If wrapped in paper, the cartridges may be dropped
into water by an automatic generator at the proper times, the liquid then
loosening the gum and so gaining access to the interior; or one spot may
be covered by a drape of porous material (felt) only, through which the
water penetrates slowly. The substance inside the cartridge may be
ordinary, granulated, or "treated" carbide. Cartridges or "sticks" of
carbide are also made without wrappings, either by moistening powdered
carbide with oil and compressing the whole into moulds, or by compressing
dry carbide dust and immersing the sticks in oil or molten grease. The
former process is said to cause the carbide to take up too much oil, so
that sticks made by the second method are reputed preferable. All these
cartridges have the advantage over common carbide of being more permanent
in damp air, of being symmetrical in shape, of decomposing at a known
speed, and of liberating acetylene in known quantity; but evidently they
are more expensive, owing to the cost of preparing them, &c. They may be
made more cheaply from the dust produced in the braking of carbide, but
in that case the yield of gas will be relatively low.
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