The communication may be established at any part of the pipe which conveys the gas into the lime machine. When the connection is made, the fluid in the guage cup C, will be driven up into the perpendicular measuring tube a, by the pressure of the gas, to an altitude equal to a column of liquid contained in the lime-trough. It is essential that the tube a, be at least 21⁄2 feet in height, if the depth of the lime-trough is one foot, for without this precaution, the fluid will rise out of the tube in consequence of the oscillation which it suffers. By this means the overseer of the works will be enabled, by mere inspection, to know whether the workmen have charged the lime trough with the mixture of quick-lime and water to the requisite height, which should never be less than from ten to fifteen inches. Because the abstraction of the sulphuretted hydrogen and carbonic acid gas, from the carburetted hydrogen with which it is combined, is greatly facilitated by pressure, and there is no inconvenience whatever in operating under a pressure of a column of fluid of even double the height that has been stated, provided the apparatus is properly constructed. From experiments made on this subject, I am justified in stating that one half of the quantity of quick-lime that is required for the purification of coal gas in the ordinary way, is sufficient, if the column of the liquid opposed to the gas is raised to twenty inches high, nor is the evolution of the gas in any degree retarded under such a pressure.
The curved tube d d, which is cemented air-tight into the gauge cup c, has a free communication with the gas in the guage cup c. It serves to enable the workmen to form some notion of the chemical constitution of the crude gas, before it passes into the lime machine. For if the stop cock e of the tube be opened, and the descending leg a of the bended tube d be immersed in a glass containing a solution of super acetate of lead, some notion may be formed by a little practice of the quantity of lime requisite for the purification of the gas, from the quantity of (black precipitate) hydrosulphuret of lead produced. Two per cent of quick-lime to the coal employed (if Newcastle coal) is usually sufficient for the complete abstraction of all the sulphuretted hydrogen and carbonic acid, contained in the crude gas, provided the operation be carried on under a pressure of not less than a column of water twelve inches in height.
The test tube f, properly so called, may be adapted to any part of the pipe which conveys the purified gas to its place of destination. It serves to ascertain the purity of the gas, after it has been acted on by quick-lime, by suffering the gas to pass from the tube into a solution of super acetate of lead, which speedily becomes discoloured, if the gas contains sulphuretted hydrogen. The presence of carbonic acid is rendered obvious, by a white precipitate being produced when the gas is made to pass through barytic water. The precipitate, which is carbonate of barytes, effervesces with acids.
It must be obvious that the apparatus which we have now been describing does not require to be placed in the immediate vicinity of the gas light machinery. It may be arranged in the counting-house of the overseer, who, by mere inspection, can then at all times detect the slightest irregularity or insufficiency in the process thus given to the gas light manufacture, a degree of scientific controul of which few arts can boast.
The following method has been found economical and convenient, for preserving quick-lime in a ready state, fit for the purification of coal gas.
Take the lime as soon as possible after it is burnt; put it into a pit eight or ten feet long, five or six wide, and five or six deep, constructed of brick-work and level with the ground. By this pit set a wooden trough about six feet long, three feet broad, and two feet deep. The trough should have at one end a hole about six inches square, covered with an iron grating, the bars of which are a quarter of an inch distant. Let this grating be provided with a slider, which can occasionally be drawn up to uncover, or pushed down to cover, the grating. Put three or four bushels of lime at a time into the trough; throw water on it, and mix it up into a thick fluid mass with a hoe perforated with holes. When there is a good quantity of liquid, draw up the slider and let the slacked lime run into the pit. Throw more water on the remaining unslacked lime, and lastly reject those pieces which will not slack. The trough should have a small inclination and project over the pit.
After the lime thus slacked has been five or six hours in the pit, it will take the consistence of a stiff paste, which it retains for years. It should then be kept covered to keep it clean and to exclude the free contact of the air. For those who use larger quantities of lime, several pits should be constructed in preference to one larger reservoir. When the lime is wanted for use it may be dug out with a spade, and readily diluted with a sufficient quantity of water.
The quick-lime thus prepared forms a perfect homogeneous mixture. The practice of throwing lime simply slackened into the lime cistern is a wasteful and slovenly process, as will becomes obvious on examining the waste hydrosulphuret of lime discharged from the machine, which will be found to abound with lime in a concrete form, unacted on by the substances with which it was intended to combine.