170 PROPERTIES OF THE RADIATIONS [CH. 
appears that the spectrum is due to the action of the radium rays 
either on occluded nitrogen or the nitrogen in the atmosphere 
surrounding the radium. . 
It is very remarkable that a phosphorescent light, like that of 
radium bromide, should show a bright line spectrum of nitrogen. 
It shows that radium at ordinary temperatures is able to set up 
radiations which are produced only by the electric discharge under 
special conditions. 
Sir Wilham and Lady Huggins were led to examine the 
spectrum of the natural phosphorescent light of radium with the 
hope that some indications might thereby be obtained of the 
processes occurring in the radium atom. Since the main radiation 
from radium consists of positively charged atoms projected with 
great velocity, radiations must be set up both in the expelled body 
and in the system from which it escapes. Further experiments in 
this direction are much to be desired at the present time. 
111. Thermo-luminescence. KE. Wiedemann and Schmidt? 
have shown that certain bodies after exposure to the cathode rays 
or the electric spark become luminous when they are heated to 
a temperature much below that required to cause icandescence. 
This property of thermo-luminescence is most strikingly exhibited 
in certain cases where two salts, one of which is much in excess of 
the other, are precipitated together. It 1s to be expected that 
such bodies would also acquire the property when exposed to the 
8 or cathodic rays of radium. This has been found to be the case 
by Wiedemann”. Becquerel showed that fluor-spar, exposed to the 
radium rays, was luminous when heated. The glass tubes im which 
radium is kept are rapidly blackened. On heating the tube, a 
strong luminosity is observed, and the coloration to a large extent 
disappears. The peculiarity of many of these bodies hes in the 
fact that the property of becoming luminous when heated is retained 
for a long interval of time after the body is removed from the 
influence of the exciting cause. It appears probable that the rays 
cause chemical changes in these bodies, which are permanent until 
heat is applied. A portion of the chemical energy is then released 
in the form of visible light. 
1 Wied. Annal. 59, p. 604, 1895. 2 Phys. Zeit. 2, p. 269, 1901. 
