Untitled

22 MODERN MAGIC LANTERNS. HOME-MADE OXYGEN, GAS HOLDERS, ETC. 23
iron, removes. A prepared cake of manganese oxide
and potassium chlorate is introduced, the top put on, the
handle replaced, and the gas lit at the Bunsen burner, which
is enclosed in the hollow part underneath
(only the stopcock is
visible in the figure) ; gas rapidly
comes off until, in a quarter of an
hour or less, the cake is exhausted,
and may be replaced by another.
In case of any excess of pressure
in the vessel, the springs at the
side allow the lid to rise and the
gas to escape. The cakes are
made by mixing four parts of
manganese oxide to one part of
the powdered chlorate, enough
water being added to just moisten
the mass, which is then filled into
a mould. The cakes are allowed
to dry, when the bottom of each
is coated with a thin paste of
manganese oxide and water
to prevent it sticking to the
Fig. 16. OXYGEN RETORT.
retort. The use of the cakes entirely prevents the choking
up of the retort with the decomposed oxygen mixture, the
bye-product coming away in a mass just as the cake was
inserted. We recently saw one of these working in conjunction
with a large gas holder of the type shown in Fig. 14.
It had been in use for many years; making oxygen for
enlarging purposes, without the slightest hitch. Another
form designed by Mr. Chadwick has a weighted lid at the
end, which acts when necessary as a dead-weight safety
valve.
For oxygen making, as we said before, the manganese
oxide should be got from a reliable source. The chlorate is
best bought ready powdered if to be made up into cakes as
above described ; if to be used loose, the crystals should be
employed (it is best not to attempt to powder it oneself), and
the manganese used should be the granulated form. Both
ingredients should be carefully picked over for scraps of
cork, straw, etc. When the cakes are not employed, two
parts by weight of chlorate to one of manganese are those
we used in the days when oxygen was not so readily obtainable.
Some workers, however, prefer three, or even four parts
of chlorate.
Whatever retort is used, a gas stove is the most convenient
way of heating it ; the heat should be left on until
at least four cubic feet of oxygen have come off per pound of
chlorate. This is mentioned because the emission of gas is
often irregular, ceasing almost and then coming off with full
force again.
When the gas is to be received in a bag, it should be
washed by allowing it to bubble up, as shown in Fig. 17,
through a Woolff's bottle containing a fairly strong solution
of caustic soda, which will last for several operations, and
then through an empty flask to remove as much as possible
of the water. This washing is not necessary when a
pneumatic holder is to be employed. When no more gas is
coming off, the stopcock on the bag should be closed and the
tube 'immediately disconnected between the retort and the
first bottle ; if this is not done at once and before the retort
is allowed in any degree to cool, the liquid in the flask will
be sucked back and an explosion
of steam in the retort is
sure to ensue. There should
be no fear of this in any but
the most incompetent hands,
which latter had best leave
limelight alone altogether.
When a gas bag has got hard
through cold, it should be
warmed a little before filling
to soften it. If this is not done
the bag will rapidly perish.
In filling a bag with oxy-
Fig. 17.
APPARATUS FOR MAKING AND
WASHING OXYGEN.
gen the sequence of operations should be as follows
(1) Having connected the retort and flasks as shown,
and having the empty bag with its stopcock open and
tube ready to connect to the last flask, apply heat to
the retort until gas bubbles off freely.