but subacid fruits give a larger quantity of juice and of finer quality, when they are allowed to stand some days in a wooden or earthen vessel after they are bruised. To some vegetables which are not juicy enough of themselves, the addition of a little water is necessary. Lemons and oranges must be peeled, as their skins contain a great deal of essential oil, which would mix with the juice. The oil itself may be obtained separately, by expression with the fingers against a plate of glass.

For unctuous seeds iron plates are used; and it is customary not only to heat the plates, but to warm the bruised seeds in a kettle over the fire, after they have been sprinkled with some water, as by these means the product is increased, and the oil obtained is more limped. But as their disposition to rancidity is increased by it, if possible this practice should be laid aside, or confined to exposing the bruised seeds, inclosed in a bag, to the steam of hot

water.

Despumation is generally practised on thick and clammy liquors, which contain much slimy and other impurities, not easily separable by filtration. The scum arises either by simply heating the liquor, or by clarifying it, which is done by mixing with the liquor, when cold, whites of eggs well beaten with a little water, which on being heated coagulates, and entangling the impurities of the liquor, rises with them to the surface, and may be easily removed by a perforated ladle; or the liquor may now be filtered with ease. Spirituous liquors are clarified by means of isinglass dissolved in water, or any albuminous fluid, such as milk, which coagulates by the action of alcohol without the assistance of heat. Some expressed juices, such as those of the antiscorbutic plants, are instantly clarified by the addition of vegetable acid, such as the juice of bitter oranges.

Fluids can only be separated from each other when they have no tendency to combine, and when they differ in specific gravity. The separation may be effected by skimming off the lighter fluid with a silver or glass spoon; or by drawing it off by a syringe or syphon; or by means of a glass separatory, which is an instrument having a projecting tube, terminating in a very slender point, through which the heavier fluid alone is permitted to run; or by means of the capillary attraction of a spongy woollen thread; for no fluid will enter a substance whose pores are filled by another, for which

it has no attraction; and, lastly, upon the same principle, by means of a filter of unsized paper, previously soaked in one of the fluids, which in this way readily passes through it, while the other remains behind.

Mechanical mixture is performed by agitation, trituration, or kneading; but these will be best considered in treating of the forms in which medicines are exhibited.

Chemical Operations and Results.

Under this chapter we have to consider the apparatus employed, the changes produced, and the general analyses that ensue.

The apparatus consists of vessels, fuel, or heat; and the different modes by which such fuel or heat is applied, whether lamps, furnaces, &c.

The vessels must necessarily vary in their form and materials; upon the first of which it will be more convenient to enlarge as we proceed to treat of the particular operations in which they are employed. In choosing the materials for the construction of our vessels, the properties most generally required are a power of resisting chemical agents, transparency, compactness, strength, fixity, and infusibility, and an ability to sustain sudden variations of temperature, without breaking.

Generally speaking, metals possess the four last properties in considerable perfection; but they are all opaque. Iron and copper are apt to be corroded by chemical agents; and a solution of the last is often followed by dangerous affections. Tinning them will sometimes, but not always, answer; for tin and lead are often too fusible. Platinum, gold, and silver, resist most of the chemical agents, but are too expensive for general use.

Good earthenware resists the greatest intensity of heat, but has no other property to recommend it. Clay, the basis of all such wares, is plastic when worked with water, and sufficiently hard when burnt with an intense heat. But intense heat contracts it unduly, and it is apt to split and crack upon exposure to sudden changes of temperature; whence it is necessary to counteract this property by the addition of some other substance. Siliceous sand, clay reduced to powder, and then burnt with a very intense heat, and plumbago, are occa sionally used. These additions, however, are attended with other inconveniences; plumbago especially is liable to combustion, and sand diminishes the compactness; so that when not glazed they are porous, and

tures, but they become so hard that they can scarcely be removed, and often harden so quickly after they are mixed up, that they must be applied immediately. Examples :

Quick-lime well incorporated with a sixth part of muriate of soda.

Burnt gypsum, made up with water. One ounce of borax dissolved in a pound of boiling water, mixed with a sufficient quantity of powdered clay. Mr. Watts's

fire lute.

One part of clay with four of sand formed into a paste with water. This is also used for coating glass vessels, in order to render them stronger and capable of resisting violent degrees of heat. It is then made into a very thin mass, and applied in successive layers, taking care that each coat be perfectly dry before another be laid on.

The lutes for lining furnaces will be described when treating of furnaces.

The junctures of vessels which are to be luted to each other, must previously be accurately and firmly fitted, by introducing between them, when necessary, short bits of wood or cork, or, if the disproportion be very great, by means of a cork fitted to the one vessel, having a circular hole bored through it, through which the neck of the other vessel or tube passes. After being thus fitted, the lute is either applied very thin, by spreading it on slips of linen or paper, and securing it with thread, or, if it is a paste lute, it is formed into small cylin ders, which are successively applied to the junctures, taking care that each piece be made to adhere firmly and perfectly close in every part, before another is put on. Lastly, the whole is secured by slips of linen or bladder. In many cases, to permit the escape of elastic vapours, a small hole is made through the lute with a pin, or the lute is perforated by a small quill, fitted with a stopper.

