It is not easy to assign a limit to the magnitude of this part of a coal gas apparatus ; the slower the passage of the gas through it, and the larger• capacity of the tar-vault and condensing tubes, the more perfect will the separation of the tar and gas be. But, in fixing the opposite limit, it will be ad visable to make these of such capacity, that each portion of the gas may he detained at least three quarters of an hour in its passage through the same.
The quantity of tar, and other liquid products, yielded by a given weight of coal, varies considera bly, not only according to the quality of the coal, but from the manner in which the decomposition has been effected, and which has already been spoken of. One cwt. of good cannel coal gives, when dis tilled at a bright red heat, from six to seven lbs. of tar and oils, and commonly about half that weight of water, making together one ale gallon or there abouts.
Separation 42f the Gases unfitted for the purposes of Illumination.
These, in general, consist of sulphureted hydrogen, carbonic acid, carbonic oxide, and hydrogen gases.
To separate them entirely from the carburet ed hydrogen and ole6ant gases forms no easy task, and in gas-light establishments attention has princi pally been paid to the first of these ; and though the means used to detach the sulphureted hydrogen have also served to take away the carbonic acid, neither the separation of this acid gas, nor that of the other gases above enumerated, have been con sidered of much importance.
It has been already noticed, that the sulphureted hydrogen and. carbonic acid gases are produced principally at the commencement of the distillatory process, and disappear almost entirely towards the end. From the experiments of Dr Henry, • it appears, that gas from Wigan cannel contained about 5 per cent. of each of the above, and other varieties of coal from different parts of the kingdom, furnished results of a similar nature ; but unless care be taken to make use of coal, separated as much as possible from the common pyrites with which it frequently abounds, the quantity of sulphu. reted hydrogen gas will be increased very consider ably, and its separation be but partially effected by the application of cream of lime, as commonly ap plied. Potash, and some other substances, on ac count of their costliness, have not been made use of, at least on any large scale ; and though various plans have been proposed for absorbing the sulphureted hydrogen gas, It does not appear that any have been found to answer in which lime was not the principal agent, and used either in the shape above mention ed, or more or less combined with water.
Chlorine, from its property of uniting with sul phureted hydrogen gas, has been proposed as a fit substance for the purification of coal gas. The im possibility, however, of presenting the two gases to gether, in proper proportions, would be a sufficient reason for not using it, were others wanting. Chlo rine, however, acts upon olefiant gas, as appears from Dr Henry's account (Man. Mem. Vol. III. new se ries), to which objections are also mentioned to ano. ther proposed plan of purification, by passing the gas through red hot iron tubes.
Washing the crude coal gas with water alone has frequently been practised, but very little be nefit can result. A simple, but very ineffectual mode of purifying the gas by the action of lime cream, and which was practised for a considerable time, consists in merely forcing the gas through a tube, or tubes, terminating a few inches beneath the surface of the fluid, as represented in fig. 9 - a a, a close vessel having pipes b, c, d, e, attached to it ; the first of these serves to introduce the lime cream, the second to admit the gas, the third to take it away, and the remaining one to draw off the lime and water. The vessel being about half filled with the liquid, gas is forced in at the pipe c, and on making its escape at the lower it rises up in bubbles, exposed to the action of the lime, and is thereby somewhat purified ; and when the liquid is supposed to .be saturated to a certain degree, it is drawn off, and a fresh supply introduced.
Another variety of purifier is represented in fig. 4 : a a is an oblong close vessel, having a number of vertical partitions made fast to the top and sides, but not reaching to the bottom ; these are pierced near their lower edges with a number of small holes; c, pipe to introduce gas, and d, one for conveying it away ; lime cream being put in by means of the ves sel b, to the height shown, the gas is forced forward, and passes, exposed to the action of the lime, through the perforated plates one after another, and finally escapes at d.