porcelain dish, pouring over it a mixture of one part of hydrochloric acid or nitric acid with about nine parts of water, and letting it digest for several hours at a moderate heat. The fluid is then poured off, and the paper is repeatedly washed with water (finally with distilled water), until litmus paper is no longer reddened by the washings: the water is then drained off, and the entire layer is carefully transferred to a quire of blotting-paper, and left there until they can be taken off singly without injury; they are then hung up to dry on lines in a place free from dust. With the finer sorts of paper (Swedish paper) I prefer washing the filters in the funnel. To this end they are first sprinkled with a little moderately diluted hydrochloric or nitric acid, and then thoroughly washed with water, finally with distilled water. Filtering paper, to be considered good, must, besides being pure, also let fluids pass readily through, whilst yet completely retaining even the finest pulverulent precipitates, such as sulphate of baryta, oxalate of lime, &c. If a paper satisfying these requirements cannot be readily procured, it is advisable to keep two sorts, one of greater density for the tion of very finely divided precipitates, and one of greater porosity for the speedy separation of grosser particles. The funnels must be of glass or porcelain (§ 16, 10); they are usually placed on an appropriate stand, to keep them in a fixed position. The stand shown in Fig. 1 is particularly well adapted for the reception of the small-sized funnels used in qualitative analyses. 8 6. 5. DECANTATION. FIG. 1. separa. This operation is frequently resorted to instead of filtration, in cases where the solid particles to be removed are of considerably greater specific gravity than the liquid in which they are suspended, as they will in such cases speedily subside to the bottom, thereby rendering it easy either to decant the supernatant fluid by simply inclining the vessel, or to draw it off by means of a syphon or pipette. In cases where filtration or decantation are resorted to for the purpose of obtaining the solid substance, the latter has to be freed afterwards by repeated washing from the liquid still adhering to it. The operation is called washing or edulcoration. The washing of precipitates collected on a filter is usually effected by means of a washing bottle, such as is shown in Fig. 2. This consists of a flask or bottle, closed with a twice-perforated, snugly-fitting cork, through which pass two glass tubes, as in the figure. The outer end of the tube, a, is drawn to a moderately fine point. By blowing into the other tube, a stream of water is driven out from a with considerable force, which adapts the apparatus to removing precipitates from the sides of vessels as well as to washing them on filters. This form of washing-bottle serves for edulcoration with warm or even boiling water, provided the vessel itself has a uniformly thin bottom, so that it can be heated without fear of breaking. It is improved by binding about the neck a ring of cork, or winding it closely with smooth cord. It then may be handled with convenience when its contents are hot. [As it is usually inclined when in use, it is further improved by bending the lower part of the tube a (after it is fitted through the cork), so that its end closely approaches the bottom of the bottle at B. Its contents may then be used nearly to the last drop without need of so frequent replenishing.] Fig. 2. As the success of an analysis often depends upon the complete or proper washing of a precipitate, we may here remark that the operator must accustom himself to continue the process until he is certain that the object in view has been actually accomplished. In general, this is not the case until the precipitate has been perfectly freed from the liquid in which it was formed. The analyst must not be content to guess that a precipitate is thoroughly washed, but must prove that it is so, by applying appropriate tests. If the body to be removed is non-volatile, slow evaporation of a few drops of the last portions of the washings on a clean surface of glass or platinum, will usually serve to indicate the point at which the process may terminate. There are four operations which serve to separate volatile subsances from less volatile or from fixed bodies, viz., evaporation, distillation, ignition, and sublimation. The two former of these operations are applied exclusively to fluids, the two latter exclusively to solids. § 7. 6. EVAPORATION. This operation is of very frequent occurrence. It serves to separate volatile fluids from less volatile or from fixed bodies (no matter whether solid or fluid), in cases where the residuary sub stance alone is of importance, whilst the evaporating matter is entirely disregarded; thus, for instance, we have recourse to evaporation for the purpose of removing from a saline solution part of the water, in order to bring about crystallization of the salt; we resort to this process also for the purpose of removing the whole of the water of the menstruum from the solution of a non-crystallizable substance, so as to obtain the latter in a solid. form, &c. The evaporated water is entirely disregarded in either of these cases, the only object in view being to obtain, in the former case, a more concentrated fluid, and in the latter a dry substance. These objects are invariably attained by converting the fluid which is to be removed to the gaseous state. This is generally done by the application of heat; sometimes also by