The introduction of hot air into the central portion of the sheet of flame is advantageously supplemented by the spreading out of the flame by means of the metal disk, without any possibility of its being divided. In this way a more intense heat is obtained, and consequently the illuminating power is considerably increased, by the uncombined carbon being more readily set free, and being thus kept longer in the flame, F. This burner, which may be constructed for a moderate gas consumption, gives remarkable results as regards illuminating power and steadiness; the abstraction of heat in no way impairing the luminosity of the flame, which preserves all its brightness. The Argand burner with double chimney, shown in Fig. 7, is also an economical one for a small consumption of gas. The air admitted to both the inside and the outside is raised to a high temperature by passing along the spirals of a second and transparent chimney, C¹, which surrounds the cylindrical glass, C. The gas itself is heated by passing through this hot chamber before reaching the outlet orifices; so that the mixture of air and gas takes place under the most favorable conditions for their perfect combustion. The burner is an ordinary Argand, which may terminate below in a small chamber for the gas and air to mingle. But this is not necessary; and the usual arrangement for mixing the air and gas may be adopted. The outer air enters at the top of the central chimney, C and passes into the annular space between the two glasses; then descends by the two spiral passages, which surround the cylindrical glass and terminate in a portion hermetically sealed by a brass plate attached to the supply-pipe. All the parts of the burner are thus surrounded by a highly-heated atmosphere, especially at the bottom of the double chimney; and it will be readily understood that, if the branches which lead the gas to the burner are constructed of a highly conductive metal, the gas will become heated in its turn by passing through passages raised to a high temperature. The elements are therefore dissociated or separated before their final combination; thereby raising the calorific and luminous effect to the highest possible degree. Such a burner can, of course, be made as small as may be required; thus lending itself admirably to the subdivision of illumination. The only precaution required is to properly proportion the sectional area of the hot-air passages to the radiant surface of the flame, so that the heat does not become too intense at the lower portion of the burner.