be reduced to powder. Metals which are scarcely brittle enough to be powdered, and yet are too soft to be filed, as zinc for instance, may be powdered while hot in a heated iron mortar; or metals may be rendered brittle by alloying them with a small quantity of mercury; but as metals are not required to be reduced to the state of very fine powder for pharmaceutical purposes, those processes are seldom performed.'

Trituration as to effect is the same as pulverisation by beating; it is produced by a rotatory motion of the pestle, or upon a large scale by rollers. Dr. Thomson remarks that the fine powders kept in the shops are generall ground in this manner; but there appears to be an error in reducing vegetable matters to the state of impalpable powder; for in this state, both during the process of grinding and afterwards, the air and light act powerfully upon them, and produce changes which, although they are not well understood, yet appear to alter the medicinal virtues of the substances.

Levigation is in fact trituration assisted by the addition of a fluid, which does not act chemically, or as a solvent, upon the material. This process is usually performed upon a flat stone, and the rubbing is effected by a muller of the same material with the stone. Earths are thus prepared for pharmaceutical purposes, and also some of the metals. Water or spirits of wine, or some unctuous material, are usually employed in levigation, and the substances used in the operation are for the most part previously pulverised.

Granulation is used for the mechanical division of some of the metals. The substance is melted or beaten into fine leaves, and then stirred briskly until it cools; or it is poured in its melted state into water, and stirred or agitated till it cools. The process is called granulation, on account of the metallic particles being separated by it in the form of small grains.

Rasping and filing scarcely need be mentioned as modes of dividing substances.

Sifting consists in separating the finer from the coarser parts of substances, by causing the former to pass through apertures in sieves formed of iron wire, hair cloth, or gauze. For very light and valuable powders it is necessary to employ close sieves, otherwise a great deal of the matter would be lost in the agitation.

Washing or elutriation consists in agitating the material in a fluid which does not act upon it as a solvent, but merely suspends it. The liquor containing thus the finer particles of the material is poured off from the sediment, and suffered to remain at rest for some time, when these fine and washed particles gradually settle, and the supernatant water is poured off, or drawn from the material by a syphon.

Filtration is a species of fluid sifting; filters are generally made of fine close flannel, or linen, or unsized paper, called filtering paper. When these are large it is usual to form them into a conical bag, and suspend this bag from a hoop or frame. When paper is employed it is doubled up in the shape of a cone, and inserted into a funnel; it is often requisite to introduce glass rods between the paper and the funnel, in order

to prevent the sides of the paper and funne! from being so closely in contact as to interfere with the percolation. When very acrid liquors, such as the strong acids and alkalis, require filtration, the glass funnel that is employed is partly filled with powdered quartz, and sometimes sand placed over this, so arranged that the coarser materials shall be at the bottom of the funnel, and the finer on the surface. The substance to be filtered is poured on the surface of the sand slowly, which it passes through, as also the lower substrata, and thus are the impurities of the liquor left behind.

Expression is a species of filtration: but in this last case force is employed. Expression is principally employed to obtain the juices of fresh vegetables, and the unctuous vegetable oils. The material is first beaten or bruised, then enclosed in a bag, not completely filled, and subjected to pressure between the plates of a screw press. The pressure should be applied gradually.

Vegetables treated in this manner ought to be perfectly fresh; and they should for the most part be subjected to the pressure immediately upon being bruised, for the bruising disposes them to ferment. But subacid fruits yield more juice, and of a better quality, when they are permitted to stand a few days in a wooden or earthen vessel previous to pressure. Sometimes, when the vegetable matter to be expressed is not very juicy, the addition of a little water is necessary. It is proper to peel oranges and lemons before they are pressed, in order to prevent the essential oil of the rind from mixing with the expressed juice. For unctuous seeds iron plates are used in expression, and the bruised seeds may be previously subjected in a bag to the steam of hot water.

Despumation is employed in the instance of fluids being so thick and clammy as not to pass with facility through a filter. The liquor is heated, and thus throws up a scum, which is to be carefully removed; or more generally whites of eggs are employed in the process of despumation; this entangles the impurities of the fluid, and rises up with them to the surface. In the case of clarifying spirituous liquors isinglass may be employed, for the process which coagulates in the spirit without the assistance of heat, and forms a scum which descends to the bottom of the vessel, and carries the impurities with it.

'Besides the above methods of mechanically separating the parts of substances from one another, fluids of different specific gravities, mixed together, are separated by means of the separatory funnel. It is chiefly used for separating the essential oils from the water with which they are entangled during their distillation. The funnel is first stopped at the bottom, and then filled with the mixed fluids, the heaviest of which gradually subsides into the narrow part below; and when the cork at the bottom is taken out, and the stopper above a little loosened, it flows out; by which means the lighter is easily obtained in a separate state. Some of the essential oils are heavier, others lighter than water; but both can be thus separated with equal facility.'