leaving the fluid for a certain time in contact with the atmosphere, or with an enclosed volume of air constantly kept dry by hygroscopic substances, such as concentrated sulphuric acid, chloride of calcium, &c.; or, lastly, in many cases, by placing the fluid in rarefied air, with simultaneous application of hygroscopic substances. As it is of the utmost importance in qualitative analyses to guard against the least contamination, and as an evaporating fluid is the more liable to this the longer the operation lasts, the process is usually conducted, with proper expedition, over the flame of a spirit or gas-lamp, in a separate place free from dust and not exposed to draughts of air. If the operator has no place of the kind, he must have recourse to the much less suitable proceeding of covering the dish; the best way of doing this is to place over the dish a large glass funnel secured by a retort holder, in a manner to leave sufficient space between the rim of the funnel and the border of the dish; the funnel is placed slightly aslant that the drops running down its sides may be received in a beaker. Or the dish may also be covered with a sheet of filter paper previously freed from inorganic substances by washing with dilute hydrochloric or nitric acid (see a 27 cially if acid), and the solution dripping Fig. 8. down into the evaporating fluid would speedily contaminate it. These precautions are necessary of course only in accurate analyses. [In most ordinary cases, vessels may be covered with the very thin white paper used by grocers (tea-paper) without detriment, since the impurities that can find their way into solutions from so small a weight of paper, are too slight to affect common analyses.] Larger quantities of fluid are evaporated best in glass flasks standing aslant, covered with a cap of paper, over a charcoal fire or gas; or also in retorts. Evaporating processes at 212° are conducted in an appropriate steam apparatus, or in the water-baths shown in Fig. 3. Evaporation to dryness, instead of being conducted over a free fire or lamp-flame, is best carried on either in a water or sand-bath or on a heated iron plate. § 8. 7. DISTILLATION. This operation serves to separate a volatile liquid from a less volatile or a fixed substance (no matter whether solid or fluid), where the object is to recover the evaporating fluid. In order to attain this object, it is necessary to reconvert the liquid from the gaseous form in which it evaporates, into the fluid state. A distilling apparatus consists consequently always of three parts, no matter whether admitting of separation or not. These three parts are-1st, a vessel in which the liquid to be distilled is heated, and thus converted into vapor; 2nd, an apparatus in which this vapor is cooled again or condensed, and thus reconverted to the fluid state; and 3rd, a vessel to receive the fluid thus reproduced by the condensation of the vapor (the distillate). For the distillation of Fig. 4. large quantities we use either a metallic apparatus (a copper still with head and condenser made of tin or pewter), or large glas retorts; in analytical investigations we generally employ the appa ratus shown in Fig. 4. § 9. Ignition is, in a certain measure, for solid bodies what evaporation is with regard to fluids; since it serves (at least generally) to separate volatile substances from less volatile or from fixed bodies, in cases where the residuary substance alone is of importance. The process of ignition always presupposes the application of a high temperature, in which it differs from that of drying or exsiccation. The form or state which the eliminated substance assumes on cooling-whether it remains gaseous, as in the ignition of carbonate of lime; or assumes the liquid state, as in the ignition of hydrate of lime; or solidifies, as in the ignition of a mixture containing chloride of ammonium-is a matter of perfect indiffe rence as regards the name given to the operation. The process of ignition is mostly employed, as has just been said, to effect the elimination of a volatile body. In some instances, however, substances are ignited simply for the purpose of modifying their state, without any volatilization taking place; thus the sesquioxide of chromium is converted by ignition into its insoluble modification, &c. In analytical investigations substances under examination are often ignited also, that the operator may from their deportment at a red heat draw a conclusion as to their nature in general; their fixity, their fusibility, the presence or absence of organic matter, &c. Crucibles are the vessels made use of in ignition. In operations on a large scale Hessian or black-lead crucibles are used, heated by charcoal or coke; in analytical experiments small-sized crucibles or dishes are selected, of porcelain, platinum, silver or iron, or glass tubes sealed at one end, according to the nature of the substances to be ignited; these crucibles, dishes, or tubes are heated over a Berzelius spirit-lamp or a properly-constructed gaslamp. § 10. The term sublimation designates the process which serves to expand solid bodies into vapor by the application of heat, and ubsequently to recondense the vapor to the solid state by refrigera tion; the substance volatilized and recondensed is called a subli mate. Sublimation is consequently a distillation of solid bodies We have recourse to this process mostly to effect the separation of substances possessed of different degrees of volatility. Its application is of the highest importance in analysis for the detection |