st be made in the shape of a cool
bottle between the air pump and the box. If this precaution be
omitted, and if any paraffin splashes on to the hot surface of the
box, it volatilises with decomposition and the products go to stop up
the pump. Paraffin with a melting-point of 50 deg. C. or upwards does
well.

The bath should be allowed to cool to about 60 deg. C. before the papers
are removed, so that enough paraffin may be carried out to thoroughly
coat the paper and prevent the entrance of air.

Fig. 85.

Fig. 85 is a section of a vacuum vessel which has been found very
convenient. It measures about two feet in diameter at the top. It is
round, because it is much easier to turn one circular surface than to
plane up four surfaces, which has to be done if the box is square.
Both the rim of the vessel and the approximating part of the cover
require to be truly turned and smoothly finished. A very good packing
is made of solid indiarubber core about half an inch thick. This is
carefully spliced--cemented by means of a solution of rubber in
naphtha, and the splice sewed by thick thread. The lid ought to have
a rim fitting inside the vessel, for this keeps the rubber packing in
place; the rim has been accidentally omitted in Fig. 85. The bolts
should not be more than five inches apart, and should lie at least
half an inch in diameter, and the rim and lid should be each half an
inch thick.

Condensers may now be built up of sheets of this prepared paper
interleaved with tin-foil in the ordinary way. If good results are
required, the condenser when finished is compressed between wooden or
glass end-pieces by means of suitable clamps. It can then be put in a
box of melted paraffin, heated up to 140 deg. C, and exhausted by means
of the water pump for several hours.

In this process the air rushes out from between the paper and foils
with such vehemence that the paraffin is generally thrown entirely out
of the box. To guard against this the box must be provided with a
loosely fitting and temporary lid, pierced with several holes.

The real test as to when exhaustion is complete would be the cessation
of any yield of air or water. Since it is not generally convenient to
make the vacuum box so air-tight that there are absolutely no leaks at
all, and as the paraffin itself is, I think, inclined to "crack"
slightly at the temperature of 140 deg. C, this test or criterion cannot
be conveniently applied.

T