It is easy to show the effect of supplying a common gas flame with warm air by holding it over a cylinder packed with wire gauze which has been made red hot.  A common burner held over such a hot air shaft burns far more brightly and whitely.  There is no question but that this is the plan to get good illumination out of gas combustion; and many regenerative burners are now in the market, all depending on this principle, and utilizing the waste heat to make a high temperature flame.  But although it is evidently the right way to get light, it was by no means evidently the right way to get heat.  Yet so it turns out, not by warming solid objects or by dull warm surfaces, but by the brilliant radiation of the hottest flame that can be procured, will rooms be warmed in the future.  And if one wants to boil a kettle, it will be done, not by putting it into a non-luminous flame, and so interfering with the combustion, but by holding it near to a freely burning regenerated flame, and using the radiation only.  Making toast is the symbol of all the heating of the future, provided we regard Mr. Siemens’ view as well established.

The ideas are founded on something like the following considerations:  Flame cannot touch a cold surface, i.e., one below the temperature of combustion, because by the contact it would be put out.  Hence, between a flame and the surface to be heated by it there always intervenes a comparatively cool 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.