stopper of B is perforated and fitted with additional tubes and wash-bottles, or a system of distributing pipes or T-pieces connected with the tube from the cork in B may be employed. board with a glass front and small doors: the cupboard must be furnished with a flue in which a strong draught is created by an argand or ring gas-burner, made of fire-clay or steatite and not of metal; if the cupboard doors are kept closed as much as possible, and students when using the gas are careful to avoid allowing its unnecessary escape, the atmosphere of the laboratory may be kept tolerably free from this badly smelling gas. The cupboard should have within it a drain into which are emptied the spent acid and washings of the apparatus. 500 a. Passing sulphuretted hydrogen into a liquid.-Each student must keep a bent glass tube (D, figs. 39 and 40), made according to the directions in par. 9: when the gas has to be passed into a liquid, the shorter end of this tube is fitted into the india-rubber joint E, and the other end is passed to the bottom of the liquid; on slightly opening the clamp E, a stream of the gas will bubble up through the liquid and may be regulated by the clamp; when the gas has passed for about five minutes the liquid will generally be saturated; if this is the case it will smell of the gas strongly after the air above the liquid has been blown out of the vessel and the vessel has been well shaken. Great care must be taken to close the clamp at E completely when the gas is stopped. The tube D must be thoroughly cleansed after use, employing a tobacco-pipe cleaner if necessary for its inside. 501. A small agate pestle and mortar (fig. 32, p. 57).This is required for powdering very hard substances, more particularly minerals. The substance must have been already broken into small fragments, and these are then crushed to powder by pressure and "trituration,"* until a powder is produced in which no particles or grains are felt when it is pressed or rubbed with the pestle or the finger; such a powder is commonly termed an "impalpable powder." Caution.-On no account must a substance be broken by placing it in the mortar and striking it blows with the pestle, as this is very liable to cause the fracture of the mortar. 502. Several small leaden cups or a platinum crucible should be kept in readiness for testing F by paragraph (295) or (296). The leaden cups are easily made by beating out thick sheet-lead into the required shape, the mouth being of such a size as to be readily covered by a watch-glass; the small leaden inkpots often employed in school-desks serve well for this purpose. 503. A steam oven.-This is a small oven of sheet-copper, heated to the temperature of boiling water (100° C.) by water contained in the space between the exterior of the oven and a copper casing which surrounds it (fig. 41): the outer casing Rubbing round and round under the pestle. often has a circular opening cut in the top, covered by a lid when not in use, which serves as a water-bath (504). The level of water in the jacket is maintained constant by a little contrivance shown in section at c: a small reservoir communicating by a lateral tube with the space between the oven and its jacket is constantly replenished by water which drops into it from the supply tube (d), the overflow of this reservoir is a tube rising in its centre whose end terminates inside above the level of the lateral tube. The outlet for steam (ƒ) may be turned downwards, so that any water formed by the condensation of the steam drops into the reservoir (c), or it may be made to communicate with the worm of the still (506), the escape steam being thus condensed into distilled water. Distilled water should always be supplied to the steam-oven, as this prevents the formation of a troublesome incrustation in the interior; and since the above arrangement, which is all that can be desired when tap-water is supplied, would occasion much waste of distilled water, a better plan of supplying the oven is to place a vessel of distilled water beside the oven and connect them with a syphon, having a bulb or vertical closed tube at its bend, to prevent the air boiled out from the water from stopping the action of the syphon. The steam-oven is employed to drive off the moisture from solid substances which require to be dried at a gentle heat. 504 Several copper water-baths.-The water-bath consists of a hemispherical copper dish with its edges turned over inwards; a series of flat copper rings of gradually diminishing diameters are made to rest upon the edges of the bath and upon one another's edges, so that the opening at the top can be made as small as may be desired. The bath is about two-thirds filled with water, and is heated on a tripod-stand until the water boils, the excess of steam escaping by a small hole made just below the edge of the bath: the flame should be so regulated that the water is kept boiling quietly. A small-sized saucepan is a homely substitute for the copper water-bath. Any liquid requiring to be evaporated at a gentle heat is placed in an evaporating basin upon this bath, its temperature during evaporation cannot then exceed the temperature of boiling water (100° C.). The top of the water-oven is frequently made to serve as a water-bath, see (503). If the water-bath requires to be used for any considerable length of time, it may be supplied with water in the same way as the steam oven (503). 504 a. The following cheap and simple device serves the purpose of both water-bath and steam-oven on a small scale. Two circular plates of sheet zinc are obtained,* about four inches in diameter, so as to cover the larger-sized porcelain dishes used by each student: in one of these a circular hole concentric with the edge of the plate and two and threequarters inches in diameter is cut, in the other a similar hole is made one and a-half inches across. These covers when placed on one of the larger porcelain dishes, nearly filled with water and boiled by a lamp, convert it into a water-bath, a small dish containing the liquid to be evaporated being supported in the hole of the plates: small quantities of liquid may be evaporated or solid substances may be dried by placing them on a watch-glass supported on the smaller perforated plate. In a large laboratory, however, steam-ovens and waterbaths are usually kept constantly heated by steam or boiling water, and are used in common by all students. The steam from a large water-bath may advantageously be condensed to distilled water by connecting the escape-pipe of the bath with a long tube of the still-worm. 505. Several small tubutated flasks.-The small flask shown in fig. 38 (p. 132) is very convenient for the reactions described in pars. 260, 264, and 273. It is perhaps the form of apparatus best suited for the test for a chloride described in (260), because the K,Cr,O, powder is not introduced through the same tube as the CrO2Cl2 escapes by, and the risk of the mixture in the flask spirting into the delivery tube is rendered very small. Its neck should be closed by an india-rubber or glass stopper. * Any working tinman will make them to order. DISTILLATION OF WATER. As has been already shown (Exp. 31, p. 24), water ordinarily contains certain solid substances dissolved in it which render it in a chemical sense impure; such water is therefore unfit to be employed for the processes of solution and washing, since any impurity thus introduced into a substance during analysis would be considered when detected to have been present in the original substance. The quantity of these impurities present in any particular water-supply will depend upon the nature of the soil with which the water has been in contact previous to its collection. The water supplied in some districts will be found when subjected to the tests given in [(526) Remark 41] to be almost perfectly pure; carefully collected rain-water will invariably yield no indication of dissolved impurity; the water supplied to the majority of laboratories will, however, be found to be unfit for analytical purposes, until it has been freed from the solid substances dissolved in it by being subjected to the process of distillation. In most towns distilled water can be purchased, but it is preferable to distil in the laboratory or its immediate neighbourhood all the water required for use. For this purpose the steam obtained either from a steam-boiler or from a copper-still heated by a furnace or gas-burner, or from the water-baths and steam-ovens in the laboratory, is condensed by a tin worm-pipe immersed in a tub through which a constant stream of cold water runs. 506. The following constant apparatus has proved most serviceable in the author's laboratory, and may be used wherever gas and water are laid on in a suitable way. Its advantage lies in the fact that the still is constantly supplied with the hottest portion of the water from the condenser through a lateral tube. When the apparatus has been properly fitted up and the supply adjusted, it requires no attention whatever except lighting and extinguishing the gas under the still, and turning the water supply on and off when distillation is |