be poured into a heated mould anointed with tallow, never with oil, or what is still better, covered with a thin coating of chalk, which is applied by laying it over with a mixture of chalk diffused in water, and then evaporating the water completely by heat. To prevent the crucible from being broken by cooling too rapidly, it is to be either replaced in the furnace, to cool gradually with it, or covered with some vessel to prevent its being exposed immediately to the air.

Fusion is performed with the intention of weakening the attraction of aggregation; or of separating substances of different de grees of fusibility from each other.

Vaporization is the conversion of a solid or fluid into vapour by the agency of caloric. Although vaporability be merely a relative term, substances are said to be permanently elastic, volatile or fixed. The permanently elastic fluids or gases are those which cannot be condensed into a fluid or solid form by any abstraction of caloric we are capable of producing. Fixed substances, on the contrary, are those which cannot be converted into vapour by great increase of temperature. The pressure of the atmosphere has very considerable effect in varying the degree at which substances are converted into vapour. Some solids, unless subjected to very great pressure, are at once converted into vapour, although most of them pass through the intermediate state of fluidity.

Vaporization is employed to separate substances differing in volatility; and to promote chemical action, by disaggregating them.

When employed with either of these views, no regard is paid to the substances volatilized, whether from solids, as in ustulation and charring; or from fluids, as in evaporation; or whether the substances vaporized are condensed in proper vessels; for example, in a liquid form, as in distillation; or in a solid form, as in sublimation. Or whether the substances vaporized are permanently elastic, and are collected in their gaseous form, in a pneumatic appa

ratus.

Ustulation is almost entirely a metallurgic operation, and is employed to expel the sulphur and arsenic contained in some metallic ores. It is performed on small quantities in tests placed within a muffle. Tests are shallow vessels made of bone ashes or baked clay. Muffles are vessels of baked clay, of a semi-cylindrical form, the flat side forming the floor, and the arched portion

the roof and sides. The end and sides aré perforated with holes for the free transmis sion of air, and the open extremity is placed at the door of the furnace, for the inspec tion and manipulation of the process. The reverberatory furnace is commonly employed for roasting, and the heat is at first very gentle, and slowly raised to redness. It is accelerated by exposing as large a surface of the substance to be roasted as possible, and by stirring it frequently, so as to prevent any agglutination, and to bring every part in succession to the surface.

Charring may be performed on any of the compound oxides, by subjecting them to a degree of heat sufficient to expel all their hydrogen, nitrogen, and superabundant oxygen, while the carbon, being a fixed principle, remains behind in the state of charcoal. The temperature necessary for the operation may be produced either by the combustion of other substances, or by the partial combustion of the substance to be charred. In the former case, the operation may be performed in any vessel which excludes the ac cess of air, while it permits the escape of the vapours formed. In the latter, the access of air must be regulated in such a manner, that it may be suppressed whenever the combustion has reached the requisite degree; for if continued to be admitted, the charcoal itself would be dissipated in the form of carbonic acid gas, and nothing would remain but the alkaline and earthy matter, which these substances always contain. When combustion is carried this length, the process is termed incineration. The vapours which arise in the operation of charring, are sometimes condensed, as in the manufacture of tar.

Evaporation is the conversion of a fluid into vapour, by its combination with caloric. In this process, the atmosphere is not a necessary agent, but rather a hindrance, by its pressure. This forms a criterion between chemical evaporation and spontaneous evaporation, which is merely the solution of a fluid in air. It is performed in open, shallow, or hemispherical vessels of silver, tinned copper or iron, earthenware or glass. The necessary caloric may be furnished by means of an open fire, a lamp, or a furnace, either immediately, or with the intervention of sand, water, or vapour. The degree of heat must be regulated by the nature of the substance operated on. In ge neral, it should not be greater than what is absolutely necessary.

Evaporation may be partial; producing

from saline fluids, concentration; and from viscid fluids, inspissation; or it may be total and produce exsiccation. Concentration is employed to lessen the quantity of dilating fluids, which is called dephlegmation; or as a preliminary step to crystalli zation. Inspissation is almost confined to animal and vegetable substances; and as these are apt to be partially decomposed by heat, or to become empyreumatic, it should always be performed, especially towards the end of the process, in a water or vapour bath. Exsiccation is here taken in a very Jimited sense; for the term is also with propriety used to express the drying of vegetables by a gentle heat, the efflorescence of salts, and the abstraction of moisture from mixtures of insoluble powders with water, by means of chalk-stones, or powdered chalk pressed into a smooth mass. At present, we limit its meaning to the total expulsion of moisture from any body by means of caloric. The exsiccation of compound oxides should always be performed in the water bath. Salts are deprived of their water of crystallization by exposing them to the action of heat in a glass vessel or iron ladle. Sometimes they first dissolve in their water of crystallization, or undergo what is called the watery fusion, and are after wards converted into a dry mass by its total expulsion; as in the calcination of borax or burning of alum. When exsiccation is at tended with a crackling noise, and splitting of the salt, as in muriate of soda, it is termed decrepitation, and is performed by throwing into a heated iron vessel, small quantities of the salt at a time, covering it up, and waiting until the decrepitation be over, before a fresh quantity is thrown in. Exsiccation is performed on saline bodies, to render them more acrid or pulverulent, or to prepare them for chemical operations. Animal and vegetable substances are exsiccated to give them a solid form, and to prevent their fermentation.

