calcium oxide in equation (1) so as to produce the 74 parts of calcium hydroxide appearing on the right-hand side of equation (2). The matter is perhaps rendered more intelligible by employing the old name for calcium hydroxide or slaked lime, viz., hydrated oxide of calcium, and by writing its formula in the corresponding form, when equation (2) becomes CaC_2 + 2H_2O = C_2H_2 + CaO.H_2O. It is, therefore, absolutely correct to state that if the amount of calcium carbide present in an acetylene generator is more than chemically sufficient to decompose all the water introduced, no more, and theoretically speaking no less, acetylene can ever be liberated than 26 parts by weight of gas for every 18 parts by weight of water attacked. This, it must be distinctly understood, is the condition of affairs obtaining in the ideal acetylene generator only; since, for reasons which will be immediately explained, when the output of gas is measured in terms of the water decomposed, in no commercial apparatus, and indeed in no generator which can be imagined fit for actual employment, does that output of gas ever approach the quantitative amount; but the volume of water used, if not actually disappearing, is always vastly in excess of the requirements of equation (2). On the contrary, when the make of gas is measured in terms of the calcium carbide consumed, the said make may, and frequently does, reach 80, 90, or even 99 per cent. of what is theoretically possible. Inasmuch as calcium carbide is the one costly ingredient in the manufacture of acetylene, so long as it is not wasted-- so long, that is to say, as nearly the theoretical yield of gas is obtained from it--an acetylene generator is satisfactory or efficient in this particular; and except for the matter of solubility discussed in the following chapter, the quantity of water consumed is of no importance whatever.