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20 MODERN MAGIC LANTERNS,
Gas bags are at the best evils, which in some cases perhaps
are still necessary ones. Where portability is not important
they yield the palm, or as much of it as is left them, to the
pneumatic gas holder, which may take the form either of
Figs. 14 or 15. Fig. 14 represents a gas holder which is
virtually a model of the huge miscalled " gasometers " which
are so striking a feature of the landscape in most towns.
The diagram sufficiently indicates its construction : A is a
cylinder, the top of which
(r.
Fig. 14. GAS HOLDF.R
OF THE "GASOMETER" TYPE.
must be perfectly gas-tight,
and must carry the stopcock
B, which acts as the inlet and
outlet for the gas ; c is a
similar cylinder without a
stopcock, a little larger than
A, which carries two or
more uprights with pullies,
over which pass cords from
A to the counter weights D.
In holders of any size, the
space inside the smaller
cylinder is filled, to a large
extent, by an empty metal
drum, as shown in the
figure, to avoid the necessity
for so large a bulk of
water. To charge this gas
holder, c is filled to within
an inch or two of its top
with water, and A, with its
open end downwards, immersed until the stopcock, which
must be kept open while depressing A, is under water. Before
use it should be seen that the counterbalance weights, D, are
just sufficient to allow A to move up or down with perfect
freedom while the stopcock B is open. As soon as A is completely
immersed, the cock may be connected with the oxygen
apparatus from which the holder is to be filled. As the
oxygen enters it will gradually raise the upper cylinder ; the
supply must be turned off before the bottom edge of the
latter is within an inch or two of the surface of the water.
HOME-MADE OXYGEN, GAS HOLDERS, ETC. 21
To use the gas, the jet can be connected with B by indiarubber
pipe, or a second stopcock can be fitted to the upper
cylinder as the gas outlet. Weights must be placed on the
flat top of A until the desired
pressure is obtained. Small
gas holders of this pattern are
easily constructed of galvanised
iron and will last a lifetime.
We have seen a very
efficient one made of two of
the large cylinders in which
paint is supplied. Another
form of gas holder is shown
at Fig. 15, in which A is the
inlet and outlet for the gas ;
B, a stopcock which is opened
during the filling of the holder
to allow the displaced water
to escape ; c, a pipe through
Fig. 15. A GAS HOLDER.
which water, under more or less pressure, is supplied to
drive out the gas when it is required for use.
One or other of these forms of gas holder are still largely
used where the limelight is constantly wanted in the same
place, as, for instance, in theatres, in photographic enlarging
establishments, etc.
Oxygen gas is generally prepared in small quantities by
heating together in an iron vessel a mixture of chlorate of
potassium and oxide of manganese. Both these compounds
should be fairly pure, it being particularly important that
the manganese oxide is not adulterated in any way ; the
presence of, f or example, carbon in any form in it being most
dangerous. The cheapest (and worst) form of generator
takes the form of a conical sheet-iron or copper vessel, which
is very difficult to properly clean, and is unprovided with
any safety-valve arrangement. Mr. Chadwick, to whom
lanternists are indebted for many ingenious and practical
improvements, has introduced two forms, either of which
are much more suitable. One of these is shown in Fig. 16.
In this, the handle at the top being pulled over, the bellshaped
top, which, like the rest of the apparatus, is of cast