lightly
soluble in water. The sodium, ammonium, and barium salts are, however,
easily soluble in water. Picric acid, when heated, burns with a luminous
and smoky flame, and may be burnt away in large quantity without
explosion; but the mere contact of certain metallic oxides, with picric
acid, in the presence of heat, develops powerful explosives, which are
capable of acting as detonators to an indefinite amount of the acid, wet
or dry, which is within reach of their detonative influence. The formula
of picric acid is

C_{6}H_{2}|(NO_{2})_{3}
|OH.

which shows its formation from phenol (C_{6}H_{5}OH.), three hydrogen
atoms being displaced by the NO_{2} group. The equation of its formation
from phenol is as follows:--

C_{6}H_{5}.OH + 3HNO_{3} = C_{6}H_{2}(NO_{2})_{3}OH + 3H_{2}O.

According to Berthelot, its heat of formation from its elements equals
49.1 calories, and its heat of total combustion by free oxygen is equal to
+618.4 cals. It hardly contains more than half the oxygen necessary for
its complete combustion.

2C_{6}H_{2}(NO_{2})_{3}OH + O_{10} = 12CO_{2} + 3H_{2} + 3N_{2}.

The percentage composition of picric acid is--Nitrogen, 18.34; oxygen,
49.22; hydrogen, 1.00; and carbon, 31.44, equal to 60.26 per cent. of
NO_{2}. The products of decomposition are carbonic acid, carbonic oxide,
carbon, hydrogen, and nitrogen, and the heat liberated, according to
Berthelot, would be 130.6 cals., or 570 cals. per kilogramme. The reduced
volume of the gases would be 190 litres per equivalent, or 829 litres per
kilogramme. To obtain a total combustion of picric acid it is necessary
to mix with it an oxidising agent, such as a nitrate, chlorate, &c. It has
been proposed to mix picric acid (10 parts) with sodium nitrate (10 parts)
and potassium bichromate (8.3 parts). These proportions would furnish a
third of oxygen in excess of the necessary proportion.

Picric acid was not considered to be an explosive, properly so called, for
a long time after its discovery, but the disastrous accident which
occurred at Manchester (_vide_ Gov. Rep. No. LXXXI., by Colonel (now Sir
V.D.) Majendie, C.B.), and some experiments made by Dr Dupre and Colonel
Majendie to ascertain the cause of the accident, conclusively proved that
this view was wrong. The experiments of Berthelot (_Bull. de la Soc. Chim.
de Paris_, xlix., p. 456) on the explosive decomposition of picric acid
are also deserving of attention in this connection. I