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Phosphas calcis, Edin. magnesia usta, Lond. burnt on calcined magnesia.

Sulphas aluminae exsiccatus, Edin. dried sulphate of alumine, burnt alum, Lond.

In the Dublin process for making magnesia, there is a mutual decomposition of the two salts employed. The potash unites itself to the sulphuric acid, while the carbonic acid combines with the magnesia. The large quantity of water used is necessary for the solution of the sulphate of potash formed; and the boiling is indispensably requisite for the expulsion of a portion of the carbonic acid, which retains a part of the magnesia in solution. Sulphate of potash may be obtained from the liquor which passes through the filter, by evaporation. This is not pure, however, but mixed with undecomposed carbonate of potash; for one hundred parts of crystallized carbonate of potash are sufficient for the decomposition of one hundred and twenty-five parts of sulphate of magnesia; and as the carbonate of potash of commerce contains a larger proportion of alkali than the crystallized carbonate, a still less proportion should be used. From these quantities about forty-five parts of carbonate of magnesia are obtained.

The ablutions should be made with very pure water; for nicer purposes, distilled water may be used, and soft water is in every case necessary. Hard water for this process is peculiarly inadmissible, as the principle in waters, giving the property called hardness, is generally a salt of lime, which decomposes the carbonate of magnesia, by compound affinity, giving rise to carbonate of lime, while the magnesia unites itself to the acid of calcareous salt, by which the quantity of the carbonate is not lessened, but is rendered impure by the admixture of carbonate of lime. Another source of impurity is the silica which the sub-carbonate of potash generally contains. It is most easily got rid of by exposing the alkaline solution to the air for several days before it is used. In proportion as it becomes saturated with carbonic acid, the silica is precipitated, and may be separated by filtration.

The carbonate of magnesia thus prepared, is a very light, white, opaque substance, without smell'ortaste, effervescing with acids. It is not, however, saturated with carbonic acid. By decomposing sulphate of magnesia by an alkaline carbonate, without the application of heat, carbonate of magnesia is gradually deposited,

in transparent, brilliant, hexagonal crystals, terminated by an oblique hexagonal plane, and soluble in about four hundred and eighty times its weight of water. The crystallized carbonate of magnesia consists of fifty acid, twenty-five magnesia, and twenty-five water; the sub-carbonate consists of forty-eight acid, forty magnesia, and twelve water; and the carbonate of commerce of thirty-four acid, forty-five magnesia, and twenty-one water. It is decomposed by all the acids, potash, soda, baryte, lime, and strontian, the .sulphate, phosphate, nitrate, and muriate of alumina, and the super-phosphate of lime.

CLAss IV. SULPHUREA.

The preparations under this head are few; we need only enumerate the two following: Sulphur lotum, Lond. washed flowers of sulphur. Sulphur praecipitatum, Lond, precipitated sulphur. In preparing this last, instead of dissolving sulphuret of potash in water, we may gradually add sublimed sulphur to a boiling solution of potash, until it be saturated. When the sulphuretted potash is thrown into water, it is entirely dissolved, but not without decomposition, for it is converted into sulphate of potash, hydroguretted sulphuret of potash, and sulphuretted hydroguret of potash. The two last compounds are again decomposed on the addition of any acid. The acid combines with the potash, sulphuretted hydrogen flies off in the form of gas, while sulphur is precipitated. It is of little consequence what acid is employed to precipitate the sulphur. The London College order the sulphuric; while the Dublin College use nitrous acid; probably because the nitrate of potash formed is more easily washed away than sulphate of potash. Precipitated sulphur does not differ from well-washed sublimed sulphur, except in being much dearer. Its paler colour is owing to its more minute division, or, according to Dr. Thomson, to the presence of a little water; but from either circumstance it derives no superiority to compensate for the disagreeableness of its preparation. These are all the more simple preparations of sulphur in common use. There are various preparations into which sulphur enters as an ingredient; but such as constitute compounds of the general mature of metals, alkalies, oils, &c. will be found under those classes.

