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 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 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;

d

g h

FIC.7.

viz. 1st, a tube (a b), fitted, for greater convenience, with a horn or ivory mouthpiece, through which air is blown from the mouth; 2nd, a small cylindrical vessel (c d), into which a b is screwed airtight, and which serves as an air-chamber, and to retain the moisture of the air blown into the tube; and, 3rd, a smaller tube (f g), also fitted into the vessel (cd). This small tube, which forms a right angle with the larger one, is fitted at its aperture either simply with a finely perforated platinum plate, or more conveniently with a finely perforated platinum cap (h) screwed in air-tight. The construction of the cap is shown in Fig. 7. It is, indeed, a little dearer than a simple plate, but it is also much more durable. Whenever the opening of the cap happens to be stopped up, the obstruction. may be removed by heating the cap to redness before the blowpipe.

The proper length of the blowpipe depends upon the distance to which the operator can see with distinctness; it is usually from twenty to twenty-five centimètres. The form of the mouthpieces varies. Some chemists like them of a shape to be encircled by the lips; others prefer the form of a trumpet Figs. 6, 7. mouthpiece, which is only pressed against the lips. The latter require less exertion on the part of the operator, and are accordingly generally chosen by those who have a great deal of blowpipe work.

The blowpipe serves to conduct a continuous fine current of air into a gas-flame, or into the flame of a candle or lamp. The flame of a candle (and equally so that of gas or of an oil lamp), burning under ordinary circumstances, is seen to consist of three distinct parts, as shown in Fig. 8, viz., 1st, a dark nucleus in the centre (a); 2nd, a luminous cone surrounding this nucleus (e fg); and, 3rd, a feebly luminous mantle encircling the whole flame (b c d). The dark nucleus is formed by the gases which the heat evolves from the wax or fat, and which cannot burn here for want of oxygen. In the luminous cone these gases come in contact with a certain amount of air insufficient for their complete combustion. In this part, therefore, it is principally the hydrogen of the carbides of hydrogen evolved which burns, whilst the carbon separates in a state of intense ignition, which imparts to the flame the luminous appearance observed in this part In the outer coat, the access of air is no longer limited, and all the gases not yet burned are consumed here. This part of the flame is the hottest; oxidizable bodies oxidize therefore with the greatest possible rapidity when placed in it, since all the conditions of oxidation are here united, viz. high temperature, and an unlimited supply of oxygen. This outer part of the flame is therefore called the oxidizing flame.

Fig. 8.

On the other hand, oxides having a tendency to yield up their oxygen, suffer reduction when placed within the luminous part of the flame, the

oxygen being withdrawn from them by the carbon and the still unconsumed carbide of hydrogen present in this sphere. The luminous part of the flame is therefore called the reducing flame.

Now the effect of blowing a fine current of air across a flame, is first to alter the shape of the latter, which, from tending upward, is now driven sideways in the direction of the blast, and at the same time lengthened and narrowed; and, in the second place, to extend the sphere of combustion from the outer to the inner part. As the latter circumstance causes an extraordinary increase of the heat of the flame, and the former a concentration of that heat within narrower limits, it is easy to understand the exceedingly energetic action of the blowpipe flame. The way of holding the blowpipe and the nature of the current, will always depend upon the precise object in view, viz., whether the operator wants a reducing or an oxidizing flame. The easiest way of producing most efficient flames of both kinds is by means of coal-gas delivered from a tube, terminating in a flat top with a somewhat slantingly downwardturned slit 1 centimètre long and 1 to 2 millimètres wide; as with the use of gas the operator is enabled to control and regulate not only the blowpipe flame, but the gas stream also. The task of keeping the blowpipe steadily in the proper position may be greatly facilitated by firmly resting that instrument upon some moveable metallic support, such as, for instance, the ring of Bunsen's gas lamp for supporting dishes, &c. Fig. 9 shows the flame for reducing, Fig. 10 the flame for oxidizing. The luminous parts are shaded.

Fig. 9.

The reducing flame is produced by keeping the jet of the blowpipe just on the border of a tolerably strong gas flame, and driving a moderate blast across it. The resulting mixture of the air with the gas is only imperfect, and there remains between the inner bluish part of the flame and the outer barely visible part a luminous and reducing zone, of which the hottest point lies somewhat beyond the apex of the inner cone. To produce the oxidizing flame, the gas is lowered, the jet of the blowpipe pushed a little further into the flame, and the strength of the current somewhat increased. This serves to effect an intimate mixture of the air and gas, and an inner pointed, bluish cone, slightly luminous towards the apex is formed, and surrounded by a thin, pointed, light-bluish, barely visible mantle. The hottest part of the flame is at the apex of the inner cone. Difficultly fusible bodies are exposed to this part to effect