428 Molecular Physics in High Vacua. [June, 
which I am indebted to my friend Mr. Blogg, of the 
firm of Blogg and Martin, who placed a small sackful 
at my disposal. As soon as I turn on the induction 
spark you will see these rubies shining with a brilliant 
rich red colour, as if they were glowing hot. Now the 
ruby is nothing but crystallised alumina with a little 
colouring-matter, and it became of great interest to ascer- 
tain 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 
all sizes, and from those I have selected the mass in this 
tube which I now place under the discharge : they phos- 
phoresce 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 in- 
strument by which the duration of phosphorescence in the 
sunlight can be examined). Here is some chemically pure pre- 
cipitated 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 dis- 
charge in a vacuum, corundum phosphoresces of a rose-pink 
colour. There is another curious faCt in which I think 
chemists will feel interested. The sapphire is also crystal- 
lised 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 
* Annales de Chimie et de Physique, 3rd series, vol. lvii., p. 50, 1859. 
