CH. VI] PROPERTIES OF THE RADIATIONS 167 
uranium and potassium is more luminous than hexagonal blende 
under X rays, but the reverse is true for radium rays; under the 
influence of these rays, sulphide of calcium gave a blue luminosity 
but was hardly affected by X rays. 
The following table shows the relative phosphorescence excited 
in various bodies. 

| 



| Without screen Across screen | 
Substance | Intensity } of black | 
paper | 
Hexagonal blende _... wee nao 300 | 13°36 “04 
| Platino-cyanide of barium ... 366 ree 1-99 05 | 
| Diamond a ae ee vee S00 | 114 | ‘Ol | 
| Double sulphate of Uranium and Potassium | 1:00 31 | 
Calcium Fluoride __... ane see 290 | “30 02 | 
— 


In the last column the intensity without the screen is in each 
case taken as unity. The great diminution of intensity after the 
rays have passed through black paper shows that most of the phos- 
phorescence developed without the screen is, in the majority of 
cases, due to the a rays. 
Bary! has made a very complete examination of the class of 
substances which become luminous under radium rays. He found 
that the great majority of substances belong to the alkali metals 
and alkaline earths. All these substances were also phosphorescent 
under the action of X rays. 
Zinc sulphide (Sidot’s blende) phosphoresces very brightly 
under the influence of the rays from radium and other very active 
substances. This was observed by Curie and Debierne in their 
study of the radium emanation and the excited activity produced 
by it. It has also been largely used by Giesel as an optical means 
of detecting the presence of emanations from very active sub- 
stances. It is an especially sensitive means of detecting the 
presence of a rays, when it exhibits the “scintillating ” property 
already discussed in section 88. In order to show the luminosity 
due to the a rays, the screen should be held close to the active 
substance, as the rays are absorbed in their passage through a few 
1 C. R. 130, p. 776, 1900. 
