It has also been proposed to utilise the readiness with which acetylene polymerises on heating to form benzene, for the production of benzene commercially; but the relative prices of acetylene and benzene would have to be greatly changed from those now obtaining to make such a scheme successful. Acetylene also lends itself to the synthesis of phenol or carbolic acid. If the dry gas is passed slowly into fuming sulphuric acid, a sulpho-derivative results, of which the potash salt may be thrown down by means of alcohol. This salt has the formula C_2H_4O_2,S_2O_6K_2, and on heating it with caustic potash in an atmosphere of hydrogen, decomposing with excess of sulphuric acid, and distilling, phenol results and may be isolated. The product is, however, generally much contaminated with carbon, and the process, which was devised by Berthelot, does not appear to have been pursued commercially. Berthelot has also investigated the action of ordinary concentrated sulphuric acid on acetylene, and obtained various sulphonic derivatives. Schröter has made similar investigations on the action of strongly fuming sulphuric acid on acetylene. These investigations have not yet acquired any commercial significance. If a mixture of acetylene with either of the oxides of carbon is led through a red-hot tube, or if a similar mixture is submitted to the action of electric sparks when confined within a closed vessel at some pressure, a decomposition occurs, the whole of the carbon is liberated in the free state, while the hydrogen and oxygen combine to form water. Analogous reactions take place when either oxide of carbon is led over calcium carbide heated to a temperature of 200° or 250° C., the second product in this case being calcium oxide. The equations representing these actions are: C_2H_2 + CO = H_2O + 3C 2C_2H_2 + CO_2 = 2H_2O + 5C CaC_2 + CO = CaO + 3C 2CaC_2 + CO_2 = 2CaO + 5C By urging the temperature, or by increasing the pressure at which the gases are led over the carbide, the free carbon appears in the graphitic condition; at lower temperatures and pressures, it is separated in the amorphous state. These reactions are utilised in Frank's process for preparing a carbon pigment or an artificial graphite (_cf._ Chapter XII.). Parallel decompositions occur between carbon bisulphide and either acetylene or calcium carbide, all the carbon of both substances being eliminated, while the by-product is either sulphuretted hydrogen or calcium (penta) sulphide. Other organic bodies containing sulphur are decomposed in the same fashion, and it has been suggested by Ditz that if carbide could be obtained at a suitable price, the process might be made useful in removing sulphur (_i.e._, carbon bisulphide and thiophen) from crude benzol, in purifying the natural petroleum oil which contains sulphur, and possibly in removing "sulphur compounds" from coal-gas.