The above quotation is from Dr. Thomson; and in what follows respecting the chemical

operations of pharmacy we shall be principally indebted to the publication of that author.

The changes of which we have above spoken, as in some sort holding an intermediate place between mere formative and absolutely essential alteration, are effected by 1st, Caloric, inducing liquefaction, fusion, evaporation, exsiccation, distillation, rectification, dephlegmation, sublimation; by 2d, Water, and other fluids, causing solution, lixiviation, maceration, digestion, infusion, decoction, extraction; 3d, By other chemical agents inducing coagulation.

Calorific changes. Liquefaction. This term is applied to that operation by which certain bodies, when exposed to a moderate heat, are rendered fluid after passing through several intermediate states of softness. Fat, lard, wax, resin, and many other similar bodies, undergo liquefaction; which is therefore employed in pharmacy to facilitate the combination of these bodies in the formation of ointment. The best vessels for conducting the process of liquefaction are earthenware pans.

Fusion differs from liquefaction in the sudden change from the solid to the fluid state, which those bodies which are liable to it suffer on exposure to heat. There are no intermediate states of softness; but the fusible body, when heated to a certain point, immediately assumes the fluid form. This point differs very considerably in different solids; but in general simple substances are less fusible than compounds; and some of the simple earths cannot be fused without the addition of some other substances to promote their fusion. These are generally saline bodies, and are denominated fluxes. Fusion is usually performed in crucibles, the best of which are made of very pure clay or potter's earth. They are made of various forms, three cornered, or round, and fitted with covers.

Crucibles are also made of cast-iron, of fine silver, and of platina. The first, however, are destroyed when saline substances are melted in them; and when made red hot in a current of air are apt to suffer oxidation; but in other respects they are durable, and can sustain sudden alternations of heat and cold without cracking. Some of the metallic crucibles combine many of the best qualities necessary for this set of instruments; particularly those of platina: which, however, are too expensive for ordinary

use.

Evaporation is the dissipation of a liquid by means of heat, and is employed in pharmacy generally with the view of obtaining in a separate state any fixed substance which may be combined with water or some other evaporable fluid. Thus, by exposing an aqueous solution of a salt to a certain degree of heat, the caloric which combines with the water renders it volatile, and disperses it in the form of an elastic aeriform fluid; while the particles of the salt, being brought nearer to each other, and within the sphere of their mutual attraction, reunite, and the salt is obtained in its concrete state. In spontaneous evaporation the air is the principal agent, the dissipation being independent on artificial caloric or increased temperature. It is easy to conceive that the process of evaporation

is only had recourse to when the part of the body which is thus dissipated is of little value; when a solid is to be separated from a more valuable fluid, such as alcohol, distillation and not evaporation is employed.

Evaporating dishes are made of different materials; the best are those of biscuit porcelain Inade by Wedgewood, When glass or earthenware dishes are employed the heat is best applied through the medium of sand; or, if a still more moderate heat be necessary, by means of boiling water over which the evaporating dish should be placed. The first is named a sand-bath; the second a water-bath. Evaporating dishes in the general way should be flat-bottomed and shallow, so as to expose a large surface to the action of the applied heat.

Exsiccation is a variety of evaporation. It is generally employed for depriving salts of their water of crystallisation. The bodies to be exsiccated are heated in an iron ladle or pot, and undergo first what is called the watery fusion, that is, are heated and dissolved in their own fluid; this fluid, by continuing the process, is evaporated, and the body is left in the form of a dry mass. When the substances to be exsiccated are liable to decomposition in a temperature above 212°, as is the case with some of the compound oxides, the process must be conducted by the heat of a water-bath.

Distillation is also a species of evaporation, differing from it only in this, that the vapor of volatile matter, instead of being dissipated and lost in the air, is collected and condensed in close vessels. Some of the vessels used in this process will be found in the plates appended to the article CHEMISTRY. The simplest is the retort and receiver. The common still is a well known apparatus. It consists of two parts, the boiler, and the head or capital. The boiler, which is the part to which the fire is applied and contains the materials, is of a cylindrical shape, and may be sunk in a furnace or immersed in a water-bath when the temperature requires to be nicely regulated. The head or capital is a large hollow globe, the upper part of which is drawn out into a tapering pipe bent to a curve or arch, and terminating in the serpentine or worm. These parts are generally made of copper; but the worm is a long pewter pipe of a decreasing diameter which winds in a spiral direction obliquely through a deep tub filled with cold water. The body, head, and worm require to be luted together; but in general slips of paper dipped in flour, paste, or pieces of wet bladder, are sufficient for this purpose. In this apparatus the vapors are raised into the head, whence they pass into the worm, where they are condensed and issue in drops from the lower end of the pipe. By degrees the water in the refrigeratory becomes warmed and requires to be renewed, and thence the necessity of the tube being furnished with a stop-cock, by which the heated water may be drawn off without disturbing the apparatus. As in this species of distillation the vapor ascends before it is condensed, it is named distillation per ascensum. The distillation by the retort or cucurbit is named distillation per latus.

