346 History of Luminescence 



The preparation of a strontium sulphide phosphor appears to have 

 been first made in 1817 by Johann Friedrich John (1782-1847) , at 

 one time a professor of chemistry at the University of Frankfurt 

 a. d O., later a physician in Berlin. He found that his SrS lumi- 

 nesced sky blue, while the BaS which he also prepared luminesced 

 reddish violet. 



One of the points emphasized by the very early workers for suc- 

 cess in obtaining a phosphor was the character of the raw material, 

 i. e., its physical properties. The Bolognian stone to be calcined 

 should have a certain size (that of a walnut) or surface quality 

 (smooth and silvery) or grain (fibrous) , if the phosphor was to 

 absorb the light of the sun. Also important was the temperature of 

 calcination or length of heating. Later Lemery held that the use of 

 a brass mortar for grinding the mass to be heated was an absolute 

 necessity. Wilson's experience with oyster shells indicated that traces 

 of metals affected the color of the luminescence, but contamination 

 of the material was too great to come to any definite conclusion, 

 and Wilson was forced to admit that chance played a large part in 

 the result. Neverthless, the necessity of metal impurities was cor- 

 rect and important. De Saussure (fils) , in 1792, in connection with 

 his study of dolomite, made the suggestion that impurities like iron 

 were necessary for thermoluminescence, and pointed out that the 

 more colored a specimen of fluorspar is, the more intense thermo- 

 luminescence it will show. 



Following Benjamin Wilson (1775) , attempts to increase the 

 intensity and duration of light and to control the color of the light 

 of phosphors by adding other materials were made by Gottfried 

 Wilhelm Osann (1797-1866), professor of physics and chemistry 

 at the University of Wurzburg, and by G. F. Wach. 



Osann (1825) obtained phosphors with blue or green lumines- 

 cence from oyster shells to which arsenic, antimony, or mercury 

 sulphide had been added. 



In a second paper Osann (1834) described a quantitative photo- 

 metric study of luminescence intensity after exposure to light 

 through various colored filters. He also discussed the three recog- 

 nized theories of phosphorescence— that a combustion takes place;- 

 that light is absorbed and then slowly liberated; and that light 

 already present is set free by excitation (insolation or slight heat- 

 ing) . He favored a combination of the absorption and excitation 

 theory. When exposed to light phosphors absorb a certain amount 

 and then give up a part of what they have absorbed. The remainder 

 can be liberated by moderate heating. The combustion theory was 

 ruled out by the fact that many non-combustible substances are 



