space, across which heat must pass by radiation. It is by radiation
ultimately, therefore, that all bodies get heated. This being so, it is
well to increase the radiating power of flame as much as possible. Now,
radiating power depends on two things: the presence of solid matter in the
flame in a fine state of subdivision, and the temperature to which it is
heated. Solid matter is most easily provided by burning a gas rich in dense
hydrocarbons, not a poor and non-luminous gas. To mix the gas with air so
as to destroy and burn up these hydrocarbons seems therefore to be a
retrograde step, useful undoubtedly in certain cases, as in the Bunsen
flame of the laboratory, but not the ideal method of combustion. The ideal
method looks to the use of a very rich gas, and the burning of it with a
maximum of luminosity. The hot products of combustion must give up their
heat by contact. It is for them that cross tubes in boilers are useful.
They have no combustion to be interfered with by cold contacts. The _flame_
only should be free.

The second condition of radiation was high temperature. What limits the
temperature of a flame? Dissociation or splitting up of a compound by heat.
So soon as the temperature reaches 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