There is, nevertheless, one modification of calcium carbide which, in a
small but important sphere, finds a useful _role_. It has been
pointed out that a carbide containing much calcium phosphide is usually
objectionable, because the gas evolved from it requires extra
purification, and because there is the (somewhat unlikely) possibility
that the acetylene obtained from such material before purification may be
spontaneously inflammable. If, now, to the usual furnace charge of lime
and coke a sufficient quantity of calcium phosphate is purposely added,
it is possible to win a mixture of calcium phosphide and carbide, or, as
Bradley, Read, and Jacobs call it, a "carbophosphide of calcium," having
the formula Ca_5C_6P_2, which yields a spontaneously inflammable mixture
of acetylene, gaseous phosphine, and liquid phosphine when treated with
water, and which, therefore, automatically gives a flame when brought
into contact with the liquid. The value of this material will be
described in Chapter XIII.
GAS-ENRICHING.--Other methods of diluting acetylene consist in adding a
comparatively small proportion of it to some other gas, and may be
considered rather as processes for enriching that other gas with
acetylene.
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