oform, dry, and weigh: then extract with absolute
methyl-alcohol, evaporate, weigh, and examine the nitro-cellulose in the
nitrometer.

~Picric Acid and Picrates.~--Picric acid is soluble in hot water, and to
the extent of 1 part in 100 in cold water, also in ether, chloroform,
glycerine, 10 per cent. soda solution, alcohol, amylic alcohol, carbon
bisulphide, benzene, and petroleum. If a solution of picric acid be boiled
with a strong solution of potassium cyanide, a deep red liquid is
produced, owing to the formation of potassium iso-purpurate, which
crystallises in small reddish-brown plates with a beetle-green lustre.
This, by reaction with ammonium chloride, gives ammonium iso-purpurate
(NH_{4}C_{8}H_{4}N_{5}O_{6}), or artificial murexide, which dies silk and
wool a beautiful red colour. On adding barium chloride to either of the
above salts, a vermilion-red precipitate was formed, consisting of barium
iso-purpurate. With ammonio-sulphate of copper, solutions of picric acid
give a bright green precipitate. Mr A.H. Allen gives the following methods
for the assay of commercial picric acid, in his "Commercial Organic
Analysis":--

~Resinous and Tarry matters~ are not unfrequently present. They are left
insoluble on dissolving the sample in boiling water. The separation is
more perfect if the hot solution be exactly neutralised by caustic soda.

~Sulphuric Acid, Hydrochloric Acid, and Oxalic Acid~, and their salts are
detected by adding to the filtered aqueous solution of the sample
solutions of the picrates of barium, silver, and calcium. These salts are
readily made by boiling picric acid with the carbonates of the respective
metals and filtering: other soluble salts of these methods may be
substituted for the picrates, but they are less satisfactory.

~Nitric Acid~ may be detected by the red fumes evolved on warming the
sample with copper turnings.

~Inorganic Impurities and Picrates of Potash and Sodium~, &c., leave
residues on cautious ignition.

~General Impurities and Adulterations~ may be detected and determined by
shaking 1 grm. of the sample of acid in a graduated tube with 25 c.c. of
ether, the pure acid dissolves, while any oxalic acid, nitrates, picrates,
boric acid, alum, sugar, &c., will be left insoluble, and after removal of
the ethereal liquid, may be readily identified and determined. For the
detection and determination of water and of oxalic acid, 50 c.c. of warm
benzene may be advantageously s