1848.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



15T 



COMPOSITION OF COAL GAS. 



Extracts from a lecture, by Dr. A. \V. Hofmann, delivered at the Royal 

 College of Chemistry, Hunorer-square, 



The composition of gas evolved in tlie distillation of coal is by no 

 means constant ; on the contrary, it vanes to a considerable extent, de- 

 pending principally on the nature of the coals, the presence or absence of 

 moisture, and the temperature at which the distillation takes place. The 

 chief elements which constitute coal are carbon and hydrogen, with 

 small quantities of nitrogen and oxygen ; and, according to the quality of 

 the coal, a larger or smaller amount of earthy matter. Another frequent 

 ingredient is sulphur. This sulphur occurs almost invariably in combina- 

 tion with iron, in the form of iron pyrites. The quantity in which it exists 

 varies very considerably ; many kinds of coal contain so large an amount 

 that they become altogether useless for the purpose of distilling gas. If 

 coal be ignited, and atmospheric air excluded, a portion of its elements are 

 evolved as gas, and the remainder become coke. The gases thus evolved 

 contain carbon, hydrogen, nitrogen, oxygen, and sulphur. None, however, 

 of these elements, except nitrogen, are found in an uncombined state among 

 the products of the distillation of coal. 



The following tables show the different combinations into which these 

 elements enter during distillation. These combinations are very numerous, 

 and are divided into two groups — viz. : substances which are solids or 

 liquids at the ordinary temperatures, and compounds which present them- 

 selves at the common temperature in the form of gas. 

 COAL GAS NAPTHA. 

 Acid Portion. 

 Hydrate of phenyle. . .. C12 .. H6..02 



Neutral Portion. 



NOTE. — The letters represent — C, carbon: H, hydrogen; O, oxygen; N, nitrogen, 

 and so on; the figures designate the number of atoms of xrhich each volume is com- 

 posed — thus, one atom of aniline contains 12 atoms of carbon, 7 of hydrogen, and I of 

 nitrogen. 



The above series of substances are each of them highly interesting to 

 the scientific chemist, whilst several are likely to become of high practical 

 utility. These substances, along with others which are little known, 

 constitute the complex viscid mixture called "tar;" and it is rather sin- 

 gular that many of them, in their separate form, are oils, possessing the 

 most delightful odours. 



The second table exhibits the different constituents of the gaseous pro- 

 ducts of the distillation of coal, as follows : — 



CONSTITUENTS OF COAL GAS. 



Name of Constituent. Proportions. Spec. Gray. Products of Combustion. 



0-OUOl 



Water. 



V'ater and carbonic acid. 



Incombustible gases. 



Hydrogen . . . . H 



Light carburetted hydrogen C H 2 0-5J28 .. "] 



Oliliant gas .. .. C H l o;i6"4 r^^' 



Volatile hydrocarbons.. CnHnJ ' ' .. J 



Carbonic oxide.. .. CO 0-;tf.r4 .. Carbonic acid. 



Cyanogen .. .. C2N l"bolO .. Carbonic acid and nitrogen. 



Sulphide of carbon .. C S2 .. Ciirbonic acid and sulph. acid. 



Sulphuretted hydrogen H S 2'0620 .. Water and sulphureous acid. 



Ammonia .. .. H H3 0"58i)7 .. M'ater and nitrogen. 



Sulphureous acid .. SO 2'2114 .. ^—^ 



Hydrochloric acid . . H C 1 



Aqueous vapour .. HO 



Nitrogen .. .. N Oft'.'O 



Carbonic acid .. .. C 02 1-5203 



In this table, the first is hydrogen, one of the constituents of water. 

 From it is obtained a colourless transparent gas, remarkable for its low 

 specific gravity, being one of the lightest substances known. It burns 

 with a pale IJame, requiring I a volume of oxygen, or 2i volumes of at- 

 mospheric air, for its combustion. It is not, however, the luminous prin- 

 ciple of coal gas. The next, light carburetted hydrogen, or tnarsk gas, is 

 a compound, combining a proportion of carbon with two equivalents of 

 hydrogen (C H 2). This gas, along with carbonic acid, is produced by 

 the putrefaction of vegetable substances under water — hence its name of 

 7narshffas. It burns with a pale bluish Hanie, rather more substantial than 

 that of hydrogen — though it is also evident that it could not be, any more 

 than the othei, the illuminating principle of coal gas. The chief consti- 

 tuent of coal gas is oiejiant gas — a name derived from its property 

 of producing, when in contact with chlorine at the common temperature, a 

 peculiar aromatic oil (of which a specimen was exhibited). It very much 

 resembles chloroform, and no doubt but that it has also the same remark- 

 able properties. It is far richer in carbon than marsh gas, the per centage 

 of the latter being only 73, while that of the former is more than 85. 

 Oleliant gas burns with a beautifully brilliant flame, and constitutes the 

 true illuminatiug principle of coal gas. It requires for combustion to one 

 volume of olefiant gas, three of oxygen, or 15 of atmospheric air. Marsh 

 gas is composed of one atom of carbon, and two of hydrogen ; while 

 olefiant gas combines the two in equal quantities. There is, therefore, a 



large amount of catbon in this gas, which may be proved in a striking 

 manner, by lighting an admixture of one volume of olefiant gas with two 

 of chlorine, which will produce hydrochloric acid, and deposit all the 

 carbon contained in the gas in a cylinder, in the form of a dense smoke, 

 which renders the gas perfectly opaque. 



