OF THE NATURE AND PROPERTIES OF CAR

BURETTED HYDROGEN GAS.

THE Customs and amusements of society having rendered the use of artificial light indispensable, ingenuity has devised a variety of contrivances for obtaining and displaying it. That denominated coal gas derives the name from its being one of the component parts of pit-coal; but different animal, vegetable, and mineral substances, yield the inflammable elastic fluids which afford light; and whether it is emitted by a tallow or wax candle, the oil in a lamp, or coal gas, it proceeds from similar elements; hence a short explanation of the process by which it is produced may tend to manifest the nature of the gas.*

* The terms carburetted hydrogen gas equally apply to the gas obtained from tallow, oil, wax, or coal; but Dr. Henry observes, that "the general name of coal gas is quite indefinite. It is in fact a mixture of the two varieties of carburetted hydrogen with a third, which remains to be more fully investigated, as well as with hydrogen gas, carbonic oxide, carbonic acid, nitrogen and sulphuretted hydrogen gases in ever varying proportions."-Henry's Elements of Chemistry, Vol. I. p. 429, 1826.

Tallow, oil, and wax, are chiefly composed of carbon and hydrogen; and when a candle is first lighted, the heat communicated to the wick melts a small quantity of the tallow or wax, and forms a kind of cup for the tallow or wax which the heat renders fluid. The flame proceeds from the burning of the carburetted hydrogen gas, and to procure this it is essential that the combustible substances containing it should be decomposed; therefore the wick serves an important purpose in the process, by exposing them to the action of the heat in very small portions only. In proportion as the heat increases, the melted matter rises by capillary attraction, between the fibres of the wick, and as it comes into contact with that part which is ignited, its decomposition is gradually effected ;*-this operation causing the carburetted hydrogen to be given out, the burning wick exposes it, so that it may combine with the oxy

* The wick of a candle, or a lamp may, in some respects, be compared to a bundle of capillary tubes; and a small lamp, recently invented, to burn without a wick, very happily illustrates the principle on which they act. It consists of a brass cup, with a brass tube fixed in its centre; the bore of the tube is similar to that of the finest glass tubes, and the cup is placed in a basin filled with oil, which ascends to the top of the tube, when the oil being inflamed by a taper, it continues to burn as long as any remains to furnish matter for combustion.

gen contained in the atmospheric air, and hence the brilliant flame is produced which yields the light. The principle and process are similar in the burning of oil in a lamp, but the oil, being in a fluid state at the usual temperature of the atmosphere, may spread itself more readily in the wick, and therefore conduce to the greater rapidity of combustion.

No person can be unacquainted with the resemblance between the flame of a candle or a lamp, and a common fire; the great degree of brightness also which occasionally proceeds from the latter, when made with good pit-coal, must have attracted particular notice, and both are the effects of the same cause. However, the burning

of coal in a common fire-place, not only expels the carburetted hydrogen, but also various other substances that are mixed with it. The mixture of these different substances occasions the alternate appearance of bright light, dense vapour, and smoke, as well as the undulating motion and changes of colour which are usually observable in its flame. But the effects are similar, as far as regards the light and heat, whether the flame is produced by the use of wax, oil, tallow, or coal, for each possesses the same properties to afford them. Nevertheless, in the open fire-place a great part of the carburetted hydrogen gas escapes during the combustion of the coal, and as a small

portion of it only becomes inflamed, the light and heat, which otherwise might be derived from the gas, are consequently lost.* This illustration, it is presumed, will render it evident, that the elementary substances that yield them are precisely the same, whatever difference there may be in the mode of producing, collecting, or exhibiting them.

In' the different attempts to obtain heat and light, the successive production of vivid lustre and varied colour, besides the vapour and smoke, were such remarkable changes as forcibly attracted the attention of men of science, and excited an ardent curiosity to ascertain their causes.

* The quantity of heat produced by a common fire, in a great degree depends upon the mode of managing it. When a fire burns clear, it is observed, that a great deal of heat is thrown out; but if too large a quantity of coal be put upon a fire, or if it be placed in such a manner as not to allow a current of air to pass freely through it, the heat is not given out, because the combustion of the coal is but partially effected; and the hydrogen gas, which produces the heat, escapes without being inflamed, from its not being properly supplied with air. If the cause of this were more generally understood by servants, the knowledge would not only save them a great reat deal of trouble, but would also materially lessen expense in an important article of constant and necessary domestic use.

Hence they were led to important discoveries, and their ingenuity suggested those useful processes by which the greater part of the substances derived from the combustion of pit-coal are now collected, analyzed, and applied to valuable purposes, though they were formerly allowed to be dispersed and considered of no avail. But farther, to elucidate the subject, we shall endeavour to describe and explain the apparatus and different operations employed in procuring and preparing coal gas for use, that the various means by which the objects of gas lighting are accomplished, may be easily understood by those who may possess but little or no knowledge of chemistry.

OF THE DECOMPOSITION OF COAL FOR THE PURPOSE OF MAKING GAS.

THE process by which the decomposition of pit-coal is effected is sometimes designated by the term carbonization as well as distillation. The products of this operation are various in their qualities and quantities, according to the different degrees of heat which may be applied to the coal, but the carburetted hydrogen is that which is produced in the largest proportion.