Indeed it would be endless to enumerate the various sources of this gazeous fluid. A most curious variety of carburetted hidrogen gas has been discovered by the associated Dutch chemists (Van Dieman, Troostwyck, and others) which is procured from ether or alcohol, and has the remarkable property of generating a heavy oil when in contact with chlorine gas. Hence it has been termed oily carburetted hidrogen, or olifiant gas—it consists of carburetted hydrogen, supersaturated with carbon. The oil generated is heavier than water, whitish, and semi-transparent. By keeping, it becomes yellow and limpid; its smell is highly fragrant and penetrating—its taste somewhat sweet—it is partly soluble in water, imparting to it, its peculiar smell. A portion of this gas always accompanies the common carburetted hidrogen obtained from coal, and those sorts of coal that afford the largest quantity of it are best suited for the production of gas-light.
The nature of carburetted hidrogen obtained from coal varies considerably according to the conditions under which it is obtained. The first part is always much heavier than the last, though still lighter than common air, and holds in solution a portion of oil, for on standing for some time over water it becomes lighter, and is found to require less oxygen for saturation than before. The oil which it held suspended, then becomes precipitated. The average specific gravity of the first and last gas mixed, which may be taken as an average of the whole specific gravity is to that of common air as 2 to 3—112lb. of common cannel coal produce at its minimum, from 350 to 360 cubic feet of carburetted hidrogen gas; but the same quantity of the best Newcastle coal, that is to say, such as coke, which, when laid on the fire readily undergoes a kind of semi-fusion, and sends out brilliant streams of flame, produces upon an average from 300 to 360 cubic feet of this gazeous fluid, besides a large portion of sulphuretted hidrogen, carbonic oxid and carbonic acid. Half a cubic foot of this carburetted hidrogen, fresh prepared, that is to say, holding in solution or suspension, a portion of the essential oil, which is generated during the evolution of the gas, is equal in illuminating power to from 170 to 180 grains of tallow, (being the quantity consumed by a candle six to the pound in one hour.) Now, one pound avoirdupoise is equal to 7000 grains, and consequently one pound of candles of six in the pound, burning one at a time in succession, would last (if we take 175 grains of tallow to be consumed in an hour) 7000175 = 40 hours. To produce the same light we must burn one half of a cubic foot of coal-gas per hour; therefore, one-half multiplied by forty hours is equal to twenty cubic feet of gas in 40 hours, consequently equal to one pound of candles, six to the pound, provided they were burnt one after another. One hundred and twelve pounds of cannel-coal, produce, at its minimum, three hundred and fifty cubic feet of gas; and are equal to three hundred and fifty, divided by twenty, which last is equivalent to one pound of tallow, making one hundred and twelve pounds of cannel-coal, equal to 35020 = 171⁄2lbs. of tallow. Further, one hundred and twelve pounds of cannel-coal, divided by seventeen and a half of tallow make six and four-tenths of cannel-coal, equal to one pound of tallow.
With regard to Newcastle coals[17], it may be stated that one chaldron of Wall’s-End coal may be made to produce in the large way upwards of 11,000 cubic feet of crude gas; which, when properly purified, diminishes to nearly 10,000 cubic feet.
[17] One chaldron of Newcastle coal weighs from 2850 to upwards of 2978lb.
The production of carburetted hydrogen, both with regard to quantity and quality from the same kind of coal depends much upon the degree of temperature employed in the distillatory process. If the tar and oil produced during the evolution of the gas in its nascent state, be made to come in contact with the sides of the red hot retorts, or if it be made to pass through an iron cylinder or other vessel heated red hot, a large portion becomes decomposed into carburetted hydrogen gas and olifiant gas, and thus a much larger quantity of gas is produced than would be obtained without such precaution from the same quantity of coal.[18]
[18] One pound of coal-tar produces 15 cubic feet of carburetted hidrogen abounding in olifiant gas.
The distillation of the coal, (if gas be the chief object) should therefore not be carried on too rapidly. Most of the retorts used in the large way, are calculated for containing about one hundred weight of coal, and in general, when previously heated, produce from two and one-half to three cubic feet of gas, in four hours for each pound of coal they contain; but when the layer of coals in them does not exceed four inches in depth, three and one-half to four feet of gas may be obtained in the same time.
The retorts best calculated for large gas-light works are seven or eight foot long (without the mouth-piece) and twelve inches in diameter, tapering down to ten inches—if they are larger the coal which they contain cannot be heated properly. The advantages that may be derived from the circumstances before stated are of greater value in the gas-light manufacture than is often imagined, and the quantity as well as the quality of the gas is very much influenced by such circumstances. If coal be distilled with a very low red heat scarcely observable by daylight, the gas produced gives a feeble light—if the temperature be increased so that the distillatory vessel is of a dull redness, the light is more brilliant and of a better colour—if a bright or cherry-red heat be employed the gas produced, burns with a brilliant white flame, and if the heat be increased so far that the retort is almost white hot, and consequently in danger of melting, the gas given out, has little illuminating power, and burns with a clear blueish flame;[19] or if the coal abounds in pyrites or sulphuret of iron, as is sometimes the case with Newcastle coal, a large quantity of sulphuretted hidrogen is likewise evolved, which although it increases the illuminating power of the coal-gas, has the capital disadvantage, of producing an intolerable suffocating odour, when the gas is burnt which is particularly perceptible in low rooms illuminated with such gas.
[19] It is chiefly a mixture of carbonic oxid, and hydrogen gas.
These observations also apply to the distillation of tar, which when distilled either in a vaporous or nascent state, during its first production from coal in the ordinary process, or if it be submitted to a second distillation, mingled with a fresh portion of pit-coal, a practice usually had recourse to when this product cannot be disposed of more advantageously. The best depth of coal in the retort for procuring excellent gas, and at the same time for yielding the greatest quantity from the same weight in the shortest possible time, is about six inches.