Volatile hydro-carbons was the next constituent, but with respect to 

 which, at present, there was not much known. The first table contained 

 the names of several substances which had been extracted from the liquid 

 products of the distillation of coal, called tar. These substances differed 

 much with regard to their physical properties — some of them boiling only 

 at very high temperatures, while others volatilised at a heat far below that 

 of boiling water. It was evident, then, that the gas, generated along with these 

 liquids in the retort, would carry off a certain quantity of these hydro-carbons 

 — varying with the distance from the works at which the gas was examined. 

 The great importance of these hydro-carbons in the luminous effects of coal 

 gas would become obvious if their comnosition were considered. Benzol, for 

 instance, contained not less than 92 per cent, of carbon, a far greater 

 amount than that of even olefiant gas itself. This was proved by in- 

 flaming a small quantity of the liquid, so as to allow it an insuflicient 

 quantity of oxygen for complete combustion ; and, in this way, a large 

 portion of the carbon was separated. When mixed with a due amount 

 of oxygen, the combustion of this liquid afforded a splendid light. — 

 — [The talented lecturer showed this, by passing a current of atmospheric 

 air through the lighted benzol ; and also illustrated the peculiarly rich il- 

 luminating power of this vapour, by passing it through the pale and almost 

 invisible flame of hydrogen, which, when thus combined, gave out a volume 

 of light, which gradually and steadily increased in vividness, until the eye 

 could no longer bear its dazzling brightness.] 



Carbonic oxide was the next constituent. Carbon combined with oxygen 

 in two proportions, forming two compound gases ; the one containing the 

 smallest proportion of oxygen was called carbonic oxide; and the other, 

 containing the largest proportion, carbonic acid, which appeared as the last 

 item in the table. Both these gases were colourless, but their properties ex- 

 hibited a striking difference. Carbonic acid was not inflammable, whilst 

 carbonic oxide burnt with a pale blue flame, of little or no luminous power. 

 Again, the latter was quite insoluble in water, while the former dissolved, 

 particularly when the water contained a little alkali, so rapidly as to form a 

 vacuum. — [This was illustrated by experiment.] This solubility rendered 

 easy the removal of carbonic acid from coal gas ; but no method had been 

 discovered of separating carbonic oxide, which burns with gas, though it 

 adds nothing to its illuminating power. 



The other gases were produced in the distillation of coal gas in very 

 small quantities ; he should, therefore, only briefly notice them. There 

 were two more compounds of carbons — the one with nitrogen, and the 

 other with sulphur; the former of these were called cyanogen and the latter 

 sulphide of carbon. Cyanogen was distinguished by its beautiful violet 

 flame — carbonic acid being produced in its combustion, and nitrogen set 

 free. It was also remarkable for its solubility in alkalies — cyanide of 

 potassium being produced, which, with ironsalts, yielded Prussian blue. 

 This gas occurred in coal gas in such small quantities, that its presence 

 might, for a long time, have remained unknown, but for the very delicate 

 test chemists possessed for cyanogen, by which the smallest traces could he 

 detected. Sulphide of carbon was highly inflammable, burning with a blue 

 flame, and producing carbonic and sulphureous acids. These substances had 

 been actually found in coal gas, though they were by no means produced 

 from every kind of coal. 



Sulphuretted hydrogen viislhe next substanceon thelist; it was,however, 

 invaiiably generated, and that too frequently inconsiderable quantities. Sul- 

 phurets of iron, or iron pyrites, which were disseminated through the mass 

 of the coal, was the source of this gas; and its quantity, therefore, de- 

 pended upon the amount of that mineral in the coal. Sulphuretted hydro- 

 gen was also the offensive piinciple in the exhalations from putrefying sub- 

 stances containing sulphur. Sulphuretted hydrogen was a colourless gas, 

 burning with a pale blue flame ; it had not only a most offensive odour, but 

 produced a most deleterious effect upon health, even when mixed with a 

 large proportion of atmospheric air. He had frequently witnessed, in the 

 laboratory, fainting produced by the inspiration of this gas incautiously. 

 Professor Faraday iiad proved, that a dog would die in an atmosphere of which 

 l-800th only was tliis gas ; and that a bird coidd not exist if the gas formed 

 only l-lj06th of its breathing medium. Fortunately, this gas was con- 

 verted, by combustion, into sulphureous acid, which was very much less 

 dangerous and offensive. It was necessary, however, in making coal gas, 

 to obliterate every trace of sulphuretted hydrogen, for the sulphureous acid 

 it produced, although far less injurious, independently of its effect upon 

 health, attacked very readily every metallic surface. Besides, the small 

 quantity of sulphuretted hydrogen which would escape unburnt between the 

 turning of the cock and the ignition of the gas at the burner, and by leak- 

 age, was sufficient to destroy lead, painted, gilt, or silvered articles, in a very 

 short time. The presence of sulphuretted hydrogen might easily be de- 

 tected in gas, by submitting a piece of paper, moistened with the solution 

 of acetate of lead, to the gas uninflamed. 



.Immonia was another product, largely found in the distillation of coal 

 gas, into which nearly all the nitrogen contained in coal was converted. 

 Ammonia was a colourless gas, which, of itself, was very diflicult to inflame, 

 though, when mixed with other combustible gases, it was entirely consum- 

 able. Respecting the products of the combustion of ammonia, accurate 



