In the
second place, commercial carbide exists in masses of highly irregular
shape, so that when they are packed into any vessel they only touch at
their angles and edges; and accordingly, even if the material were a
fairly good heat conductor of itself, the air or gas present between each
lump would act as an insulator, protecting the second piece from the heat
generated in the first. In the third place, the calcium hydroxide
produced as the by-product when calcium carbide is decomposed by water
occupies considerably more space than the original carbide--usually two
or three times as much space, the exact figures depending upon the
conditions in which it is formed--and therefore a carbide container
cannot advisedly be charged with more than one-third the quantity of
solid which it is apparently capable of holding. The remaining two-thirds
of the space is naturally full of air when the container is first put
into the generator, but the air is displaced by acetylene as soon as gas
production begins. Whether that space, however, is occupied by air, by
acetylene, or by a gradually growing loose mass of slaked lime, each
separate lump of hot carbide is isolated from its neighbours by a
material which is also a very bad heat conductor; and the heat has but
little opportunity of distributing itself evenly.
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