judge from incandescent lamps, this is not essential--a fact which
entirely coincides with my own experience.

On the other hand, when sealing electrodes into German glass I have
often used a drop of enamel with perfect results, though this is not
always done in Germany. In all cases, however, in which electrodes
have to be sealed in--especially when they are liable to heat--I
recommend flint glass, and in this have the support of Mr. Rain (The
Incandescent Lamp and its Manufacture, p. 131). The exact details
for the preparation of eudiometer tubes are given by Faraday (Chemical
Manipulation, Sec. 1200).

In view of what has preceded, however, I will content myself with the
following notes. Make the hole through which the wire is to protrude
only slightly larger than the wire itself, and be sure that the latter
is clean. Allow the glass to cool sufficiently not to stick to the
wire when the latter is pushed in. Be sure that, on heating, the
glass does not get reduced, and that it flows up to the wire all
round; pull and push the wire a little with a pair of pincers, to
ensure this.

It is not a bad plan to get the glass exceedingly fluid round the
wire--even if the lump has to be blown out a little afterwards--as it
cools. The seal should finally be well annealed in asbestos, but
first by gradually moving it into the hot air in front of the flame.

It was observed by Professor J. J. Thomson and the author some years
ago (Proc. Roy. Soc. 40. 331. 1886) that when very violent
discharges are taken through lightly sealed-in electrodes in
lead-glass tubes--say from a large battery of Leyden jars--gas
appears to be carried into the tube over and above that naturally
given off by the platinum, and this without there being any apparent
want of perfection in the seal. This observation has since been
confirmed by others. Consequently in experiments on violent
discharges in vacuo where certainty is required as to the exclusion of
air, the seals should be protected by a guard tube or cap containing
mercury; this must, of course, be put in hot and clean, on hot and
clean glass, and in special cases should be boiled in situ.

A well-known German physicist (Warburg, I think) recommends putting
the seals under water, but I cannot think that this is a good plan,
for if air can get in, why not water? which has its surface tension
in its favour. The same reasoning prevents my recommending a layer of
sulphuric acid