acid being then added to the equivalent amount of nitrate of soda. Nitric
acid is then distilled over and condensed in the usual way. Very often,
however, the waste acid is added direct to the charge of nitrate without
previously eliminating the nitric acid. The treatment of the waste acid
from the manufacture of nitro-glycerine is somewhat different. The small
amount of nitro-glycerine in this acid must always be eliminated. This is
effected either by allowing the waste acid to stand for at least twenty-
four hours in a big vessel with a conical top, where all the nitro-
glycerine which will have separated to the surface is removed by skimming;
or, better still, the "watering down process" of Col. Nathan may be
employed. In Nathan's nitrator every existing trace of nitro-glycerine is
separated from the acids in a few hours after the nitration, and any
further formation of nitro-glycerine is prevented by adding about 2 per
cent. of water to the waste acids, which are kept agitated during the
addition. The waste acid, now free from nitro-glycerine, but which may
still contain organic matter, is denitrated by bringing it into contact
with a jet of steam. The waste acid is passed in a small stream down
through a tower of acid-resisting stoneware (volvic stone), which is
closely packed with earthenware, and at the bottom of which is the steam
jet. Decomposition proceeds as the acid meets the steam, nitric and
nitrous acids are disengaged and are passed out at the top of the tower
through a pipe to a series of condensers and towers, where the nitric acid
is collected. The nitrous acid may be converted into nitric acid by
introducing a hot compressed air jet into the gases before they pass into
the condensers. Weak sulphuric acid of sp. gr. 1.6 collects in a saucer in
which the tower stands, and is then passed through a cooling worm. The
weak sulphuric acid, now entirely free from nitric and nitrous acids, may
be concentrated to sp. gr. 1.842 and 96 per cent. H_{2}SO_{4} by any of
the well-known processes, e.g., Kessler, Webb, Benker, Delplace, &c., and
it may be used again in the manufacture of nitro-glycerine or gun-cotton.