washing, and it will be well to describe these
operations in the above order.

~Nitration.~--The most essential condition of nitrating is the correct
composition and strength of the mixed acids. The best proportions have
been found to be three parts by weight of nitric acid of a specific
gravity 1.525 to 1.530, and containing as small a portion of the oxides of
nitrogen as possible, to five parts by weight of sulphuric acid of a
specific gravity of 1.840 at 15 deg. C., and about 97 per cent. of mono-
hydrate. It is of the very greatest importance that the nitric acid should
be as strong as possible. Nothing under a gravity of 1.52 should ever be
used even to mix with stronger acid, and the nitration will be
proportional to the strength of the acid used, provided the sulphuric acid
is also strong enough. It is also of great importance that the oxides of
nitrogen should be low, and that they should be kept down to as low as 1
per cent., or even lower. It is also very desirable that the nitric acid
should contain as little chlorine as possible. The following is the
analysis of a sample of nitric acid, which gave very good results upon the
commercial scale:--Specific gravity, 1.525, N_{2}O_{4}, 1.03 per cent.;
nitric acid (HNO_{3}), 95.58 per cent.

The amount of real nitric acid (mono-hydrate) and the amount of nitric
peroxide present in any sample should always be determined before it is
used for nitrating purposes. The specific gravity is not a sufficient
guide to the strength of the acid, as an acid having a high gravity, due
to some 3 or 4 per cent of nitric oxides in solution, will give very poor
nitration results. A tenth normal solution of sodium hydroxide (NaOH),
with phenol-phthalein as indicator, will be found the most convenient
method of determining the total acid present. The following method will be
found to be very rapid and reliable:--Weigh a 100 c.c. flask, containing a
few cubic centimetres of distilled water, and then add from a pipette 1
c.c. of the nitric acid to be examined, and reweigh (this gives the weight
of acid taken). Now make up to 100 c.c. at 15 deg. C.; shake well, and take
out 10 c.c. with a pipette; drain into a small Erlenmeyer flask, and add a
little of the phenol-phthalein solution, and titrate with the tenth normal
soda solution.

The nitric peroxide can be determined with a solution of potassium
permanganate of N/10 strength, thus: Take a small conical flask,
containing about 10 c.c