which signifies that the loss of acetylene by leakage under the same conditions of pressure, &c., will be only 0.663 times that of the loss of coal-gas. In practice, however, the pressures at which the gases are usually sent through mains are not identical, being greater in the case of acetylene than in that of coal-gas. Formula (1) therefore requires correction whenever the pressures are different, and calling the pressure at which the acetylene exists in the main _p_, and the corresponding pressure of the coal-gas _p'_, the relative losses by leakage are-- (2) (_v_/_v'_) = (0.40 / 0.91)^(1/2) x (_p_/_p'_)^(1/2) _v_ = 0.663_v'_ x (_p_/_p'_)^(1/2) It will be evident that whenever the value of the fraction (_p_/_p'_)^(1/2), is less than 1.5, _i.e._, whenever the pressure of the acetylene does not exceed double that of the coal-gas present in pipes of given porosity or unsoundness, the loss of acetylene will be less than that of coal-gas. This is important, especially in the case of large village acetylene installations, where after a time it would be impossible to avoid some imperfect joints, fractured pipes, &c., 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, _cœteris 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° C., and the critical pressure, _i.e._, the pressure under which that change takes place at that temperature, is nearly 68 atmospheres. Below the critical temperature, a lower pressure than this effects liquefaction of the gas, _i.e._, at 13.5° C. a pressure of 32.77 atmospheres, at 0° C., 21.53 atmospheres (Ansdell, _cf._ Chapter XI.). These data are of comparatively little practical importance, owing to the fact that, as explained in Chapter XI., liquefied acetylene cannot be safely utilised.