Condensation is the reverse of expansion, and is produced either by mechanical pressure forcing out the caloric in a sensible form, as water is squeezed out of a sponge, or, by the chemical abstraction of caloric, which is followed by an approximation of the particles of the substance. This latter kind alone is the object of our investigation at present. In this way we may be supposed to condense substances existing naturally as gases or vapours; or substances, naturally solid or fluid, converted into vaponrs by adventitious circumstances. The VOL. V.

former instance is almost supposititious; for we are not able, by any diminution of temperature, to reduce the permanently elastic fluids, to a fluid or solid state. The latter instance is always preceded by vaporization, and comprehends those operations in which the substances vaporized are condensed in proper vessels.

When the product is a fluid, it is termed distillation; when solid, sublimation. Distillation is said to be performed, viâ humidâ, when fluids are the subjects of the operation. Viâ siccâ, when solids are subjected to the operation, and the fluid product arises from decomposition, and a new ar rangement of the constituent principles. The objects of distillation are, to separate more volatile fluids from less volatile fluids or solids. To promote the union of different substances and to generate new products by the action of fire.

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In all distillations, the heat applied should not be greater than what is necessary for the formation of the vapour, and even to this degree it should be gradually raised. The vessels also in which the distillation is performed should never be filled above one-half, and sometimes not above one fourth, lest the substance contained in them should boil over.

As distillation is a combination of evaporation and condensation, the apparatus consists of two principal parts: the vessels in which the vapours are formed; and those in which they are condensed. The vessels employed for both purposes are very various in their shapes, according to the manner in which the operation is conducted. The first difference depends on the direction of the vapour after its formation. It either descends, ascends, or passes off by the side, constituting a distillation per descensum, per ascensum, or per latus.

In the distillation per descensum, a perfo. rated plate of tinned iron, or other mate. rials, is fixed within any convenient vessel, so as to leave a space beneath it. On this the subject of the operation is laid, and over it is placed another plate, accurately closing the mouth of the vessel, and suffi. ciently strong to support the fuel: thus the heat is applied from above, and the vapour is forced to descend into the inferior cavity, where it is condensed. In this way the oil of cloves is prepared, and on the same principles tar is manufactured, and mercury and zinc are separated from their ores.

In the distillation per ascensum, the vapour is allowed to arise to some height,

and then is conveyed away to be condensed. The vessel most commonly employed for this purpose is the common copper still, which consists of a body for containing the materials, and a head into which the vapour ascends. From the middle of the head a tube rises for a short way, and is then reflected downwards, through which the steam passes to be condensed. Another kind of head, rising to a great height before it is reflected, is sometimes used for separating fluids, which differ little in volatility, as it was supposed that the less volatile vapours would be condensed and fall back into the still, while only the more volatile vapours would arise to the top, so as to pass to the refrigeratory. The same object may be more conveniently attained by managing the fire with caution and address. The greater the surface exposed, and the less the height the vapours have to ascend, the more rapidly does the distillation proceed; and so well are these principles understood by the Scotch distillers, that they do not take more than three minutes to discharge a still containing gallons of fluid.

The condensing apparatus used with the common still is very simple. The tube in which the head terminates is inserted into the upper end of a pipe, which is kept cool by passing through a vessel filled with water, called the refrigeratory. This pipe is commonly made of a serpentine form; but as this renders it difficult to be cleaned, Dr. Black recommends a sigmoid pipe. The refrigeratory may be furnished with a stop-cock, that when the water it contains becomes too hot, and does not condense all the vapour produced, it may be changed for cold water. From the lower end of the pipe, the product of the distillation drops into the vessel destined to receive it; and we may observe, that when any vapour issues along with it, we should either diminish the power of the fire, or change the water in the refrigeratory. There was a process of this kind, called circulation. It consisted in arranging the apparatus, so that the vapours were no sooner condensed into a fluid form, than this fluid returned back into the distilling vessels, to be again vaporised; and was effected by distilling in a glass vessel, with so long a neck that the vapours were condensed before they escaped at the upper extremity, or by inverting one matrass within another. When corrosive substances are distilled in this way, the cucurbit and alembic are used; but these substances are more conveniently distilled

per latus.

