,
throughout the extensive distributing mains. The same loss of gas by
leakage would represent a far higher pecuniary value with acetylene than
with coal-gas, because the former must always be more costly per unit of
volume than the latter. Hence it is important to recognise that the rate
of leakage, _coeteris paribus_, is less with acetylene, and it is
also important to observe the economical advantage, at least in terms of
gas or calcium carbide, of sending the acetylene into the mains at as low
a pressure as is compatible with the length of those mains and the
character of the consumers' burners. As follows from what will be said in
Chapter VII., a high initial pressure makes for economy in the prime cost
of, and in the expense of laying, the mains, by enabling the diameter of
those mains to be diminished; but the purchase and erection of the
distributing system are capital expenses, while a constant expenditure
upon carbide to meet loss by leakage falls upon revenue.
The critical temperature of acetylene, _i.e._, the temperature below
which an abrupt change from the gaseous to the liquid state takes place
if the pressure is sufficiently high, is 37 deg.
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