lity where the shop is situated and the nature
and quantity of the work to be done. The furnaces can be heated with
producer gas in most cases, but when space is of value illuminating
gas from a separate source of supply has some compensations. When
the latter is used it is well to install a governor if the pressure
is likely to fluctuate, particularly where the shop is at a high
altitude or at a long distance from the gas supply.

Many furnaces are coal-fired, and although greater care is required
in maintaining a uniform temperature good results have been obtained.
The use of electricity as a means of reaching the requisite temperature
is receiving some attention, and no doubt it would make the control
of temperature comparatively simple. However, the cost when applied
to large quantities of work will, for the present at least, prevent
this method from becoming popular. It is believed that the results
obtainable \with the electric furnace would surpass any others; but
the apparatus is expensive, and unless handled with intelligence
would not last long.

The most elementary medium of carburization is pure carbon, but
the rate of carburization induced by this material is very low,
and other components are necessary to accelerate the process. Many
mixtures have been marketed, each possessing its individual merits,
and as the prices vary considerably it is difficult to decide which
is the most advantageous.

Absorption from actual contact with solid carbon is decidedly slow,
and it is necessary to employ a compound from which gases are liberated,
and the steel will absorb the carbon from the gases much more readily.

Both bone and leather charcoal give off more carburizing gases
than wood charcoal, and although the high sulphur content of the
leather is objectionable as being injurious to the steel, as also
is the high phosphorus content of the bone charcoal, they are both
preferable to the wood charcoal.

By mixing bone charcoal with barium carbonate in the proportions
of 60 per cent of the former to 40 per cent of the latter a very
reliable compound is obtained.

The temperature to which this compound is subjected causes the
liberation of carbon monoxide when in contact with hot charcoal.

Many more elaborate explanations may be given of the actions and
reactions taking place, but the above is a satisfactory guide to
indicate that it is not the actual compound which causes carburization,
but the gases released