dents also happen occasionally, in consequence of the evolution of fire-damp from the cavities existing in coal stored on board ship.

It is also the product of the destructive distillation of coal, and of the action of a high temperature on the vapor of alcohol.

Preparation. The best method of obtaining this gas is to heat in a copper or coated glass retort, an intimate mixture of 4 parts of dried acetate of soda, 4 parts of fused potassa, and 6 parts of quicklime.

It may be collected over water.

NaO.C ̧H2O2+KO.HO=C,H,+KO.CO,+NaO.CO,.

Properties.-Light carburetted hydrogen is a colorless and inodorous gas, burning with a pale blue and white flame; the results of the combustion are carbonic acid and water. It is but very slightly soluble in water. Chlorine has no action upon it in the dark, but when a mixture of the gases is exposed to diffused daylight in the presence of moisture, hydrochloric and carbonic acids are produced :—

CH2+Cl2+4HO=2CO,+8HCI.

When mixed with a small quantity of air, light carburetted hydrogen explodes but feebly or not at all; but if the proportion of air be about the quantity necessary for its complete combustion (about 10 volumes to 1 of the gas), the mixture explodes very violently. As the quantity of air increases beyond that proportion, the explosive power diminishes, until at last the gaseous mixture merely burns round the flame of a candle or lamp. The frequent and lamentable accidents occurring in coal mines in consequence of the formation of fire-damp, or explosive mixture of air and light carburetted hydrogen, led Sir H. Davy to make a close examination of this gas, with reference to its combustible properties, in connection with some beautiful researches of his on the power possessed by the metals, in consequence of their conducting properties, of cooling down gases in a state of combustion to such an extent as to extinguish flame.

Davy found that flame could not be communicated through a narrow glass tube to an explosive mixture of marsh-gas and air, the cooling power of the sides of the tube preventing the gas from attaining a sufficiently high temperature. Metallic tubes naturally possessed this power to a higher degree, in consequence of their good conducting properties, and it was found that even metallic wires, held within a certain distance of each other, or, more conveniently, wire gauze of a certain fineness, possessed the power of obstructing the passage of flame, or of protecting an explosive mixture from ignition by flame. Thus, by allowing a stream of coal-gas to pass through a piece of moderately fine wire gauze (containing not less than 400 meshes to the square inch), it may be kindled on the upper side without the flame being thereby communicated to it below the gauze. The development of these principles led Davy to the construction of the safety-lamp, by the use of which the miner is protected from danger, and at the same time warned of the existence of a pernicious atmosphere. It consists merely of an oil lamp, the wick of which is inclosed in an iron wire gauze cage, of which the upper part is double; a wire, bent into a hook at the upper extremity, passes upwards through the lamp, by which it may be trimmed to some extent without removing the gauze cage.

When this lamp is introduced, while burning, into an inflammable atmosphere, the flame of the lamp will be extinguished, while a bluish flame will be seen

1 The carbonic acid formed by the explosion of fire-damp in coal mines is technically called "after-damp," and is more fatal to the miners than even the explosion itself.

2 No explosion takes place if the volume of air be less than 6 or more than 14 times the volume of the gas.

within the gauze cage, arising from the combustion of the gas as it penetrates; by this, therefore, the miner is immediately made aware of the presence of firedamp. The lamp may even be allowed to remain in this state in an atmosphere of light carburetted hydrogen until the wire gauze is heated to dull redness by the heat of the burning gases inside; for, although kindled immediately by flame, this gas requires a higher temperature for its ignition than most other inflammable gases. An iron rod heated to dull redness will set fire to olefiant gas, hydrogen, carbonic oxide, &c., while the temperature of iron must be almost a white heat before it will inflame light carburetted hydrogen.

COAL-GAS.

$126. The gaseous product of the distillation of coal contains light carburetted hydrogen in much larger proportion than any other gas; it will, therefore, not be out of place to enter here into a brief account of the preparation and nature of coal-gas.

By submitting coal to distillation in closed iron retorts, three principal products are obtained: a dark, oily, viscid fluid, known as coal-tar, containing a variety of substances; an aqueous liquid, containing ammonia and its salts; and a gaseous mixture, the principal constituent of which is light carburetted hydrogen, but which contains besides, variable quantities of olefiant gas, carbonic oxide, hydrogen, carbonic acid, and nitrogen, and smaller quantities of sulphuretted hydrogen, bisulphide of carbon, vapor of volatile liquid hydrocarbons, ammonia, cyanogen, hydrochloric acid, and aqueous vapor. The four first-named products form the main bulk of the gas, together with the vapors of the volatile liquids; the hydrogen and carbonic oxide are evidently produced by the decomposition of moisture in the coal, or of water produced in the distillation, the vapor of which, passing over the redhot coal, yields these two gases. All coal contains more or less sulphur (generally existing as iron-pyrites), which is converted during the distillation into hydrosulphuric acid, and vapor of bisulphide of carbon.