In some cases, as in the distillation of several essential oils, the vapor, instead of passing laterally or ascending, is forced to descend. To produce this effect a plate of tinned iron is fixed within any convenient vessel so as to leave a space beneath it; and, the materials to be distilled being laid upon this, they are covered by another plate, accurately fitted to the sides of the vessel, and strong enough to support the fuel which is burnt upon it. By this means the volatilised matter of the materials under the fire is forced into the lower cavity of the vessel and there condensed. This mode of distillation is denominated distillation per descensum. For an account of Woulfe's apparatus, and its modifications, see plates of CHEMISTRY.

Rectification is the repeated distillation of any product obtained by distillation, when it is not perfectly pure. This second operation is carried on at a lower temperature, so that the more volatile parts only are raised and pass over into the receiver, leaving the impurities behind. When the fluid is simply rendered stronger, as in the case of alcohol, by bringing over the spirit and leaving behind the superfluous water, the operation is named dephlegmation, or concentration, When the liquid is distilled off from any substance the process is called abstraction; and cohobation if the product be redistilled from the same materials, or from a fresh parcel of the same materials.

Sublimation is a species of distillation in which the product of volatilisation is condensed in a solid form; but, as this condensation takes place at a higher temperature than that of watery vapor, a much more simple apparatus is required. The process is conducted sometimes in a crucible, with a cone of paper or another crucible inverted over it, in which the product is condensed; and, as in this case it is light and spongy, it was formerly denominated flowers. For other matters which are less volatile a cucurbit and capital, or a flask and phial, are employed, and sunk about two-thirds in a sand-bath. In these cases the product is a solid, and is denominated a sublimate.

CHANGES BY THE ACTION OF FLUIDS. Solution. For an account of the laws and circumstances of this process, see the article CHEMISTRY.

Lixiviation. When a saline body consists of both soluble and insoluble ingredients, the solvent of the former, acting upon the salt, produces lixiviation, which process, when conducted on a great scale, is generally performed in large tubs or vats having a hole near the bottom containing a wooden spigot and faucet. A layer of straw is placed at the bottom of the tub, over which the substance is spread, and covered by a cloth; after which hot or cold water, according as the salt is more or less soluble, is poured on. The water, which soon takes up some of the soluble parts of the saline body, is after a little while drawn off by the spigot; and a fresh portion of water is successively added and drawn off until the whole of the soluble matter is dissolved. The straw in this operation acts as a filter, and the cloth prevents the water from making a hollow in the in

gredients when it is poured on, by which it might escape without acting on the whole of the ingredients.

In smaller operations lixiviation may be conducted in glass matrasses, and the lie, which is the name given to the solution, filtered through paper in a glass funnel.

By maceration soluble portions of substances, chiefly of a vegetable nature, are obtained in solution; these substances being immersed in cold water or in spirituous fluids for a sufficient length of time. It is frequently employed as a preparation for infusion and decoction, which are always rendered more effective by the previous maceration of the materials.

Digestion is similar to maceration, except that in this last case the power of the fluid is assisted by a gentle heat. It is usually performed in a glass matrass, and the evaporation of the liquid impeded by stopping the mouth of the matrass slightly with a plug of tow, or tying over it a piece of wet bladder perforated with small holes. When the menstruum is valuable, as alcohol for instance, another matrass with a smaller mouth may be inverted over the former and the joinings secured by a piece of wet bladder; or, what is perhaps preferable, a long open glass tube may be luted to the mouth of the matrass which contains the materials. By these means any part of the liquor which is resolved into steam by the heat is condensed and conveyed back upon the materials. The matrass may be either heated by a common fire, or a lamp, a waterbath, or a sand-bath; and, when either of the latter is used, the matrass should not be sunk deeper in the water or the sand than the portion that is filled. The process has been denominated circulation, when the condensed vapors are returned upon the ingredients.

Infusion is intended chiefly to extract the volatile and aromatic principle of vegetables, which would be dissipated by decoction or digestion; and also those parts of vegetables which are more readily soluble in water, as gum, sugar, extract, tannin, the salts and part of the resin from the insoluble parts. The water is poured boiling hot on the materials, sliced, or reduced to a coarse powder, and kept in a closely covered vessel till they are cold, when the infusion or liquor is decanted off for use. Infusions are sometimes effected with cold water; but for the most part these though more grateful are weaker than the infusions by heat.