Heat and Fuel. As caloric is an agent of the most extensive utility in the chemical operations of pharmacy, it is necessary that we should be acquainted with the means of employing it in the most economical and efficient manner. The rays of the sun are used in the drying of many vegetable substances, and the only attentions necessary are to expose as large a surface as possible, and to turn them frequently, that every part may be dried alike. They are also sometimes used for promoting spontaneous evaporation.

The combustion of different substances

is a much more powerful and certain source of heat. The substances employed for this purpose, are either fluid or solid. Alcohol, oil, tallow, wood, turf, coal, charcoal, and coke, are all occasionally employed. Alcohol, oil, and melted tallow, fluid inflammables, must be burnt on porous wicks. These act merely mechanically, by drawing up a portion of the fluid to be volatilized and inflamed. They are therefore burnt in lamps of various constructions. But althongh commonly used to produce light, they af ford a very uniform, though not very high temperature: it may, however, be increased by increasing the number of the wicks, and their size. Alcohol produces a steady heat, no soot, and, if strong, leaves no residuum. Oil gives a higher temperature, but on a common wick produces much smoke and soot. These are diminished, and the light and heat increased, by making the surface of the flame bear a large proportion to the centre, which is best done by a cylindrical wick, so contrived that the air has free access both to the outside and to the inside of the cylinder, as in Argand's lamp, invented by Mr. Boulton of Birmingham. In this way, oil may be made to produce a considerable temperature, of great uniformity, and without the inconvenience of smoke.

Wicks have the inconvenience of being charred by the high temperature to which they are subjected, and of becoming so clogged as to prevent the fluid from rising in them: they must then be trimmed, but this is seldomer necessary with alcohol and fine oils than with the coarser oils. Lamps are also improved by adding a chimney to them it must admit the free access of air to the flame, and then it increases the current, confines the heat, and steadies the flame. The intensity of the temperature of flame may be increased astonishingly, by forcing a small current of hot air through it as by the blow-pipe. Wood, turf, coal, charcoal, and coke, solid combustibles, are burnt in grates and furnaces. Wood has the advantage of kindling readily, but af fords a very unsteady temperature, is inconvenient from its flame, smoke, and soot, and requires much attention. The heavy and dense woods give the greatest heat, burn longest, and leave a dense charcoal. Dry turf gives a steady heat, and does not require so much attention as wood; but it consumes fast, its smoke is copious and penetrating, and the empyreumatic smell which it imparts to every thing it comes in contact with, adheres with great obstinacy.

The heavy turf of marshes is preferable to the light, superficial turf. Coal is the fuel most commonly used in this country: its heat is considerable, and sufficiently permanent, but it produces much flame and smoke. Charcoal, especially of the dense woods, is a very convenient and excellent fael: it burns without flame or smoke, and gives a strong, uniform, and permanent heat, which may be easily regulated, especially when it is not in too large pieces, and is a little damp; but it is costly, and burns quickly. Coke, or charred coal, possesses similar properties to charcoal; it is less easily kindled, but is capable of producing a higher temperature, and burns more slowly.

When an open grate is used for chemical purposes, it should be provided with cranes, to support the vessels operated in, that they may not be overturned by the burning away of the fuel,

Furnaces. In all these, the principal objects are, to produce a sufficient degree of heat, with little consumption of fuel, and to be able to regulate the degree of heat. An unnecessary expenditure of fuel is prevented by forming the sides of the furnace of very imperfect conductors of caloric, and by constructing it so, that the subject operated on may be exposed to the full action of the fire. The degree of heat is regulated by the quantity of air which comes in contact with the burning fuel. The quantity of air is in the compound ratio of the size of the aperture through which it enters, and its velocity. The velocity is increased by mechanical means, as by bellows, or by increasing the height and width of the chimney. The size and form of furnaces, and the materials of which they are constructed, are various, according to the purposes for which they are intended.

The essential parts of a furnace are, a body for the fuel to burn in; a grate for it to burn upon; an ash-pit to admit air, and receive the ashes; a chimney for carrying off the smoke and vapours.

The ash-pit should be perfectly close, and furnished with a door and registerplate, to regulate the quantity of air admitted. The bars of the grate should be triangular, and placed with an angle pointed downwards, and not above half an inch distant. The grate should be fixed on the outside of the body. The body may be cylindrical or elliptical, and it must have apertures for introducing the fuel and the

subjects of the operation, and for convey. ing away the smoke and vapours. When the combustion is supported by the current of air naturally excited by the burning of the fuel, it is called a wind-furnace; when it is accelerated by increasing the velocity of the current by bellows, it forms a blast. furnace; and when the body of the furnace is covered with a dome, which terminates in the chimney, it constitutes a reverbera. tory furnace.

