extent), by what is called "depression in trade." In fact, it has in many
places reached so low a commercial value that it is profitable to burn it
as a fuel. Happily, this is not the case at Nottingham; and our interest in
tar as a fuel is more experimental, in view of what may happen if a further
fall in tar products sets in. I have abandoned the use of steam injection
for our experimental tar fires in favor of another system. The steam
injectors produce excellent heats, but are rather intermittent in their
action, and the steam they require is a serious item, and not always
available.

[Illustration]

Tar being a _pseudo_ liquid fuel, in arranging for its combustion one has
to provide for the 20 to 25 per cent. of solid carbon which it contains,
and which is deposited in the furnace as a kind of coke or breeze on the
distillation of the volatile portions, which are much more easily consumed
than the tar coke.


THE TAR FIRE

I have adopted is one that can be readily adapted to an ordinary coke
furnace, and be as readily removed, leaving the furnace as before. The
diagram conveys some idea of the method adopted. An iron frame, d, standing
on legs on the floor just in front of the furnace door, carries three fire
tiles on iron bearers. The top one, a, is not moved, and serves to shield
the upper face of the tile, b, from the fierce heat radiated from the
furnace, and also causes the air that rushes into the furnace between the
tiles, a and b, to travel over the upper face of the tile, b, on which the
tar flows, thereby keeping it cool, and preventing the tar from bursting