The uncondensable gases still require a further purifica-
tion to remove the carbonic acid and any volatile sulphur
compounds. For this purpose they are passed through
perforated trays holding a layer of slaked lime some three
inches deep, the trays being enclosed in boxes with a mov-
able top, and built of f-inch steel plates. They are generally
constructed 10 feet square, with a depth of 4 feet. The
gases pass in at the bottom and ascend through the various
layers of lime, which absorb the impurities, finally passing out
at the top direct to the gasometer.
The volume of gas yielded is considerable, and in its use
for lighting the works, or for motive or heating power, it
plays a very important part in the successful working of a
concern. The bones passing into the retort have about 6 per
cent, of nitrogen. A portion of this goes to form the
complex bodies of the tar ; another portion is found in the
illuminating gases ; a further quantity is lost in actual work-
ing ; and the remainder goes to build up the various ammonia
compounds forming the ammoniacal liquor.
BONE PRODUCTS AND MANURES 27

In the manufacture of cyanide of potash nitrogen forms
the principal factor, and any cheap source for providing it
and making it available for this purpose would be welcomed.
