Cedercreutz's figures, given in the above table, show that it would be
very unfair to determine the gas-making capacity of a given parcel of
carbide in which the lumps happened to vary considerably in size by
analysing only the smalls, results so obtained being possibly 15 per
cent. too low. This is due to two causes: first, however carefully it be
stored, carbide deteriorates somewhat by the attack of atmospheric
moisture; and since the superficies of a lump (where the attack occurs)
is larger in proportion to the weight of the lump as the lump itself is
smaller, small lumps deteriorate more on keeping than large ones. The
second reason, however, is more important. Not being a pure chemical
substance, the commercial material calcium carbide varies in hardness;
and when it is merely crushed (not reduced altogether to powder) the
softer portions tend to fall into smaller fragments than the hard
portions. As the hard portions are different in composition from the soft
portions, if a parcel is sampled by taking only the smalls, practically
that sample contains an excess of the softer part of the original
material, and as such is not representative. Originally the German
Acetylene Association did not lay down any rules as to the crushing of
samples by the analyst, but subsequently they specified that the material
should be tested in the size (or sizes) in which it was received.
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