70 THE NEW KNOWLEDGE. 



light when exposed to a bombardment of the corpuscles in 

 cathode rays. This phosphorescence is very beautiful, its 

 color depending on the nature of the substance struck. The 

 spectrum given out by these bodies is generally a continuous 

 one, but certain rare earths, such as yttrium, yield, on cor- 

 puscular bombardment, a bright-line spectrum ; and this 

 fact has been of great importance in the study of the rare 

 earths. 



The power of glass to phosphoresce under these rays has 

 already been referred to. It ought to be stated, however, 

 that the glass will in time "grow tired." This may be 

 seen with the apparatus shown in Fig. 15, where, if the 

 cross, which is on hinges, be thrown down, the phosphores- 

 cence of that part of the glass which has hitherto been in 

 the shadow of the cross, is now much brighter than that of 

 the surrounding glass. 



The substances barium platino-cyanide and calcium tung- 

 state are exceedingly sensitive to the impact of corpuscles. 

 Their phosphorescence is, in fact, used to detect the pres- 

 ence of corpuscles. 



CHEMICAL EFFECT. 



Corpuscles cause chemical changes in certain bodies on 

 which they fall. Thus, rock salt takes a beautiful violet 

 color, which, unless exposed to moisture, it will retain for 

 years. Lithium chloride is remarkably sensitive to the im- 

 pact of corpuscles. If a beam of corpuscles be slowly moved 

 over the salt by a magnet, the path of the beam traces out 

 a colored band on the surface of the salt. 



HEATING EFFECT. 



Corpuscles heat a body on which they fall. They may 

 be concentrated by a bowl-shaped cathode, such as shown 



