| 1. Per cent. | 2. Per cent. | 4. Per cent. | |
| Hydrogen | 38.09 | 43.77 | 48.02 |
| Marsh gas | 42.72 | 34.50 | 30.70 |
| Olefines | 7.55 | 5.83 | 4.51 |
| Carbon monoxide | 8.72 | 12.50 | 13.96 |
| Nitrogen | 2.92 | 3.40 | 2.81 |
| 100.00 | 100.00 | 100.00 |
The gas analysis of No. 3 was lost, but the illuminating power shows that it was intermediate in composition between Nos. 2 and 4. From this it will be seen that, with the increase of temperature, the hydrocarbons—the olefines and marsh gas series—gradually break up, depositing carbon in the crown of the retort, and liberating hydrogen, the percentage of which steadily increases with the rise of temperature.
The tar formed is affected to an even greater extent than the gas by alterations in the temperature at which the destructive distillation takes place. The lower the temperature, the smaller will be the volume of gas produced, and the lighter the specific gravity of the tar, whilst with increase of temperature, the volume of gas rapidly rises, and so does the specific gravity of the tar. Working with a caking coal Wright obtained the following results:—
| Yield of Gas per ton, Cub. ft. | Specific Gravity of Tar. |
| 6,600 | 1.086 |
| 7,200 | 1.120 |
| 8,900 | 1.140 |
| 10,162 | 1.154 |
| 11,700 | 1.206 |
Analysis of the tar showed that the increase of the specific gravity was due to the increase in the quantity of pitch, which rose from 28.89 to 64.08% in the residuals; whilst the ammonia, naphtha and light oils steadily fell in quantity, the creosote and anthracene oils doing the same, but to a smaller extent. Naphthalene also begins to show in quantity in the tar as soon as the yield of gas reaches 10,000 cub. ft. per ton of coal carbonized.
In spite of these variations, however, the products in their main characteristics will remain the same. They may be divided into—(a) Solids, such as the coke and retort carbon; (b) liquids, consisting of the tar and ammoniacal liquor; and (c) gases, consisting of the unpurified coal gas. The proportions in which the products are approximately obtained from a ton of gas coal have been given as follows:—
| 10,000 cub. ft. of gas | = | 380 | ℔ = | 17.0 | per cent. |
| 10 gallons of tar | = | 115 | ” = | 5.1 | ” |
| Gas liquor[1] | = | 177 | ” = | 7.9 | ” |
| Coke | = | 1568 | ” = | 70.0 | ” |
| —— | —— | ||||
| 2240 | 100.0 |
The chief solid residue, coke, is not absolutely pure carbon, as it contains the mineral non-volatile constituents which remain behind as ash when the original coal is burnt, and which, to a Solid products. great extent, existed in the sap that filled the cells of the plant from which the coal was formed. The retort carbon formed as a dense deposit on the crown of the retort by the action of the high temperature on the hydrocarbons is, however, carbon in a very pure form, and, on account of its density, is largely used for electrical purposes.
The liquid products of the destructive distillation of coal are tar and ammoniacal liquor. Tar derived from ordinary bituminous Liquid products. coal is a black, somewhat viscid liquid, varying in specific gravity from 1.1 to 1.2. The ultimate composition of tar made in the London Gas Works is approximately as follows:—