212 THE POPULAR SCIENCE MONTHLY. 



For luminif erous 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 luminif erous 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 im- 

 mense advantage over coal-gas, its illuminating powers being as much 

 as fifteen-fold. And when we are speaking, not of an organized sys- 

 tem 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 tAvo 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 inves- 

 tigation. The latest patents for electric lighting are now found, in 

 many cases, to be reproductions of methods long since introduced and 



