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object is to obtain the soluble substance in a state of purity, the operation is termed lixiviation, and as small a quantity of the menstruum as is possible is used. When, however, it is employed to free an insoluble substance from soluble impurities, it is termed edulcoration, which is best performed by using a very large quantity of the menstruum. Organic products being generally composed of heterogeneous substances, are only partially soluble in the different menstrua. To the solution of any of these substances, while the others remain undissolved, the term extraction is applied; and when, by evaporation, the substance extracted is reduced to a solid form, it is termed an extract, which is hard or soft, watery or spirituous, according to the degree of consistency it acquires, and the nature of the menstruum employed. Infusion is employed to extract the virtues of aromatic and volatile substances, which would be dissipated by decoction, and destroyed by maceration, and to separate substances of easy solution from others which are less soluble. The process consists in pouring upon the substance to be infused, placed in a proper vessel, the menstruum, either hot or cold, according to the direction, covering it up, agitating it frequently, and after a due time, straining or decanting off the liquor, which is now termed the infusion. Maceration differs from infusion, in being continued for a longer time, and can only be employed for substances which do not easily ferment or spoil. Digestion, on the other hand, differs from maceration only in the activity of the menstruum being promoted by a gentle degree of heat. It is commonly performed in a glass matrass, which should only be filled one-third, and covered with a piece of wet bladder, pierced with one or more small holes, so that the evaporation of the menstruum may be prevented as much as possible, without risk of bursting the vessel. The vessel may be heated, either by means of the sun's rays, of a common fire, or of the sand-bath: and when the last is employed, the vessel should not be sunk deeper in the sand than the portion that is filled. Sometimes when the menstruum employed is valuable, a distilling apparatus is used to prevent any waste of it. At other times, a blind capital is luted on the matrass, or a smaller matrass is inverted within a larger one; and as the vapour which arises is condensed in it, and runs
back into the larger, the process in this form has got the name of circulation, upon which we have observed already. Decoction is performed by subjecting the substances operated on to a degree of heat, which is sufficient to convert the menstruum into vapour, and can only be employed with advantage for extracting principles which are not volatile, and from substances whose texture is so dense and compact as to resist the less active methods of solution. When the menstruum is valuable, that portion of it which is converted into vapour is generally saved by condensing it in a distilling apparatus. Solutions in alcohol, if coloured, are termed tinctures, and in vinegar or wine, medicated vinegars or wines. The solution of metals in mercury is termed amalgamation. The combinations of other metals with each other form alloys. Absorption is the condensation of a gas into a fluid or solid form, in consequence of its combination with a fluid or solid. It is facilitated by increase of surface and agitation; and the power of absorption in fluids is much increased by compression and diminution of temperature, although in every instance it be limited and determinate. Dr. Nooth invented an ingenious apparatus for combining gases with fluids, and Messrs. Schweppe, Paul, and Cuthbertson, have very advantageously employed compression. Fluids often become solid by entering into combination with solids, and this change is always accompanied by considerable increase of temperature, as in the slaking of lime. Chemical Decomposition is the separation of the elementary parts of bodies which were chemically combined, and can only be effected by the agency of substances possessing a stronger affinity for one or more of the constituents of the compound, than these possess for each other. Decomposition has acquired various appellations, according to the phenomena which accompany it. Dissolution differs from solution in being accompanied by a decomposition, or change in the nature of the substance dissolved. Thus, we correctly say, a solution of lime in muriatic acid, and a dissolution of chalk in muriatic acid. Sometimes a gas is separated during the action of bodies on each other. When this escapes with considerable violence and agitation of the fluid, it is termed ef. fervescence. The gas is very frequently allowed to escape into the atmosphere, but at otlier times is either collected in a pneumatic apparatus, or made to enter into some new combination. The vessels in which an effervescing mixture is made, should be high, and .#. large to prevent any loss of the materials from their running over, and in some cases the mixture must by made slowly and gradually. Precipitation is the reverse of solution. It comprehends all those processes in which a solid is obtained by the decomPosition of a solution. The substance separated is termed a precipitate, if it sink to the bottom of the fluid ; or a cream, if it swim above it. Precipitation, like soHution, is performed either via humida, or via sicca; and is effected by lessening the quantity of the solvent by evaporation; by diminishing its powers, as by reduction of temperature, or dilution; or by the addition of some chemical agent, which, from its more powerful affinities, either combines with the solvent, and precipitates the solvend, or forms itself an insoluble compound with some constituent of the solution. The object of precipitation are, the separation of substances from solutions in which they are contained; the purification of solutions from precipitable impurities; or the formation of new combinations. The two