Phosphorescence 337 



spark to excite phosphorescence. The transparency of quartz for 

 uhraviolet radiation has turned out to be one of the most important 

 practical discoveries for work in this region of the spectrum. In 

 1845 Draper confirmed the opacity of glass for phosphorogenic rays 

 of the electric spark but found that it was transparent for those 

 from incandescent lime. In 1890 V. Schumann found that quartz 

 was opaque to very short ultraviolet wave-lengths, whereas fluorspar 

 was quite transparent in this now designated Schumann region. 



Phosphors and Combustion 



In the last quarter of the century a number of publications on 

 phosphors appeared in which the light was regarded as resulting 

 from combustion, as this process was understood at that time. In 

 Wilson's day, the phlogiston theory was at the zenith, with both 

 Priestley and Scheele strong adherents. One might expect that these 

 two men would be prejudiced in favor of a combustion theory, or 

 at least a phlogiston theory of phosphor light, but such was not the 

 case. Priestley advocated no views of his own, but it is clear from 

 the opening sentence of his chapter on the Bolognian phosphorus 

 in the History of Light and Colours (1772: 360) that he accepted 

 the light absorption (sponge) theory. He wrote: 



The hypothesis of the materiality of light is peculiarly agreeable to the 

 phenomenon of the Bolognian stone, which has the remarkable property 

 of imbibing light, of retaining it for some time, and of emitting it again; 

 and more especially of the emitting it more copiously, according to the 

 degree of heat applied to it. By this hypothesis, also, it is easiest to 

 account for some facts, by which it appears that the colour and internal 

 texture of bodies are changed by light only, exclusive of heat, or any 

 other circumstances. 



Scheele's ideas were essentially similar to Priestley, although he 

 did refer to phlogiston in connection with heat. His remarks will 

 be found in his Chemical Observations and Experiments on Air and 

 Fire (1777) translated by John Reinhold Forster in 1780 and dedi- 

 cated by Forster to Priestley. Scheele wrote (p. 121) : 



In regard to the Phosphorus Balduini, and the Bononian Phosphorus, it 

 is most probable, that these two substances attract the light from the 

 sun or fire. I cannot find the cause in anything else than in a certain 

 magnitude of the subtle pores, into which the particles of light pene- 

 trate, without being very strongly attracted by the matter of these bodies; 

 to which the acid of nitre or sulphur existing in these bodies may con- 

 tribute something. Heat which necessarily ought to be somewhat coarser 

 than light, when made very elastic by the addition of more phlogiston. 



