LECTURE VI.

Fourth term in the series of typical hydrogen-compounds--marsh-gas, or light carbonetted hydrogen-reasons for its separate consideration—its occurrence in marshes-in coal-mines-in coal-gas-its preparation—its distinctive characters-its qualitative analysis-separation therefrom of carbon by chlorine.—Decomposition of marsh-gas by heat-its quantitative analysis-its synthesis not yet accomplished-combining weight of its constituent, carbon-its analogues.-Silicetted hydrogen-probability of the construction thereof upon the type of marsh-gas.

In the series of typical hydrogen compounds which we have now studied, we have found one volume each of chlorine, oxygen, and nitrogen, united respectively with one, two, and three volumes of hydrogen; the condensation increasing in direct ratio with the increasing proportion of hydrogen, so that, in each case, two volumes only of the resultant compound are produced.

This typical series does not end here, however. It comprises a fourth member-a compound containing, in two volumes, four volumes of hydrogen condensed; the other constituent being the body familiar to all as carbon. This compound is the light, inflammable gas, too well known to coal-miners as Fire-damp; and also termed, on account of its frequent emanation from boggy ground, Marsh-gas.

The description of this fourth typical hydrogen-compound might have been included with that of the rest of the series, but we have preferred to reserve it as a matter for separate study, chiefly on account of this signal difference; that whereas in all the other members of the series both constituents of the compound are gaseous, in this fourth one the hydrogen is united with a body which is not only solid at common temperatures, but incapable of volatilization by any, the intensest means at our present command. Hence, while the first three compounds, studied collectively, apart from the fourth, illustrated, with an

admirable symmetry, and in an unbroken ascending scale, the laws of combination and progressive condensation, by volume as well as by weight, the fourth exemplifies volume combination and condensation only so far as its gaseous ingredient is concerned; our positive knowledge, as to its solid constituent, extending only to the weight-ratio, and any views we may entertain as to the volume-ratio of this element being of necessity speculative. It is, indeed, maintained by many chemists, and not without some show of reason, that analogy affords a fair basis for such speculation; but not until carbon shall have been actually volatilized, and its vapour actually weighed, shall we be entitled to rank marsh-gas, with respect to its volumetric constitution, on the same certain footing as experiment already assigns to hydrochloric acid, water, and ammonia.

In describing this as the chief, we imply that it is not the only difference which justifies us in separating the study of marshgas from that of the three other typical compounds. A peculiarity in its chemical deportment also distinguishes marsh-gas from the other members of the typical series, in a manner which will call for our special attention hereafter, but of which it would be premature to say more, at present, than that its effect is to prevent our employing, in the case of marsh-gas,* certain modes of investigation, which we have adopted with advantage in studying the other typical compounds.

Thus much premised, let us proceed to make acquaintance with this fourth member of our typical series.

There escapes from the fissures of the great coal-measures a transparent, colourless, inflammable gas, which frequently accumulates in the galleries of ill-ventilated coal-mines; and, when ignited by the miner's candle, through neglect of the protective precautions provided by science, gives rise to the explosions so much dreaded and deplored on account of their life-destroying violence. Many varieties of coal so abound in this gas, that it may be seen rising therefrom in bubbles, when newly-dug fragments are thrown into water. The same gas, as we have already men

100

SOURCES OF MARSH-GAS.

tioned, is developed in marshy lands, and is often observed bubbling up from stagnant pools and swamps, where vegetal matters are in process of gradual decay. In summer, when decay is most active, the development of this gas is most abundant; and it may be readily collected in a glass cylinder, inverted over, and plunging into, stagnant waters. The gas thus obtained may be at once distinguished from common air by applying a light, when it is found to be inflammable.

It would, of course, be inconvenient to procure this compound from either of its natural sources, even if they furnished it in a state of purity, instead of, as always happens, yielding it in admixture with common air and other gases. The ordinary illuminating gas, distilled from coal by artificial heat, always contains a large percentage of marsh-gas; but here also it is mixed with other gases, scarcely separable from it; so that we are debarred from this source of supply, otherwise so accessible and abundant. Chemists have, however, devised a simple process by which marsh-gas may be readily prepared in any quantity requisite for laboratory purposes, from well-known materials, procurable at relatively moderate cost. In a flask (of glass, or better, of copper or iron), arranged for gas disengagement, strong vinegar is heated with a mixture of lime and the caustic soda of commerce; after a short time a transparent colourless gas is evolved, which is collected over water in the usual way (Fig. 60). We have, for the present, no interest in dwelling on the reaction which, under these circumstances, gives rise to marsh-gas. Suffice it to say that a portion of the carbon contained in the vinegar combines with the hydrogen present, so as to form this gas; which is also frequently called light carburetted or carbonetted hydrogen. For brevity's sake, however, we will, on the present occasion, retain the name of marsh-gas.

From the hydrogen compounds previously examined, marshgas is readily distinguishable by its inflammability. Kindled at a taper it burns with a feebly-luminous flame. Independently of this character, however, marsh-gas is strikingly dis

DISTINCTIVE CHARACTERS OF MARSH-GAS.

101

tinguished from hydrochloric acid and ammonia gases, by its possessing neither odour nor action on vegetal colours; which negative characters it shares with water-gas.

Not less easily may marsh-gas be distinguished from the three elementary gases which we have submitted to examination. Indeed, this readily inflammable gas cannot possibly be confounded either with chlorine, oxygen, or nitrogen, which are all incombustible bodies; the two former being supporters of combustion, but themselves not inflammable, while the last-named neither burns, itself, nor sustains the combustion of other bodies. From chlorine, moreover, marsh-gas differs by its lack of colour, odour, and bleaching property; from oxygen, marsh-gas, like nitrogen, is distinguishable by its total incapacity for supporting the process of combustion. The only elementary gas for which, at the first glance, the hydrogen compound of carbon is at all likely to be mistaken, is hydrogen itself; seeing that inflammability, coupled with the absence of colour, of odour, of bleaching power, and of the property of supporting combustion, characterise both these gases. The difference, however, of

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ACTION OF CHLORINE ON MARSH-GAS.

hydrogen from carbonetted hydrogen becomes at once obvious if both gases be burned side by side: for, while hydrogen burns with a non-luminous and scarcely-visible flame, the combustion of marsh-gas is attended by the evolution of feeble, but unmistakeable rays of light.

The chemical difference of the two gases may be demonstrated by a simple experiment. We remember that a mixture of hydrogen and chlorine, when ignited, merely gave rise to the formation of hydrochloric acid. A similar experiment performed with a mixture of marsh gas and chlorine produces, as we shall see, a further and very striking result.

For this purpose, we fill a tall cylinder with warm water, invert it over the pneumatic trough, and pass marsh-gas into it until a little more than one-third of the water is displaced; which done, we fill the two remaining thirds of the cylinder with

FIG. 61.

chlorine, taking care to protect the vessel, during manipulation, from exposure to sunlight. The cylinder thus filled we close, by slipping a glass plate beneath its mouth; then, raising it from the trough, we agitate it to mix the gases thoroughly; and finally, we apply a light to the mixture. Ignition takes place, with production of hydrochloric acid, by the union of the chlorine with the hydrogen present, as in our former experiment with pure hydrogen and chlorine. But the marsh-gas betrays its additional constituent, by a copious separation of carbon

as the flame descends into the cylinder; the sides of which become coated with a dense black layer of soot (Fig. 61).