128 NATURE OF THE RADIATIONS [CH. 
screen was kept at a constant temperature, but the radium cooled 
down to the temperature of liquid air, no appreciable difference m 
the number of scintillations was observed. If, however, the screen 
was gradually cooled to the temperature of liquid air, the scintilla- 
tions diminished in number and finally ceased altogether. This is | 
due to the fact that the screen loses to a large extent its power of 
phosphorescence at such a low temperature. 
The scintillations are produced not only by radium and 
polonium, but also by a negatively charged wire made active by 
exposure in the open air or in a vessel containing the emanations 
of thorium or radium. As far as observations have yet gone, the 
production of scintillations appears to be a general property of the 
a rays from all radio-active substances. The scintillations are best 
shown with a zine sulphide screen. If a screen of barium platino- 
cyanide is exposed to the a rays from radium, the scintillations are 
difficult to observe, and the luminosity is far more persistent than 
for a zinc sulphide screen exposed under the same conditions. The 
duration of the phosphorescence probably accounts for the absence 
of visible scintillations. 
In the scintillations of zine sulphide, we are actually witnessing 
the effect produced by the impact on the screen of single atoms of 
matter projected with enormous velocity. Each of the particles 
carries an amount of energy corresponding to 59 x 10 ergs. On 
account of the ease with which these particles are stopped, most of 
this energy is given up at the surface of the screen, and a portion 
of the energy is transformed into light. Zine sulphide is very 
sensitive to mechanical shocks. Luminosity is observed if a pen- 
knife is drawn across the screen, or if a current of air is directed on 
to the screen. The disturbance effected by the impact of the a 
particle extends over a distance very large compared with the size 
of the impinging particle, so that the spots of hight produced have 
an appreciable area. Becquerel! recently has made an examination 
of the scintillations produced by different substances and_ has 
concluded that the scintillations are due to irregular cleavages 
in the crystals composing the screen, produced by the action 
of the a rays. Scintillations can be mechanically produced by 
crushing a crystal. Tommasina? found that a zinc sulphide sereen 
1 C. R. 137, Oct. 27, 1903. 2 C. R. 137, Nov. 9, 1903. 

