visually to such as follow them, that certain explosives, especially
those which are slow-burning like black powder, or produce high
temperature in connection with comparative slow burning, will ignite
mixtures of gas and air, or mixtures of coal dust and air, and cause
explosions. The results point out clearly to all concerned, the danger
of using such explosives. The remedy is also made available by the
announcement of the names of a large number of explosives now on the
market at reasonable cost, which will not cause explosions under these
conditions. It is believed that when permissible explosives are
generally adopted in coal mines, this source of danger will have been
greatly minimized.

_Explosives Investigations._--Questions have arisen on the part of
miners or of mine operators as to the greater cost in using permissible
explosives due to their expense, which is slightly in excess of that of
other explosives; as to their greater shattering effect in breaking down
the coal, and in giving a smaller percentage of lump and a larger
percentage of slack; and as to the possible danger of breathing the
gases produced.

Observations made in mines by Mr. J. J. Rutledge, an experienced coal
miner and careful mining engineer connected with the Geological Survey,
as to the amount of coal obtained by the use of permissible and other
explosives, tend to indicate that the permissible explosives are not
more, but perhaps less expensive than others, in view of the fact that,
because of their greater relative power, a smaller quantity is required
to do the work than is the case, say, with black powder. On the other
hand, for safety and for certainty of detonation, stronger detonators
are recommended for use with permissible explosives, preferably electric
detonators. These may cost a few cents more per blast than the squib or