Radioluminescence 419 



compound, whether a phosphor or not, would have emitted the 

 penetrating radiation. Although uranium was discovered by M. H. 

 Klaproth in 1789, over one hundred years were to elapse before its 

 radioactivity was suspected, and over one hundred and fifty years 

 before the full significance of its potentialities was to be recognized. 



A test of various uranium minerals by Madame Curie led to the 

 discovery of some which had a much greater action than could be 

 explained by their uranium content. Chemical purification of one 

 of these, pitchblende, resulted in the isolation of polonium and 

 radium in 1898. Not only did radium isolated by the Curies, Marie 

 (1867-1934) and Pierre (1859-1906), excite luminescence when 

 brought near a barium platinocyanide screen, but the radium salt 

 itself emitted blue light, a perpetual self-luminosity. 



Later, radium was found to give off three sorts of invisible radia- 

 tion: (1) the alpha rays which turned out to be charged helium 

 atoms, corresponding to anode rays in a helium vacuum tube, (2) 

 the beta rays or electrons, corresponding to the cathode rays of a 

 vacuum tube, and (3) gamma rays of high penetrating power, which 

 were identified as X-rays of very short wave length. All these radia- 

 tions on striking certain compounds may excite a light to which 

 the name radioluminescence has been applied. Strictly speaking, 

 only the gamma ray excitation is a radioluminescence; the beta rays 

 cause anodo- and the alpha rays excite cathodoluminescence. 



It is not surprising to find that H. Becquerel (1899) made an 

 early study of " les phenomenes de phosphorescence produits par la 

 rayonnement du radium." Soon after radium was isolated he found 

 that some minerals (ruby and calcite) which luminesced in visible 

 light did not luminesce in the presence of radium, but that in 

 general those excited by ultraviolet and X-rays did respond to 

 radium rays, with a few exceptions. Some diamonds which were 

 not excited by X-rays would luminesce in radium rays. A fluorspar 

 exposed to radium luminesced for twenty-hour hours, whereas the 

 luminescence of fluorspar after exposure to sunlight is momentary. 

 He also found that fluorspar which exhibited thermoluminescence 

 after exposure to visible light would likewise luminesce on heating 

 after exposure to radium rays. 



Another radioactive material was thorium, discovered independ- 

 ently in 1898 by Madame Curie (1898) and G. C. Schmidt (1898) . 

 It was also noticed that the activity of thorium compounds appeared 

 to vary greatly, a peculiarity which was traced by R. B. Owens 

 (1899) to the action of air currents, and by E. Rutherford (1900) 

 to the formation of a gas. In fact, a radioactive gas called the emana- 

 tion, which excited luminescence, could be drawn from the ma- 



