Calculi composed entirely of uric acid are common; a minute portion heated on platinum foil chars, burns, and leaves scarcely a trace of ash. The phosphates frequently occur together, forming what is known as the fusible calculus, from the readiness with which a fragment aggregates, and even fuses to a bead, when heated on a loop of platinum wire in the blowpipe-flame. The phosphates may, if necessary, be further examined by the method described in connexion with urinary deposits. Calcic oxalate often occurs alone, forming a dark-coloured calculus having a very rough surface, hence termed the mulberry calculus. Smaller calculi of the same substance are called, from their appearance, hempseed calculi. Calculi of cystin are rarely met with. Xanthin (from Eavoòs, xanthos, yellow, in allusion to the colour it yields with nitric acid) still less often occurs as a calculus.

The earthy concretions, or chalk-stones, which frequently form in the joints of gouty persons, are composed chiefly of urates, the sodium salt being that most commonly met with. Gall-stones, or biliary calculi, occasionally form in the gallbladder: they contain cholesterin (from xoλn, chole, bile, and oTepeòs, stereos, solid), a fatty substance soluble in alcohol or ether, and crystallizing from such solutions in well-defined, square, scaly crystals. Calculi of many pounds weight are often found in the stomach and larger intestines of animals.

Experience in manipulation having been gained by the analysis of fragments weighing half a grain or a grain, still smaller quantities of a calculus may be examined by directly testing for each common ingredient, always beginning by heating a minute portion on platinum foil to ascertain whether organic matter, inorganic matter, or both, are present. If organic matter only, the calculus will in nearly all cases be uric acid, the indication being confirmed by applying the murexid test in a watchglass to another fragment, half the size of a small pin's head. If inorganic, the ash on the platinum foil may be examined for phosphates, and a separate portion of the calculus for oxalates. Even a single drop of liquid obtained in any of these experiments may be filtered by placing on a filter not larger than a sixpence, and previously moistened with water, and adding three or four drops of water one after the other as each passes through the paper.

Knowledge of the composition of a calculus or urinary deposit affords valuable diagnostic aid to the physician, hence the importance of a correct analysis of these substances.

THE GALENICAL PREPARATIONS OF THE BRITISH PHARMACOPOEIA.

The preparation of Confections, Decoctions, Enemas, Extracts, Glycerines, Infusions, Inhalations, Juices, Liniments, Lozenges, Mixtures, Fixed and Volatile Oils, Ointments, Pills, Plasters, Poultices, Powders, Spirits, Suppositories, Syrups, Tinctures, Waters, and Wines, includes a number of mechanical rather than chemical operations, and belong to the domain of pure Pharmacy. The medical or pharmaceutical student will doubtless have had ample opportunity of practically studying these compounds before working at experimental chemistry, and will probably have prepared many of them according to the directions of the Pharmacopoeia; if not, he is referred to the pages of the last edition of that work for details.

Among the extracts, however, there are five (namely, those of Aconite, Belladonna, Hemlock, Henbane, and Lettuce) which are not simply evaporated infusions, decoctions, or tinctures, like most others, but are evaporated juices from which vegetable albumen, the supposed source of fermentation and decay, has been removed, and chlorophyll (the green colouring-matter of plant-juice) retained practically unimpaired in tint. In order that attention may be concentrated on the process by which these are prepared, rather than on the extracts themselves, it is advisable to make an extract of some ordinary green vegetable, such as cabbage or turnip-tops. Bruise the green leaves of a good-sized cabbage in a mortar, and press out the juice; heat it gradually to 130°, and remove the green flocks of chlorophyll which separate, by filtration through calico. When the liquor

has all passed through the filter, set the chlorophyll aside for a time, heat the strained liquor to 200° to coagulate albumen; remove the latter by filtration and throw away; evaporate the filtrate by a water-bath to the consistence of thin syrup; then add to it the chlorophyll, and, stirring the whole together assiduously, continue the evaporation at a temperature not exceeding 140°, until the extract is of a suitable consistence for forming pills. A higher temperature than that indicated would cause the alteration of the chlorophyll to a dark-brown substance, the extract no longer having the green tint which custom and the Pharmacopoeia demand.

The process by which volatile oils are usually obtained from herbs, flowers, fruits, or seeds, may be imitated on the small scale by placing the material (bruised cloves or caraways for instance) in a tubulated retort, adapting the retort to a Liebig's condenser, and passing steam, generated in a Florence flask, through a glass tube to the bottom of the retort. The steam in its passage upward through the substance will carry the oil over the neck of the retort into the condenser, and thence, liquefied and cooled, into the receiving vessel, where the oil will be found floating on the water. It may be collected by running off the distillate through a glass funnel, having a stopcock in the neck; or by letting the water from the condenser run into an old testtube which has a small hole in the bottom, or any similar tube placed in a larger vessel, the water and oil being subsequently separately run off from the tube as from a pipette. The water will in most cases be the ordinary officinal medicated water (Aqua Carui, Aqua Cinnamomi) of the material operated on. Many volatile oils rapidly absorb oxygen from the air and yield stearoptens or camphor-like bodies; hence they must be kept in wellclosed bottles.

THE CHEMICAL PREPARATIONS OF THE BRITISH PHARMACOPOEIA.

The process by which every officinal chemical substance is prepared has already been described, and the strict chemical character of the processes illustrated by experiments and explained by aid of equations. Should the student, in addition, desire an intimate acquaintance with those details of manipulation on which the successful and economic manufacture of chemical substances depends, he is advised to prepare a few ounces of each of the salts mentioned in the British Pharmacopoeia or commonly used in Pharmacy. His guide in these operations should be the Pharmacopoeia itself, as well as any chemical works that may be at his disposal.

The production of many chemical and galenical substances on a commercial scale can only be successfully carried on in manufacturing-laboratories and with some knowledge of the circumstances of supply, demand, value of raw material, and of by-products, &c. These are points which, during pupilage, may be regarded as of secondary importance, the acquirement of chemical principles being considered of highest necessity.

The next subjects of experimental study will be determined by the nature of the student's future pursuits. In most cases the operations of quantitative analysis will engage his attention. These should be of a volumetric and gravimetric character; details concerning them he is referred to the following pages.

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QUANTITATIVE ANALYSIS.

INTRODUCTORY REMARKS.

The proportions in which chemical substances unite with each other in forming compounds are definite and invariable. Quantitative analysis is based on this law. When, for example, aqueous solutions of a salt of silver and a chloride are mixed, a white curdy precipitate is produced containing chlorine and silver in atomic proportions, that is, 35-5 parts of chlorine to 108 of silver. No matter what the chloride or what the salt of silver, the resulting chloride of silver is invariable in composition. The formula AgCl is a convenient picture of this compound in these proportions. The weight of a definite compound being given, therefore, the proportional amounts of its constituents can be ascertained by simple calculation. Thus, for instance, 8.53 parts of chloride of silver contain 2·11 parts of chlorine and 6.42 of silver; for if 143.5 (the molecular weight) of chloride of silver contain 35.5 of chlorine, 8·53 of chloride of silver will be found to contain 2.11 of chlorine :

And if 143.5 of chloride of silver contain 108 of silver, 8·53 of chloride of silver will contain 6·42 of silver. To ascertain, for example, the amount of silver in a substance, all that is neces