 

 in the discharge tube. Judging from the length of spark observed, I 

 have, in my researches, generally employed tubes of about equal exhaus- 

 tion and only seldom those of much greater or less density. Using the 

 jfliotometer of L. Weber, the best at my command, I compared the 

 intensity of fluorescence on the screen in two positions distant 100 and 

 200 millimeters, respectively, from the discharge tube. From the results 

 of these experiments, agreeing well with each other, it appeared that 

 the intensity varies inversely as the square of the distance. Hence the 

 ah- absorbs a much smaller portion of the X-rays passing through it 

 than of cathode rays. Tliis result is in accord with the observation 

 above mentioned, that it is possible to distinguish fluorescence at 2 

 meters distance from the discharge tube. 



