PHYSIOLOGICAL CLASSES OF MEDICINES.

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slightly soluble only in water. The distilled waters of the Pharmacopoeia are saturated solutions of them. If the oils be previously rubbed withsugar they dissolve more readily in water. The mixtures or compoundsof volatile oils and sugar are called elceosacchara. According to thePrussian Pharmacopoeia they consist of one drop of oil to a scruple ofsugar. Volatile oils dissolve readily in alcohol, ether, pyroxilic spirit,and naphtha, and easily mix with the fixed oils and resins.

The volatile oils, as ordinarily met with, usually consist of two oils—the one liquid, at ordinary temperatures ( volatile oil, properly so called ;the eleoptene of Berzelius ; the hygrusin of Bizio)—the other solid(stearoptene of Bezelius ; stereusin of Bizio ; camphor of the Germanchemists). When the latter predominates, the oil readily concretes incold weather—as the oil of anise and the oil of star-anise. The camphorof the shops is the solid volatile oil ( stearoptene ) of Camphora officinarum.

In regard to ultimate composition the volatile oils vary. Some consistof carbon and hydrogen only—as those of turpentine, juniper, savin,lemon, and bergamot. Others contain also oxygen—as iavender, anise,mint, and rosemary: while a third class contain no less than five ingre-dients ; namely, carbon, hydrogen, oxygen, sulphur, and nitrogen ; as thevolatile oil of mustard. It is remarkable that all the volatile oils whichcontain carbon and hydrogen only, (10 C + 8 II) have the same ultimatecomposition; or, at least, they consist of the same elements in thesame relative proportion.

The volatile oils undergo chemical changes when exposed to the air.They become deeper coloured and thicker, absorb oxygen, and give riseto the formation of carbonic acid and resin. The resins of turpentine andcopaiva appear to be simple oxides of their respective oils.

2. Resin ( resina ).—This is rarely found in the mineral kingdom, orin animal substances ; but is common in vegetables. In the latter itexists almost invariably, if not universally, in combination with volatileoil, from which, perhaps, it may be formed by the action of the oxygen ofthe air. It is a transparent or partially opaque, hard, soft, or elastic solid ;coloured or colourless ; lighter or heavier than water, its sp. gr. varyingfrom O'93 to 1'2 ( Gmelin ); fusible and combustible. It is a bad con-ductor of electricity, and becomes strongly negatively electrical by fric-tion. As commonly met with it is odorous, but probably, if completelydeprived of volatile oil, would be inodorous. Its taste is usually moreor less acrid ; sometimes bitter, and, occasionally, is not perceptible. Itis not soluble in water, though some resins form hydrates with thisliquid. It is soluble in ether and volatile oil, and frequently more or lessso in alcohol; and on the addition of water to the alcoholic solution theresin is thrown down as a white powder, which gives a milky appearanceto the fluid.

Most resins possess acid properties ; that is, they redden litmus, andcombine with alkalies and other metallic oxides. This is the case withthe two. resins (pinic and sylvic acids) of which colophony is. composed ;as well as the resin of copaiva (copaivic acid); of guaiaemn (guaiacicacid); of gamboge (gambogic acid), &c. The compounds formed bythe union of resins with alkalies, or other basic substances, are calledresinous soaps. The sapo-guajacinus and sapo-jalapinus of the PrussianPharmacopoeia, as well as the savon de terebinthine (Starkey's soap) ofthe French Codex , are soaps of this kind, and will be noticed hereafter.