obtainable, and the field of our investigations is greatly extended.
Think we as we may, of all the possible directions to develop a
practical illuminant, the line of high vacua seems to be the most
promising at present. But to reach extreme vacua the appliances must
be much more improved, and ultimate perfection will not be attained
until we shall have discarded the mechanical and perfected an
_electrical_ vacuum pump. Molecules and atoms can be thrown out of a
bulb under the action of an enormous potential: _this_ will be the
principle of the vacuum pump of the future. For the present, we must
secure the best results we can with mechanical appliances. In this
respect, it might not be out of the way to say a few words about the
method of, and apparatus for, producing excessively high degrees of
exhaustion of which I have availed myself in the course of these
investigations. It is very probable that other experimenters have used
similar arrangements; but as it is possible that there may be an item
of interest in their description, a few remarks, which will render
this investigation more complete, might be permitted.

[Illustration: FIG. 30.--APPARATUS USED FOR OBTAINING HIGH DEGREES OF
EXHAUSTION.]

The apparatus is illustrated in a drawing shown in Fig. 30. S
represents a Sprengel pump, which has been specially constructed to
better suit the work required. The stop-cock which is usually employed
has been omitted, and instead of it a hollow stopper s has been fitted
in the neck of the reservoir R. This stopper has a small hole h,
through which the mercury descends; the size of the outlet o being
properly determined with respect to the section of the fall tube t,
which is sealed to the reservoir instead of being connected to it in
the usual manner. This arrangement overcomes the imperfections and