418 Molecular Physics in High Vacua. [June, 
end of the tube is another terminal, d. The induCtion-coil 
is connected so that the hemi-cylinder is negative and the 
upper pole positive, and when exhausted to a sufficient 
extent, as is the case with this tube, the projection 
of the molecular rays to a focus is very beautifully 
shown. The rays are driven from the hemi-cylinder in a 
direction normal to its surface; they come to a focus and 
then diverge, tracing their path in brilliant green phosphor- 
escence on the surface of the glass. 
You will notice that the rays which project from the 
negative pole and cross in the centre have a bright green ap- 
pearance ; that colour is entirely due to the phosphorescence 
of the glass. At a very high exhaustion the phenomena no- 
ticed in ordinary vacuum tubes when the induction spark 
passes through them — an appearance of cloudy luminosity 
and of stratifications — disappears entirely. No cloud or fog 
whatever is seen in the body of the tube, and with such a 
vacuum as I am working with in these experiments — about a 
Fig. 4. 
millionth part of an atmosphere — the inner surface of the 
glass glows with a rich green phosphorescence, the intensity 
of colour varying with the perfection of the vacuum. 
It scarcely begins to show much before the 800,000th of 
an atmosphere. At about a millionth of an atmosphere 
the phosphorescence is very strong, and after that it begins 
to diminish until there are not enough molecules left to 
allow the spark to pass.* 
I have here a tube which will serve to illustrate the 
dependence of the green phosphorescence of the glass on 
the degree of perfection of the vacuum (Fig. 4). The two 
poles are at a and b, and at the end ( c ) is a small supple- 
mentary tube connected with the other by a narrow aperture, 
and containing solid caustic potash. The tube has been 
exhausted to a very high point, and the potash heated 
so as to drive off moisture and deteriorate the vacuum. 
i*o millionth of an atmosphere = 0*00076 millim. 
1315 789 millionths of an atmosphere== i*o millim. 
1,000,000- ,, ,, ,, = 760*0 millims. 
„ „ „ = 1 atmosphere. 
