Another product of the oxyhydrogen flame is the quartz fibres which are used for suspending the needles in the finest galvanometers. The quartz is melted, in this case a crucible being employed. An arrow is then dipped in the liquid quartz and immediately "fired" into the air. The thick treacly liquid is thus drawn out into a thread of such fineness that a microscope is necessary to find it with. Hotter even than oxyhydrogen is the oxyacetylene flame, which at its hottest point reaches nearly 3500° C. The gas, which is another of the combinations of carbon and hydrogen (its molecules containing two atoms of each), is easily made by allowing water to come into contact with calcium carbide. The latter, which is CaC_{2}, is made by heating coke and lime together in the intense heat of an electric furnace. This accounts largely for the great heating power of acetylene, for since great heat is necessary to cause the elements to combine great heat is given out by them when they ultimately separate. Here again is the conservation of energy. The heat energy of the electric furnace is largely expended in forcing these two elements into partnership. They are, as it were, given a large amount of capital in the form of heat. It ceases to be sensible heat, becoming latent in the compound, but still it is there. So a lump of calcium carbide, with which many readers are familiar, has vast stores of heat locked up within it. When water comes into contact with the carbide the partnership is broken, but the heat is not liberated then, since another partnership is formed, which still retains the old heat-capital. The calcium in the carbide is displaced by the hydrogen from the water, and so C_{2}H_{2} comes into being, while the rejected calcium consoles itself by entering into combination with the equally forsaken oxygen from the water, forming CaO, which is but another name for lime. Then the acetylene (C_{2}H_{2}) is mixed with oxygen in the blowpipe and burnt, under which conditions the pent-up heat, borrowed originally from the electric furnace, is brought into play. With this flame the harder metals can be fused and welded. Wrought iron, cast-iron, steel in all its forms, all can be melted by the oxyacetylene flame, almost as easily as snow by a hot iron. The fusion welding of these metals is then carried on just as already described for brass. By means of a special blowpipe, wherein an excess of oxygen is introduced at the hot point, hard steel plates can be cut to pieces almost as easily as a grocer cuts cheese. Even thick, hard armour-plate can thus be cut, almost the only way, indeed, in which it can be cut.