In any case the position of a rising holder is the most
obvious sign of the degree of exhaustion of a generator; and therefore,
to render absolutely impossible a failure of the light during an evening,
a non-automatic generator fitted with a rising holder is best.
Since calcium carbide is a solid body having a specific gravity of 2.2,
water being unity, and since 1 cubic foot of water weighs 62.4 lb., in
round numbers 137 lb. of _compact_ carbide only occupy 1 cubic foot
of space. Again, since acetylene is a gas having a specific gravity of
0.91, air being unity, and since the specific gravity of air, water being
unity, is 0.0013, the specific gravity of acetylene, water being unity,
is roughly O.00116. Hence 1 cubic foot of acetylene weighs roughly 0.07
lb. Furthermore, since 1 lb. of good carbide evolves 5 cubic feet of gas
on decomposition with water, acetylene stored at atmospheric pressure
occupies roundly 680 times as much space as the carbide from which it has
been evolved. This figure by no means represents the actual state of
affairs in a generator, because, as was explained in the previous
chapter, a carbide vessel cannot be filled completely with solid; and,
indeed, were it so "filled," in ordinary language, much of its space
would be still occupied with air.
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