of the reaction--as already stated it is desirable that water shall be so vaporised--and will remain safely vaporised as long as the pipes are kept warm inside or near the generator; but directly the pipes pass away from the hot generator the cooling action of the air begins, and some liquid water will be immediately produced. Like the phenomenon of after- generation, this equally inevitable phenomenon of water condensation will be either an inconvenience or source of positive danger, or will be a matter of no consequence whatever, simply as the whole acetylene installation, including the service-pipes, is ignorantly or intelligently built. As long as nothing but pure polymerisation happens to the acetylene, as long, that is to say, as it is merely converted into other hydrocarbons also having the general formula C_(2n)H_(2n), no harm will be done to the gas as regards illuminating power, for benzene burns with a still more luminous flame than acetylene itself; nor will any injury result to the gas if it is required for combustion in heating or cooking stoves beyond the fact that the burners, luminous or atmospheric, will be delivering a material for the consumption of which they are not properly designed. But if the temperature should rise much above the point at which benzene is the most conspicuous product of polymerisation, other far more complicated changes occur, and harmful effects may be produced in two separate ways. Some of the new hydrocarbons formed may interact to yield a mixture of one or more other hydrocarbons containing a higher proportion of carbon than that which is present in acetylene and benzene, together with a corresponding proportion of free hydrogen; the former will probably be either liquids or solids, while the latter burns with a perfectly non-luminous flame. Thus the quantity of gas evolved from the carbide and passed into the holder is less than it should be, owing to the condensation of its non-gaseous constituents. To quote an instance of this, Haber has found 15 litres of acetylene to be reduced in volume to 10 litres when the gas was heated to 638° C. By other changes, some "saturated hydrocarbons," _i.e._, bodies having the general formula C_nH_(2n+2), of which methane or marsh-gas, CH_4 is the best known, may be produced; and those all possess lower illuminating powers than acetylene. In two of those experiments already described, where Lewes observed maximum temperatures ranging from 703° to 807° C., samples of the gas which issued when the heat was greatest were submitted to chemical analysis, and their illuminating powers were determined. The figures he gives are as follows: I. II. Per Cent. Per Cent. Acetylene 70.0 69.7 Saturated hydrocarbons 11.3 11.4 Hydrogen 18.7 18.9 _____ _____