Phosphorescence 359 



tion decreases with rise in temperature, a relation confirmed by 

 G. A. Bardetscher (1889). In fact E. Wiedemann (1892) later 

 demonstrated that eosin in dry gelatin, which phosphoresces at room 

 temperature, becomes at 140° a fluorescence of so short a duration 

 that the time could not be measured in his phosphoroscope. 



After the preparation of liquid oxygen by L. Cailletet and R. P. 

 Pictet, independently in 1877, and commercial manufacture of 

 liquid air in sufficient quantities in the eighteen nineties, it was 

 found that phosphorescent light in some substances may disappear 

 at this temperature and reappear on warming, as shown by R. Pictet 

 and by J. Dewar, both in 1894. The fact that exposure to any type 

 of exciting radiation at liquid air temperatures would result in phos- 

 phorescence when the temperature of the phosphor was raised, even 

 though no light could be seen in liquid air, appears to have been 

 first observed by James Dewar (1894) . Low temperatures were also 

 studied by R. Cusack (1897), A. and L. Lumiere (1899), C. C. 

 Trowbridge (1899) , and others during the twentieth century. 



The color of the light of phosphors changes with temperature 

 also, as noted by E. Becquerel in 1859. He found a SrS phosphor 

 to give off dark violet light at —20° C, blue violet at 20°, bright 

 blue at 40°, green at 70°, yellow green at 90°, yellow at 100°, and 

 orange at 200° The phenomenon is a particularly striking one as 

 certain phosphors warm up from liquid air temperatures. Similar 

 changes were noted by G. A. Bardetscher (1889) and by E. Wiede- 

 mann and G. A. Schmidt (1895) . 



It is apparent that in place of a heterogeneous mass of informa- 

 tion, Becquerel was responsible for what can be designated a planned 

 approach, based on quantitative measurement of intensity and dura- 

 tion of phosphorescence and spectral emission in relation to the 

 characteristics of the exciting light. Becquerel did not attempt to 

 explain phosphorescence. He was content to ascribe the effects of 

 light to an upsetting of the molecular equilibrium. His greatness 

 lies in the collection of precise data on which a theory could be 

 based when generalization became possible. 



POLARIZATION OF PHOSPHORESCENT LIGHT 



Becquerel's investigations were so thorough that he practically 

 monopolized the study of phosphorescence during his most produc- 

 tive years. However, a few scattered discoveries by other workers 

 not previously described, can be called " firsts." An early published 

 test of polarized light in exciting phosphorescence of prepared non- 

 crystalline phosphors was made by H. W. Dove in 1861. He found 

 that polarized light was quite effective in excitation and that the 



