. of water, and add from a burette 10 to 16 c.c. of
the permanganate solution; then add 2 c.c. of the acid to be tested, and
shake gently, and continue to add permanganate solution as long as it is
decolourised, and until a faint pink colour is permanent.

_Example._ N/10 permanganate 3.16 grms. per litre, 1 c.c. = O.0046 grm.
N_{2}O_{4}, 2 c.c. of sample of acid specific gravity 1.52 = 3.04 grms.
taken for analysis. Took 20 c.c. permanganate solution, O.0046 x 20 =.092
grm. N_{2}O_{4}, and (.092 x 100)/3.04 = 3.02 per cent. N_{2}O_{4}. The
specific gravity should be taken with an hydrometer that gives the
specific gravity directly, or, if preferred, the 2 c.c. of acid may be
weighed.

A very good method of rapidly determining the strength of the sulphuric
acid is as follows:--Weigh out in a small weighing bottle, as nearly as
possible, 2.45 grms. This is best done by running in 1.33 c.c. of the acid
(1.33 x 1.84 = 2.447). Wash into a large Erlenmeyer flask, carefully
washing out the bottle, and also the stopper, &c. Add a drop of phenol-
phthalein solution and titrate, with a half normal solution of sodium
hydrate (use a 100 c.c. burette). Then if 2.45 grms. exactly have been
taken, the readings on the burette will equal percentages of H_{2}SO_{4}
(mono-hydrate) if not, calculate thus:--2.444 grms. weighed, required 95.4
c.c. NaOH. Then--

2.444 : 95.4 :: 2.45 : _x_ = 95.64 per cent. H_{2}SO_{4}.

It has been proposed to free nitric acid from the oxides of nitrogen by
blowing compressed air through it, and thus driving the gases in solution
out. The acid was contained in a closed lead tank, from which the escaping
fumes were conducted into the chimney shaft, and on the bottom of which
was a lead pipe, bent in the form of a circle, and pierced with holes,
through which the compressed air was made to pass; but the process was not
found to be of a very satisfactory nature, and it is certainly better not
to allow the formation of these compounds in the manufacture of the acid
in the first instance. Another plan, however, is to heat the acid gently,
and thus drive out the nitrous gases. Both processes involve loss of
nitric acid.

Having obtained nitric and sulphuric acids as pure as possible, the next
operation is to mix them. This is best done by weighing the carboys in
which the acids are generally stored before the acids are drawn off into
them from the condensers, and keeping their weights constantly attached to
t