Decoction, or boiling, is employed with advantage to extract the mucilaginous parts of plants, as well as their bitterness and several other vegetable principles which do not easily yield to mere infusion. It is generally performed in slightly covered vessels; but when the menstruum is valuable, as alcohol for instance, a retort and receiver, or the common still may be used, in the body of which the decoction is prepared, while the vapors, which otherwise would escape, are condensed and preserved.

Extraction.-If the liquor obtained by either infusion or decoction be subjected to evaporation the watery part is dissipated, and the part which is extracted by it is obtained in the solid form, and is denominated an extract. It is obvious

enough that extract of some materials will not contain the whole of the principles of those meterials.

CHANGES BY CHEMICAL AGENTS. Coagulation. This is the conversion of a fluid into a solid of more or less consistency. The means employed to effect this are increase of temperature, alcohol, acids, and runnets. The effect appears to arise from a new arrangement of particles produced by the affinity exerted between the solid particles contained in the fluid and the coagulating substance.

CHANGES PRODUCED BY THE CHEMICAL ACTION OF ONE SET OF BODIES ON ANOTHER.

Decomposition.-This implies the separation of the component, parts of bodies from one another. It is produced in some cases by heat so as to overcome the affinity of aggregation. Electricity or Galvanism may also effect decomposition; but in the greater number of instances it is the result of a superior affinity that holds the principles of the substance about to be decomposed in union, and produces new compounds. In pharmaceutical operations decomposition is very frequent, and it is of the utmost importance for the prescribers of medicine to be acquainted with its general circumstances.

Dissolution. This differs from mere solution by its being necessarily accompanied by some change in the nature of the dissolved body. In general effervescence is caused by the process, the disengaged materials becoming extracted in a gaseous form. The making a common saline draught is an instance of dissolution.

Precipitation. Here also decomposition occurs, but the substance extricated is thrown

down instead of otherwise separating itself. The material used to produce this separation is called the precipitant, and the separated substance the precipitate. It is necessary for the prescriber to be acquainted with those substances which, when mixed with others, produce precipitation, otherwise he will often be foiled in attempting the combination of incompatible principles. The following table of precipitants is extracted by Dr. A. T. Thomson, from the System of Chemistry by Dr. Thomas Thomson; all the substances not employed in pharmacy being omitted:1. ALKALIES.

Potash
Soda

Ammonia

2. ALKALINE EARTHS.

Barytes

Lime

Magnesia

3. EARTHS PROper. Alumina

4. METALLIC OXIDES.
Silver
Mercury
Copper
Iron

Lead

PRECIPITANTS. Tartaric acid. 0

Fixed alkalies.

Sulphuric acid, sulphates.
Oxalic acid, oxalates.
Phosphoric acid, phos-
phate of soda (not direct).

Ammonia, hydro-sulphu-
ret of potassa.
Muriate of soda.
Muriate of soda.
Iron.
Succinate of soda, benzo-
ate of soda.
Sulphate of soda.

In some cases where decomposition is effected by the addition of another substance, the separated body is not precipitated, but rises to the surface, and is thence denominated a cream: thus, by the addition of any acid to a solution of soap, the alkali unites with the acid, while the oil is separated and swims on the surface of the liquor.

Crystallisation.-See CHEMISTRY.

Fermentation.-For an account of those changes which vegetable substances undergo when separated from the living plant, and placed

under certain circumstances so as to act upon one another; and for the different compounds the vinous, acetous, and putrefactive fermentaand principles which are severally the results of tions, whether naturally occurring or artificially produced, see also the article CHEMISTRY.

Having thus premised an account of the general principles of pharmaceutic science, with its application to medicinal purposes, we now proceed to detail the several processes ordered and in this, the main part of the present article, we in the London and Edinburgh Pharmacopoeias; shall follow the plan adopted by Dr. A. T. Thomson, giving first the translations of the directions ordered by the colleges, and then some few remarks on the qualities of the composition. In respect to that portion of the treatise which relates to the incompatibility of one substance with another, Dr. Thomson will be wholly our guide. It should, however, be premised that chemical niceties may in some few instances suc

cumb before actual observations on the effects of compounds.

We do not include the articles of the Dublin Pharmacopoeia, fearing too great extension of the paper; and we may here take occasion to say that it is much to be lamented that one general prescription is not adopted throughout the whole of the united kingdom. In an appendix, however, will be found some of the more recent remedies, principally of the French school, which the Dublin college has recently adopted.

PART II. ACIDS.

In the article CHEMISTRY, and under the word ACID, will be found remarks on the principle of acidification and the composition of acids. It will there be seen that all acids having been supposed compounds of oxygen with certain bases, the name of the particular acid was taken from