The Chemistry of Powder and Explosives - Sciencemadness Dot Org

348 DYNAMITE AND OTHER HIGH EXPLOSIVES
sive mixtures of methane and air would always be exploded by
them if it were not for the circumstance, discovered by Mallard
and Le Chatelier, 23 that there is a certain delay or period of
induction before the gaseous mixture actually explodes. At 650°
this amounts to about 10 seconds, at 1000° to about 1 second, and
at 2200° there is no appreciable delay and the explosion is presumed
to follow instantaneously after the application of this temperature
however momentary. Mallard and Le Chatelier concluded
that an explosive having a temperature of explosion of 2200° or
higher would invariably ignite fire damp. The French commission
which was studying these questions at first decided that the
essential characteristic of a permissible explosive should be that
its calculated temperature of explosion should be not greater than
2200°, and later designated a temperature of 1500° as the maximum
for explosives permissible in coal seams and 1900° for those
intended to be used in the accompanying rock.
The flame which is produced by the explosion of a brisant
explosive is of extremely short duration, and its high temperature
continues only for a small fraction of a second, for the hot gases
by expanding and by doing work immediately commence to cool
themselves. If they are produced in the first place at a temperature
below that of the instantaneous inflammation of fire damp,
they may be cooled to such an extent that they are not sufficiently
warm for a sufHciently long time to ignite fire damp at
all. Black powder, burning slowly, always ignites explosive gas
mixtures. But any high explosive may be made safe for use in
gaseous mines by the addition to it of materials which reduce the
initial temperature of the products of its explosion. Or, in cases
where this initial temperature is not too high, the same safety
may be secured by limiting the size of the charge and by firing
the shot in a well-tamped bore hole under such conditions that
the gases are obliged to do more mechanical work and are cooled
the more in consequence.
Permissible explosives may be divided into two principal
classes: (1) those which are and (2) those which are not based
upon a high explosive which is cool in itself, such as ammonium
nitrate, or guanidine nitrate, or nitroguanidine. The second class
may be subdivided further, according to composition, into as
Ann. Ik, [8] 11, 274 (1887).