A retort constructed so that the coal (broken; small) could be exposed in very limited quantities at once to the action of the fire, and, at the same time scattered so as to cover a large surface of the heat. ed vessel, would be desirable. The constant char ging and discharging of such a retort, however, where each process of distillation would be com pleted in a few minutes, is a serious objection. Forms have been devised for effecting these ends during the carbonization of a number of succes sive portions of coal, without opening the doors of the retort ; but the internal machinery requisite for this purpose is liable to derangement and uncertain. ty of action, owing to the great heat it must necessa rily be exposed to. We have not hitherto seen or heard of any constructions wherein the requisites of simplicity, durability, and certainty of effect are at tained to such a degree as to warrant their being substituted for others, the results of which are known from long experience.
The quality of coal gas varies considerably during the period of distillation, the first products usually containing- carbonic. acid, olefiant, and sulphuretect hydrogen gases-1 while those obtained towards the, end of the process contain hydrogen gas and carbo nic oxide. The quantity of gas produced in a given time varies also very considerably : such a retort, as is shown in fig. 8, will generate it at the rate of about three cubic feet per minute at the commence ment, and continue to do so, or with little variation, for nearly two hours, when the quantity rapidly de creases till the end of the operation, which will oc cupy from three and a half to nearly four hours, when the retort is kept at a proper temperature.
Separation of the Gaseous and Liquid Products ob. tainedfrom Coal.
It has already been stated, that the liquid substan ces obtained from coal by distillation are tar, oil, and water ; the latter generally. holding in solution a portion of hydrosulphurct and subcarbonate of ammonia. As. all of •these leave the retort in a va porous. state, the, condensation of this vapour is an. object necessary to be attended to, and it is more or less perfectly effected by passing the same through talbes•r vessels surrounded with cold water, or other wise exposed to a cooling medium.
Refrigatories for this purpose have been variously constructed ; some differing, in no respect, from the worm of a still-tub ;.others so arranged as to present broad, flat, or curved surfaces to the action of the cooling, body ;—all these, and similar contrivances, do not, however, effect the separation of the tar and oils;—a. minute portion remains suspended in the gas, of which it is not easily deprived. Time and
stillness, or the absence of agitation or currents, have a considerable influence in inducing a deposi tion ; and by constructing the internal parts of the condensing apparatus very large and roomy, as re gards their transverse section (but, at the same time, of forms exposing a great surface), the gas is requir ed, to travel through it with a proportionably slow. current, thereby giving the tar a better opportunity of depositing, than if the vessels were-of a less area of section, and greater in extent otherwise. It is therefore of little consequence of what particular forms the condensing_vessels are made ; a flat tube, of considerable area, placed perpendicular, or nearly horizontal (end surrounded with water), and extend ed in one continued line or otherwise, as circum stances will admit of, and having proper apertures for the admission and discharge of the gas, and a suitable reservoir and outlets for the tar and other fluids, will be found to answer the purpose as well as constructions of greater complexity.
Such an apparatus is represented in Plate L XX XII. fig. 1, where a a.is a water-tight cistern ; b b, a range of tubes placed vertical, connecting with each other as .shown; c,.entrance.pipe for the gas, and d. that where it makes its exit; e, a pipe to convey away the tar and other condensible liquids which fall down in to the lower part of the vessel,ff Another construction is represented in fig. 2, where a a is a close cistern or .vault of.brick, or-iron, having an opening b, for admitting the whole of, the volatile products of tho..distillation ; another at c, for drawing off the tar and other fluids at any particular level, the pipe there attached being made to slide vertically through a stuffing box. A verti cal partition is placed in the vault, extending nearly the whole of its length, to cause the gas to circulate through it previous to its passing into a third opening d, which conveys away the crude gas, and so much of the tar, &c. as has not got deposited in the vault a ; e e, a water-tight cistern of iron or other material, through which a current of cold water is continually circulating : it may either be placed over the former, or in any other convenient situation, and the pipe ffff, traverses this vessel as shown, sloping gra dually upwards till its termination at k, where it passes forward to the purifier. The gas in its ascent along this pipe is exposed to the action of the cold surface, and the tar and oils which are thereby con densed run back into the vault a a.