Phosphorescence 343 



Nevertheless, Heinrich (1811) described an extraordinary number 

 of phosphors, including a particularly bright one prepared from 

 white alabaster by mixing it with acid potassium oxalate and heat- 

 ing in a coal fire. 



The three men are of special interest because they all proposed 

 universal but quite different theories to account for the light pro- 

 duction. For Dessaignes, water was all important, for Heinrich, it 

 was acid, and for Grotthus, electricity. 



As we have seen (Chapter VI) , theories of light emission at the 

 beginning of the nineteenth century assumed light to be a material 

 but imponderable substance, and to be held in some way between 

 the molecules of the phosphor. Dessaignes believed that the light 

 substance was associated with water but there were also electrical 

 undertones in his view. He cited the fact that strongly heated earths 

 which were freed of water by high temperature failed to phos- 

 phoresce but acquired the ability if slightly dampened. Since glass 

 and porcelain are phosphorescent and certainly contain no water, 

 Dessaignes was forced to assume that in hard bodies there was a 

 phosphoric fluid (" fluide de la phosphorescence ") , which would 

 luminesce if it were " concentrated " by moderate heat or other 

 physical or mechanical forces. To explain the light that appeared 

 from friction or collision, the fluid was believed to be of an elec- 

 trical nature. In favor of this view, Dessaignes cited an experiment 

 in which he heated glass powder until it was no longer phospho- 

 rescent and then sent electric discharges through the powder, when 

 it again became phosphorescent. This result was of course due to 

 the ultraviolet light in the spark but Dessaignes cited additional 

 experiments which convinced him that the electricity of the spark 

 and not the light was responsible. He concluded that good con- 

 ductors of electricity were never phosphorescent even after the dis- 

 charge of a Leyden jar; insulators were only weakly phosphorescent 

 and required the discharge of a Leyden jar, but medium conduc- 

 tors were very good phosphors, luminescing after exposure to 

 sunlight. Had Dessaignes used the word semiconductor, his phrase- 

 ology would have sounded quite modern and correct. 



Dessaignes also confirmed the fact that the color of the emitted 

 light was independent of the exciting light, thus disposing (in his 

 mind) of the idea that the light substance was absorbed on illumina- 

 tion. He held the light substance to be already present in the phos- 

 phor and to be set in vibration by the repulsive force of the light. 

 But if the phosphor is heated to a high temperature, the fluid is 

 driven out and illumination no longer excites to phosphorescence. 



Dessaignes distinguished two kinds of water in non-metallic bodies 



