The quality of coal-gas is largely dependent upon the temperature employed in its manufacture. If the retorts are insufficiently heated, the result will be the formation of certain easily-condensable hydrocarbons, which not only diminish the bulk of the gas, but cause considerable inconvenience by collecting in and blocking up the pipes. On the contrary, should too much heat be used, the gas becomes partially decomposed by contact with the red-hot retort, and deposits on its sides the substance known as “gas carbon,” thus not only removing to a certain extent the constituent to which the gas owes its illuminating power, but impoverishing its lighting qualities still more, by diluting it with an unnecessary quantity of liberated hydrogen. These latter effects are forcibly illustrated in the following analysis of the gas collected from Wigan cannel coal at different periods of the distillation.
The best gas is said to be produced when the retorts are heated to a bright cherry red.
| In 100 Volumes. | 1st Hour. | 5th Hour. | 10th Hour. |
| Olefiant gas and volatile hydro-carbons | 13·0 | 7·0 | 0·0 |
| Marsh gas | 82·5 | 56·0 | 20·0 |
| Carbonic oxide | 3·2 | 11·0 | 10·0 |
| Hydrogen | 0·0 | 21·3 | 60·0 |
| Nitrogen | 1·3 | 4·7 | 10·0 |
“The value of gas as an illuminating agent may be said to depend on the amount of hydrocarbons present, and on the relation which the carbon bears to the hydrogen in these substances. In marsh gas, CH4, which is, practically speaking, non-luminous, the per-centage composition is, carbon 75, and hydrogen 25. In olefiant gas, C2H4, the carbon is 85·7, and the hydrogen 14·3, and the gas possesses a correspondingly greater amount of illuminating value. In acetylene, C2H2, we have a gas of still greater illuminating value, the proportion of carbon to hydrogen being also greater, the per-centage composition being, carbon 92·3, and hydrogen 7·7. In benzol, C6H6, we have the same per-centages; while in naphthalene, C10H8, a still higher ratio between the carbon and hydrogen exists, and a corresponding increased value in light-giving power. It was formerly taken as an axiom that the illuminating value of a mixture of gases was also proportionate to the relation between the carbon and hydrogen, but although this is very good as a rough criterion in practice, the statement must not be accepted as strictly true. The illuminating power of a mixture of gases is known now to depend far more on the nature of the particular compounds present, than upon the absolute proportion between the hydrogen and carbon; for while on the one hand it is possible to have a gas (marsh gas) containing as much as 75·4 per cent. of carbon, and yet which is valueless for illuminating purposes; it is also possible to have a mixture of gases in which the per-centage of carbon is far less, although the illuminating value is much greater.”[326]
[326] ‘Chemistry, Theoretical, Practical, and Analytical,’—Mackenzie.
Coal gas consists of a mixture of the following bodies:
Marsh gas (light carbonetted hydrogen). Olefiant gas (heavy carbonetted hydrogen). Hydrogen. Carbonic oxide. Nitrogen. Vapours of liquid hydro-carbons. Vapour of bisulphide of carbon.
The yield of gas, and also the illuminating power of the product, vary greatly with different kinds of coal. The average yield may be roughly estimated at 10,000 cubic feet of gas per ton of coal.[327]
[327] For practical details respecting the manufacture of this product, see the article Coal gas in ‘Ure’s Dictionary of Arts, Manufactures, &c.,’ ‘Wagner’s Chemical Technology,’ and ‘Chemistry, Theoretical, Practical, and Analytical.’
Anthracite is by no means suited for a gas coal. The best coals for this purpose are those which are bituminous; they comprise caking coal, parrot coal, and certain varieties of cannel coal. London gas (which is generally deficient in