bottom out, a and b are glass plates, between them lie the filters which have been previously cut and folded, d is a glass tube fitted into the cork c, e is a piece of flexible tube, which is closed by a piece of glass rod or a clip. The bottle is filled with a mixture of one part hydrochloric acid sp. gr. 1.12 and two parts water, in which the filters are allowed to soak twelve hours, the acid being then run off and replaced by ordinary water. After an hour this is replaced by fresh water, and so on till the washings are barely acid. The washing is continued with distilled water till the washings are free from hydrochloric acid-that is, till they cease to give any turbidity when mixed with a few drops of solution of nitrate of silver. Finally, the filters are drained, turned out onto blotting paper, covered with the same, and dried in a sieve in a warm place. When we merely want to wash two or three filters, we place them in a funnel, as in filtering, one inside the other, moisten them with dilute hydrochloric or nitric acid, and after some time wash them well with distilled water. Filtering paper, to be considered good, must, besides being pure, also let Hluids pass readily through, yet completely retain even the finest Fig. 2. pulverulent precipitates, such as sulphate of baryta, oxalate of lime, &c. Where a paper satisfying these requirements cannot be readily procured it is advisable to keep two sorts, one of closer texture, for the separation 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 (§ 18, 10); they are usually placed on an appropriate stand, to keep them in a fixed position. The stand shown in fig. 2 is particularly well adapted for the reception of the small-sized funnels used. in qualitative analyses. The method of rapid filtration by means of an exhausting apparatus will be described in the Quantitative Analysis. § 6. 5. DECANTATION. 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 many cases we are obliged to employ decantation for the complete separation of precipitates from fluids, especially when a precipitate is of so gelatinous or slimy a nature that it would immediately stop the pores of a filter, and prevent the passage of the fluid. Occasionally we combine decantation and filtration together by allowing the precipitate to settle, and then pouring the fluid as clear as possible onto a filter. § 7. 6. WASHING. 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 washing of precipitates collected on a filter is usually effected by means of the washing-bottle (fig. 3). The drawing needs no elaborate explanation. The outer end of the tube a is drawn out to a fine point. By blowing air into the flask through the other tube, a fine jet of water is expelled through a, with a certain degree of force, which is particularly well suited for washing precipitates. Washing-bottles of this construction afford also the advantage that they do equally well for washing with hot water. For this purpose they are either furnished with a handle, or some cork is bound round the neck. The best way of washing by decantation is, after the supernatant fluid has been poured off, to stir up the precipitate with water or whatever fluid may be used for the washing, to allow to settle, to pour off again, and so on. Fig. 3. As the success of an analytical operation often depends absolutely upon the proper washing of a precipitate, it may as well be mentioned at once that the operation ought never to be considered complete before the object of it has been really attained. And this is usually the case only when the precipitate has been absolutely freed from the fluid adhering to it. The operator should, in this respect, never trust to mere belief or guessing, but should always make quite sure by properly testing the last washings. With fixed bodies it generally suffices to slowly evaporate a drop of the last washings on platinumfoil, when complete volatilization will show that the end in view has been fully attained. § 8. Dialysis is an operation which is occasionally employed for the separation of certain bodies from each other when they are in solution together; at first sight it appears to have some resemblance to filtration, but in reality it differs essentially from filtration. This operation has been recently introduced to the scientific world by GRAHAM (Phil. Mag. Fourth Series, Nos. 153-155), and depends upon the different behavior of bodies dissolved in water towards moist membranes. A certain class of bodies, the crystalloids, have the power of penetrating suitable membranes with which their solution may be placed in contact; whilst another class, the colloids, do not possess that property. Hence the two classes may be separated by taking advantage of this action. To the crystalloids belong all crystallizable bodies; to the colloids all non-crystallizable bodies, as gelatine, gum, dextrin, caramel, tannin, albumen, extractive matters, hydrated silicic acid, &c. The septum must consist of a colloid material, as for instance an animal membrane, or, better still, parchment paper, and it must on the other side