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