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
III. The treatment is of two kinds, or of a combination of both. In one
process the lumps are coated with a strong solution of glucose, with the
object of assisting in the removal of spent lime from their surface when
the carbide is immersed in water. Lime is comparatively much more soluble
in solutions of sugar (to which class of substances glucose belongs) than
in plain water; so that carbide treated with glucose is not so likely to
be covered with a closely adherent skin of spent lime when decomposed by
the addition of water to it. In the other process, the carbide is coated
with or immersed in some oil or grease to protect it from premature
decomposition. The latter idea, at least, fulfils its promises, and does
keep the carbide to a large extent unchanged if the lumps are exposed to
damp air, while solving certain troubles otherwise met with in some
generators (cf. Chapter III.
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