hes the dissociation point at which the
compound can no longer exist, combustion ceases. Anything short of this may
theoretically be obtained.

But Mr. Siemens believes, and adduces some evidence to prove, that the
dissociation point is not a constant and definite temperature for a given
compound; it depends entirely upon whether solid or foreign surfaces are
present or not. These it is which appear to be an efficient cause of
dissociation, and which, therefore, limit the temperature of flame. In the
absence of all solid contact, Mr. Siemens believes that dissociation, if it
occur at all, occurs at an enormously higher temperature, and that the
temperature of free flame can be raised to almost any extent. Whether this
be so or not, his radiating flames are most successful, and the fact that
large quantities of steel are now melted by mere flame radiation speaks
well for the correctness of the theory upon which his practice has been
based.

_Use of Small Coal_.--Meanwhile, we may just consider how we ought to deal
with solid fuel, whether for the purpose of making gas from it or for
burning it _in situ_. The question arises, In what form ought solid fuel to
be--ought it to be in lumps or in powder? Universal practice says lumps,
but some theoretical considerations would have suggested powder. Remember,
combustion is a chemical action, and when a chemist wishes to act on a
solid easily, he always pulverizes it as a first step.

Is it not possible that compacting small coal into lumps is a wrong
operation, and that we ought rather to think of breaking big coal down into
slack? The idea was suggested to me by Sir W. Thomson in a chance
conversation, and it struck me at once as a brilliant one. The amount of
coal wasted by being in the form of slack is very great. Thousands of tons
are never raised from the pits because the price is too low to pay for the
raising--in some places it is only 1s. 6d. a ton. Mr. McMillan calculates
that 130,000 tons of breeze, or powdered coke, is produced every year by
the Gas Light and Coke Company alone, and its price is 3s. a ton at the
works, or 5s. delivered.

The low price and refuse character of small coal is, of course, owing to
the fact that no ordinary furnace can burn it. But picture to yourself a
blast of hot air into which powdered coal is sifted from above like ground
coffee, or like chaff in a thrashing mill, and see how rapidly and
completely it might burn. Fine dust in a flo