30 On the Crimson Line of Phosphorescent Alumina. [Jan. 13, 



melting point of platinum for about five minutes. He-tested in a 

 vacuum tube this alumina was seen to glow at the points and edges 

 of the lumps where the heat had been fiercest, with a red light, 

 giving a faint line spectrum. The bulk of the mass, however, gave 

 out the original green glow. 



To get the crimson line most brilliantly, it is necessary to ignite the 

 earth to the highest temperature of the blowpipe flame. With a 

 slightly less heat the phosphorescence is not strong, and the line is 

 faint. When the temperature has not been raised high enough the 

 colour of the emitted light in most aluminas is green, and no line is 

 visible ; whilst the same earth raised to a higher temperature glows 

 with a red light, and the red line comes into view. The most brilliant 

 crimson line, when seen at all, has always been obtained when the 

 alumina has been kept near the melting point of platinum for some 

 time. 



Physical differences, or perhaps even difference in molecular com- 

 position, also exert a great influence on the phosphorescence of 

 alumina. In this connexion, I ask permission to quote a sentence 

 from my paper of May, 1881, already mentioned : — " Two earthen 

 crucibles were tightly packed, the one with sulphate of alumina, the 

 other with acetate of alumina. They were then exposed, side by side, 

 to the most intense heat of a wind-furnace — a heat little short of 

 the melting point of platinum. The resulting aluminas were then 

 tested in the molecular stream. The alumina from the sulphate 

 gave the crimson glow and the spectrum line. The alumina from 

 the acetate gave no red glow or line, but a pale green phosphor- 

 escence." 



Experience gained in the yttria research has taught me that the 

 possibility of the molecule of aluminium being composed of two or 

 more submolecules, only one of which is capable of giving the crimson 

 line phosphorescence, must not be overlooked. To test this hypo- 

 thesis alumina, as pure as I could prepare it, was submitted to three 

 separate processes of fractionation, the operations in each case being 

 repeated from twenty to thirty times. Alumina giving the crimson 

 line always concentrated towards one end of the fractionations, 

 whilst at the other end the alumina sometimes phosphoresced of a 

 green tint, and at others scarcely phosphoresced at all, the crimson 

 line being either very feeble or entirely absent in the spectrum. The 

 earths were always ignited for the same time and, as nearly as 

 possible, to the same temperature. 



In no case could chromium be detected at either extremity of the 

 fractionations. 



These experiments are perhaps too few to permit any important 

 inference being drawn from them. There seem, however, to be four 

 possible explanations of the phenomena observed : — 



