This is due to the fact that throughout the whole period of
reaction heat is escaping from the mass, and passing into the atmosphere
at a fairly constant speed; so that, clearly, the more slowly heat is
produced, the better opportunity has it to pass away, and the less of it
is left to collect in the material under consideration. During the action
of an acetylene generator, there is a current of gas constantly
travelling away from the carbide, there is vapour of water constantly
escaping with the gas, there are the walls of the generator itself
constantly exposed to the cooling action of the atmosphere, and there is
either a mass of calcium carbide or of water within the generator. It is
essential for good working that the temperature of both the acetylene and
the carbide shall be prevented from rising to any noteworthy extent;
while the amount of heat capable of being dissipated into the air through
the walls of the apparatus in a given time is narrowly limited, depending
upon the size and shape of the generator, and the temperature of the
surrounding air. If, then, a small, suitably designed generator is
working quite slowly, the loss of heat through the external walls of the
apparatus may easily be rapid enough to prevent the internal temperature
from rising objectionably high; but the larger the generator, and the
more rapidly it is evolving gas, the less does this become possible.
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