be in contact with water. GRAHAM explains the action on the assumption that the crystalloids appropriate to themselves the water absorbed by the colloid septum, acquiring thereby a medium for diffusion, whilst the dissolved colloids are unable to separate the water from the septum, and fail therefore to penetrate the latter. Figs. 4 and 5 exhibit suitable forms of apparatus for this opera tion. In fig. 4 the dialyser consists of the top of a bottle closed below with parchment paper; in fig. 5 it consists of a hoop covered like a sieve with parchment paper. The hoop may be of wood, or better of gutta-percha. The disc of parchment paper used should measure three or four inches in diameter more than the space to be covered; it is moistened, stretched over and fastened by a string or by an elastic band, but it should not be secured too firmly. The parchment paper must not be porous; its soundness may be tested by sponging the upper side with water, and observing whether wet spots show on the other side. Defects may be remedied by applying liquid albumen and coagulating this by heat. When the dialyser has thus been got ready, the mass to be examined is poured into it. If the mass is quite fluid the apparatus fig. 4 may be used, but if it contains undissolved solid matter, the hoop is to be preferred. The depth of fluid in the dialyser should not be more than half an inch, and the membrane should dip a little way below the surface of the water in the outer vessel, which should amount to at least four times the quantity of the fluid to be dialysed. The dialyser in fig. 4 is suspended in the manner indicated the hoop is simply floated on the water. After twenty-four hours half or three-fourths of the crystalloids will be found in the external water, while the colloids remain in the dialyser-at most only traces pass into the external fluid. If the dialyser is brought successively in contact with fresh supplies of water, the whole of the crystalloids may be finally separated from the colloids. This operation is often of considerable service in chemico-legal investigations for the extraction of poisonous crystalloids from parts of a dead body, food, vomit, &c. There are four operations which serve to separate volatile substances from less volatile or from fixed bodies, viz., evaporation, distillation, ignition, and sublimation. The two former of these operations refer exclusively to fluids, the two latter exclusively to solids. § 9. 8. EVAPORATION. This is one of the most common operations in analytical chemistry. It serves to separate volatile fluids from less volatile or from fixed bodies (no matter whether solid or fluid), in cases where the residuary substance 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 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, in porcelain or platinum dishes, over the flame of a spirit or gas-lamp, in a separate place free from dust, preferably in a cupboard provided with a draught. 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 § 5); were common and unwashed filterpaper used for the purpose, the sesquioxide of iron, lime, &c., contained in it would dissolve in the vapors evolved (more especially if acid), and the solution dripping down into the evaporating fluid would speedily contaminate it. These precautions are necessary of course only in accurate analyses. Large quantities of fluid are evaporated best in flasks standing aslant, covered with a cap of pure filtering paper, over a charcoal fire or gas; or also in tubulated retorts with neck rising obliquely upward, and open tubulure. Evaporating processes at 100° are conducted in a suitable steam apparatus, or in the water-bath shown in fig. 6. Evaporation to dryness is not usually conducted over the naked flame, but generally either on the waterbath, or the sand-bath, or on an iron plate. a Fig. 6. There is a circumstance connected with the subject which should be noted here; namely, that porcelain and glass vessels-which we can hardly avoid using for the evaporation of large quantities of fluids are slightly attacked, and so their contents become more or less contaminated with their constituents. This will be more fully discussed in the Quantitative Analysis, but it becomes a source of great trouble in delicate work. I will now be content to advise the student never to evaporate alkaline fluids in glass, as at a boiling temperature they attack it considerably. $10. 9. DISTILLATION. This operation serves to separate a volatile liquid from a less volatile or a non-volatile substance (no matter whether solid or fluid) where the object is to recover the evaporating fluid. In order to attain this end, 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 separable or not. These three parts are-1st, a vessel in which the liquid to be distilled d g A Fig. 7. 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 large quantities metallic apparatus are used (copper stills with head and condenser of tin), or large glass retorts; in analytical investigations we either use small retorts with receivers, or more usually an apparatus such |