LENARD'S RESEARCHES ON PHOSPHORESCENCE 57 



covered with the powdered phosphors and observing the 

 luminosity produced in various parts of the spectrum ; he 

 found, as previous observers had done, that the nature of the 

 emitted light was independent of the wave-length of the 

 exciting light. Becquerel also made important observations on 

 the temperature effects and the law of decay of the phosphores- 

 cent light with time which will be dealt with later on ; by 

 systematically using the spectroscope in this work he placed 

 the study of the whole question on a new footing. But he did 

 not put forward any general theory of phosphorescence. 



At this time, the latter half of the nineteenth century, one 

 of the chief obstacles in the way of the study of the subject was 

 the difficulty of preparing artificial phosphoroids which would 

 behave in a definite way : for instance, calcium sulphide could 

 be prepared so that it would phosphoresce either yellow or 

 green. A first step in the direction of a solution of this problem 

 was made by Lecoq de Boisbaudran, who showed that certain 

 substances, which did not phosphoresce in the cathode rays 

 when pure, were rendered phosphorescent by the addition of 

 traces of foreign metals ; that, for instance, the luminosity 

 of many substances was due to traces of manganese. About 

 this time, Crookes, working on the rare earths, showed that 

 their presence, for example in salts of calcium, gave rise to 

 definite phosphorescence spectra under the influence of cathode 

 rays ; both he and Lecoq de Boisbaudran did much work on 

 these phosphorescent spectra. Verneuil traced the phos- 

 phorescence of calcium sulphide to the presence of traces of 

 bismuth. It is at this point that the researches of Lenard 

 begin, to whose work I shall now devote special attention. 



Lenard, in conjunction with Klatt, first stated in detail the 

 conditions to be observed in preparing phosphoroids from the 

 alkaline earths and systematically prepared a large number of 

 substances of this class, which includes nearly all those which 

 remain luminous during a considerable period after the exciting 

 light has ceased ; a form of luminosity which it is convenient to 

 call the after-glow. Three components are necessary : the 

 sulphide of an alkaline metal ; a small quantity — generally less 

 than a ten-thousandth of the whole — of a foreign metal ; and 

 a fusible component or flux. The action of the flux, which 

 may be any one of a large number of colourless fusible salts, 

 sodium sulphate for instance, is principally to bind the loose 



