as is shown in fig. 7.

The fluid to be distilled is boiled in A, and the vapor escapes through the tube which is fitted into the cork. The tube is surrounded with a wider tube which is filled with cold water. In proportion as the water cools the inner tube, it becomes heated itself, and consequently requires renewing continually or occasionally, which is done by pouring cold water in through d, after placing a vessel under g to catch the hot water which will run out. A small flask serves as a receiver.

§ 11.
10. IGNITION.

Ignition is, in a certain manner, 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 respect it differs from drying. 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 chromium is converted by ignition into the so-called 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 generally made use of in ignition. In operations on a large scale Hessian or black-lead crucibles are used, heated by charcoal or gas; 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 gas-lamp, or a bellows blowpipe.

§ 12.

11. SUBLIMATION.

The term sublimation designates the process which serves to convert 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. vessels used in sublimation are of various shapes, according to the different degrees of volatility of the substances operated upon. In

The

sublimations for analytical purposes we generally employ sealed glass tubes. When the sublimation is performed with the aid of a current of hydrogen or carbonic acid we use open glass tubes, which are usually made narrower just behind the part to which the heat is applied.

§ 13.

12. FUSION.

Simple fusion is the conversion of a solid substance into the fluid form by the application of heat; it is most frequently resorted to for the purpose of effecting the combination or the decomposition of bodies. The term is also applied 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 is 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 iron ring of the Berzelius spirit-lamp or the gas-lamp. Triangles of thick iron wire, especially when laid upon the 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 fusing is especially required for the analysis of the sulphates of the alkaline earths, and also for that of many silicates and alumina compounds. 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 § 76). In certain cases hydrate of baryta is used instead of the alkaline carbonates.

For the fusion of aluminates, bisulphate of potassa or bisulphate of soda is frequently used.

A platinum crucible is used for the fusion, whether alkaline carbonates, hydrate of baryta, or alkaline bisulphates are employed.

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 nitrates of the alkalies, hydrate of potassa and soda, metals, or sulphides of metals or cyanides of the alkali metals should be fused in such vessels; nor should readily deoxidizable metallic oxides, or salts of the heavy metals with organic acids be ignited in them, or phosphates in presence of organic compounds. It is also detrimental to platinum crucibles, and especially to their covers, to expose them direct to an intense charcoal fire, as the action of the ash is likely to lead to the formation of silicide of platinum, which renders the vessel brittle. It is always advisable to support platinum crucibles used in ignition or fusion on triangles of platinum wire. When a platinum crucible has been made white hot over the bellows blowpipe, it is unwise to cool it too quickly by suddenly turning off the gas, and allowing the cold blast to play upon it, since the crucible is under these circumstances very liable to become slightly cracked. Platinum crucibles are cleaned by rubbing with wet sea-sand, the round grains of which do not scratch the metal. Where this fails to remove the stains the desired object may be attained by

fusing bisulphate of potassa or borax in the crucible, boiling subsequently with water, and polishing finally with sea-sand.

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

$14.

13. 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 being 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 sulphide 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 with cyanide of potassium, and observing whether they deflagrate, &c. To attain the former object the perfectly dry mixture of the substance and the deflagrating agent is projected in small portions at a time into a red-hot crucible. Experiments of the latter description are invariably made with minute quantities preferably on a piece of thin platinum foil, or in a small spoon.

§ 15.

14. 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 the apparatus required, the mode of its application, and the results of the operation.

The blowpipe fig. 8 is a small instrument, usually made of brass or German silver. It was originally used for soldering, whence it derived the name of soldering pipe (Löthrohr), by which the Germans designate it. It consists of three distinct parts; 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 e d, into which a b is screwed air-tight, and which serves as an air-chamber and to retain the moisture of the air blown into the tube; and 3rd, a smaller tube fg, also fitted into e d. 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). The construction of the cap is shown in fig. 9. It is,

a

Fig. 8.

9 h

Fig. 9.

indeed, a little dearer than a simple plate, but it is also much more durable. If the opening of the cap gets stopped up, the obstruction may generally be removed by heating it 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 centimetres. The form of the mouthpiece varies. Some chemists like it of a shape to be encircled by the lips; others prefer the form of a trumpet mouthpiece, which is only pressed against the lips. The latter requires less exertion on the part of the operator, and is accordingly generally chosen by those who have a great deal of blowpipe work.

Fig. 10.

The blowpipe serves to conduct a continuous fine current of air into a gas-flame, or into the flame of a candle or lamp, or sometimes into a spirit-of-wine flame. 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. 10, viz., 1st, a dark nucleus in the centre (a); 2nd, a luminous cone surrounding this nucleus (efg); and, 3rd, a feebly luminous mantle encircling the whole flame (bed). 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 matter not yet burned is consumed here. This part of the flame is the hottest, and the extreme apex is the hottest point of it. 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. On the other hand, oxides having a tendency to yield up their oxygen suffer reduction when placed within the luminous part of the flume, the oxygen being withdrawn from them by the carbon and the still unconsumed carbide of hydrogen there present. The luminous part of the flame is therefore called the reducing flame.

Now the effect of blowing a fine stream of air across a flame is, first, to alter the shape of the flame, as, from tending upward, it is now driven sideways in the direction of the blast, being 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 blast will depend upon 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 jet, shaped as in fig. 11, the slit being 1 centimetre long and 1 to 2 millimetres wide; as with the

use of gas the operator is enabled to regulate not only the current of air, but that of the gas 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 intended for supporting dishes, &c.

Fig. 11 shows the flame for reducing, fig. 12 the flame for oxidizing. The luminous parts are shaded.

Fig. 11.

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

尋

Fig. 12.

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 their fusion; but bodies to be oxidized are held a little beyond the apex, that there may be no want of air for their combustion.