frequently observed. An electric alarm thermometer is also a useful
adjunct to the cotton drying house. Great care must also be taken that
there are no exposed hot-water pipes or stoves in the drying house, as the
fine gun-cotton dust produced by the turning or moving of the material
upon the shelves would settle upon such pipes or stoves, and becoming hot,
would be very sensitive to the least friction. The floor also should be
covered with linoleum or indiarubber. When hot currents of air are made to
pass over the surface of gun-cotton, the gun-cotton becomes electrified.
It is important, therefore, to provide some means to carry it away. Mr
W.F. Reid, F.I.C., was the first to use metal frames, carriers, and
sieves, upon which is secured the cloth holding the gun-cotton, and to
earth them.

The compression of gun-cotton into blocks, discs, &c., is also attended
with considerable risk. Mr O. Guttmann, in an interesting paper upon "The
Dangers in the Manufacture of Explosives" (_Jour. Soc. Chem. Ind._, No. 3,
vol. xi., 1892), says: "The compression of gun-cotton into cartridges
requires far more care than that of gunpowder, as this is done in a warm
state, and gun-cotton even when cold, is more sensitive than gunpowder.
When coming out of the centrifugal machines, the gun-cotton should always
pass first through a sieve, in order to detect nails or matches which may
by chance have got into it. What has been said as to gunpowder presses
applies still more to those for gun-cotton, although the latter are always
hydraulic presses. Generally the pistons fit the mould perfectly, that is
to say, they make aspiration like the piston of a pump. But there is no
metal as yet known which for any length of time will stand the constant
friction of compression, and after some time the mould will be wider in
that part where the greatest compression takes place. The best metal for
this purpose has proved to be a special steel made by Krupp, but this also
is only relatively better; for pistons I prefer hard cast iron. If the