The nitrogen existing in coal is expelled as ammonia, cyanogen (C,N), and a combination of the latter substance with sulphur, sulphocyanogen (CNS); any free nitrogen is due to the atmospheric air contained in the retorts at the commencement of the operation. The only absolutely essential constituents of coalgas, obtained in the ordinary process of manufacture, are light carburetted hydrogen and olefiant gas. Its illuminating power is mainly owing to the latter, and to the vapors of liquid hydrocarbons which the gas contains, while the light carburetted hydrogen, though burning with but a pale flame, evolves much more heat by its combustion than the olefiant gas, and thus, by imparting a very high temperature to the small particles of carbon separated during the combustion of the illuminating constituents (§ 65), contributes considerably, though indirectly, to the luminosity of the gas-flame.

There are, however, considerable objections to the presence of large quantities of both light carburetted hydrogen and carbonic oxide, as diluents in coal-gas, in consequence of the large amount of carbonic acid generated by their combustion; the excessive amount of heat evolved during the combustion of light carburetted hydrogen is also a great drawback to the use of such gas, containing a large quantity of this constituent, for the illumination of apartments; though, on the other hand, when the gas is employed solely as a source of heat, its value is much increased from the same cause. It would, however, greatly add to the general utility of gas, if the light carburetted hydrogen could be entirely, or in part, replaced by hydrogen, since this gas evolves much less heat in proportion to its volume, and its product of combustion is perfectly innocuous. This object appears to be to a great extent attained, by the application of a new process for the manufacture of gas, which we shall presently notice.

The peculiar property which chlorine possesses, of forming, when mixed with coal-gas, oily substances with those of its compounds to which it owes its illuminating properties, namely, olefiant gas, and the vapors of hydrocarbons, affords a ready means of testing its illuminating power, which is, of course, in direct proportion to the decrease in volume of the gas when mixed with chlorine. From experiments made by Henry, it has been proved that the gaseous products of the distillation of coal differ very considerably at the various periods of the operation. Before the coal arrives at a cherry-red heat, the products consist almost entirely of hydrogen and tar, together with the atmospheric air from the retort. When the retort has attained the stated temperature, illuminating gas passes over, the value of which decreases in proportion to the length of time for which the coal has been heated.

We see from the above that the gas evolved towards the termination of the process, only serves to increase the total volume obtained from the coal, and to deteriorate by dilution the illuminating power of the gas, since that obtained at the expiration of the tenth hour burns with merely a pale blue flame. The large amount of hydrogen found in the last stage of the operation can evidently no longer be produced by the decomposition of water, but must be due to the action of the high temperature upon the compounds of carbon and hydrogen which have been proved to be resolved into their elements at a high red heat (§ 129). The carbon thus liberated is found deposited on the sides of the retort as an exceedingly hard and dense crust, known by the name of gas-carbon ($ 121). This kind of decomposition cannot, under any circumstances, be entirely avoided, since the gas, as it is evolved from coal in large retorts, must always pass over a redhot surface; it may, however, be much diminished by a careful moderation of the temperature.

The cannel coal is found to be the best for the manufacture, and the Scotch parrot coal the next. The coal should be as dry as possible, and when subjected to distillation, should be raised as rapidly as possible to a cherry-red heat, at which temperature it should be uniformly maintained throughout the distillation. The operation should be stopped when experiment has shown that the illuminating power of the gas evolved decreases rapidly.1

The retorts used are of cast-iron; their forms elliptical, cylindrical, or slight modifications of these, and are connected with necks, from which conductingpipes pass into a large horizontal tube, termed the hydraulic main, where a large

1 The period of the distillation varies from five to eight hours, according to the nature of the conl and the form of the retort used.

2 Stone-ware retorts are now very frequently substituted for those of cast-iron.

quantity of tar and ammoniacal liquor is deposited from the gas as it passes from the retorts. From the hydraulic main the gas is conducted into a series of iron tubes, which are kept cool by water, and in which the chief part of the easilycondensable impurities is separated from the gas.

In some of the large works the gas is allowed to pass from the condenser into a washing-apparatus, consisting of a very long iron case furnished with three or four shelves, placed at intervals of about one foot, and attached alternately to either side of the case; water is allowed to trickle over these shelves, and a large surface being thus exposed to the action of the gas flowing in an opposite direction, a large quantity of the gaseous impurities is absorbed by it.

In another ingenious arrangement, termed the scrubber, the gas is allowed to enter at the bottom of a tall cylindrical case, containing perforated shelves charged with fragments of coke, over which water is allowed to trickle; in this way a still further purification of the gas may be effected.

