In the lime-machine, until lately in use, the gas was made to pass in the apparatus, through passages which could not be guarded from being stopped up in the course of time by the concretion of a quantity of carbonate and hydro-sulphuret of lime, formed during the purification of the gas, so that when the stoppage occurred, a prodigious pressure was produced in the machine, in consequence of which, it was either found impossible to keep the distillatory apparatus air-tight, or if this was accomplished, a great part of the gas was forced through the purifying apparatus, without coming in contact with the lime, by driving the column of mixture of lime and water before it, and of course without being rendered fit for use, previous to its passing into the gas reservoir. This effect was unavoidable without the precaution of employing a very dilute mixture of quick-lime and water.
Numerous instances have also occurred where from the increased pressure which the gas exerted in the lime apparatus, the tar from the hydraulic main was driven up with a prodigious force through the dip pipe, P, fig. 2, [plate IV.], into the retort when the retort was opened, where it took fire to the imminent danger of the whole establishment.
The apparatus originally employed was composed of a large vessel closed on all sides to receive the gas; within this was a smaller vessel or lime trough open at top containing the quick-lime and water; and there was also a third vessel, or inverted trough into which the gas was received.
This inverted trough was open at bottom, and the edge of the open part was immersed beneath the surface of the mixture of lime and water contained in the lime-trough, so that the gas which was introduced in the last-mentioned inverted trough could not escape therefrom, except rising up through the lime and water. To facilitate this, holes or openings were made in the inverted trough near the bottom edge thereof, and beneath the surface of the purifying mixture, so that the bubbles of gas were obliged to rise up through these openings. From this construction of the machine the apertures through which the gas had to pass, were extremely liable to become stopped up, and dangerous consequences ensued.
In order to remedy in some measure the evil, a plan was adopted by Mr. Malam, for making the gas to pass in thin strata underneath a series of shelves, placed horizontally in the machine so as to expose the gas in as large a surface as possible to the contact of the lime and water, and employing the purifying mixture at the same time in a more dilute state:—this arrangement is as follows.
Fig. 4; [plate V.], represents a vertical section of the machine; it is made of cast-iron plates, rendered air-tight by screws, bolts, and iron cement. It consists of three separate chambers, a, a, a, destined to contain the mixture of quick-lime and water. At the under side of each chamber, is bolted a cylinder, h, h, h, the lower extremity of which is furnished with a large flanch, extending nearly to within the whole inner diameter of the machine.
From the bottom of each of the chambers, a, a, a, proceeds a pipe curved upwards, and communicating with a circular vessel, C, C, C, which serve for the purpose of charging the chambers, a, a, a, with the mixture of quick-lime and water, and regulating the level of the fluid within the chambers. The curved pipe likewise prevents the escape of the gas when the contents of the chambers a, a, a, are discharged.
The vessels, C, C, C, are provided with a waste pipe and stop-cock, as shown in the sketch, for discharging the contents of one chamber into the chamber placed below it, and lastly into the reservoir e.
b b, are pipes which convey the gas into the chambers, one extremity of each pipe communicates with the cylinders h, h, h, and the other with the chamber below it, and the lower pipe communicates with the valve M, so that by this means a communication is formed from the lower cylinder h, to the middle cylinder h, and from the middle to the uppermost cylinder. K, is the exit pipe which conveys the purified gas from the uppermost chamber into the reservoir destined to receive it. Through the centre of the machine passes a wrought-iron shaft, furnished with agitators or arms, to stir up the mixture of quick-lime and water. The arms are not immediately connected with the shaft, but proceed from cast-iron hydraulic cups, of the usual construction, by this means the escape of the gas is prevented, nor can the fluid pass from one chamber into another. The axis is put in motion, by wheel-work as shown in the design e, the handle for turning the shaft.
g, is a receiver to collect the condensible products. The contents of this vessel may be discharged by a hand pump being attached to the upper extremity of the pipe f, after the cap with which it is closed is removed.