CLAss W. JMetallica. METALLINE PRE- PAir ATIONS.

The metalline preparations are very numerous, especially those of antimony and quicksilver. Sulphuretum antimonii praeparatum, Edin. prepared antimony. Oxidum antim. cum sulphure per nitratum potassae, Edin. crocus of antimony, Lond. Oxidum antimonii, cum sulphure, vitrificatum, vitrified antimony, Lond. glass of antimony. Sulphuretum antimonii praecipitatum, precipitated sulphuret, or sulphur of antimony, Lond. Murias antimonii, Edin, muriated antimony, Lond, butter of antimony. Oxidum antimonii cum phosphate calcis, Edin. pulvis antimonialis, Lond. antimonial powder. Tartris antimonii, tartarised, or tartrite of antimony. Vinum tartritis antimonii, Edin. tartar emetic, antimonial wine, Lond. Nitras argenti, Edin. argentum nitratum, Lond. nitrate of silver, lunar caustic. Frugo praeparata, Lond. Dub. prepared verdigrise, or carbonate of copper. Solutio sulphatis cupri composita, Edin. styptic water. Ammoniaretum cupri, Edin. ammoniacal copper. Aqua cupri ammoniati, Lond. water of the same. Ferri limatura purificata, Edin. purified iron filings. Carbonis ferri, Edin. rubigo ferri, Lond. carbonate, or rust of iron. Sulphas ferri, Edin. vitriolated iron, Lond. sulphate of iron. Tinctura muriatis ferri, tincture of muriate of iron, Lond. Murias ammoniae et ferri, martial flowers, ammoniacal iron, Lond. Tinctura ejusdem, tincture of the same. Tartris ferri, tartrite of, or tartarised, iron, Lond. Vinum ferri, Lond. wine of iron. Hydrargyrus purificatus, Lond. purified quicksilver. Acetis hydrargyri, Edin. acetite of quicksilver. Murias hydrargyri, Edin. Lond. muriate of quicksilver, corrosive sublimate. Submurias hydrargyri, Edin. calomel, Lond,

Submurias hydrargyri praecipitatus, Edin. mild muriated quicksilver, Lond. Calx hydrargyri alba, Lond, white precipitate. Hydrargyrus calcinatus, Dub. Lond. calcined quicksilver. Oxydum hydrargyri rubrum, Edin. red precipitate. Subsulphas hydrargyri flavus, Edin. vi. triolated quicksilver, Lond. Sulphuretum hydrargyri nigrum, Edin. aethiops mineral, turpeth mineral. Hydrargyrum sulphuratum nibrum, Lond. Dub. factitious cinnabar. Acetis plumbi, Edin. acetite of lead, sugar of lead. Aqua lythargyri acetata, Lond. extract of lead. Cerussa acetata, Lond. acetated ceruse. Stanni pulvis, Lond. powder of tin. Oxydum zinci, Edin. oxide of zinc, calcined zinc, Lond. Carbonas zinci, Edin. impurus praeparatus, prepared calamine. Oxydum zinci impurum praeparatum, Edin. prepared tutty. Sulphas zinci, Edin. vitriolated zinc, Lond. The antimonial powder of the London College is supposed to be nearly the same with the celebrated nostrum of Dr. James, the composition of which was ascertained by Dr. Pearson of London, to whom we are also indebted for the above formula. By burning sulphuret of antimony and shavings of hartshorn in a white heat, the sulphur is entirely expelled, and the antimony is oxydized, while the gelatine of the hartshorn is destroyed, and nothing is left but phosphate of lime, combined with a little lime. Therefore the mass which results is a mixture of oxide of antimony and phosphate of lime, which corresponds, at least as to the nature of the ingredients, with James's powder, which, by Dr. Pearson’s analysis, was found to consist of 43 phosphate of lime, and 57 oxide of antimony. Another excellent chemist, M. Chenevix, has lately proposed a method of forming the same combination in the humid way, with the view of obtaining a preparation always similar in its composition and properties. He was led to this proposal, by considering the uncertainty of the application, and the precarious nature of the agency of fire, by which means a variable portion of the oxide of antimony may be volatilized, and that which remains may be oxydized in various degrees. M. Chenevix, therefore, proposes to prepare a substitute for James’s powder, by dissolving together equal weights of

