HOMOGENEOUS CARGOES

393

It is recommended that colliers and vessels carrying coal on long
passages should unship the side dunnage battens and so remove this
avoidable source of providing air pockets and the supply of oxygen
necessary for combustion.

All kinds of coal, even anthracite, are liable to spontaneous heating
and combustion, though some are more dangerous than others

All coal gives off inflammable gas when freshly worked or when
freshly broken, and the gas becomes explosive when mixed with certain
proportions of air *

Heating of coal does not proceed from the presence of gas, but is
caused by the absorption of oxygen from the air

This absorption and the accompanying development of heat is
greater at high than at low temperatures, so that when once commenced
it proceeds at an increasing rate if the supply of air is maintained.

Danger of over heating and spontaneous combustion increases with
the length of time the coal remains in the ship, 77° Fahr. being a
critical temperature.

There is no risk entailed by coal being wet when taken on board
It is not in any way more dangerous to carry than coal which is
perfectly dry.

When loading a cargo of coal the dunnage wood is stowed at the
ends of the holds and covered up to keep it clean and clear of the coal.

The first few tons of coal are lowered into the hold, instead of being
dropped from a height, and some planks laid over the ceiling in the
square of the hatch. Provision is made so that when loaded the temper-
ature of the cargo below the surface can be taken in different parts of the
hold. The trimming of the cargo should also be carefully superintended
and an efficient system of surface ventilation maintained.

Fig. 2.—Section of Ore Carrier.
Ores.—Iron, manga,nesea copper and other ores are also canied ia
bulk* but owing to the heavy nature of such cargoes a great amount