188 RADIUM. 



II. 



The radiations of radium produce pliotographic impressions very 

 quickly, and are able to penetrate any screen whatsoever. Bodies 

 differ in transparency, but no screen is absolutely opaque to radium ' 

 ra3's. 



The radiations of radium excite phosphorescence in a great numl>er 

 of bodies, including, among others, the following: Alkaline salts, 

 alkaline earths, organic substances, the skin, glass, paper, salts of ura- 

 nium, etc., while diamond, plati no-cyanide of barium, and the phos- 

 phorescent sulphide of zinc of Sidot are particularly sensitive. The 

 luminescence of phosphorescent sulphide of zinc persists for some 

 time after the removal of the radium which excites it. 



Radium emits its ra3^s with equal intensity whether immersed in 

 liquid air at —180° C. or at ordinar}^ temperatures. When a ]nt of 

 radium salt is placed with a little screen of platino-cyanide of barium 

 in a test tube and the whole plunged into liquid air, the screen appears 

 to glow at least as strongly as before. Under the same circumstances 

 a screen of sulphide of zinc loses some of its luminosity, but this is 

 owing to the diminished phosphorescent power of this substance at 

 low temperatures. 



Little by little phosphorescent substances are altered under the pro- 

 longed action of radium and become less readily excited and less 

 luminous. 



The salts of radium are spontaneously luminous, and it ma}' be pre- 

 sumed that they render themselves phosphorescent })y their own radio- 

 activit}'. Radium-chloride and radium-bromide are the most intensely 

 luminous of these salts, and may even appear visibly bright in open 

 daylight. In these circumstances the light emitted by the radium 

 recalls to mind the color of that given by the iirefl}" or glowworm. 

 The luminosity of radium salts diminishes with lapse of time, but 

 never wholly disappears, and salts at lirst uncolored become at length 

 tinged with gray, 3'ellow, or violet. 



III. 



The radiations of radium inqjart electrical conductivity to the air 

 through ^hich they pass. When a fragment of radium salt is brought 

 near a charged electroscope the latter is inunediately discharged. If 

 the electroscope is inclosed by a thick, solid wall the discharge still 

 takes place, though more slowly. Lead and platinum are strongly 

 absorl)ent, but aluminum is the most transparent of the metals, and 

 organic su])stances al)soi'l> relatively little of the Becquerel rays. 



Noncon(hicting li(|ui(ls, such as petroleum ether, sulphide of carbon, 

 benzine, and Tuiuid air are rendered conducting under the influence of 

 radium." 



«P. Curie, C. R., February 17, 1902. 



