The gas, as evolved, passes directly from the generator through a pipe iuto another vessel called the purifier, in which it is washed. This vessel is exactly similar in shape to the gas holder, shown in Fig. 293, but is smaller, and has, of course, no rising bell. The pipe which con ducts the gas into this vessel reaches nearly to the bottom, and another pipe, for the exit of the gas, reaches to within a few inches of the surface of the water. The object of the purifier is not so much to trash the gas as to act as a catch-box, to retain any whiting or vitriol that may be blown over by the force of the escaping gas. A few pieces of marble or other limestone should be intro duced into it, in order that any acid carried over may not be wasted. Various arrangements of pipes in the purifier are adopted by different manufacturers to cleanse the gas as thoroughly as possible. As it is difficult, however, to effect a thorough purification of the gas in this vessel, it is again washed in its passage through the gas-holder, which will now he described.
The Gas-holder.—As it leaves the purifier, the gas is conducted by a leaden pipe into the gas holder, a vessel consisting simply of a wooden tub, containing a rising bell, as shown in Fig. 293. The tub is filled with water up to the dotted line, the bent tube from the purifier dipping well 2 n underneath the surface. The other, or drawing-off pipe, stands above the water, and at sueh a height, that it is impossible for the latter to be drawn into it. If it be suspected that this is the case, the pipe should at once be examined for a crack or split. The water must be changed when it has become too impure for the purpose ; generally the holder is cleaned out once a month The copper holder, or bell, is steadied by means of counter-weights. To commence the process, the top cock is opened to allow the air to escape as the bell descends ; the latter should not touch the bottom, but remain within a few inches of it. The drawing-off pipe, by which the gas is conducted from the gas-holder to the filling machine, is bent in the shape shown in the figure.
The Mixing machine, in which the gas and water are mixed together, consists essentially of a cylinder containing an agitator. The gas and water are forced in, and the mixture is effected by the aid of the agitation, the gas being, of course, under a high pressure. To force in the gas and water, a pump is attached to the machine, having connection with the gas-holder and with a vessel containing water placed at the side. There are two arrangements of the pump io common use ; in the first of these, each stroke of the piston drives in the gas first and then the water ; in the second this order is reversed, the water going in first and the gas following. When the water is ejected from the pump-barrel before the gas, all the passages of the valve-box are filled with the latter under compression, which must expand before the pump can draw a new supply ; this expansion is equal to a loss of fully one-third of the labour employed, since the plunger is half-way up the barrel before it begins to draw. But with the other
arrangement of the pump, in which the valve box is at the top and the plunger underneath, the gas is ejected before the water, and all the passages being empty, the pump begins to draw from the commencement of the descent of the plunger.
In Fig. 285 is shown what is termed a double pump machine, having one cylinder and two pumps, of the first of the two classes mentioned above. If two cylinders are affixed to this machine, it will produce two kinds of aerated waters at the same time ; or it may be arranged to yield a water doubly charged with gas. The cylinder, which is shown partly in section in Fig. 286, is made of copper thickly lined with tin, and is in two parts, firmly bolted together. The advantage of this is that if anything goes wrong in the interior, it may be readily discovered by taking the cylinder to pieces ; machines which have the cylinder soldered up often occasion a good deal of trouble. The agitator should have a bracket support outside the cylinder, in order that it may work in leather instead of metal, since the latter imparts a metallic taint to the water in the interior. The cylinder in the figure is provided with a water gauge, dial indicator, and safety valve ; the necessity for these will be obvious. As will be seen, the pumps are placed beneath the cylinder ; they are connected by means of tinned pipes, with the gas-holder placed alongside the machine, and with the water or solution pan, which is also placed at the side or behind the machine on a stand, as shown in the figure. Another pipe made of pure tin connects the cylinder with the bottling machine, to be described later. The agitator should be driven by toothed wheels, not by belts or bands. Belts are always liable to run off or to slip, oeourrences which seriously interfere with the regular working.
Before setting to work, the machine should be turned round a few times, and all the parts of the machine carefully examined to see that the water and pressure gauges are in working order, index cocks properly set, the bolts of bearings at proper tension, and the valves free from If it has been standing some time, the machine will probably leak, on account of the washers being dry, in which case, time should bo allowed for them to soak thoroughly before tightening up; this, when necessary, is done gradually and evenly all round. As the cylinder should not be moro than about half-full of water during working, the gas cock is turned on full, and the water cock half way. The earthenware pan at the side of the machine, which supplies the solution or water, should be kept cool and covered over.