ness, is drawn down to
a point. The larger bulb, about 5 centimetres in diameter, is blown
at one end of this tube. The thinner the bulb the better, provided
that it does not collapse under atmospheric pressure. A very good
idea of a proper thickness may be obtained from the statement that
about 4 centimetres length of the tubing should be blown out to form
the bulb. This would give a bulb of about the thickness of an
ordinary fractionating bulb. Before going any further it is as well
to test the bulb by tapping on the table and by exhausting it by means
of an ordinary water-velocity pump.

The side tube is next prepared out of narrower tubing, and is provided
with a smaller bulb, a blowing-out tube, and a terminal, to be made as
will be described. This side tube is next fused on to the main tube,
special care being taken about the annealing, and the cathode terminal
is then sealed into the main tube. After using clean glass it is in
general only necessary to rinse the tube out with clean alcohol, after
which it may be dried and exhausted.

The success of the operation will depend primarily on the attention
given to the preparation and sealing-in of the electrode facing the
large bulb.

Preparation of Terminals. Some platinum wire of about No. 26
B.W.G--the exact size is unimportant--must be provided, also some
sheet aluminium about 1 millimetre thick, some white enamel cement
glass, and a "cane" of flint-glass tube of a few millimetres bore.

The electrodes are prepared by cutting discs of aluminium of from 1 to
1.5 centimetres diameter. The discs of aluminium are bored in the
centre, so as to admit the "stems" which are made of aluminium wire
of about 1 millimetre diameter. The stems are then riveted into the
discs. The "stems" are about I centimetre long, and are drilled to a
depth of about 3 millimetres, the drill used being about double the
diameter of the platinum wire to be used for making the connections.
The faces of the electrodes--i.e. the free surfaces of the aluminium
discs--are then hammered flat and brought to a burnished surface by
being placed on a bit of highly polished steel and struck by a "set"
provided with a hole to allow of the "stem" escaping damage. The
operation will be obvious after a reference to Figs. 39 and 40; it
is referred to again on page 96.

The platinum wires may be most conveniently attached by melting one
end of the piece of platinum wire in the oxygas blow-p