To regulate the proportions in which the air and gas should mingle, in order that the combination should be as intimate as possible, the air inlet is made variable by a perforated collar, which permits of the orifices, O, being more or less covered. The other proportions of the burner--that is to say, the relative capacity of the two compartments and the length of the hollow needle--are determined by the sectional area of the supply-pipe for the gas, which is admitted under moderate pressure. Instead of a wire-gauze cap, impregnated with a solution of metals or of salts, two fine platinum wires may be used--one bent into the form of a semicircle of about an inch radius, and the other (of slightly larger diameter) rolled spirally round the former. When both ends of the two wires are connected with the upper portion of the tubular arrangement (which in this case is flattened), and the gas is ignited at the burner, the metallic arc becomes red hot, and then brightly incandescent, emitting a light, less brilliant indeed than with magnesia, but of remarkable steadiness. In this case the production of light is chiefly due to the fact that calorific condensation, caused by the use of the helicoidal coil surrounding the curved wire, prevents loss of heat in this conductor. In these forms of high-power burner, in which the gas is used directly for the production of light, the difficulty generally encountered of heating the air (present in a larger volume than the gas) has been successfully overcome. Fig. 3 shows the straight and outspread flame burner with a special heater. In this arrangement the gas and air are heated before combustion, in the compartment, G, directly exposed to the action of a small Bunsen burner, R, which is placed (in an opaque glass) in the middle of a lyre-shaped figure formed by the two gas-pipes, AA. The burner proper consists of two fine annular passages meeting above, and emitting a thin annular sheet of gas over the guide, T, made of a white refractory substance placed between the two annular jets. The object of this guide is to stretch the incandescent sheet of flame, composed of several jets, and interpose friction, so as to prevent a too rapid ascent of hot gases. [Illustration: FIG. 3 and FIG. 4 REGENERATIVE BURNERS WITH INVERTED FLAMES.]