will be immediately explained, when the output of gas is measured in
terms of the water decomposed, in no commercial apparatus, and indeed in
no generator which can be imagined fit for actual employment, does that
output of gas ever approach the quantitative amount; but the volume of
water used, if not actually disappearing, is always vastly in excess of
the requirements of equation (2). On the contrary, when the make of gas
is measured in terms of the calcium carbide consumed, the said make may,
and frequently does, reach 80, 90, or even 99 per cent. of what is
theoretically possible. Inasmuch as calcium carbide is the one costly
ingredient in the manufacture of acetylene, so long as it is not wasted--
so long, that is to say, as nearly the theoretical yield of gas is
obtained from it--an acetylene generator is satisfactory or efficient in
this particular; and except for the matter of solubility discussed in the
following chapter, the quantity of water consumed is of no importance
whatever.

HEAT EVOLVED IN THE REACTION.--The chemical reaction between calcium
carbide and water is accompanied by a large evolution of heat, which,
unless due precautions are taken to prevent it, raises the temperature of
the substances employed, and of the apparatus containing them, to a
serious and often inconvenient extent. This phenomenon is the most
important of all in connexion with acetylene manufacture; for upon a
proper recognition of it, and upon the character of the precautions taken
to avoid its numerous evil effects, depend the actual value and capacity
for smooth working of any acetylene generator. Just as, by an immutable
law of chemistry, a given weight of calcium carbide yields a given weight
of acetylene, and by no amount of ingenuity can be made to produce either
more or less; so, by an equally immutable law of physics, the
decomposition of a given weight of calcium carbide by water, or the
decomposition of a given weight of water by calcium carbide, yields a