208 THE POPULAR SCIENCE MONTHLY. 



intensity of the radiant center is inversely proportionate to the square 

 of the distance from the screen, a very reliable comparison is attain- 

 able. 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 com- 

 bustion. 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 thermome- 

 ter, 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 defi- 

 nite quantity of carbon would prove a better measure of light than 

 any that has yet been tried. It would, at all events, link the phenom- 

 ena 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 Har- 

 court, 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 analy- 

 ses 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 