When the gas leaves the washing-apparatus, it has still to be purified from the remainder of the incondensable constituents, which are highly detrimental to its quality, such as ammonia, sulphuretted hydrogen, &c. The method exclusively employed, till very recently, for the removal of these obnoxious impurities, was that of bringing the gas into contact with slaked lime, or milk of lime. When milk of lime is used, the gas is allowed to pass through a considerable bulk in very small bubbles, the liquid being, at the same time, continually agitated with a rouser or stirrer. The chemical action of this substance is, however, insufficient to effect perfect purification of the gas; the most effectual means of retaining not only the sulphur compounds, but the ammoniacal salts, is evidently the use, as purifiers, of salts of earths, or other metallic oxides, which will effect double decomposition, and produce non-volatile compounds. Sulphate of lead has been employed as a purifier, sulphate of ammonia and sulphide of lead being the chief results of its action on the gas; it is, however, necessary to use lime together with this salt, in order to remove the carbonic acid.

The sulphates of manganese and iron are more efficacious, but the best purifying medium known at present appears to be that which is now introduced into several large metropolitan gas-works, namely, sawdust, saturated with chloride of calcium. The purifying material may, after it has been employed, be worked with advantage for several important commercial chemicals, such as sulphate and muriate of ammonia, and Prussian blue, &c.

After this purification, the gas passes directly into the gasometers, whence it is distributed to the various mains.

The secondary products in the manufacture of gas are of considerable commercial importance. The tar, by distillation, furnishes a liquid known as coaltar-naphtha, which is a mixture of various acid, basic, and neutral hydrocarbons, the description of which falls within the province of organic chemistry. The most important of these substances is a volatile liquid known as benzol (C1H), which has lately been employed as an illuminating material, and for several other purposes. The residue after the distillation of tar is termed pitch.

The ammoniacal liquor, which is very rich in carbonate of ammonia, is employed for the preparation of sulphate of ammonia, and chloride of ammonium. • Other chemical products (e.g. Prussian blue) are also obtained from this liquor. When lime is used as a purifying agent, it is withdrawn from the purifiers after a certain time, and is used as a manure under the name of gas-lime.

§ 127. ANALYSIS OF COAL-GAS.-The rigorous analysis of gases requires so much accuracy of manipulation, and involves so many operations peculiar to itself, that it almost deserves to be considered a separate branch of practical chemistry. Sufficient has been said in the article on the measurement and absorption of gases (§ 36), to give a general idea as to the manner in which analyses of gas are con

ducted. We shall therefore confine ourselves in this place to an outline of the method usually pursued in the examination of coal-gas.

The following are the substances usually determined :

Olefiant gas

Vapors of hydrocarbons

Light carburetted hydrogen.

Hydrogen.

Carbonic acid.

Carbonic oxide.

Nitrogen.

Illuminating constituents.

A measured volume of the gas standing over mercury, is agitated with solution of potassa, allowed to stand for some time, and again measured.

The difference of volume indicates the amount of carbonic acid.

The residual gas is transferred, in the pneumatic (water-) trough to a graduated tube, carefully measured, mixed with half its volume of pure chlorine, and allowed to stand for twenty-four hours in a dark place. The tube is then placed in a jar containing solution of potassa, and agitated for some time to remove the excess of chlorine. The residue is then measured.

The difference between this volume and the preceding, indicates the volume of olefiant gas and vapors of hydrocarbons (and hence is a measure of the illuminating value of the gas).

Another method consists in passing up a pellet of coke, moistened with fuming sulphuric acid (in the manner described, § 36), into a known volume of the dry gas over mercury, and afterwards removing any acid vapor by a similar pellet of fused potassa.

The remaining gas, or a measured portion of it, is introduced into a small tube, the closed end of which is bent downwards, standing over mercury. (All moisture must be removed from the mercury with blotting paper, and from the gas by prolonged contact with pellets of fused hydrate of potassa.) A small pellet of potassium is introduced into the closed extremity of the tube, and gently heated, so that it may absorb the carbonic oxide.

The gas is then again transferred to a graduated tube, over water, and measured. The difference in volume shows the amount of carbonic oxide.

The remaining gas, after the removal of the carbonic oxide, is introduced into an eudiometer (§ 32), mixed with a measured quantity (about twice the volume of the gas) of oxygen, and exploded by the electric spark, over mercury.

The volume of the residual gas (oxygen, nitrogen, carbonic acid) is carefully noted, and agitated with solution of potassa, which absorbs the carbonic acid, the volume of which is equal to that of the light carburetted hydrogen.

The remainder is exposed to the action of phosphorus, over water, for 24 hours; the amount of oxygen absorbed is then deducted from the total quantity employed, in order to ascertain the amount consumed in the combustion. By deducting from this quantity twice the volume of the light carburetted hydrogen, we obtain the measure of oxygen consumed by the free hydrogen, the volume of which is of course double that of the oxygen.

Finally, the residue consists of nitrogen.

In the above sketch, we have, for the sake of simplicity, omitted to mention the corrections for temperature, barometric pressure, and aqueous vapor, so necessary in all analyses of gaseous mixtures. For the methods of effecting these corrections, we refer to § 8.

§ 128. OIL-GAS.-Refuse fat, train oil, and impure oils of various kinds, which are not adapted for burning, are sometimes used for the production of gas. Experiments made by Henry, have shown that the gas obtained by distilling various oils at a comparatively low temperature (a dull red heat) possesses illu