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only about 2 per cent. of water-soluble matter, according to Prof. Markoe.

Resin of scammony is wholly soluble in ether and oil of turpentine, and is not precipitated by acids from its solution in caustic alkalies. It is very slowly acted upon by sulphuric acid, whereas common resin is immediately turned intensely red; the presence of the latter can thus be detected. The resin of scammony can be distinguished from the gum-resin by not yielding a green emulsion when triturated with water.

CHAPTER XXV.

COLLODIONS.

UNDER this head are recognized in the Pharmacopoeia four solutions, the base of which is pyroxylin, or soluble gun-cotton (see Cellulose, Part III.), and the solvent, a mixture of alcohol and ether. Collodions are employed only for external medication, and owing to the very volatile character of the solvent, they rapidly form a skinlike covering, or pellicle, when applied, which is impervious to water. Where a strong contractile coating is desired, the plain collodion is preferred, otherwise a less constringent and more comfortable covering is obtained by the addition of castor oil and Canada turpentine, as in the case of the official flexible collodion. For the purpose of medication, any substance soluble in ether may be added, such as iodine, iodoform, extract of Indian cannabis, salicylic acid, croton oil, mercuric chloride, veratrine, atropine, resorcin, pyrogallol, etc. Since pyroxylin is insoluble in water, the addition of the latter to collodion would cause immediate precipitation, hence all substances soluble only in water or alcohol and water, such as extract of belladonna, morphine sulphate, etc., are excluded from admixture. Collodions should always be preserved in tightly cork-stoppered bottles, in a cool place, remote from fire, on account of the ether present; care should also be taken that no collodion be allowed to remain on the lip or in the neck of the bottle after pouring out the liquid, to avoid "fixing" of the cork as the menstruum evaporates.

Collodions are best dispensed in small, round-shouldered vials provided with a cork through which a camel-hair pencil has been passed and securely fastened; this avoids loss of material and drying of the collodion in the brush-a very annoying occurrence.

ALPHABETICAL LIST OF OFFICIAL COLLODIONS.

Collodium. If the pyroxylin has been carefully prepared, it should be perfectly soluble in the official menstruum, although a slight sedi

ment of dirt, etc., occurs after the solution has been set aside for a few hours; from this the liquid can be carefully poured off, as filtration is impracticable. Anthony's collodion cotton, specially prepared for photographers' use, I have always found very satisfactory.

Collodium Cantharidatum. The value of cantharidal collodion will depend upon the quality of the powdered cantharides used and the care with which they are exhausted. Since chloroform is a very volatile menstruum, percolation must be conducted in a special apparatus (see Oleoresins), to avoid loss; the liquid is easily recovered by distillation on a water-bath, as chloroform boils at about 60° C. (140° F.). The extract dissolves readily in flexible collodion, by agitation, the finished product representing 60 per cent. of its weight of powdered cantharides, which makes it nearly twice as strong as the official cerate of cantharides.

Collodium Flexile. The addition of Canada turpentine (Canada balsam or balsam of fir) and castor-oil lends to collodion the property of forming a flexible pellicle which, while serving as a perfect covering to the part affected, yet permits perfect freedom of motion.

Collodium Stypticum. Owing to the large proportion of tannin ordered in the formula for styptic collodion, it is necessary to use a small quantity of diluent, ether and alcohol, with which the tannin is thoroughly mixed before the final addition of collodion. Flexible collodion is not suitable in this case, as a constringent pellicle is desired. Any impurities from the tannin, may be removed by rapidly straining the solution through gauze previously moistened with a little ether and alcohol.

CHAPTER XXVI.

EMULSIONS.

THE term "emulsion" is applied to a more or less permanent homogeneous liquid mixture composed of fatty, ethereal, or resinous substances and water, the former being suspended in a minutely divided state, which gives rise to the peculiar opaque and milk-like appearance. Nature provides types of true emulsions, in the form of milk, the natural food of all young mammalia, and the milk-like juices of certain plants from which the official and other gum-resins are obtained.

