for this purpose are crucibles of a conical form; they should have covers to keep out the fuel and air. Lead is commonly fused in iron, and salts in glass.

II. VESSELS for EVAPORATION; these may be divided into three classes:-1. for evaporation merely; 2. Distillation, or Sublimation; 3. Cementation.

1. EVAPORATION. Flat vessels may be used when the heat is gentle, but in a strong heat crucibles must be employed. See Pl. V. fig. 11 and 12.

2. DISTILLATION. Distillation is of three kinds:-1. Distillation per descensum; 2. per ascensum; 3. per latus.

The first is when the heat is applied above, and the vapour falls directly downward into a condensing vessel. In this manner oil of cloves used to be distilled, but it is now laid aside. The tar manufacture is, however, still carried on by this mode. The wood that produces the tar is put into an oven, which is heated from above, and this causes the oily and resinous parts to sweat out and fall into water, which is placed below to receive it. Zinc is also prepared in this way from lapis calaminaris, one of its ores.

2. By the common still per ascensum. For corrosive substances we use CUCURBITS, or glass vessels in the shape of an egg, which are called by some practical chemists matrasses or bodies. See fig. 13. The common still used for the

distillation of spirits is represented in fig. 14. It consists of a boiler covered by a metal head, A, usually of copper, for collecting the vapour, which is made to pass through a spiral tube called a worm, immersed in a tub of cold water, B, for the purpose of condensing the spirit, which comes out at the pipe C in a fluid form.

3. By the retort and receiver (see fig. 15.) we perform the lateral distillation. Those that have the neck of the retort descending into the lower parts of the receiver are more useful than those in which the neck of the receiver is inserted in the retort. Some have contrived them with an opening in the back for the mixture of bodies that emit elastic steam: this hole is stopped during the process, but not very tight, so that if the vapour is very elastic it may find its way, and the vessel be preserved. The conical receiver is accounted best. It is dangerous to restrain the steam too tight, and therefore in luting the retort and receiver do not screw them too tight. When a degree of heat capable of softening glass. is required, the retort may be coated over with clay and sand, which do not contract with heat: for very intense heat, we must have a retort of the same materials with the crucibles. In many operations of the retort we have occasion to mix two substances in order to obtain a third or new product, which is to rise in vapour upon mixture. In this operation the neck of the retort must be kept very clean and dry. In sub

limation, when the vapour condenses into a solid, we use a cucurbit with a blind head, or without a pipe, but more commonly a retort and receiver the retort must have a short and wide neck. A capital is sometimes necessary to receive the vapours that condense in a fluid, before those that condense in a solid begin to rise. Many sublimates are difficult to be converted into vapour, and, when converted into vapour, soon condense; for these a Florentine or common oil flask answers sufficiently well.

The alembic or cucurbit (fig. 16.) is used for distillation when the products are very volatile. It consists of a body, A, to which is adapted a head, B. The head is of a figure inclined to conical, and has its circumference or base depressed lower than its neck, so that the vapours which rise, and are condensed against its sides, run down, and are conveyed by the nose or beak, C, into the receiver, D. The principal object of this instrument is, that in distillation, and more particularly sublimation, the head may receive the more dense and dry products, while the more volatile pass into the receiver.

An aludel is analogous to adopters in distillation: its use is to condense bodies according to different degrees of volatility. What remains after the spirit or sublimate is taken off, is called the residuum or caput mortuum.

The 3d process is CEMENTATION, when vapours act upon one another, by laying the ma

terials in alternate layers stratum super stratum. It is thus copper is converted into brass, viz. by alternate layers of copper and lapis calaminaris mixed with other substances: the vapours from this last tinge the copper yellow. The vessels for this process are chiefly crucibles.

III. VESSELS for SOLUTION. Those of a conical form are often very convenient, as they bear sudden alterations of heat, and prevent the easy escape of vapour. To this class may be referred the pelican and Papin's digester.

Solution is often partial, in which case it is necessary to purify it by filtration. The common means are through a spongy kind of paper, such as the common blotting-paper, and to separate still farther powdery matter from fluids, chalk-stone.

Among chemical instruments may be reckoned the iron rod, or knife for cutting glass. By applying this iron red-hot we may lead a crack in a glass in any direction, moving the iron a little before the crack, which follows the iron. If there is not a crack in the vessel, the iron must be applied red-hot to the part we want to separate, till the glass is well heated, and then apply a file dipped in cold water to the part, and it will immediately crack.

The means of producing heat are seven, viz. 1. Animal Heat; 2. Friction or Percussion; 3. Electricity; 4. Mixture of Bodies; 5. Fermentation; 6. Rays of the Sun; 7. Fuel.

I. Animal Heat is only used in the regulation of the thermometer, by putting it in the mouth. II. Friction or Percussion is applied to some purposes in common life, as in fire-arms, where fire is produced by the percussion of the flint against the steel.

III. The Flash of Electricity has not been much attended to by chemists, though its effects on metals are great, as you have seen in the lecture on electricity. The justly celebrated Davy has, however, latterly applied the kindred influence Galvanism, with great success in chemical investigation; and in no instance more so than in ascertaining its powers in effecting the decomposition of different substances; supporters of combustion or acidifying principles being attracted by the positive pole of the voltaic or galvanic apparatus, from which oxygen is liberated, whilst inflammable substances are attracted by the opposite electrised or negative pole, from which hydrogen gas is liberated.

IV. Mixture. This heat, like the former, is too transient to be applied to any useful purposes. We may, however, account mixture as one of the useful means of lessening heat occasionally, and of producing cold. To obtain low degrees of heat, we have recourse to a mixture of water or pounded ice with salts; of these muriat of lime with snow is most powerful. Snow or ice mixed with strong nitrous acid produces also a great degree of cold, even such as to freeze the mer