Hitherto, for the sake of simplicity, the by-product in the preparation of acetylene has been described as calcium oxide or quicklime. It is, however, one of the leading characteristics of this body to be hygroscopic, or greedy of moisture; so that if it is brought into the presence of water, either in the form of liquid or as vapour, it immediately combines therewith to yield calcium hydroxide, or slaked lime, whose chemical formula is Ca(OH)_2. Accordingly, in actual practice, when calcium carbide is mixed with an excess of water, a secondary reaction takes place over and above that indicated by equation (1), the quicklime produced combining with one chemical part or molecule of water, thus-- CaO + H_2O = Ca(OH)_2. As these two actions occur simultaneously, it is more usual, and more in agreement with the phenomena of an acetylene generator, to represent the decomposition of calcium carbide by the combined equation-- (2) CaC_2 + 2H_2O = C_2H_2 + Ca(OH)_2. By the aid of calculations analogous to those employed in the preceding paragraph, it will be noticed that equation (2) states that 1 molecule of calcium carbide, or 64 parts by weight, combines with 2 molecules of water, or 36 parts by weight, to yield 1 molecule, or 26 parts by weight of acetylene, and 1 molecule, or 74 parts by weight of calcium hydroxide (slaked lime). Here again, if more than 36 parts of water are taken for every 64 parts of calcium carbide, the excess of water over those 36 parts is left undecomposed; and in the same fashion, if less than 36 parts of water are taken for every 64 parts of calcium carbide, some of the latter must remain unattacked, whilst, obviously, the amount of acetylene liberated cannot exceed that which corresponds with the quantity of substance suffering complete decomposition. If, for example, the quantity of water present in a generator is more than chemically sufficient to attack all the carbide added, however largo or small that excess may be, no more, and, theoretically speaking, no less, acetylene can ever be evolved than 26 parts by weight of gas for every 64 parts by weight of calcium carbide consumed. It is, however, not correct to invert the proposition, and to say that if the carbide is in excess of the water added, no more, and, theoretically speaking, no less, acetylene can ever be evolved than 26 parts by weight of gas for every 36 parts of water consumed, as might be gathered from equation (2); because equation (1) shows that 26 parts of acetylene may, on occasion, be produced by the decomposition of 18 parts by weight of water. From the purely chemical point of view this apparent anomaly is explained by the circumstance that of the 36 parts of water present on the left-hand aide of equation (2), only one-half, _i.e._, 18 parts by weight, are actually decomposed into hydrogen and oxygen, the other 18 parts remaining unattacked, and merely attaching themselves as "water of hydration" to the 56 parts of