But there is yet another direction in which the prior heating of the air supply contributes to the development of improved illuminating power. Being heated, its density is lowered; so that in any given volume of air there is less weight of oxygen than when cold. The consequence is that as less oxygen is presented to a given surface area of flame, the separated particles of carbon remain for a longer period of time in the incandescent condition before being entirely consumed. Thus there are three distinct results produced by heating the gas and air before combustion--namely, first, the particles of carbon are liberated earlier in the flame; second, they are raised to a more exalted temperature; and, third, they remain for a longer time in the incandescent condition. The combined effect of all three is the improved illuminating power developed from the gas consumed. [Illustration: FIG. 19.--BOWDITCH'S REGENERATIVE GAS-BURNER.] [Sidenote: Bowditch's regenerative burner.] So far back as the year 1854, the principle of heating the air supply to an Argand burner, by means of waste heat from the flame, was partially applied, with some success, by the Rev. W. R. Bowditch, M.A., of Wakefield. Mr. Bowditch's burner, which is shown in the accompanying diagram, contained, in addition to the ordinary chimney, an outer glass chimney, which extended for some distance below the inner one, and was closed at the bottom; so that all the air needed to support the combustion of the gas was required to pass down the annular space between the chimneys, and in its passage became intensely heated by contact with the hot surface of the inner chimney, as well as by radiation from the flame itself. This burner contained many defects. Amongst others, the inner chimney could not long withstand the intense heat to which it was subjected, and, in consequence, had to be frequently renewed; the heating of the air was not effected solely by the products of combustion, but, perhaps in a greater degree, by the abstraction of heat from the flame itself; while, at best, this heating was but partial. Yet, these defects notwithstanding, the burner showed very clearly the beneficial results attending even a partial application of the principle; as, in the illuminating power it developed from the gas consumed, a clear gain of 67 per cent. over the ordinary Argand burner was obtained. Although the drawbacks connected with the construction of Mr. Bowditch's burner prevented its ever receiving general, or even extensive adoption, its simplicity has gained for it the distinction of being freely copied by so-called inventors of a later day. [Sidenote: Invention of the Siemens regenerative burner.]