138 PHYSICAL SCIENCE 



through which they pass into conductors of 

 electricity. In ordinary circumstances, as was 

 pointed out in the earlier part of this chapter, air 

 is an almost perfect insulator ; and an electrified 

 body exposed to it, while shielded from other 

 sources of leakage, loses its charge with extreme 

 slowness. If, however, Rontgen rays are passing 

 through the air in the neighbourhood of the 

 electrified body, the charge quickly disappears. 



For several years after their discovery, the 

 physical nature of the Rontgen rays was widely 

 discussed, and, for a long time, no general con- 

 sensus of opinion was reached. Their photo- 

 graphic effects and the fluorescence they produced 

 on suitable screens suggested that, like ordinary 

 light, they were to be regarded as waves in the 

 luminiferous aether. The power they possess of 

 penetrating some opaque substances does not 

 forbid such an assumption ; for a difference in 

 the wave-length, or in the period of vibration, is 

 sufficient to produce marked differences in the 

 penetration of ordinary light. Glass, transparent 

 to the visible rays, is opaque to those invisible 

 rays of longer wave-length, which possess great 

 heating power — hence its use in fire-screens; 

 while a solution of iodine in bisulphide of carbon 

 is opaque to luminous radiation, but allows the 

 long waves to pass. 



Rontgen rays are not refracted like ordinary 

 light, and very little trace of regular reflection 

 has been detected. Moreover, it was only with 

 great difficulty that they were persuaded to show 

 signs of such a typical property as polarisation. 

 Two plates ol tourmaline seem to be as trans- 

 parent to the rays when the axes of the crystals 