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submuriate of antimony and of phosphate of lime in the smallest possible quantity of muriatic acid, and then pouring this solution gradually into water sufficiently alkalized with ammonia. For the reason mentioned in the preceding article, it is absolutely necessary that the muriatic solution be poured into the alkaline liquor. By an opposite mode of procedure, the precipitate would contain more antimony at first, and towards the end the phosphate of lime would be predominant, and the antimony would be partly in the state of a submuriate. The phosphate of lime is most conveniently obtained pure by dissolving calcined bone in muriatic acid, and by precipitating it by ammonia. If the ammonia be quite free from carbonic acid, no muriate of lime is decomposed. M. Chenevix also found that this precipitate is entirely soluble in every acid which can dissolve either phosphate of lime or oxide of antimony separately, and that

about 0.28 of James's powder, and at an average 0.44 of the pulvis antimonialis of the London Pharmacopoeia, resist the action of every acid.

CLAss VI. Olea Fiacata. FIxED OILs.

These oil are improperly denominated expressed, which is their usual characteristic name, as in some instances they are obtained without expression, and in other instances expression is employed to obtain volatile oils. The Edinburgh college have therefore distinguished these different classes of oils by the terms fixed and volatile, which accurately characterize them. Fixed oil is formed in no other part of vegetables than in their seeds. Sometimes, although very rarely, it is contained in the parenchyma of the fruit. Of this, the best known example is the olive. But it is most commonly found in the seeds of dicotyledonous vegetables, sometimes also in the fruit of monocotyledonous plants, as the cocos butyracea. It has various degrees of consistency, from the tallow of the croton sebiferum of China, and the butter of the butter-tree of Africa, to the fluidity of olive oil. Fixed oils are either, 1. Fat, easily congealed, and not inflammable by nitric acid, oil of olives, almonds, rapeseed, and ben. 2. Drying, not congealable, inflammable by nitric acid, oil of linseed, nut, and poppy. 3. Concrete oils, palm oil, &c. Fixed oil is separated from fruits and seeds which contain it, either by expresVOL. IX.

sion or decoction. Heat, by rendering the oil more limpid, increases very much the quantity obtained by expression; but as it renders it less bland, and more apt to become rancid, heat is not used in the preparation of oils which are to be employed in medicine. When obtained by expression, oils often contain a mixture of mucilage, starch, and colouring matter; but part of these separate in course of time, and fall to the bottom. When oils become rancid, they are no longer fit for internal use, but are then said to effect the killing of quicksilver, as it is called, more quickly. Decoction is principally used for the extraction of the viscid and consistent oils, which are melted out by the heat of the boiling water, and rise to its surface. Those who prepare large quantities of the oil of almonds, blanch them, by steeping them in very hot water, which causes their epidermis to swell, and separate easily. After they peel them, they dry them in a stove, then grind them in a mill like a coffee mill, and lastly, express the oil from the paste inclosed in a hempen bag. By blanching the almonds, the paste which remains within the bag is sold with greater advantage to the perfumers, and the oil obtained is perfectly colourless. But the heat employed disposes the oil to become rancid, and the colour the oil acquires from the epidermis does not injure its qualities. For pharmaceutical use, therefore, the oil should not be expressed from blanched almonds, but merely rubbed in a piece of coarse linen, to separate the brown powder adhering to the epidermis as much as possible. Sixteen ounces of sweet almonds Commonly give five ounces and a half of oilBitter almonds afford the same proportions, but the oil has a pleasant bitter taste. In this manner are to be expressed, Oleum amygdalae, almond oil, from the kernel. Oleum lini, linseed oil, from the bruised seeds. Oleum ricini, castor oil, from the seeds previously decorticated. Oleum sinapeos, oil of mustard, from the bruised seeds.

