ation near Wigan in 1895, when 8 oz. or
12 oz. charges were fired unstemmed into an admixture of coal dust and 10
per cent. of gas, without any ignition taking place. It is manufactured by
Messrs Curtis's & Harvey Ltd. at their factory, Tonbridge, Kent.

~Sprengel's Explosives.~--This is a large class of explosives. The
essential principle of them all is the admixture of an oxidising with a
combustible agent at the time of, or just before, being required for use,
the constituents of the mixture being very often non-explosive bodies.
This type of explosive is due to the late Dr Herman Sprengel, F.R.S.
Following up the idea that an explosion is a sudden combustion, he
submitted a variety of mixtures of oxidising and combustible agents to the
violent shock of a detonator of fulminate. These mixtures were made in
such proportions that the mutual oxidation or de-oxidation should be
theoretically complete. Among them are the following:--

1. One chemical equivalent of nitro-benzene to equivalents of nitric acid.

2. Five equivalents of picric acid to 13 equivalents of nitric acid.

3. Eighty-seven equivalents of nitro-naphthalene to 413 equivalents of
nitric acid.

4. Porous cakes, or lumps of chlorate of potash, exploded violently with
bisulphide of carbon, nitro-benzol, carbonic acid, sulphur, benzene, and
mixtures of these substances.

No. 1 covers the explosive known as _Hellhoffite_, and No. 2 is really
oxonite, and No. 4 resembles rack-a-rock, an explosive invented by Mr S.R.
Divine, and consisting of a mixture of chlorate of potash and nitro-
benzol. Roburite, bellite, and securite should perhaps be regarded as
belonging to the Sprengel class of explosives, otherwise this class is not
manufactured or used in England. The principal members are known as
_Hellhoffite_, consisting of a mixture of nitro-petroleum or nitro-tar
oils and nitric acid, or of meta-di-nitro-benzol and nitric acid;
_Oxonite_, consisting of picric and nitric acids; and _Panclastite_, a
name given to various mixtures, proposed by M. Turpin, such as liquid
nitric peroxide, with bisulphide of carbon, benzol, petroleum, ether, or
mineral oils.

~Picric Acid, Tri-nitro-Phenol, or Carbazotic Acid.~--Picric acid, or a
tri-nitro-phenol (C_{6}H_{2}(NO_{2})_{3}OH)[2:4:6], is produced by the
action of nitric acid on many organic substances, such as phenol, indigo,
wool, aniline, resins, &c. At one time a yellow gum from Botany Bay
(_Xanthorrhoea hastili