temperatures dynamite again becomes somewhat sensitive to shock. When
it is frozen at ordinary temperatures even the strongest detonating
caps fail to develop the full force. In thawing dynamite, care must be
exercised. The fact that a small quantity will often burn quietly has
led to the dangerously mistaken notion that mere heating will not
cause explosion. It is chiefly a question of temperature. If the
quantity ignited by flame be large enough to heat the entire mass to
the detonating point (say 360 deg. F.) before all is consumed, an
explosion will result. Furthermore, dynamite, when even moderately
heated, becomes extremely sensitive to shocks. There are several
accepted modes of thawing dynamite: (1) In a water bath, the
cartridges being placed in a vessel surrounded on the sides and bottom
by warm water contained in a larger enclosing vessel. The warm water
may be renewed from time to time, or the water bath placed over a
candle or small lamp, _not_ on a stove. (2) In two vessels, similar to
the above, with the space between them occupied by air, provided the
heat applied can be definitely limited, as by using a candle. (3) When
large quantities of dynamite are used a supply may be kept on shelves
in a wooden room or chamber, warmed by a stove, or by a coil of pipe
heated by exhaust steam from an engine. Live steam should not be used,
as the heat might become excessive. Thawing should always take place
slowly, never before an open fire or by direct contact with a stove or
steam pipes and care must be taken that the heat does not rise high
enough to cause sweating or exudation of liquid nitroglycerin from the
cartridges, which would be a source of danger.

For the storage of explosives at mines, &c., proper magazines must be
provided, situated in a safe place, not too near other buildings, and
preferably of light though fireproof construction. Masonry magazines,
though safer from some points of view, may be the cause of greater
damage in event of an explosion, because the brick or stones act as
projectiles. Isolated and abandoned mine workings, if dry, are
sometimes used as magazines.

[Illustration: FIG 11. Electrical Fuze.]

Firing blasts by electricity has a wide application for both surface
and underground work. An electrical fuze (fig. 11) consists of a pair
of fine, insulated copper wires, several feet long and about 1/40 of
an inch