consumer for himself. A generator which is perfectly satisfactory in
general behaviour, and which evolves a sufficient proportion of the
possible total make of gas to be economical, does not of necessity
decompose the carbide quantitatively; nor is it constructed in a fashion
to render an exact measurement of the gas liberated at standard
temperature and pressure easy to obtain. For obvious reasons the careful
consumer of acetylene will keep a record of the carbide decomposed and of
the acetylene generated--the latter perhaps only in terms of burner-
hours, or the like; but in the event of serious dispute as to the gas-
making capacity of his raw material, he must have a proper analysis made
by a qualified chemist.

Calcium carbide is crushed by the makers into several different sizes, in
each of which all the lumps exceed a certain size and are smaller than
another size. It is necessary to find out by experiment, or from the
maker, what particular size suits the generator best, for different types
of apparatus require different sizes of carbide. Carbide cannot well be
crushed by the consumer of acetylene. It is a difficult operation, and
fraught with the production of dust which is harmful to the eyes and
throat, and if done in open vessels the carbide deteriorates in gas-
making power by its exposure to the moisture of the atmosphere. True dust
in carbide is objectionable, and practically useless for the generation
of acetylene in any form of apparatus, but carbide exceeding 1 inch in
mesh is usually sold to satisfy the suggestions of the British Acetylene
Association, which prescribes 5 per cent, of dust as the maximum. Some
grades of carbide are softer than others, and therefore tend to yield
more dust if exposed to a long journey with frequent unloadings.

There are certain varieties of ordinary carbide known as "treated
carbide," the value of which is more particularly discussed in Chapter