CLAss VII. Aquae Distillatae. DISTILLED WATERs.

Substances which differ in volatility, may be separated from each other by applying a degree of heat capable of converting the most volatile into vapour, and I) d

by again condensing this vapour in a proper apparatus. Water is converted into vapour at 212°, and may be separated by distillation from the earthy and saline matters, which it always contains in a natural state. But it is evident, that if any substances which are as volatile as water be exposed to the same degree of heat, either by immersing them in boiling water, or exposing them to the action of its steam, they will rise with it in distillation. In this way the camphor and volatile oils of vegetable substances are separated from the more fixed principles; and as water is capable of dissolving a certain quantity of these volatile substances, it may be impregnated with a great variety of flavours by distilling it from different aromatic substances. If the subject of our distillation contain more volatile oil than the water employed is capable of dissolving, it will render the water milky, and afterwards separate from it. It is in this way that essential oils are obtained. Essential oils are obtained only from odoriferous substances; but not equally from all of this class, nor in quantity proportional to their degree of odour. Some, which, if we were to reason from analogy, should seem very well fitted for this process, yield extremely little oil, and others none at all. Roses and chamomile flowers, whose strong and lasting smell promises abundance, are found to contain but a small quantity of oil; the violet and jessamine flower, which perfume the air with their odour, lose their smell upon the gentlest coction, and do not afford any oil, on being distilled, unless immense quantities are submitted to the operation at once; while savin, whose disagreeable scent extends to no great distance, gives out the largest proportion of oil of almost any vegetable known. Nor are the same plants equally fit for this operation, when produced in different soils or seasons, or at different times of their growth. Some yield more oil, if gathered when the flowers begin to fall off, than at any other time. Of this we have examples in lavender and rue; others, as sage, afford the largest quantity when young, before they have sent forth any flowers; and others, as thyme, when the flowers have just appeared. All fragrant herbs yield a larger proportion of oil, when produced in dry soils and in warm summers, than in opposite circumstances. On the other hand, some of the disagreeable strong-scented ones, as wormwood, are said to contain most oil

