intensity of the radiant center is inversely proportionate to the square of the distance from the screen, a very reliable comparison is attainable. But the weak point is the variable and ill-defined character of the unit of comparison. In the French experiments this defect is to a great extent avoided by the use of a Carcel lamp, which not only is intended to consume a given quantity of oil per hour, but is further weighed at the commencement and at the close of each observation, so that a correction is made in case of any variation in the actual combustion. Still, the Carcel lamp is an arbitrary unit. It is equal to about 9·6 English standard sperm candles; but when we have said that, we have only compared one arbitrary unit with another. In the case of the unit of heat, although it has been arrived at in terms of capacity (as regards the water heated) and of Fahrenheit's thermometer, which is in itself an arbitrary scale, it so happens that the Joule equivalent is exactly equal to the quantity of heat that is liberated by the combustion (if chemically perfect) of half a grain of carbon. If we take the same unit for the measurement of light, it must further be specified that the combustion of the carbon must be so effected as to produce carbonic acid and not carbonic oxide, and that it must take place in atmospheric air, and not in pure oxygen, or any other medium. That being borne in mind, it is probable that the combustion of a definite quantity of carbon would prove a better measure of light than any that has yet been tried. It would, at all events, link the phenomena of luminiferous to those of calorific combustion, and afford a ready means of detecting waste of illuminative power.
Various analyses have been given of ordinary coal-gas. Indeed, not only does that gas vary according to the quality of the coal from which it is-produced, but it differs according to the process by which it is produced from coal of the same quality. Experts are divided, for example, as to the degrees of heat at which it is best to effect the distillation of coal-gas. But for our present inquiry it is enough to assume the composition of coal-gas as analyzed by Mr. Vernon Harcourt, who gives the proportions of fifty-eight per cent, of carbon and twenty-three per cent, of hydrogen. The details are given by Mr. D. K. Clark, in his invaluable work, the "Manual of Rules, Tables, and Data for Mechanical Engineers." Of this gas thirty cubic feet, at the temperature of 62° Fahr., weigh one pound. And the heating power of one pound of this gas (chemically speaking) is given by the same analyst at 22,684 British units of heat, of which sixty-three per cent. is due to the combustion of the hydrogen, and thirty-seven per cent, to that of the carbon. It thus follows that coal-gas is far more highly effective as a fuel than it is as a source of illumination. Other analyses give a yet higher proportion of hydrogen, the heat-giving element.
There is, however, a mineral fuel in which this distribution of the elements is very different. Petroleum is a natural fluid, consisting of hydrogen and carbon, which has been distilled in the great laboratory
