For luminiferous purposes we have seen that there is no comparison between the consumption of crude coal and that of coal-gas. Allowing the mean proportion of 10,000 cubic feet of gas to the ton of coal which we have before taken, the consumption of an ordinary gas-burner, whether an argand or a fish-tail, is about five cubic feet per hour, giving a light of from twelve to sixteen candles, according to the richness of the gas. If we take Mr. Vernon Harcourt's analysis, thirty cubic feet, or one pound of gas, contains 4,060 grains of carbon. Five cubic feet therefore contain 67·6 grains, which will be the hourly consumption of pure carbon in an ordinary gas-light.
Petroleum, however, contains from eighty-two to eighty-seven per cent. of carbon, and from eleven to fifteen per cent. of hydrogen. Averaging this at eighty-four per cent. of the former and thirteen of the latter, a pound of petroleum contains 6,080 grains of carbon and 910 grains of hydrogen. Its luminiferous power is thus almost exactly fifteen times that of coal-gas, taking equal weights. Its calorific power, supposing a perfect combustion, will be ten per cent. less than that according to Mr. Vernon Harcourt's estimate, and less than half that of the highest estimate given by Mr. Clark.
It is thus as clear as any deduction from chemical data can be, that while the economy in the use of coal-gas as a source of heat is so great as to render it worth while to keep up the distillation of this product, as now carried on, for calorific purposes alone, even exclusive of its use for a light, for the purposes of illumination petroleum offers an immense advantage over coal-gas, its illuminating powers being as much as fifteen-fold. And when we are speaking, not of an organized system of fixed lights, but of the convenience of a hand lamp, the price and the illuminative value of petroleum indicate it as the source of the economical light of the future. In fact, its light-giving power is ten per cent. more than that of either tallow or olive-oil, and four per cent. more than that of wax, weight for weight, notwithstanding the great difference in price.
The question of the miner's safety, then, resolves itself into the construction of a petroleum lamp, which shall have the safety of the "Geordie," while giving the light of one, or even of two or three fish-tail burners of gas, and which shall be so made as neither to empty nor to be extinguished if laid on the side.
It is desirable, in an inquiry of this nature, to avoid anything that assumes the appearance of advertisement, or of an attempt to introduce anything of a commercial bearing. For that reason less must be said than honestly and fairly might be said as to the principles on which such a lamp may be unquestionably constructed. Two or three patents exist, which would require due consideration. It is always, indeed, doubtful how far recent patents will stand the test of thorough investigation. The latest patents for electric lighting are now found, in many cases, to be reproductions of methods long since introduced and
