1 879.] 
Molecular Physics in High Vacua . 
429 
different 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 differ- 
ence between one and the other; but I connedt them with 
the indudtion-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 eledtric discharge in a vacuum, 
but whenever exposed to a strong light. 
The spedtrum of the red light emitted by all these varie- 
ties of alumina — the ruby, corundum, or artificially precipi- 
tated alumina — is the same as described by Becquerel 
twenty years ago. There is one intense red line, a little 
below the fixed line B in the spedtrum, having a wave- 
length of about 6895. There is a continuous spedtrum be- 
ginning at about B, and a few fainter lines beyond it, but 
they are so faint in comparison with this red line that they 
Fig. 12 . 
2 F 2 