first means of precipitation have been already noticed. In performing it in the last manner we may observe the following rules:–The solution and precipitant must possess the requisite degree of purity. The solution should be perfectly saturated, to avoid unnecessary expenditure of the solvent or precipitant. The one is to be added slowly and gradually to the other. After each addition, they are to be thoroughly mixed by agitation. We must allow the mixture to settle after we think that enough of the precipitant has been added, and try a little of the clear solution, by adding it to some of the precipitant; if any precipitation takes place, we have not added enough of the precipitant. This is necessary, not only to avoid loss, but in many instances the precipitant, if added in excess, re-dissolves or combines with the precipitate. After the precipitation is completed, the precipitate is to be separated from the supernatant fluid by some of the means already noticed. When the precipitate is the chief ob
ject of our process, and when it is not soluble in water, it is often advisable to dilute to a considerable degree both the solution and precipitant before performing the operation. When it is only difficultly soluble, we must content ourselves with washing the precipitate after it is separated, by filtration. In some cases the separation of the precipitate is much assisted by a gentle heat. Crystallization is a species of precipitation, in which the particles of the solvend, on separating from the solution, assume certain determinate forms. The conditions necessary for crystallization are, that the integrant particles have a tendency to arrange themselves in a determinate manner, when acted on by the attraction of aggregation; that they be disaggregated, at least so far as to possess sufficient mobility to assume their peculiar arrangement; and that the causes disaggregating them be slowly and gradually removed. Notwithstanding the immense variety in the forms of crystals, M. Hauy has rendered it probable that there are only three forms of the integrant particles; the parallelopiped, the triangular prism, and the tetrahedron. But as these particles may unite in different ways, either by their faces or edges, they will compose crystals of various forms. The primitive forms have been reduced to six; the parallelopiped, the regular tetrahedron, the octahedron with triangular faces, the six-sided prism, the dodecahedron terminated by rhombs, the dodecahedron with isosceles triangular faces. Almost all substances on crystallizin retain a portion of water combined wi them, which is essential to their existence as crystals, and is therefore denominated water of crystallization. Its quantity varies very much in different crystallized substances. The means by which the particles of bodies are disaggregated, so as to admit of crystallization, are, solution, fusion, vaporization, or mechanical division and suspension in a fluid medium. The means by which the disaggregating causes are removed are, evaporation, reduction of temperature, and rest. When bodies are merely suspended in a state of extreme mechanical division, nothing but rest is necessary for their crystallization. When they are disaggregated by fusion or vaporization, the regularity of their crystals depends on the slowness with which their temperature is reduced; for if cooled too quickly, their particles have not time to arrange themselves, and are converted at once into a confused or unvaried solid mass. Thus glass, which, when cooled quickly, is so perfectly uniform in its appearance, when cooled slowly has a crystalline texture. But in order to obtain crystals by means of fusion, it is often necessary, after the substance has begun to crystallize, to remove the part which remains fluid; for otherwise it would fill up the interstices among the crystals first formed, and give the whole the appearance of one solid mass. Thus, after a crust has formed on the top of melted sulphur, by pouring off the still fluid part we obtain regular crystals. The means by which bodies which have been disaggregated by solution are made to crystallize most regularly, vary according to the habitudes of the bodies with their solvents and caloric. Some saline substances are much more soluble in hot than in cold water. Therefore a boiling saturated solution of any of these will deposit, on cooling, the excess of salt, which it is unable to dissolve when cold. These salts commonly contain much water of crystallization. Other salts are scarcely, if at all, more soluble in hot than in cold water; and, therefore, their solutions must be evaporated either by heat or spontaneously. These salts commonly contain little water of crystallization. The beauty and size of the crystals depend upon the purity of the solution, its quantity, and the mode of conducting the evaporation and cooling. When the salt is not more soluble in hot than in cold water, by means of gentle evaporation a succession of pellicles are formed on the top of the solution, which either are removed, or permitted to sink to the bottom by their own weight; and the evaporation is continued until the crystallization be completed. But when the salt is capable of crystallizing on cooling, the evaporation is only continued until a drop of the solution, placed upon some cold body, shews a disposition to crystallize, or at furthest only until the first appearance of a pellicle. The solution is then covered up, and set aside to cool, and the more slowly it cools the more regular are the crystals. The mother-water, or solution which remains after the crystals are formed, may be repeatedly treated in the same way, as long as it is capable of furnishing any more salt. When very large and beautiful crystals
are wanted, they may be obtained by laying well-formed crystals in a saturated solution of the same salt, and turning them every day. In this way their size may be considerably increased, though not without limitation, for after a certain time they grow smaller instead of larger.
