Thus a second quantity of heat is developed, equal by theory to
that previously evolved; but a second elevation in temperature, far more
serious, and far less under control, than the former also occurs; and
this is easily sufficient to determine some of those undesirable effects
already described. Digressing for a moment, it may be admitted that the
desiccation of the acetylene produced in this manner is beneficial, even
necessary; but the advantages of drying the gas at this period of its
treatment are outweighed by the concomitant disadvantages and by the
later inevitable remoistening thereof. Suppose now (2) that both the
water inlet and the gas exit of the carbide cylinder are at the same end,
A. Again half the added water, as liquid, reacts with the carbide it
first encounters, but the hot stream of damp gas is not permitted to
travel over the rest of the lumps extending towards B: it is forced to
return upon its steps, leaving B practically untouched. The gas
accordingly escapes from the cylinder at A still loaded with water
vapour, and for a given weight of water introduced much less acetylene is
evolved than in the former case. The gas, too, needs drying somewhere
else in the plant; but these defects are preferable to the apparent
superiority of the first process because overheating is, or can be, more
thoroughly guarded against.
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