$ 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 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. This operation is termed 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 washing bottle consists of a flask closed with a perforated cork, into which a small glass tube is fitted, with the outer end drawn out to a fine point. By blowing air into the flask through this tube and, when the air is sufficiently compressed, reversing the flask, so as to place the inner aperture of the tube under water, a fine jet of that fluid is expelled with a certain degree of force; this contrivance is well adapted for washing precipitates. Fig. 3 represents a washing bottle of a different construction, which is used more especially to wash precipitates with boiling water, and affords also the advantage that it enables the operator to produce an uninterrupted jet of water. The drawing needs no explanation; the outer end of the tube a is drawn out to a fine point. An India-rubber cap fitted with two tubes may be used instead of the double-perforated cork.

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 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 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 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 § 5); were common and unwashed filter paper 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. Larger quantities of fluid are evaporated best in glass flasks standing aslant, covered with a cap of pure filtering paper, over a charcoal fire or gas; or also in retorts. Evaporating processes at 212° are conducted

a

Fig. 4.

in an appropriate steam apparatus, or in the water-baths shown in Fig. 4.

$ 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 large

Fig. 5.

quantities we use either a metallic apparatus (a copper still with head and condenser made of tin or pewter), or large glass retorts; in analytical investigations we generally employ the apparatus shown in Fig. 5.

$ 9.

8. IGNITION.

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 indifference 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 chro

mium 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 gas-lamp.

§ 10.

9. SUBLIMATION.

The term sublimation designates the process which serves to expand solid bodies into vapor by the application of heat, and subsequently to recondense the vapor to the solid state by refrigeration ;—the substance volatilized and recondensed is called a sublimate. Sublimation is con

sequently 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 of certain substances, e. g. of arsenic. The vessels used in sublimation are of various shapes, according to the different degrees of volatility of the substances operated upon. In sublimations for analytical purposes we generally employ sealed glass tubes.

§ 11.

10. FUSION AND FLUXING.

We designate by the term "fusion" the conversion of a solid substance into the fluid form by the application of heat; fusion is most frequently resorted to for the purpose of effecting the combination or the decomposition of bodies. The term "fluxing" is applied to this process in cases where substances insoluble or difficult of solution in water and acids are by fusion in conjunction with some other body modified or decomposed in such a manner, that they or the new-formed compounds will subsequently dissolve in water or acids. Fusion and fluxing are conducted either in porcelain, silver, or platinum crucibles, according to the nature of the compound. The crucible is supported on a triangle of moderately stout platinum wire, resting on, or attached to, the ring of the spirit or gas-lamp. Triangles of thick iron wire, especially when laid upon the still stouter brass ring of the lamp, carry off too much heat to allow of the production of very high temperatures. Small quantities of matter are also often fused in glass tubes sealed at one end.

Resort to fluxing is especially required for the analysis of the sulphates of the alkaline earths, and also for that of many silicates. The flux most commonly used is carbonate of soda or carbonate of potassa, or, better still, a mixture of both in equal atomic proportions (see § 75). In certain cases, hydrate of baryta is used instead of the alkaline carbonates (see § 76). But in either case the operation is conducted in pla

tinum crucibles.

I have to add here a few precautionary rules for the prevention of

damage to the platinum vessels used in these operations. No substance evolving chlorine ought to be treated in platinum vessels; no nitrate of potassa, caustic potassa, metals, sulphides of the metals, or cyanides of the alkalies, should be fused in such vessels; nor ought readily deoxidizable metallic oxides, organic metallic salts, or phosphates to be ignited in them in the presence of organic compounds. It is also detrimental to platinum crucibles, and especially to their covers, to expose them directly to an intense charcoal fire, since the action of the ash, under such circumstances, gives readily rise to the formation of silicide of platinum, which renders the vessel brittle. It is always advisable to support the platinum crucible in which a process of ignition or fusion is to be conducted, on a triangle of platinum wire. Soiled platinum crucibles are cleaned by fusing in them bisulphate of potassa, and boiling afterwards with water.

We have still to speak here of another operation which bears some affinity to fusion.

$ 12.

11. DEFLAGRATION.

We understand by the term "deflagration," in a more general sense, every process of decomposition attended with noise or detonation-(the cause of the decomposition is a matter of perfect indifference as regards the application of the term in this sense).

We use the same term, however, in a more restricted sense, to designate the oxidation of a substance in the dry way, at the expense of the oxygen of another substance mixed with it (usually a nitrate or a chlorate), and connect with it the idea of a sudden and violent combustion attended with vivid incandescence and noise or detonation. Deflagration is resorted to either to produce the desired oxide-thus tersulphide of arsenic is deflagrated with nitrate of potassa to obtain arsenate of potassa ;—or it is applied as a means to prove the presence or absence of a certain substance thus salts are tested for nitric or chloric acid, by fusing them in conjunction with cyanide of potassium, and observing whether this process will cause deflagration or not, &c.

To attain the former object, the perfectly dry mixture of the substance under examination and of the deflagrating agent is projected in small portions into a red-hot crucible. Experiments of the latter description are invariably made with very minute quantities; the process is, in such cases, best conducted on a piece of thin platinum foil, or in a small spoon.

§ 13.

12. THE USE OF THE BLOWPIPE.

This operation belongs exclusively to the province of analytical chemistry, and is of paramount importance in many analytical processes. We have to examine here, 1, the apparatus; 2, the mode of its application; and, 3, the results of the operation.

The blowpipe (Fig. 6) is a small instrument, usually made of brass or German silver. It was originally used by metallurgists for the purpose of soldering, whence it derived the name of "soldering-pipe" (Löthrohr) by which the Germans designate it. It consists of three distinct parts;