The distillation per latus is performed in a retort, or pear-shaped vessel, having the neck bent to one side. The body of a good retort is well rounded, uniform in its appearance, and of an equal thickness, and the neck is sufficiently bent to allow the vapours, when condensed, to run freely away, but not so much as to render the application of the receiver inconvenient, or to bring it too near the furnace. The passage from the body into the neck must be perfectly free and sufficiently wide, otherwise the vapours produced in the retort only circulate in its body without passing over into the receiver. For introducing liquors into the retort without soiling its neck, which would injure the product, a bent funnel is necessary. It must be sufficiently long to introduce the liquor directly into the body of the retort; and in withdrawing it, we must carefully keep it ap plied to the upper part of the retort, that the drop hanging from it may not touch the inside of the neck. In some cases, where a mixture of different substances is to be distilled, it is convenient and necessary to have the whole apparatus properly adjusted before the mixture is made, and we must therefore employ a tubulated retort, or a retort furnished with an aperture, accurate. ly closed with a ground stopper. This tubulature should be placed on the upper convex part of the retort before it bends to form the neck, so that a fluid poured through it may fall directly into the body without soiling the neck.

Retorts are made of various materials. Flint-glass is commonly used when the heat is not so great as to melt it. For distillations which require excessive degrees of heat, retorts of earthenware, or coated glass retorts are employed. Quicksilver is distilled in iron retorts.

The simplest condensing apparatus used with the retort, is the common glass receiver; which is a vessel of a conical or globular form, having a neck sufficiently wide to admit of the neck of the retort being introduced within it. To prevent the loss and dissipation of the vapours to be condensed, the retort and receiver may be accurately ground to each other, or secured by some proper lute. To prevent the receiver from being heated by the caloric evolved during the condensation of vapours in it, we must employ some means to keep it cool. It is either immersed in cold water, or covered with snow, or pounded ice, or a constant evaporation is supported from its surface, by covering it with a cloth, which

is kept moist by means of the descent of water, from a vessel placed above it, through minute syphons or spongy worsted threads. But as, during the process of distillation, permanently elastic fluids are often produced, which would endanger the breaking of the vessels, these are permitted to escape either through a tubulature, or hole, in the side of the receiver, or rather through a hole made in the luting. Receivers having a spot issuing from their side, are used when we wish to keep separate the products obtained at different periods of any distillation. For condensing very volatile vapours, a series of receivers, communicating with each other, termed adopters, were formerly used; but these are now entirely superseded by Woulfe's apparatus, which consists of a tubulated retort, adapted to a tubulated receiver. With the tubulature of the receiver, a three-necked bottle is connected by means of a bent tube, the further extremity of which is immersed, one or more inches, in some fluid contained in the bottle. A series of two or three similar bottles are connected with this first bottle in the same way. In the middle tubulature of each bottle a glass tube is fixed, having its lower extremity immersed about a quarter of an inch in the fluid. The height of the tube above the surface of the fluid must be greater than the sum of the columns of fluid standing over the further extremities of the connecting tubes, in all the bottles or vessels more remote from the retort. Tubes so adjusted are termed tubes of safety, for they prevent that reflux of fluid from the more remote into the nearer hottles, and into the receiver itself, which would otherwise inevitably happen on any condensation of vapour taking place in the retort, receiver, or nearer bottles. Different contrivances for the same purpose have been described by Messrs. Welter and Burkitt; and a very ingenious mode of connecting the vessels without lute has been invented by Citizen Girard, but they would not be easily understood without plates. The further tubulature of the last bottle is commonly connected with a pneumatic apparatus, by means of a bent tube. When the whole is properly adjusted, air blown into the retort should pass through the receiver, rise in bubbles through the fluids contained in each of the bottles, and at last escape by the bent tube. In the receiver, those products of distillation are collected which are condensable by cold alone. The first bottle is commonly filled with water,

and the others with alkaline solutions, or other active fluids; and as the permanently elastic fluids produced are successively subjected to the action of all these, only those gases will escape by the bent tube which are not absorbable by any of them.

In separating permanently elastic fluids or gases from the substances in which they are found, we are compelled to employ a distinct pneumatic apparatus; and the gas may then be received either into vessels ab, solutely empty; or, filled with some fluid, on which it exerts no action.

The first mode of collecting gases may be practised by means of a bladder, moistened sufficiently to make it perfectly pliable, and then compressed so as to press out every particle of air from its cavity. In this state it may be easily filled with any gas. An oiled silk bag will answer the same purpose, and is more convenient in some respects, as it may be made of any size or form. Glass or metallic vessels, such as balloons, may also be emptied for the purpose of receiving gases, by fitting them with a stop-cock, and exhausting the air from them by means of an air-pump.

