A HISTORY OF SCIENCE 



markedly phosphorescent at low temperatures. Thus, 

 according to Professor Dewar, "gelatine, celluloid, par- 

 affine, ivory, horn, and india-rubber become distinctly 

 luminous, with a bluish or greenish phosphorescence, 

 after cooling to 180 and being stimulated by the 

 electric light." The same thing is true, in varying 

 degrees, of alcohol, nitric acid, glycerine, and of paper, 

 leather, linen, tortoise-shell, and sponge. Pure water 

 is but slightly luminous, whereas impure water glows 

 brightly. On the other hand, alcohol loses its phos- 

 phorescence when a trace of iodine is added to it. In 

 general, colored things are but little phosphorescent. 

 Thus the white of egg is very brilliant but the yolk 

 much less so. Milk is much brighter than water, and 

 such objects as a white flower, a feather, and egg-shell 

 glow brilliantly. The most remarkable substances of 

 all, says Professor Dewar, whom I am all along quot- 

 ing, are "the platinocyanides among inorganic com- 

 pounds and the ketonic compounds among organic. 

 Ammonium platinocyanide, cooled while stimulated 

 by arc light, glows fully at 1 80 ; but on warming it 

 glows like a lamp. It seems clear," Professor Dewar 

 adds, " that the substance at this low temperature must 

 have acquired increased power of absorption, and it 

 may be that at the same time the factor of molecular 

 friction or damping may have diminished." The cau- 

 tious terms in which this partial explanation is couched 

 suggest how far we still are from a full understanding 

 of the interesting phenomena of phosphorescence. 

 That a molecule should be able to vibrate in such a way 

 as to produce the short waves of light, dissevered from 

 the usual linking with the vibrations represented by 



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