Those prepared by pharmacists, may be conveniently divided into natural and artificial emulsions; to the former class belong those which are made from seed or gum-resins, by simple trituration with water, Nature having provided the necessary emulsifying agent in intimate association with the oil or resin. Artificial emul

sions are such as require the addition of some foreign body, by means of which the suspension of the oil or resin is made possible; to this class belong the majority of the emulsions prepared at the dispensing counter. Fixed and volatile oils, as well as ether, chloroform, oleoresins and resins, are suitable for exhibition in the form of emulsion, the suspension in water being accomplished by the aid of appropriate excipients, such as acacia, tragacanth, yelk of egg, casein, dextrin, Irish moss, gelatin, soap-bark, etc. Oil-yielding seeds and natural gum-resins contain albuminous and mucilaginous matter, by means of which the oil and resin can be brought into perfect suspension in water, and such emulsions approach more closely in character and stability to cow's milk, which may be looked upon as the most perfect emulsion known. The theory of emulsification is as follows: The insoluble liquid or solid, in a state of minute division, is completely surrounded or enveloped by the vehicle consisting of water and excipient, and thus an opaque mixture is produced, from which, the particles cannot separate by mere force of cohesion; such a condition can be obtained to perfection only by choice of a proper excipient, and, for artificial emulsions, none better than acacia has yet been found. Stability of artificial emulsions, while primarily dependent upon the division of the insoluble liquid into minute globules, is also influenced, to some extent, by the density of the vehicle, thus sugar has been found to increase the suspending power of gum materially; to prevent fermentative changes likely to arise in all aqueous vegetable solutions, alcohol or glycerin is frequently added to emulsions, in the proportion of one or two fluidounces for every pint.

With the exception of those made of volatile oils or ethereal liquids, emulsions should always be made in a mortar, either of unglazed Wedgewood or hard porcelain, having a flat bottom, and, in the case of seed or gum-resin emulsions, one of deep shape provided with a hard-wood pestle is to be preferred, in order to avoid injury from the force often necessary in crushing and manipulating the material. (For making gum-resin emulsions, the cleanest and hest tears should be selected, as the commercial fine powders are unfit for this purpose, partly because they are inferior in quality, and partly because they have been so modified by drying that when triturated with water they simply form an ordinary mixture from which the powder separates rapidly on standing; this change is due to dehydration, whereby the natural association of gum and resin has been broken up and their intimate union destroyed. For seed emulsions, when no other proportions are specified, 1 part of seed is used to 10 parts of water, all dirt and dust being carefully removed, if necessary, by washing with cold water. In both cases the material is crushed into a coarse powder, and, after the addition of a small quantity of water, beaten into a perfectly smooth pasty mass; to this, the remainder of the water is then added in divided portions, triturating the mass thoroughly and keeping it well scraped down from the pestle and sides of the mortar, so that a uniform mixture may result, which is finally passed through a well-wetted strainer of loose flannel or cheese-cloth, to remove the inert woody fibre and possible impurities. In making emulsion of lycopodium, it becomes necessary to triturate the seed dry, with some pressure, in order to rupture the hard seed envelope; when the powder changes in color and becomes damp and adhesive from the oil, a little water is added, with which a smooth soft paste can be formed, to be further diluted by the addition of water as directed above. Emulsion of lycopodium should never be strained, and, if properly made, will show no particles floating on the surface; the insoluble matter which settles upon standing is readily reincorporated by agitation.

Oil emulsions, which are far more frequently used (at least in this country) than those made from seed or gum-resins, require more care in their preparation, as success depends not only on the manipulation, but also on the judicious choice of a proper excipient. As a general rule, it may be stated that acacia produces the whitest and most stable emulsions, because its perfect and ready solubility in water, enables the operator to divide the oil quickly into minute globules, which are at once surrounded by an envelope of the mucilaginous liquid and thus kept from coalescing. The oil globules of a well-made acacia emulsion, when compared with milk under a magnifying lens more closely resemble its fat globules than would be the case if made with other excipients. To insure success, it is essential that definite proportions of oil, gum, and water be used for making the primary emulsion, which can then be diluted with water as desired. Not less than one-fourth nor more than one-half as much acacia as oil Not