Crystallization is employed to obtain crystallizable substances in a state of purity; or to separate them from each other, by taking advantage of their different solubility at different temperatures.
General Analysis resulting from the Application of Chemical Powers.
The simple elementary substances into which bodies are capable of being reduced, submitted to chemical action, are, light, caloric, electricity, galvanism, magnetism, oxygen, hydrogen, nitrogen, carbon, sulphur, soda, potash, phosphorus, metals, and earths. Of these the first five have no appreciable gravity, which is evinced by all the rest. Of the latter, again, some are combustible, others incombustible; some oxygenizable, others destitute of all affinity for oxygen. . But to enter minutely into these subjects would be to carry us beyond the limits of this article, and to infringe upon those that belong to chemistry as a general science, and to which, as also to the several articles above enumerated in the alphabetical order, we refer the reader for further information. So little progress, however, have we hitherto made in the general science of chemistry, that perhaps we are even now committing a double error, in offering the above as a table of simple elementary substances. It is possible that one of these substances is, strictly speaking, a simple element, or, in other words, totally uncompounded of rudiments that are more simple. We may also be in an error in conceiving every one of them to be a distinct substance from every other : we have many reasons, for example, for supposing that galvanism and electricity are the very same substance, only in different states of modification: and some philosophers have ventured to suspect that magnetism, or the magnetic power, is, in like manner, in unity with both. Neither soda nor potash, again, are scarcely any longer to be regarded as simple substances; we have many valuable experiments of Mr. Davy before us, by which they appear to have been completely decomposed; and there can be little doubt of the full confirmation of these experiments by subsequent trials of other chemists. And in this case it is possible that metallic substances will have to be as completely struck out of the list of simple elements as potash or soda. There are also several of the acids which are still admitted into the same catalogue, but whose pretensions are every day becoming still more doubtful, and of which, on this very account, we have taken no notice, though we shall have occasion to advert to them, and especially the muriatic acid, as we proceed. See Light, CALoR1c, ELECTRICITY, &c.
PART II. PHARMACEUTICAL PRE PARATIONs.
The classes into which these are divided have a considerable difference, as well in number as in arrangement, in our different collegiate Pharmacopoeias. That of the London College in present use is become perfectly obsolete, both in order and nomenclature. To the nomenclature of the Edinburgh we have little to object, but cannot altogether approve of its order. Why the Sulphurea should lead the way, and be so far separated from the JMetallica, with which they are so intimately connected by nature, we know not. We have reason to believe, that the forthcoming Pharmacopoeia of the London College will, in this, as well as in several other respects, evince a more systematic attention. In the mean time, while we give the general heads of both, we shall take the liberty of arranging them in the following manner:
Nitrous acid is frequently impure. Sulphuric acid is easily got rid of by re-distilling the nitrous acid from a small quantity of nitrate of potash. But its presence is not indicated when nitrous acid forms a precipitate with nitrate of baryte, as affirmed by almost all chemical authors; for nitrate of baryte was discovered by Mr. Hume to be insoluble in nitrous acid. Muriatic acid is detected by the precipitate formed with nitrate of silver, and may be separated by dropping into the nitrous acid a solution of nitrate of silver, as long as it forms any precipitate, and drawing off the nitrous acid by distillation. The general properties of nitrous acid have been already noticed. Mr. Davy has shewn, that it is a compound of nitric acid and nitric oxide, and that by additional doses of the last constituent, its colour is successively changed, from yellow to orange, olive-green, and blue-green, and its specific gravity is