separate parts: the lower portion is a shallow dish of steel into which a massive hollow hemispherical cylinder, also of steel, is accurately fitted by grinding; the upper portion is a solid cylinder of the same metal, which exactly fills up the hollow cylinder. When a mineral has to be crushed, it is introduced into the bed of the mortar, the solid cylinder is then replaced and struck forcibly several times with a mallet, by which it is reduced to a coarse powder, and may afterwards be brought to an impalpable state by grinding in the agate mortar.

EVAPORATION.

23. Evaporation is an operation to which the chemist is constantly resorting; he has recourse to it for concentrating liquids previous to the application of certain tests; for separating volatile fluids from fixed substances; for inducing crystallization; or for obtaining in a solid form substances held in solution by water or other liquids. When the object is to retain the fluid evaporated, as well as the residue, the process is called distillation. The ordinary operation of evaporation is conducted in basins of earthenware, silver, or platinum: watch-glasses also occasionally form very useful vessels. The earthenware basins should be as thin as is consistent with strength, and should resist the action of acids and alkalies in solution; the silver and platinum vessels should be provided with a projecting slip of metal to serve as a handle, whereby they may be held by a pair of pincers. In ordinary cases, the object sought by evaporation, is attained by exposing the fluid to heat; sometimes, however, it is effected by leaving it for a certain time in contact with the atmosphere at common temperatures, or in confined air kept dry by hygroscopic substances. In quantitative experiments the evaporating substance should never be allowed to enter into actual ebullition, as a loss would almost unavoidably be sustained by bubbling. It is frequently advisable, therefore, to apply heat through the medium of the water bath, which may be a copper basin about six inches in diameter and three inches deep, provided with a series of rings of different diameters to suit dishes of different sizes. While a liquid is evaporating, it is requisite carefully to protect it from dirt and dust: this is best done in the manner recommended by Fresenius, viz., by providing two small, thin, wooden hoops, one of which must be made to fit loosely in the other;

a sheet of blotting-paper is spread over the smaller, and the larger pushed over it: an excellent cover is thus formed, which, while it effectually guards the liquid, does not come into contact with it, and does not in the least impede or retard the process of evaporation. Sometimes it is necessary to evaporate fluids from solid substances contained in crucibles: in such cases the crucibles should be supported in an inclined position, and the heat applied a little above the level of the liquid an equal distribution of heat is thereby secured, and the loss of a portion of the sediment by spirting avoided. The same method may be adopted in the evaporation of solutions of those salts which have a tendency to effloresce and creep up the sides of the evaporating vessel in the latter case the object may be also attained by slightly greasing the upper part of the sides of the dish. Very frequently in the course of an analysis large quantities of filtrates have to be evaporated down to perfect dryness: in such cases the fluids are first concentrated by evaporation over the naked fire, and afterwards transferred into smaller vessels, to be finished in the water or sand bath. The process of transferring liquid from one basin to another, so as to avoid the smallest loss, requires care and a steady hand; the edge of the large dish should be slightly smeared with tallow, and the liquid poured down an inclined rod, as recommended above when treating of filtration. The evaporation of a mineral water from which carbonic acid or other gases are expelled by heat, should always be commenced in a flask loosely covered with a piece of bibulous paper.

As the evaporation of a fluid in which a considerable sediment is formed draws to a conclusion, the thick mixture requires to be treated with great care; for, the circulation of the heat being now interrupted, the temperature frequently increases at the bottom of the vessel until above the boiling-point of the solution, and then the sudden evolution of small quantities of steam occasions a projection of portions of the substance from the basin. Sometimes, indeed, when the solid residue dries hard, it forms a cake on the surface, underneath which steam accumulates, and suddenly explodes with a force not merely sufficient to disperse the contents of the vessel, but even to break it in pieces. In these cases, the substance should be continually stirred with a glass rod, by which the loss by spirting is prevented.

DISTILLATION.

24. This operation is performed when it is desired to collect the evaporating substance. It must obviously, therefore, be conducted in an apparatus in which the vapour as it rises may be refrigerated, and again reduced to a liquid or solid state, and collected in a separate vessel. When the elastic fluid or vapour assumes on cooling the solid state, the process is called sublimation.

The vessel in which almost every laboratory distillation is effected is the glass retort; those made of hard green glass are

the most serviceable, as they withstand the action of caustic alkalies; it is difficult, however, to get them stoppered. Retorts made of hard German glass are now much used in this country; their high price, though lately much reduced, is the only objection to be raised against them.

The general process of distillation is represented in Fig. 30. The bend of the retort is protected from the cooling influence of the air by a thick paper or cardboard cone, with a broad notch to admit the neck; this interferes with a ready change of

air at the top of the retort, and saves much heat which would otherwise escape at that part, from the condensation of the vapour within. The end of the neck of the retort is connected with a long tube, either by means of a cork or occasionally by cement, or loosely, the junction being made tight by strips of moistened bladder or sheet caoutchouc, and the end of this tube enters the mouth of the vessel into which the distilled products are to be received. The receiver is plunged into a basin of cold water in which lumps of ice are floating. In order to liquefy the vapour rising from the retort as soon as possible, several folds of bibulous paper are wrapped round the neck, just below the bend; and lower down, just above the point where the neck enters the adapter, a ring of tow is tied, the ends of which are caused to hang down four or five inches. Cold water is supplied to this paper from a large funnel containing a filter placed immediately above it in such a position that the drops of water shall fall on and spread over the paper, which will soon become saturated, and the water will run down to the tow, from which it will descend and be caught in a vessel placed beneath, and, provided it does not flow in too rapid a stream, none will enter the flask.

In the distillation of many liquids, particularly such as contain alcohol, the vapour is frequently evolved with such difficulty as not only to endanger the sudden expulsion of part of the substance, but the safety even of the whole apparatus. A tranquil and regular evolution of vapour may always be obtained by introducing into the retort certain angular solids on which the liquid exerts no chemical action, as fragments of glass, or, still better, slips of platinum foil; but care must be taken not to introduce these promoters of evaporation while the fluid is hot, or the burst of vapour might probably be so instantaneous as to do more harm than the previous irregular boiling. The same precaution is applicable to the introduction of solids, of whatever nature, into liquids while boiling or near their boilingpoints. Sulphuric acid, which is the most difficult and dangerous of all substances to distil, may be drawn over quietly and regularly by previously dropping into the retort a few pieces of platinum foil or wire.

Various other contrivances have been resorted to for the purpose of condensing the vapour in the process of distillation. In the common still this is well known to be accomplished by causing the vapours to pass through a long spiral tube, called a

worm, fixed in a tub, and surrounded by cold water. In cases where the products are not easily condensed, Liebig's condenser is a very useful and convenient arrangement, and is applicable in all cases where an open apparatus is admissible.

The condenser is a hollow metal cylinder, through the centre of which a glass tube passes, being fixed water-tight by means of perforated corks covered with cement. This tube is connected either by a cork or by tubing with the beak of a retort;

a constant stream of water is caused to flow from the reservoir placed above, down the funnel, thus entering the cylinder at its lower part. The water, warmed by the condensation of the vapour, flows out through the vertical tube underneath the upper part of the cylinder: the whole apparatus is thus kept constantly cool, and the distillation proceeds in a uniform and steady manner. The operation is represented in Fig. 31.

It is sometimes required to conduct the gaseous products of distillation into vessels containing water or other liquids, as in