But it must not be assumed, because of the foregoing remarks, that the pressure at which the gas issues from the burner is altogether an unmixed evil. In flat-flame burners it fulfils the important function of promoting intensity of combustion, by bringing the white-hot particles of carbon into intimate and rapid contact with the air that is necessary for complete combustion. In Argand burners this duty is discharged by the glass chimney; but with flat-flame burners it devolves entirely upon the pressure at which the gas issues from the burner. It will be seen, therefore, that the pressure of the gas is a factor of considerable importance in determining the amount of light afforded by a gas flame, as it is a matter requiring careful adjustment with each and every burner. On the one hand, with an excessive pressure the intensity of combustion is increased; but the separated carbon does not remain so long in the flame. The area of luminosity is thereby decreased, and the total light yielded is reduced. On the other hand, with insufficient pressure the combustion is not energetic enough to raise the particles of carbon to a white heat; consequently, the illuminating power of the flame is feeble, or else the carbon escapes unconsumed as smoke. The thickness of the flame produced by any burner has also an important bearing upon the degree of light afforded; and this property of thickness, again, is dependent upon the width of slit, in the case of batswings (or, in the case of union-jets, upon the size of orifices), and the pressure at which the gas is supplied. The thickness of the flame yielded by any burner will obviously vary inversely with the pressure at which the gas is supplied to it. With a thin flame, all parts of the flame are so completely exposed to the air, that the particles of carbon are no sooner raised to the temperature required to enable them to give out light than they are entirely consumed. With a thicker flame the carbon separated in the midst of the flame exists for a sensibly longer period of time in the white-hot state before it reaches the outside of the flame, and meets with sufficient oxygen for its complete combustion. Thus we find that the best flat-flame burners have comparatively wide orifices; while the pressure at which the gas is delivered from the burner is carefully reduced to the lowest point at which a firm flame is obtained, without smoke. Similarly, in the best Argands the pressure is considerably diminished within the burner, and the gas allowed to issue gently through relatively large holes; while the chimney is carefully adapted to draw upon the surface of the flame just sufficient air to completely consume the quantity of gas which the burner is calculated to deliver. IMPROVEMENTS IN FLAT-FLAME BURNERS.