bituminous coal is distilled at a comparatively low temperature, the carbon passes off with but little hydrogen, liquid compounds of carbon and hydrogen are formed, consequently we get plenty of tar and little gas, the latter, however, being of a high illuminating power; but if we gradually increase the temperature, the liquid hydrocarbons decrease, while the gaseous products increase, that is, we find more gas and less tar; increasing the heat still more, we find that the gaseous products are richer in hydrogen and poorer in carbon, marsh-gas, or methane, is formed in great abundance, and finally, when the temperature becomes still higher, free hydrogen is given off, this result always occurring towards the end of the operation of gas-making.

With an increase of temperature the strength of the union between carbon and hydrogen appears to decrease. Ethylene passed over a highly heated surface deposits a portion of its carbon in the solid form, and is converted into methane and hydrogen.

Now when coal is destructively distilled in retorts heated to a proper temperature, it is exposed to two different conditions.

In the first place, the exterior surface of the coal coming into direct contact with the hot walls of the retort is immediately decomposed, such hydrocarbons as ethylene are produced, which at this temperature are partly decomposed and converted into methane and free hydrogen, with an accompanying deposition of carbon, these gaseous products, together with some undecomposed ethylene and volatile hydrocarbons, passing off from the retort.

The interior of the mass of coal is subjected to a somewhat different treatment, as for some time it is exposed to only a very moderate heat, which effects a simple distillation, with the result that those compounds that are formed at a comparatively low temperature, the heavy hydro

carbons, which would ordinarily be in a liquid state, are eliminated, a portion, rising into vapour as it reaches the hotter surface, passes off with the gases formed, and is again condensed on leaving the retort in the form of tar; but that portion of the vapour which in its passage comes into contact with the red-hot surface of the exterior mass of coal, and of the sides of the retort, deposits a portion of its carbon, and is decomposed into simple compounds, ethylene and volatile hydrocarbons, which in turn are also partly decomposed. As the heat penetrates to the centre, and a red-hot mass of charred material of considerable thickness consequently surrounds the decomposing coal in the interior, which is the case towards the end of the distillation, the whole of the hydrocarbons which are then given off are decomposed by having to pass over such an extent of heated surface, and free hydrogen is almost alone given off.

Further," there is no doubt that paraffin hydrocarbons are present in the original coal. These paraffins, under the influence of heat, split up into simpler members of the same series, and into olefines; and if we imagine the action in its simplest form, we should have the gases as they were evolved consisting of (say) ethane and ethylene. These have now to pass down the heated retort on their way to the ascension pipe, and the contact with the heated sides of the retort, and the baking from the radiant heat in the retort, set up an infinity of changes.

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Ethane when heated to this degree splits up into ethylene and hydrogen, while ethylene decomposes to methane and acetylene, and the acetylene at once polymerizes to benzene, styrolene, retene, etc. A portion also condenses, and, at the same time losing some hydrogen, becomes naphthalene; and the compounds so formed by

Professor Lewes, "Journal of Gas Lighting," vol. lxiii., p. 282.

interactions among themselves build up the remainder of the hydrocarbons present in the coal-tar, while the organic substances containing oxygen in the coal break down, and cause the formation of the phenols in the tar.

"There is very little doubt that the general course of the decompositions follow these lines; but any such simple explanation of the actions taking place is rendered impossible by the fact that, instead of the breaking down of the hydrocarbons being completed in the coal, and only secondary reactions taking place in the retort, in practice the hydrocarbons to a great extent leave the coal as the vapours of condensable hydrocarbons, and that the breaking down of these to such simple gaseous compounds as ethylene is proceeding in the retort at the same time as the breaking up of the ethylene already formed into acetylene and methane, and the polymerization of the former into higher compounds. Starting with a solid hydrocarbon of definite composition, it would be theoretically possible to entirely decompose it into carbon, ethylene, methane, and hydrogen, and, by rapidly removing these from the heating zone before any secondary actions took place, prevent formation of tar. But any such ideal is hopeless in practice, as the coal is not a definite compound, and it is impossible to subject it to a fixed temperature.

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“If the retorts are at a temperature of 1000° C. (1832° F.) when the charge of coal is put in, the temperature of the distillation will vary from about 800° C. (1472° F.) close to the walls to about 400° C. (752° F.) in the centre of the coal; and in the same way, in the space above the coal, the products which come in contact with the sides of the retort are heated to 1000° C (1832° F.), while the gas near the coal is probably only heated to 600° C. (1112° F.). Hence we find that the tar is formed of two distinct sets of products the first due to incomplete decomposition, and the second to secondary reactions due to the products of

the decomposition being kept too long in the zone of heat.

"Of the first class the light paraffin oils and pitch may be taken as examples; while benzene, naphthalene, and retort carbon represent the second. The formation of the second class of bodies is a great loss to the gas manufacturer, as, with the exception of the trace of benzene carried by the gas as vapour, these products are not only useless in the gas, but one of them-naphthalene-is a serious trouble, as any trace carried forward by the gas condenses with sudden changes of temperature, and causes obstructions in the service pipes; while their presence in the tar means the loss of a very large proportion of the illuminating constituents of the gas. Moreover, these secondary products cannot be successfully reduced, by further heating, to simpler hydrocarbons of any high illuminating value; while such bodies as naphthalene and anthracene have so great a stability that, when once formed, they resist any efforts to again decompose them by heat, short of the temperature which breaks them up into methane, carbon, and hydrogen."

The effect produced by changes of temperature on the quantity and quality of the gas produced from coal has been very carefully studied by Mr. L. T. Wright, who found that as the temperature is raised, the yield of gas from a given weight of coal increases; but with the increase of volume there is a marked decrease in the quality of the gas obtained. Mr. Wright,' in a series of experiments conducted by him, found that when four portions of the same coal were distilled at temperatures which varied from a dull red heat to the highest temperature attainable in an iron retort, he obtained the following results as to make of gas and of illuminating power :

1 "Journal Chem. Soc.," vol. xlv., 1884, p. 99.

The composition of the gases, generated as above, were as shown below, with the exception of No. 3, which was unfortunately lost, but its illuminating power would tend to show that its composition was intermediate between Nos. 2 and 4.

From a study of the figures in the above tables it will be seen that with the increase of temperature the saturated and unsaturated hydrocarbons, that is, the olefines and marsh-gas series, gradually are broken up, depositing carbon and liberating hydrogen, the percentage of which steadily increases with the rise of temperature, as pre