152 Mr. William CrooTces [April 4, 



alumina with a little colouring-matter, and it became of great interest 

 to ascertain whether the artificial ruby made by M. Feil, of Paris, 

 would glow in the same manner. I had simply to make my wants 

 known to M. Feil, and he immediately sent me a box containing 

 artificial rubies and crystals of alumina of p11 sizes, and from those I 

 have selected the mass in this tube which I now place under the 

 discharge: they phosphoresce of the same rich red colour as the 

 natural ruby. It scarcely matters what colour the ruby is, to begin 

 with. In this tube of natural rubies there are stones of all colours — 

 the deep red ruby and the pale pink ruby. There are some so 

 pale as to be almost colourless, and some of the highly-prized tint of 

 pigeon's blood ; but in the vacuum under the negative discharge they 

 all phosphoresce with about the same colour. 



As I have just mentioned, the ruby is crystallised alumina. In a 

 paper published twenty years ago by Ed. Becquerel* I find that he 

 describes the appearance of alumina as glowing with a rich red colour 

 in the phosphoroscope (an instrument by which the duration of phos- 

 phorescence in the sunlight can be examined). Here is some chemi- 

 cally pure precipitated alumina which I have prepared in the most 

 careful manner. It has been heated to whiteness, and you see it 

 glows with the rich red colour which is supposed to be characteristic 

 of alumina. The mineral known as corundum is a colourless variety 

 of crystallised alumina. Under the negative discharge in a vacuum, 

 corundum phosphoresces of a rose-pink colour. There is another 

 cui'ious fact in which I think chemists will feel interested. The 

 sapphire is also crystallised alumina, just the same as the ruby. 

 The ruby has a little colouring-matter in it, giving it a red colour ; 

 the sapphire has a colouring-matter which gives it a blue colour, 

 whilst corundum is white. I have here in a tube a very fine crystal 

 of sapphire, and, when I pass the discharge over it, it gives alternate 

 bands of red and green. The red we can easily identify with the 

 glow of alumina ; but what is the green ? If alumina is precipitated 

 and purified as carefully as in the case I have just mentioned, but in a 

 somewhat difi'erent manner, it is found to glow with a rich green 

 colour. Here are the two specimens of alumina in tubes, side by 

 side. Chemists would say that there was no difference between one 

 and the other ; but I connect them with the induction-coil, and you 

 see that one glows with a bright green colour, whilst the other glows 

 with a rich red colour. Here is a fine specimen of chemically pure 

 alumina, lent me by Messrs. Hopkin and Williams ; by ordinary 

 light it is a perfectly white powder. It is just possible that the rich 

 fire of the ruby, which has caused it to be so prized, may be due, 

 not entirely to the colouring-matter, but to its wonderful power of 

 phosphorescing with a deep red colour, not only under the electric 

 discharge in a vacuum, but whenever exposed to a strong light. 



The spectrum of the red light emitted by all these varieties of 



* Annales de Cbiinie ot de Physique, 3rd series, vol. hii. p. 50, 1859. 



