168 PROPERTIES OF THE RADIATIONS (CH. 
centimetres of air. Zine sulphide is also luminous under the action 
of the 8 rays, but the phosphorescence is far more persistent than 
when produced by the a rays. 
Platimo-cyanide of barium fluoresces under the action of all 
three kinds of rays, but is especially suitable for a study of the 
Bandy rays. With a decigram of radium, the luminosity on the 
screen can be seen at a distance of a metre from the radium. The 
rays produce quite an appreciable luminosity on the screen after 
their passage through the human body. The mineral willemite (zine 
silicate) was recently found by Kunz to be an even more sensitive 
means of detecting the presence of the radiations than platimo- 
cyanide of barium. It fluoresces a beautiful greenish colour, and 
a piece of the mineral appears quite translucent under the action 
of the rays. Baskerville! has recently shown that kunzite, a new 
variety of mineral spodumene discovered by Kunz?, becomes 
luminous when exposed to the action of radium rays and retains 
its luminosity for some time. 
Both zine sulphide and platino-cyanide of barium diminish in 
luminosity after exposure for some time to the action of the rays. 
To regenerate a screen of the latter, exposure to solar light is 
necessary. A similar phenomenon has been observed by Villard 
for a screen exposed to Réntgen rays. Guiesel made a screen of 
platino-cyanide of radio-active barium. The screen, very luminous 
at first, gradually turned brown in colour, and at the same time 
the crystals became dichroic. In this condition the luminosity 
was much less, although the active substance had increased in 
activity after preparation. Many of the substances which are 
luminous under the rays from active substances lose this property 
to a large extent at low temperatures. 
109. Luminosity of radium compounds. All radium 
compounds are spontaneously luminous. This luminosity is es- 
pecially brilliant in the dry haloid salts, and persists for long 
intervals of time. In damp air the salts lose a large amount of 
their luminosity, but they recover it on drymg. With very active 
radium chloride, the Curies have observed that the light changes 
1 Science, Sept. 4, 1903. 
2 Science, Aug. 28, 1903. 