Furnaces are either fixed, and built of fire-brick, or portable, and fabricated of plate-iron. When of iron, they must be lined with some badly conducting and refractory substance, both to prevent the dissipation of heat, and to defend the iron against the action of the fire. A mixture of scales of iron and powdered tiles worked up with blood, hair, and clay, is much recommended; and Professor Hagen says, that it is less apt to split and crack when exposed at once to a violent heat, than when dried gradually, according to the common directions. Dr. Black employed two different coatings. Next to the iron he applied a composition of three parts by weight of charcoal, and one of fine clay. These are first mixed in the state of fine pow der, and then worked up with as much water as will permit the mass to be formed into balls, which are applied to the sides of the furnace, and beat very firm and compact, with the face of a broad hammer, to the thickness of about one inch and a half in general, but so as to give an elliptical form to the cavity. Over this, another lute, composed of six or seven parts of sand, and one of clay, is to be applied in the same manner, to the thickness of about half an inch. These lutes must be allowed to become perfectly dry before the furnace is heated, which should at first be done gradually. They may also be lined with fire bricks of a proper form, accurately fitted and well cemented together before the top plate is screwed on.

The general fault of furnaces is that they admit too much air, which prevents us from regulating the temperature. It either becomes too violent and unmanageable, or when more cold air is admitted than what is necessary for supporting the combustion, it carries off heat, and prevents us from raising the temperature as high as we otherwise would. The superior merit of Dr. Black's furnace consists in the facility with which the admission of air is regulated;

and every attempt hitherto made to improve it, by increasing the number of its apertures, have in reality injured it.

Heat may be applied to vessels employed in chemical operations, directly, as in the open fire and reverberatory furnace: or through the medium of sand; the sandbath: of water; the water-bath: of steam; the vapour-bath: of air; as in the muffle.

Changes produced by chemical processes. These consists chiefly in a new mode of aggregation, combination, and decomposi

tion.

The form of aggregation may be altered by fusion, vaporization, condensation, congelation, and coagulation.

Fusion is the conversion of a solid into a liquid by the sole agency of caloric. Substances differ very much in the degrees of their fusibility; some, as water and mercury, existing as fluids in the ordinary temperatures of the atmosphere; while others, as the pure earths, cannot be melted by any heat we can produce.

Liquefaction is commonly employed to express the melting of substances, as tallow, wax, resin, &c. which pass through intermediate states of softness before they become fluid. Fusion is the melting of substances which pass immediately from the solid to the fluid state, as the salts and metals, except iron and platinum.

When, in consequence of fusion, the sub. stances operated on acquire a greater or less degree of transparency, a dense uniform texture, and great brittleness, and exhibit a conchoidal fracture, with a specular surface, and the edges of the fragments very sharp, it is termed vitrification.

In general, simple substances are less fusible than compounds; for example, the simple earths cannot be melted singly, but when mixed are easily fused. The additions which are sometimes made to refractory substances, to promote their fusion, are termed fluxes: which fluxes are generally saline bodies.

Thus, the alkalies potash and soda promote powerfully the fusion of silicious stones; but they are only used for accurate experiments. The white flux is a mixture of a little potash with carbonate of potash, and is prepared by deflagrating together equal parts of nitrate of potash and supertartrate of potash. When an oxide is at the same time to be reduced, the black flux is preferred, which is produced by the deflagration of two parts of super-tartrate of potash, and one of nitrate of potash. It differs from the former only in containing

a little charcoal. Soap promotes fusion by being converted by the fire into carbonate of soda and charcoal.

Aluminous stones have their fusion greatly promoted by the addition of sub-borate of

soda.

Muriate of soda, the mixed phosphate of soda and ammonia, and other salts, are also occasionally employed for the same purpose.

An open fire is sufficient to melt some substances, others require the heat of a furnace.

The

The vessels in which fusion is performed, must resist the heat necessary for the operation. In some instances an iron or copper ladle or pot may be used, but most commonly crucibles are employed. These are of various sizes. The large crucibles are generally conical, with a small spout for the convenience of pouring out; the small ones are truncated triangular pyramids, and are commonly sold in nests. Hessian crucibles are composed of clay and sand, and when good, will support an intense heat for many hours, without softening or melting; but they are disposed to crack when suddenly heated or cooled. This inconvenience may be on many occa. sions avoided, by using a double crucible, and filling up the interstice with sand, or by covering the crucible with a lute of clay and sand, by which means the heat is transmitted more gradually and equally. Those which ring clearly when struck, and are of an uniform thickness, and have a reddish brown colour, without black spots, are reckoned the best. Wedgewood's crucibles are made of clay mixed with baked clay finely pounded, and are in every respect superior to the Hessian, but they are very expensive. The black lead crucibles, formed of clay and plumbago, are very durable, resist sudden changes of temperature, and may be repeatedly used, but they are destroyed when saline substances are melted in them, and suffer combustion when exposed, red hot to a current of air.

When placed in a furnace, crucibles should never be set up on the bars of the grate, but always upon a support. Dr. Kennedy found the hottest part of a furnace to be about an inch above the grate. They may be covered, to prevent the fuel or ashes from falling into them, with a lid of the same materials, or with another crucible inverted over them. When the fusion is completed, the substance may be either permitted to cool in the crucible, or may