, and to 50 deg. only under exceptional
circumstances.

The tank in which the jelly is made is double-lined, in order to allow of
the passage of hot water between its inner and outer linings. A series of
such tanks are generally built in a wooden framework, and the double
linings are made to communicate, so that the hot water can flow from one
to the other consecutively. The temperature of the water should be about
60 deg. C. if it is intended to gelatinise at 45 deg. C., and about 80 deg. if at
50 deg. C.; but this point must, of course, be found by experiment for the
particular plant used. An arrangement should be made to enable the workman
to at once cut off the supply of hot water and pass cold water through the
tanks in case the explosive becomes too hot.

[Illustration: FIG. 31.--MR M'ROBERTS' MIXER FOR GELATINE EXPLOSIVES.]

The best way to keep the temperature of the water constant is to have a
large tank of water raised upon a platform, some 5 or 6 feet high, outside
the building, which is automatically supplied with water, and into which
steam is turned. A thermometer stuck through a piece of cork and floated
upon the surface of the tank will give the means of regulating the
temperature.

When the jelly in the tanks has become semi-transparent and the cotton has
entirely dissolved, the mixture should be transferred to the mixing
machine. The mixing machines are specially designed for this work, and are
built in iron, with steel or bronze kneading- and mixing-blades, according
to requirements.

A suitable machine for the purpose is that known as the Nito-Universal
Incorporator, shown in Fig. 30, which has been specially constructed by
Messrs Werner, Pfleiderer, & Perkins, Ltd., after many years' experience
in the mixing of explosive materials, and is now almost exclusively
adopted in both Government and private factories. Mr George M'Roberts'[A]
mixing machine, however, which is shown in Fig. 31, is still used in some
factories for dynamite jelly.

[Footnote A: See _Jour. Soc. Chem. Ind._, 1890, 267.]

If it is intended to make gelignite, or gelatine dynamite, it is at this
point that the proper proportions of wood-pulp[A] and potassium nitrate
should be added, and the whole well mixed for at least half an hour, until
the various ingredients are thoroughly incorporated.

[Footnote A: Most of the wood-pulp used in England is obtained from
pine-trees, but poplar, lime, birch, and beech wood are also used.