At the same time corresponding improvements were made in the type of burner, which is quite as essential to success as the mantle itself. It had been found that it was absolutely essential for such a burner to give a practically non-luminous flame, as otherwise the deposit of carbon particles will ruin the mantle. Two ways of obtaining this are possible; one by mixing a certain quantity of air with the gas before combustion, the other to burn the gas in so thin a flame that the air permeates it freely. Several burners of both types were used at first, but gradually the burners in which the air is mixed with the gas became the more popular, and most of the incandescent burners now on the market are of this type. In the construction of mantles at the present time, while the principle of their use remains the same as that of the lime-light, lime itself is not used, the oxides of certain other metals having proved better adapted for the purpose. Thus the Welsbach patent of 1886 covered the use of thoria, either alone or mixed with other substances such as zirconia, alumina, magnesia, etc.; thoria being considered as having a very high power of light emission. Later it was discovered that pure thoria emits very little light by itself, although it possesses a refractory nature that gives a stability to the mantle unequalled by any other material as yet discovered. When combined with a small trace of the oxides of certain rare metals, however, such as uranium, terbium, or cerium, thoria mantles have a very high power of light emission, most modern mantles being composed of about ninety-nine per cent. thoria with one per cent. cerium. In the ordinary method of manufacturing such mantles, a cotton-net cylinder about eight inches long, more or less according to the size of mantle required, is made, one end being contracted by an asbestos thread. A loop of the same material, or in some cases a platinum wire, is fastened across the opening, to be used for suspending the mantle when in use. The cotton-thread cylinder is soaked in a solution of the nitrates of the metals thorium and cerium, and is then wrung out to remove the excess, stretched on a conical mold, and dried. The flame of an atmospheric burner being applied to the upper part at the constricted position, the burning extends downward, converting the nitrates into oxides, and removing the organic matter. Considerable skill is required in this part of the process, as the regular shape of the mantle is largely dependent upon the regularity of the burning. As a finishing process a flame is applied to the inside of the mantle after it has cooled, to remove all traces of carbon that may remain.