glass, but it seems
probable that air would diffuse through the graphite in the course of
time.

Another method for the introduction of an electrode is illustrated by
_d_, _e_, _f_ and _g_, Fig. 11. In this case the bulb or thin-walled
tube into which the electrode is to be sealed is perforated by a quick
stab with an intensely heated wire--preferably of platinum--which is
then withdrawn before the glass has had time to harden, and thus a
minute circular hole is made. The electrode is coated with a layer of
similar glass, or of the specially made enamel which is sold for this
purpose, inserted into the bulb or tube by any convenient opening, and
adjusted by careful shaking until the platinum wire projects through the
small hole. The bulb or tube is then fused to the coating of the
electrode and the whole spot expanded slightly by blowing. The
appearance of the finished seal is shown by _g_. It is well to anneal
slightly by smoking.

_Thermometers._--Apart from the notes on page 20 with respect to the
blowing of a suitable bulb on capillary tubing there is little to say in
connection with the glass working needed in making a plain thermometer.
The size desirable for the bulb will be determined by the bore of the
capillary tube, the coefficient of expansion of the liquid used for
filling, and the range of temperature for which the thermometer is
intended.

Filling may be carried out as follows:--Fit a small funnel to the open
end of the capillary by means of a rubber tube, and pour into the funnel
rather more than enough of the liquid to be used than is required to
fill the bulb. Mercury or alcohol will be used in practice, most
probably. Warm the bulb until a few air bubbles have escaped through
the liquid and then allow to cool. This will suck a certain amount of
liquid into the bulb. Now heat the bulb again, and at the same time heat
the capillary tube over a second burner. The liquid will boil and sweep
out the residual air, but it is necessary to heat the capillary tube as
well in order to prevent condensation. Allow the bulb and tube to cool,
then repeat the heating once more. By this time the bulb and tube should
be free from air, and cooling should give a completely filled
thermometer. Remove the funnel and heat the thermometer to a few degrees
above the maximum temperature for which it is to be used; the mercury or
other filling liquid will overflow from the top, and, as the temperature
falls, will recede, th