The best furnace to be used with the char coal is represented at A, fig. 430, and is made of stout sheet-iron bent into the forin of a trough, open at one end ; the plate which closes the other is perforated with an aperture three-quarters of an inch in diameter, to allow the passage of the combusuon tube ; the fur nace is about twenty inches long, five inches at top, two inches and three-quarters at bottom, and three inches high. Transverse slits are made along the floor at intervals of two inches for draught, and between each are rivetted ver tical stiff pieces of sheet-iron one inch high, terminating in a concave edge above, for the support of the combustion tube. The appa ratus may rest on bricks during the operation, as represented in the wood-cut.
The tube in which the mixture is burned, the combustion or retort tube, (fig.430, a, b. c,) should be of difficultly fusible glass free from lead, about fifteen inches long and half a.n inch in diameter: the hard Bohemian glass answers the purpose perfectly. The tube may on cer tain occasions be drawn out into a fine but strong tall bent upwards at an obtuse angle, and the mouth should be smoothed by making it red hot in the flame of the blowpipe, so that a cork need not be torn in adjusting it.
The apparatus for containing the chloride of calcium which collects tbe water, or drying tube, is conveniently made of the shape de- " picted (fig. 430, B): it consists of a tube about half an inch in diameter and four inches long. Upon one end is blown a bulb, to contain a larger portion of the chloride, and from the bulb a strong tube of small diameter extends for an inch and a half. The chloride of cium with which it is filled must not be fus but should be prepared merely by evaporati the solution of the chloride to dryness by strong sand heat. A porous mass is thus tained, which does not crystallize by absorbi moisture, as the fused variety does, to the d struction of the tube that contains it. In ord to charge the apparatus a few fibres of cott wool are put into the bulb, and by sucking through the small end adjusted over the a ture of the fine tube to prevent any min particles from falling out : into the apparatus within three-quarters of an inch of the large end, the chloride broken into fragments about the size of peas, is put, and a loose piece of cotton-wool, occupying another quarter of an inch, thrust in; the opening is then closed by a cork, through which passes a bit of straight tube, rather larger in the bore than that at tached to the bulb, and projecting about an inch outside the whole; the cork is trimmed close to the large tube and covered neatly with melted sealing-wax; and, lastly, air is drawn through the apparatus by the mouth to ascer tain that no obstruction exists. It is novv ready
for use : after two or three experiments the chloride should be renewed, or there will be a danger of the gases being imperfectly dried. The tube when not in use must always be placed in a rack with the bulb end uppermost to prevent the loss of any small pieces of chlo ride. Concentrated sulphuric acid may be advantageously substituted for chloride of cal cium in the drying tube. In this case the tube is filled with fragments of pumice-stone; these are moistened with the oil of vitriol, and the apparatus is fitted up as usual, excepting that the employment of the cotton-wool is dis pensed with.
The combustion tube is then prepared by selecting a sound elastic cork, which is made accurately to close the mouth of the tube ; it is pierced with a round file, and fitted firmly upon the fine tube proceeding from the bulb of the drying tube ; the cork is then well dried on the sand-bath, and forms the medium of connection between the retort and drying tube.
'The potash apparatus is one invented by Liebig, represented infig. 430, e. It consists of a fine but stout tube, upon which is a series of bulbs, three in the middle horizontal part of the instrument, and one on each of the ascending limbs ; one of the latter bulbs is made considerably larger than the other; the apparatus is filled by adjusting a suction tube to one of the openings, and exhausting by the cheeks until a certain measured quantity of solution of potash has entered ; when the liquid fills each of the three lower bulbs rather more than three-quarters of their capacity, suf ficient has been introduced.
The solution of potash employed has a sp. gr. of from 1.25 to 1.27, and must be renewed for every experiment ; the portions that have been used may be put aside, and afterwards, when sufficient has been collected, may again be rendered caustic in the usual way by quicklirne.
The compound commonly used for supply ing oxygen to the substance burned is oxide of copper, which readily imparts oxygen to combustible matter in contact with it, but bears a very high temperature per se vvithout decomposition. It is best procured by dis solving copper in pure nitric acid, evaporating to dryness and decomposing the nitrate by heating it strongly in an earthen crucible. Ig nition is kept up till red fumes cease to appear; if the heat be too great the oxide becomes agglutinated, and requires strong pounding in an iron mortar to pulverize it ; it however, in this dense state, is much less hygroscopic, and therefore better adapted for the purposes of analysis. The powdered oxide is sifted through a fine copper sieve, and secured in stoppered glass bottles.