Another interesting light phenomenon is found in the observed fact that very many substances become markedly phosphorescent at low temperatures. Thus, according to Professor Dewar, "gelatine, celluloid, paraffine, 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 phosphorescence 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 quoting, are "the platinocyanides among inorganic compounds and the ketonic compounds among organic. Ammonium platinocyanide, cooled while stimulated by arc light, glows fully at--180°; 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 cautious 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 high temperature, is one of the standing puzzles of physics. And the demonstrated increase of this capacity at very low temperatures only adds to the mystery.