attained is practically limited only by the capacity of the furnace itself to withstand it.
In all or nearly all the applications of illuminating gas to heating purposes, however, a practical difficulty has to be overcome. The gas is made for illuminating purposes, and therefore burns with a bright flame, which deposits a layer of lamp-black, or soot, on everything brought into contact with it. Evidently this difficulty must be overcome before any extensive use could be made of gas as fuel, particularly for those domestic purposes to which it finds one of its chief applications. For practical purposes one of two methods is adopted. One of these is to cause the gas to burn from a number of small openings in a metal pipe. Two effects are thus produced. The large mass of metal in the tubes abstracts heat from the flame, and, as a result, the latter burns mostly blue and produces little smoke or soot; and, secondly, the distribution of the gas into several small flames allows us to place the article to be heated high enough over the flame to avoid the soot, and yet near enough to get a good heating effect. A second and more common method of preventing the deposition of soot is by the use of some form of the Bunsen lamp.
Nearly all the common gas-stoves and other arrangements for heating by gas are essentially Bunsen lamps, more or less modified to suit particular purposes. One of the simplest and most common forms of this lamp is represented in section in the annexed figure. The gas 
enters through the metallic tube a, passes through the block b, and finds an outlet through one or more small apertures at c. Surrounding c is a larger metallic tube, d d, having at its base two apertures for the admission of air. The mixed gas and air rise through the tube d d, and burn at the top with a pale-blue, smokeless flame. The lamp is supported by a heavy cast-iron foot, e.
Though this lamp is of simple construction, the explanation of its operation involves some very curious and interesting facts regarding the theory of flame, as well as some very familiar ones, and in order to comprehend it we shall need to begin with a study of flame in general, and of luminous flames, like that of ordinary gas, in particular.
When a solid combustible, like charcoal, burns in the air, it produces no flame, but simply glows. The blue flame that is often observed playing over the surface of a coal-fire is that of the carbonic-oxide gas