in rainy seasons, and when growing in moist rich grounds. Several chemists have been of opinion, 4hat herbs and flowers, moderately dried, yield a greater quantity of essential oil, than if they were distilled when fresh. It is, however, highly improbable, that the quantity of essential oil will be increased by drying; on the contrary, part of it must be dissipated and lost. But drying may sometimes be useful in other ways; either by diminishing the bulk of the subject to be distilled, or by causing it to part with its oil more easily. The choice of proper instruments is of great consequence for the performance of this process to advantage. There are some oils which pass freely over the swan-neck of the head of the common still ; others, less volatile, cannot easily be made to rise so high. For obtaining these last, we would recommend a large low head, having a rim or hollow canal round it: in this canal the oil is detained in its first ascent, and thence conveyed at once into the receiver, the advantages of which are sufficiently obviOus. With regard to the proportion of water to be employed; if whole plants, moderately dried, are used, or the shavings of woods, as much of either may be put into the vessel, as, lightly pressed, will occupy half its cavity; and as much water may be added as will fill two thirds of it. When fresh and juicy herbs are to be distilled, thrice their weight of water will be fully sufficient; but dry ones require a much larger quantity. In general, there should be so much water, that, after all intended to be distilled has come over, there may be liquor enough left to prevent the matter from burning to the still. The water and ingredients, altogether, should never take up more than three-fourths of the still ; there should be liquor enough to prevent any danger of an empyreuma, but not so much as to be apt to boil over into the receiver. The subject of distillation should be macerated in the water until it be perfectly penetrated by it. To promote this effect, woods should be thinly shaved across the grain, or sawm, roots cut transversely into thin slices, barks reduced into coarse powder, and seeds slightly bruised. Very compact and tenacious substances require the maceration to be continued a week or two, or longer; for those of a softer and looser texture, two or three days are sufficient; while some tender herbs and flowers not only stand in no need of maceration, but are even injured by it. The fermentation which was formerly prescribed in some instances is always hurtful. With regard to the fire, the operator ought to be expeditious in raising it at first, and to keep it up during the whole process, to such a degree only, that the oil may freely distil; otherwise the oil will be exposed to an unnecessary heat; a circumstance which ought as much as possible to be avoided. Fire communicates to all these oils a disagreeable impregnation, as is evident from their being much less grateful when newly distilled, than after they have stood for some time in a cool place: and the longer the heat is continued, the greater alteration it produces in them. The greater number of oils require for their distillation the heat of water strongly boiling; but there are many also which rise with a heat considerably less; such as those of lemon and citron peel; of the flowers of lavender and rosemary, and of almost all the more odoriferous kinds of flowers. We have already observed, that these flowers have their fragrance much injured, or even destroyed, by beating or bruising them; it is impaired also by the immersion in water in the present process, and the more so in proportion to the continuance of the immersion and the heat; hence oils distilled in the common manner, prove much less agreeable in smell than the subjects themselves. For the distillation of substances of this class, another method has been contrived; instead of being immersed in water, they are exposed only to its vapour. A proper quantity of water being put into the bottom of the still, the odoriferous herbs or flowers are laid lightly in a basket, of such a size that it may enter into the still, and rest against its sides just above the water. The head being then fitted on, and the water made to boil, the steam, percolating through the subject, imbibes the oil, without impairing its fragrance, and carries it over into the receiver. Oils thus obtained possess the odour of the subject in an exquisite degree, and have nothing of the disagreeable scent perceivable in those distilled by boiling them in water in the common manner. Plants differ so much according to the soil and season of which they are the produce, and likewise according to their own ages, that it is impossible to fix the quantity of water to be drawn from a certain weight of them to any invariable standard. The distillation may always be con

tinued as long as the liquor runs well flavoured off the subject, but no longer. In the distillation of essential oils, the water, as was observed in the foregoing section, imbibes always a part of the oil. The distilled liquors, here treated of, are no other than water thus impregnated with the essential oil of the subject: whatever smell, taste, or virtue, is communicated to the water, or obtained in the form of watery liquor, being found in a concentrated state in the oil. All those vegetables, therefore, which contain an essential oil, will give over some virtue to water by distillation: but the degree of the impregnation of the water, or the quantity of water which the plant is capable of saturating with its virtue, are by no means in proportion to the quantity of its oil. The oil saturates only the water that comes over at the same time with it: if there be more oil than is sufficient for this saturation, the surplus separates, and concretes in its proper form, not miscible with the water that arises afterwards. Some odoriferous flowers, whose oil is in so small quantity that scarcely any visible mark of it appears, unless fifty or an hundred pounds or more are distilled at once, give nevertheless as strong an impregnation to water as those plants which abound most with oil. Many have been of opinion, that distilled waters may be more and more impregnated with the virtues of the subject, and their strength increased to any assigned degree, by cohobation, that is, by re-distilling them repeatedly from fresh parcels of the plant; experience, however, shews the contrary. A water, skilfully drawn in the first distillation, proves, on every repeated one, not stronger, but more disagreeable. Aqueous liquors are not capable of imbibing above a certain quantity of the volatile oil of vegetables; and this they may be made to take up by one, as well as by any number of distillations: the oftener the process is repeated, the ungrateful impression which they generally receive from the fire, even at the first time, becomes greater and greater. Those plants which do not yield at first waters sufficiently strong, are not proper subjects for this process. The mixture of water and oil which comes over may either be separated immediately, by means of a separatory, or after it has been put into large narrownecked bottles, and placed in a cool place, that the portion of oil which is not dis

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