422 Molecular Physics in High Vacua . , June, 
that in the low vacuum the position of the positive pole is of 
every importance, whilst in a high vacuum it scarcely matters 
at all where the positive pole is ; the phenomena seem to 
depend entirely on the negative pole. In very high 
vacua, such as we have been using, the phenomena 
follow altogether the negative pole. If the negative pole 
points in the direction of the positive, all very well, but 
if the negative pole is entirely in the opposite direction it 
does not matter : the line of rays is projected all the same 
in a straight line from the negative. 
I have hitherto spoken of and illustrated these pheno- 
mena in connexion with green phosphorescence. It does not 
follow, however, that the phosphorescence is always of that 
colour. This colouration is a property of the particular kind 
of glass in use in my laboratory. I have here (Fig. 7) three 
bulbs composed of different glass: one is uranium glass (a), 
which phosphorescesof adark green colour; anotherisEnglish 
glass (b), which phosphorescesof a blue colour; and the third 
(c) is soft German glass, — of which most of the apparatus 
before you is made, — which phosphoresces of a bright apple- 
green colour. It is therefore plain that this particular green 
phosphorescence is solely due to the glass which I am using. 
Were I to use English glass I should have to speak of blue 
phosphorescence, but I know of no glass which is equal to 
the German in brilliancy. 
Fig. 7. 
My earlier experiments were almost entirely carried on 
by the aid of the phosphorescence which glass takes 
up when it is under the influence of the eledtric discharge 
in vacuo; but many other substances possess this phosphor- 
escent power, and some have it in a much higher degree than 
glass. For instance, here is some of the luminous sulphide 
of calcium prepared according to M. Ed. Becquerel’s descrip- 
tion. When it is exposed to light — even candlelight — it 
