376 History of Luminescence 



together with their effects in exciting fluorescence of various ma- 

 terials, it was noted that bombardment by any of these radiations 

 would make the material highly thermoluminescent. In fact the 

 extraordinary ability of discharges in a vacuum tube to render fluo- 

 rite and other substances thermoluminescent led Wiedemann (1895) 

 and Wiedemann and Schmitt (1895) to deny that light in the dis- 

 charge was responsible, and they attributed the effect to " Ent- 

 ladungsstrahlen " (cathode rays) , whose nature was at the time 

 somewhat uncertain. They published a long list of compounds 

 rendered thermoluminescent by cathode rays. Some of these re- 

 tained their ability to thermoluminesce after exposure to the dis- 

 charge for months, others lost the ability fairly quickly. Wiedemann 

 and Schmitt noted the change in color of alkali halide crystals after 

 radiation and believed that chemical changes resulted from the 

 exposure. 



M. W. Hoffmann (1897) found that no thermoluminescence 

 could be excited by " Entladungsstrahlen " if solid bodies, even 

 transparent quartz or fluorite, were placed in their path, again 

 indicating that more than ultraviolet light was present. Thus the 

 electric discharge in a vacuum turned out to be a complex phe- 

 nomenon, requiring considerable controversy and much research 

 before all of its effects were cleared up and the electron designated 

 the exciting agent. 



In 1898 J. Trowbridge and J. E. Burbank exposed fluorite, whose 

 thermoluminescence had been destroyed by heating, to X-rays in a 

 dark metal box and found that after exposure the fluorite would 

 again luminesce momentarily on warming. The paper was entitled 

 " Phosphorescence Produced by Electrification." Since it was known 

 at the time that X-rays electrify bodies, Trowbridge and Burbank 

 suggested that the electric energy gained from charging fluorite by 

 X-rays was dissipated as light. 



At the end of the nineteenth century it was recognized that energy 

 from some source must be stored in some way in a thermolumi- 

 nescent body, to be released by rise of temperature. The suggestion 

 has been made that thermoluminescence of fragments from the in- 

 terior of opaque rocks, never exposed to sunlight in recent times, 

 might be the result of energy accumulated from radioactive sub- 

 stances in the crust of the earth. ^ Recent research ^° has fully born 

 out this prediction. 



* A. S. Herschel (1899) showed that material from the inside of the Middlesborough 

 aerolite was thermoluminescent and held that this indicated the interior could never 

 have been heated. 



^^ See F. Daniels, C. A. Boyd, and D. F. Saunders, Thermoluminescence as a research 

 tool. Science 117: 343-349, 1953. 



