s_) was chiefly used. One part of phenol (carbolic
acid), C_{6}H_{5}OH, is added to 3 parts of strong fuming nitric acid,
slightly warmed, and when the violence of the reaction has subsided,
boiled till nitrous fumes are no longer evolved. The resinous mass thus
produced is boiled with water, the resulting picric acid is converted into
a sodium salt by a solution of sodium carbonate, which throws down sodium
picrate in crystals.

Phenol-sulphuric acid is now, however, more generally used, and the
apparatus employed for producing it closely resembles that used in making
nitro-benzol. It is also made commercially by melting carbolic acid, and
mixing it with strong sulphuric acid, then diluting the "sulpho-
carbolic"[A] acid with water, and afterwards running it slowly into a
stone tank containing nitric acid. This is allowed to cool, where the
crude picric acid crystallises out, and the acid liquid (which contains
practically no picric acid, but only sulphuric acid, with some nitric
acid) being poured down the drains. The crude picric acid is then
dissolved in water by the aid of steam, and allowed to cool when most of
the picric acid recrystallises. The mother liquor is transferred to a tank
and treated with sulphuric acid, when a further crop of picric acid
crystals is obtained. The crystals of picric acid are further purified by
recrystallisation, drained, and dried at 100 deg. F. on glazed earthenware
trays by the aid of steam. It can also be obtained by the action of nitric
acid on ortho-nitro-phenol, para-nitro-phenol, and di-nitro-phenol (2:4
and 2:6), but not from meta-nitro-phenol, a fact which indicates its
constitution.[B]

[Footnote A: O. and p. phenolsulphonic acids.

C_{3}H_{4}(OH).SO_{3}H + 3HNO_{3} = C_{6}H_{2}(NO_{2})_{3}OH + H_{2}SO_{4}
+ 2H_{2}O. (Picric acid).]

[Footnote B: Carey Lea, _Amer. Jour. Sci._, (ii.), xxxii. 180.]

Picric acid crystallises in yellow shining prisms or laminae having an
intensely bitter taste, and is poisonous. It melts at 122.5 deg. C., sublimes
when cautiously heated, dissolves sparingly in cold water, more easily in
hot water, still more in alcohol. It stains the skin an intense yellow
colour, and is used as a dye for wool and silk. It is a strong acid,
forming well crystallised yellow salts, which detonate violently when
heated, some of them also by percussion. The potassium salt,
C_{6}H_{2}(NO_{2})_{3}OK, crystallises in long needles very s