But the second mode of collecting gases is the most convenient and common. In which case the vessels may be filled either with a fluid lighter, or heavier, than the gas to be received into it.

The former method is seldom employed; but if we conduct a stream of any gas heavier than atmospheric air, such as carbonic acid gas, muriatic acid gas, &c. to the bottom of any vessel, it will gradually displace the air, and fill the vessel. On the contrary, a gas lighter than atmospheric air, such as hydrogen, may be collected in an inverted vessel by conducting a stream of it to the top. But gases are most commonly collected by conducting the stream of gas into an inverted glass-jar, or any other vessel filled with water or mercury. The gas ascends to the upper part of the vessel, and displaces the fluid. In this way gas may be kept a very long time, provided a small quantity of the fluid be left in the vessels, which prevents both the escape of the gas, and the admission of atmospheric air.

The vessels may be made of various shapes; but those most commonly employed are cylindrical. They may be either open only at one extremity, or furnished at the other with a stop-cock. The manner of filling them with fluid, is to immerse them completely in it, with the open extremity directed a little upwards, so that the whole

ing them with their mouths downwards. For filling them with convenience, a trough or cistern is commonly used. This should either be hollowed out of a solid block of wood or marble; or, if it be constructed of wood simply, be well painted or lined with lead or tinned copper. Its size may vary very much; but it must contain a sufficient depth of fluid to cover the largest transverse diameter of the vessels to be filled in it. At one end or side, there should be a shelf for holding the vessels after they are filled. This shelf should be placed about an inch and a half below the surface of the fluid, and should be perforated with several holes, forming the apices of corresponding conical excavations on the lower side, through which, as through inverted funnels, gaseous fluids may be more easily introduced into the vessels placed over them. In general the vessels used with a mercurial apparatus should be stronger and smaller than those for a water-cistern, and we must have a variety of glass and elastic tubes for conveying the gases from the vessels in which they are formed to the funnels under the shelf.

The repeated distillation of any fluid is denominated rectification. When distillation renders the fluid stronger, or abstracts water from it, it is termed dephlegmation. When a fluid is distilled off from any substance, it is called abstraction; and if the product be redistilled from the same substance, or a fresh quantity of the same substance, it is denominated cohobation.

The difference between distillation and sublimation is only in the form of the product. When it is compact, it is termed a sublimate; when loose and spongy, it formerly had the appellation of flowers. Sublimation is sometimes performed in a crucible, and the vapours are condensed in a paper cone, or in another crucible inverted over it; sometimes in the lower part of a glass flask, cucurbit, or phial, and the condensation is effected in the upper part or capital, and sometimes in a retort with a very short and wide neck, to which a conical receiver is fitted. The heat is most commonly applied through the medium of a sand-bath; and the degree of heat, and the depth to which the vessel is inserted in it, are regulated by the nature of the sublimation.

Congelation is the reduction of a fluid to a solid form, in consequence of the abstraction of caloric. The means employed for abstracting the caloric, are the evaporation

the contact of cold bodies.

Coagulation is the conversion of a fluid into a solid of greater or less consistence, merely in consequence of a new arrange. ment of its particles, as during the process there is no separation of caloric or any other substance. The means of producing coagulation are, increase of temperature, and the addition of certain substances, as acids and rennets.

Chemical Combination, is the intimate union of the particles of at least two heterogeneons bodies. It is the effect resulting from the exertion of the attraction of athnity, and is therefore subjected to all the laws of affinity.

To produce the chemical union of any two or more bodies, it is necessary, that they possess an affinity for each other; that their particles come into actual contact; that the strength of the affinity be greater than any counteracting causes which may be present.

The principal counteracting causes are, the attraction of aggregation; and affinities for other substances. The means to be employed for overcoming the action of other affinities, will be treated of under Decomposition. The attraction of aggregation is overcome by means of mechanical division; or the action of caloric.

Combination is facilitated by increasing the points of actual contact, by the means of mechanical agitation; by condensation and compression; and the processes employed for producing combination may be considered, with regard to the nature of the substances combined; and to the nature of the compound produced. Gases combine with gases, and dissolve fluids or solids, or are absorbed by them. Fluids are dissolved in gases, or absorb them; they combine with fluids, and dissolve solids, or are rendered solid by them. Solids are dissolved in fluids and in gases, or absorb gases, and solidify fluids.

The combination of gases with each other, in some instances, takes place when simply mixed together: thus nitrous and oxygen gases combine as soon as they come into contact; in other instances, it is necessary to elevate their temperature to a degree sufficient for their inflammation, either by means of the electric spark, or the contact of an ignited body, as in the combination of oxygen gas with hydrogen or nitrogen gas.

When gases combine with each other