| Carbon | 77.53 |
| Hydrogen | 6.33 |
| Nitrogen | 1.03 |
| Sulphur | 0.61 |
| Oxygen | 14.50 |
| ——— | |
| 100.00 |
These elements in tar are built up into an enormous number of compounds (see [Coal Tar]), and its value as a by-product may be gathered from the fact that on fractional distillation it yields—(1) benzene and its homologues, from which aniline, the source of most of the coal-tar colours, can be derived; (2) carbolic acid, from which picric acid, used as a dye, a powerful explosive, and to give the bitter flavour to some kinds of beer, is made, also many most valuable disinfectants; (3) naphthalene, used for disinfecting, and also as the “Albo-carbon” employed in an enriching burner for gas; (4) pitch, extensively used in path-making, from which such bodies as anthracene and saccharin can be extracted.
The second liquid product of the destructive distillation of coal is the ammoniacal or gas liquor, which consists of water containing ammonia salts in solution, partly condensed from the hot gas, and partly added to wash the gas in the scrubbers. It contains, as its principal constituents, ammonia, partly combined with carbonic acid and sulphuretted hydrogen to form compounds which are decomposed on boiling, with evolution of ammonia gas, and partly combined with stronger acids to form compounds which require to be acted upon by a strong alkali before the ammonia contained in them can be liberated. The ammonia in the first class of compounds is technically spoken of as “free”; that present in the latter as “fixed.” The following analysis by L.T. Wright will give an idea of the relative quantities in which these compounds exist in the liquor:—
| Grammes per litre. | ||
| Free | Ammonium sulphide | 3.03 |
| Ammonium carbonate | 39.16 | |
| Ammonium chloride | 14.23 | |
| Fixed | Ammonium thiocyanate | 1.80 |
| Ammonium sulphate | 0.19 | |
| Ammonium thiosulphate | 2.80 | |
| Ammonium ferrocyanide | 0.41 |
From a scientific point of view, the term “free” is absolutely incorrect, and in using it the fact must be clearly borne in mind that in this case it merely stands for ammonia, which can be liberated on simply boiling the liquor.
The gas which is obtained by the destructive distillation of coal, and which we employ as our chief illuminant, is not a definite compound, but a mechanical mixture of several gases, some Gaseous products. of which are reduced to the lowest limit, in order to develop as fully as possible the light-giving properties of the most important constituents of the gas. The following analysis gives a fair idea of the composition of an average sample of gas made from coal, purified but without enrichment:—
| Hydrogen | 52.22 |
| Unsaturated hydrocarbons | 3.47 |
| Saturated hydrocarbons | 34.76 |
| Carbon monoxide | 4.23 |
| Carbon dioxide | 0.60 |
| Nitrogen | 4.23 |
| Oxygen | 0.49 |
| ——— | |
| 100.00 |
These constituents may be divided into—(a) light-yielding hydrocarbons, (b) combustible diluents and (c) impurities. The hydrocarbons, upon which the luminosity of the flame entirely depends, are divided in the analysis into two groups, saturated and unsaturated, according to their behaviour with a solution of bromine in potassium bromide, which has the power of absorbing those termed “unsaturated,” but does not affect in diffused daylight the gaseous members of the “saturated” series of hydrocarbons. They may be separated in a similar way by concentrated sulphuric acid, which has the same absorbent effect on the one class, and not on the other. The chief unsaturated hydrocarbons present in coal gas are: ethylene, C2H4, butylene, C4H8, acetylene, C2H2, benzene, C6H6, and naphthalene, C10H8, and the saturated hydrocarbons consist chiefly of methane, CH4, and ethane, C2H6.
The light-giving power of coal gas is undoubtedly entirely due to the hydrocarbons. The idea held up to about 1890 was that the illuminating value depended upon the amount of ethylene present. This, however, is manifestly incorrect, as, if it were true, 4% of ethylene mixed with 96% of a combustible diluent such as hydrogen should give 16- to 17-candle gas, whereas a mixture of 10% of ethylene and 90% of hydrogen is devoid of luminosity. In 1876 M.P.E. Berthelot came to the conclusion that the illuminating value of the Paris coal gas was almost entirely due to benzene vapour. But here again another mistaken idea arose, owing to a faulty method of estimating the benzene, and there is no doubt that methane is one of the most important of the hydrocarbons present, when the gas is burnt in such a way as to evolve from it the proper illuminating power, whilst the benzene vapour, small as the quantity is, comes next in importance and the ethylene last. It is the combined action of the hydrocarbons which gives the effect, not any one of them acting alone.
The series of operations connected with the manufacture and distribution of coal gas embraces the processes of distillation, condensation, exhaustion, wet purification by washing and scrubbing, dry purification, measuring, storing and distribution to the mains whence the consumer’s supply is drawn.