POPULAR SCIENCE REVIEW. 
arranged concentrically, eacli group being arranged in the 
form of a star, and the whole forming a brilliant and steady 
volume of light. This latter is, beyond all comparison, the 
most pleasant and the brightest light that has yet been obtained 
artificially. It requires, however, a chamber and large chimney 
communicating directly with the outer air, and must be placed 
at the ceiling or roof of the room to be lighted. It is compara- 
tively expensive, consuming a large quantity of gas compared 
with the available light yielded, and is thus little adapted for 
general use where economy is considered. 
The quantity of good illuminating gas procured from a ton 
of coal varies greatly according to the nature of the coal and 
the method of manufacture. By the old process, the yield of 
gas rarely exceeded 10,000 cubic feet per ton of coal, except 
from some Cannel coals, especially rich in hydrogen ; whereas, 
by what is called White’s process, as much as 30,000 cubic 
feet have been obtained from ordinary kinds, and 50,000 from 
Boghead coal. The illuminating power of the gas made has 
also been increased by modern improvements, the increase 
amounting to from twelve to upwards of a hundred per cent, 
on the old method, according to the nature of the coal. 
To give an idea of the value of the improvement in artificial 
light by the introduction of gas, we must enter into some small 
calculations. Taking sperm candles as the unit (each candle 
burning ten hours, at the rate of 120 grains per hour, and the 
value being about 4 cl.), the quantity of ordinary coal required to 
produce light equal to 1,000 such candles (value £16. 13s. 4 cl.), 
according to the old method of making gas, varied from four to 
seven hundredweight ; while, if Cannel coal were used, about 
half that weight would be needed. At present, however, the 
consumption of coal for this quantity of gas would not exceed 
from 350 to 400 pounds of ordinary kinds, and of Cannel, from 
105 to 160. With this quantity of coal (value about three 
shillings in London) from two to three thousand cubic feet of 
gas are manufactured, so that, under any circumstances, the 
cost of gas-light, compared with that of sperm candles, is not 
more than one-fiftieth part. In point of fact, however, with the 
methods of manufacture now adopted, and the increased illumi- 
nating power of the gas, it is estimated that the actual cost of 
1,000 feet of gas of the best quality is little more than one 
shilling ; so that artificial fight really costs not more than one- 
hundredth part the price that it did fifty years ago. 
In countries where coal is scarce and dear, wood, peat, and 
brown-coal all yield, on distillation at very high temperatures, 
certain illuminating gases, which can be purified for burning’, 
and thus rendered available for general use. It is only very 
lately that a method of doing this has been adopted with 
