Apparent proof that heat is not accumulating unduly in a water-jacketed
carbide container even when the generator is evolving gas at a fair speed
is easy to obtain; for if, as usually happens, the end of the container
through which the carbide is inserted is exposed to the air, the hand may
be placed upon it, and it will be found to be only slightly warm to the
touch. Such a test, however, is inconclusive, and frequently misleading,
because if more than a pound or two of carbide is present as an undivided
mass, and if water is allowed to attack one portion of it, that
particular portion may attain a high temperature while the rest is
comparatively cool: and if the bulk of the carbide is comparatively cool,
naturally the walls of the containing vessel themselves remain
practically unheated. Three causes work together to prevent this heat
being dissipated through the walls of the carbide vessel with sufficient
rapidity. In the first place, calcium carbide itself is a very bad
conductor of heat. So deficient in heat-conducting power is it that a
lump a few inches in diameter may be raised to redness in a gas flame at
one spot, and kept hot for some minutes, while the rest of the mass
remains sufficiently cool to be held comfortably in the fingers.
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