diminished. Vinegar may be distilled either in a common still or in a retort. The better kinds of wine vinegar should be used. Indeed, with the best kind of vinegar, if the distillation be carried on to any great length, it is extremely difficult to avoid empyreuma. The best method of preventing this inconvenience is, if a retort be used, to place the sand but a little way up its sides, and when somewhat more than half the liquor is come over, to pour on the remainder a quantity of fresh vinegar equal to the liquor drawn off. This may be repeated three or four times: the vinegar supplied at each time being previously heated. The addition of cold liquor would not only prolong the operation, but also endanger the breaking of the retort. Lowitz recommends the addition of half an ounce of recently burnt and powdered charcoal to each pound of vinegar in the still, as the best means of avoiding empyreuma. If the common still be employed, it should likewise be occasionally supplied with fresh vinegar, in proportion as the acid runs off, and this continued until the process can be conveniently carried no further. The distilled acid must be rectified by a second distillation in a retort or glass alembic; for although the head and receiver be of glass or stone ware, the acid will contract a metallic taint from the pewter worm. The residuum of this process is commonly thrown away as useless, although, if skilfully managed, it may be made to turn to good account, the strongest acid still remaining in it. Mixed with about three times its weight of fine dry sand, and committed to distillation in a retort, with a well regulated fire, it yields an exceedingly strong empyreumatic acid. It is, nevertheless, without any rectification, better for some purposes, as being stronger than the pure acid; particularly for making acetate of potash or soda: for then the empyreumatic oil is burnt out. Distilled vinegar should be colourless and transparent; have a pungent smell, and purely acid taste, totally free from acrimony and empyreuma, and should be entirely volatile. It should not form a black precipitate on the addition of a solution of baryte, or of water saturated with sulphuretted hydrogen ; or change its colour when super-saturated with ammonia. These circumstances show that it is adulterated with sulphuric acid, or contains lead, copper, or tin. Distilled acetous acid, in its effects on the animal economy, does not differ from vinegar, and as it is less pleasant to the taste, it is only used for pharmaceutical preparations.
CLAss II. Alkalina. ALKALINEs. The following are the chief preparations under this head: Carbonas potassae, carbonate of potash, prepared kali, mild vegetable alkali, salt of tartar. Potassa, pure kali, caustic vegetable alkali. Potassa cum calce, lime with pure kali, mild caustic. Aqua potassae, Edin. aq. kalipuri, Lond. water of potash, caustic ley. Acetis potassae, Edin. acetite of potash, acetated kali, Lond. Sulphas potassae, Edin. sulphate of potash, vitriolated tartar, vitriolated kali, Lond. Sulphas potrssae c. sulphure, Edin. sulphate of potash with sulphur, sal polychrest, Lond. Sulphuretum potassae, Edin. sulphuret of potash, liver of sulphur. Tartris potassae, tartrite of potash, Edin. soluble tartar, tartarised kali, Lond. ' Carbonas sodae, carbonate of soda, Edin. prepared natron, Lond. Phosphas sodac, Edin. phosphate of soda. Murias sodae, muriate of soda, sea salt. Sulphas soda, Edin, sulphate of soda, natron vitriolatum, Lond. Glauber's salt. Tartris sodae, Edin. tartrite of soda, natron tartarisatum. Lon. Rochelle salt. Alcohol ammoniatum, Edin, ammoniated alcohol, spirit of ammonia, Lond. Carbonas ammoniae, Edin. carbonate of ammonia, prepared ammonia, Lond. Aqua carbonatis ammoniae, Edin, water of carbonate of ammonia. Aqua acetis ammoniae, Edin, water of acetite of ammonia, spirit of mindereSuS. Hydro sulphuretum ammoniae, hydro sulphuret of ammonia. Liquor volutilis cornu cervi, salet oleum, Lond. spirit oil, and salt of hartshorn. CLAss III. Terrea. EARTHs, AND EARTHY SALTs.
The following are the preparations chiefly in use: Murias barytae, muriate of baryte, Edin. Aqua calcis, lime water, Edin. Lond. Dubl. Carbonas calcis praeparatus, prepared chalk, Dond. carbonate of lime, Edin. Phosphas calcis, Edin. phosphate of lime, burnt hartshorn, Lond. Carbonas magnesiae, Edin. magnesia alba, Lond, Dubl. carbonate of magnesia.