is above or below
these limits it burns quietly with a pale blue flame.

Coal dust.

The danger arising from the presence of coal dust in the air of dry
mines, with or without the addition of fire-damp, has, since it was
first pointed out by Professor W. Galloway, been made the subject of
special inquiries in the principal European countries interested in coal
mining; and although certain points are still debatable, the fact is
generally admitted as one calling for special precautions. The
conclusions arrived at by the royal commission of 1891, which may be
taken as generally representative of the views of British colliery
engineers, are as follows:--

1. The danger of explosion when gas exists in very small quantities is
greatly increased by the presence of coal dust.

2. A gas explosion in a fiery mine may be intensified or indefinitely
propagated by the dust raised by the explosion itself.

3. Coal dust alone, without any gas, may cause a dangerous explosion
if ignited by a blown-out shot; but such cases are likely to be
exceptional.

4. The inflammability of coal dust varies with different coals, but
none can be said to be entirely free from risk.

5. There is no probability of a dangerous explosion being produced by
the ignition of coal dust by a naked light or ordinary flame.

Danger arising from coal dust is best guarded against by systematically
sprinkling or watering the main roads leading from the working faces to
the shaft, where the dust falling from the trams in transit is liable to
accumulate. This may be done by water-carts or hose and jet, but
preferably by finely divided water and compressed air distributed from a
network of pipes carried through the workings. This is now generally
done, and in some countries is compulsory, when the rocks are deficient
in natural moisture. In one instance the quantity of water required to
keep down the dust in a mine raising 850 tons of coal in a single shift
was 28.8 tons, apart from that required by the jets and motors. The
distributing network extended to more than 30 m. of pipes, varying from
3-1/2 in. to 1 in. in diameter.

Safety explosives.

In all British coal-mines, when gas in dangerous quantities has appeared
within three months, and in all places that are dry and dusty, blasting
is prohibited, except with "permitted" explosives, whose composition and
properties have been examined at the testing station at the Royal