Untitled

CHAPTER IV.
lbonic,inabe expgen, (Sas lbotbco, Etc.
BEFORE the introduction of compressedgases and the use
of steel cylinders had rendered the purchase and transmission
of forty feet of oxygen from London to John o' Groats as easy
and as cheap as the forwarding of, say, a churn of milk for
the same distance, lanternists were compelled to use either
the delicate, expensive, and rapidly-ruined gas bag, or the
anything but portable pneumatic gas holder. As both of
these forms are still in use to some extent, and as the
latter, at any rate for certain purposes, can still hold its own,
a word as to their use will not be out of place. At the same
time, we can appropriately bracket with these forms of
apparatus the manufacture of oxygen at home.
Gas bags, which are generally wedge-shaped, are made, at
least the better qualities are, of a
stout fabric bag enclosed in another
of a close-textured twill, between
which is a third bag of thin sheet
india-rubber (Fig. 12). The outlet of
the bag, a brass stopcock, is fixed in
the middle of the edge formed by
the acute angle of the bag. This stopcock,
in the case of bags used to re-
Fig. 12. A GAS BAG.
tain oxygen made from potassium
chlorate, rapidly becomes corroded
by chlorine, which is generally present in small quantity in such
gas, unless care is taken to get rid of it (see page 23), which
HOME-MADE OXYGEN, GAS HOLDERS, ETC. 19
should always be done. The wedge-shaped bag, when being
filled, is allowed to lie freely on the table, but when thegas is
being used, is inserted between pressure boards (Fig. 13), the
upper one of which is weighted
to the requisite degree to expel
the gas. Weights are placed
at the extremity of the board,
against the shelf provided to
prevent them slipping down,
and may vary in amount from
a half - hundredweight to
three or even four times that
amount. For most purposes,
where a moderate size of disc
Fig. 13. DOUBLE PRESSURE BOARDS
is required, which in these
days will probably be the
only case in which gas bags
are used at all, a single 56 lb. weight will be sufficient
to commence with on a full bag, being supplemented with
another when the bag is more than half empty. When
using oxygen from a bag with house gas there is very little
danger of the latter entering the oxygen bag and forming
with its contents an explosive mixture, since the pressure
in the gas mains is but slight. It must be borne in mind,
however, that if at any time the two gases were mixed in a
bag, the result would in all probability be an explosion
which might endanger life and limb. This is far more likely
to occur when a mixed jet is being worked with both gases
in bags (this form of jet cannot be used with house gas
taken direct from the mains to the lantern, on account of
insufficient pressure), and the pressure in the two bags is not
equal. To get over the difficulty of maintaining the pressure
in two sets of bags and boards equal, double pressure boards
were invented, in which the same weights are imployed to
press down the two bags. When this forth is used, the apex
of the boards at A, Fig. 13, which when the bags are full is
raised, must be lowered by folding in the strut B before the
top board has sunk so low as either to overturn the entire
frame or to cause the weights to slide off. By the use of pressure
boards longer than the bags this can also be prevented.
B2