As the ratio of cubical contents to superficial area of
a lump is greater as the lump itself is larger, and as only the outer
surface of the lump can be attacked by the water in the shoot during its
descent, carbide for a hand-fed carbide-to-water generator should be in
fairly large masses--granulated material being wholly unsuitable--and
this quite apart from the fact that large carbide is superior to small in
gas-making capacity, inasmuch as it has not suffered the inevitable
slight deterioration while being crushed and graded to size. If carbide
is dropped too rapidly into such a generator which is not provided with a
false bottom or grid for the lumps to rest upon, the solid is apt to
descend among a mass of thick lime sludge produced at a former operation,
which lies at the bottom of the decomposing chamber; and here it may be
protected from the cooling action of fresh water to such an extent that
its surface is baked or coated with a hard layer of lime, while
overheating to a degree far exceeding the boiling-point of water may
occur locally. When, however, it falls upon a grid placed some distance
above the bottom of the water vessel, the various convection currents set
up as parts of the liquid become warm, and the mechanical agitations
produced by the upward current of gas rinse the spent lime from the
carbide, and entirely prevent overheating, unless the lumps are
excessively large in size.
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