Page:Radio-activity.djvu/222

 The following table shows the relative phosphorescence excited in various bodies.

++++ ++++ ++++
 * |                 | Across screen  |
 * Substance                  |  Without screen. |   of black     |
 * |   Intensity     |    paper       |
 * Hexagonal blende                        |      13·36       |      ·04       |
 * Platino-cyanide of barium               |       1·99       |      ·05       |
 * Diamond                                 |       1·14       |      ·01       |
 * Double sulphate of Uranium and Potassium |      1·00       |      ·31       |
 * Calcium Fluoride                        |        ·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 phosphorescence developed without the screen is, in the majority of cases, due to the α 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.

Crystalline 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 substances. It is an especially sensitive means of detecting the presence of α rays, when it exhibits the "scintillating" property already discussed in section 96. In order to show the luminosity due to the α rays, the screen should be held close to the active substance, as the rays are absorbed in their passage through a few centimetres of air. Zinc sulphide is also luminous under the action of the β rays, but the phosphorescence is far more persistent than when produced by the α rays.

Very beautiful luminous effects are produced by large crystals of the platino-cyanides exposed to the radium rays. Those