tration, for separating solid particles which are diffused through liquors. These are allowed to settle to the bottom, and the clear fluid is gently poured off. If the sediment be extremely light, and apt to mix again with the fluid, by the slightest motion, a syphon is used for drawing off the clear fluid.

Lixiviation is the separation by means of water, or other fluid, of such substances as are soluble in it, from other substances that are not soluble in it. Thus, if a certain mineral consists of salt and sand, or salt and clay, &c. the given body being broken to powder, is placed in water, which will dissolve the salt, and keep it suspended, whilst the earthy matter falls to the bottom of the vessel, and, by means of filtration, may be separated from the fluid.

Evaporation separates a fluid from a solid, or a more volatile fluid from another which is less volatile.

Simple evaporation is used when the more vola-' tile or fluid substance is not to be preserved. Various degrees of heat are employed for this purpose, according to the nature of the substances. it is performed in vessels of wood, glass, metal, porcelain, &c. Basons made of WEDGEWOOD'S ware are very convenient, as they are not apt to break by sudden changes of heat. Small flasks of thin glass also: these are placed either over the naked fire, or in a vessel filled with sand, which is then called a sand-bath. This affords a more regular degree of heat, and renders the vessels less liable to be broken.

When the fluid which is evaporated must be preserved, then the operation is called distillation.

Distillation is evaporating in close vessels, when

we wish to separate two fluids of different degrees of volatility, and to preserve the most volatile, or both of them. The substance to be subjected to distillation is put into some vessel that will resist the action of heat, called a retort, an alembic, or a still, having a beak or neck projecting from it, to which is attached another vessel, to receive the fluid that rises first, which is called the recipient, or receiver. The vessel that contains the liquor to be distilled is placed upon the fire, or in a sandbath, or over a lamp: the heat causes the most volatile fluid to rise in the form of vapour, and to pass into the receiver, where it is again condensed by cold. This condensation is sometimes assisted by making the vapour pass through a tube which is immersed in a vessel containing cold

water.

A (Plate 1. fig. 1.), represents a retort used for distillation. It is a vessel, either of glass or baked earth, for containing the liquid to be distilled. When it has a small neck, a, with a stopple fitted to it, for introducing the materials through, it is called a tubulated retort. B is the receiver for condensing the vapour which is raised, and into which the neck of the retort is inserted. The joining, b, is made air-tight by means of some substance applied to it, called a lute. Various methods are used for supporting both the retort and receiver, according to the degree of heat employed in the process, and several other circumstances.

When great heat is employed, earthen retorts are used, which are placed on or in the fire. When a less heat is wanted, glass retorts are generally employed, which must not not be placed immediately on the fire, unless they are coated over with a composition of clay and sand, which is

sometimes done. Glass retorts are generally placed in a sand-bath, or suspended over a lamp, for which ARGAND'S lamp is the best. The receiver is placed upon some stand convenient for the purpose, with a ring made of hay under it, or some such contrivance, to keep it steady.

A (Fig. 2.), is a vessel called a mattrass, for the same purpose, having a vessel, B, called an alembic, fitted to the head. The liquid raised by heat into the state of vapour, is condensed in the alembic, and falls into a groove all round its inside, from whence it runs out by the spout, C, into the receiver, D.

Fig. 3. are conical tubes that fit into another, for lengthening the necks of retorts, &c. to connect them with the receivers at any distance: they are called adopters.

Fig. 4. are phials with bent glass tubes fitted in them, for disengaging gases, and similar experiments they were used by PRIESTLEY, and are hence called Priestley's bottles, and sometimes proofs: they are either tubulated or plain.

A (Fig. 5.), represents a common still. It is a large vessel of copper, into which the materials to be distilled are put. The still is built up in brick-work, which covers it up to the neck; the fire is applied underneath, and runs round it in a spiral manner. B is the head of the still. This head is connected with the worm, which is a spiral tube, immersed in a vessel of cold water, called the refrigeratory, or cooling tube, C. The liquor being condensed in its passage through the worm, runs out at the cock, D, into the vessel placed there to receive it.

This is the construction of the common still for distilling spirituous liquors; but a very great im

provement has been made upon this instrument, in Scotland, within these few years. This improved apparatus is known by the name of the Scotch still, a section of which is represented, Fig. 6. The principle of the improvement consists in exposing a great quantity of the surface of the fluid to the action of the fire, and affording a more ready means for the escape of the vapour or gas.

A, is the body of the still, made very shallow and concave at the bottom, in order that the fire may act better upon it; bb, are a number of tubes opening into the still, and communicating with the neck of the still B, in order to convey the vapour off as soon as it is formed; cc, is a cover that shuts down over the pipes and top of the still, to keep it warm, by preventing the loss of heat which would be occasioned by the contact of the cold air. This is effected by the quantity of air that is confined between the cover and the top of the still; for it is a fact which is now well known, that confined air is a non-conductor of heat. In general, the heads of stills are kept warm by laying blankets upon them, at least when this is attended to, as it ought always to be; but this metallic covering, by surrounding the still with a quantity of confined air, answers the purpose still better.

When the materials which are evaporated concrete in a solid form, within the neck of the distilling vessels, then the distillation is more properly called sublimation.

By the above means, one fluid may be separated from other materials; but it often happens, that in distillation the substances which are subjected to this process have a chemical action upon each other; new combinations take place, and perma

nently elastic fluids or gases are disengaged, which are required to be preserved and examined. For this purpose, a very useful apparatus is employed, called the

PNEUMATO-CHEMICAL APPARATUS.

Fig. 7, represents an improved pneumato-chemical apparatus and lamp-furnace connected with it. A, is a vessel filled with water. In this vessel a shelf is placed, so as to be a little under the surface of the fluid, having several holes bored through it, to which small funnels are attached underneath.

The glass air-jar, or receiver, B, which is to receive and contain the gas, is filled with water, and being inverted with its mouth under water, it is raised up gently till its mouth is nearly out of the water, but not quite; and it is then placed upon the shelf over one of the holes. The receiver will remain full of water, which is kept up by the pressure of the atmosphere upon the principle of the barometer described under pneumatics.

The materials from which the gas is to be disengaged, are now to be put into a glass retort, C, which is put through, and suspended in one of the rings of the lamp-furnace, D. An improved ARGAND'S lamp, E, having two concentric wicks,* affords a much greáter degree of heat than the common ARGAND's lamp, which has only a single circular wick; this is placed upon the shelf, F.

* This lamp with two concentric wicks was first contrived by the editor some years ago, and is extremely useful in some chemical operations, as it gives a much greater heat than the common Argand's lamp with one circular wick.