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; for details concerning them he is referred to the following pages.

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

sary is to take a weighed quantity of the substance, dissolve it, precipitate the whole of the silver by adding hydrochloric acid or other chloride till no more chloride of silver falls, collect the precipitate on a filter, wash, dry, and weigh. The amount of silver in the dried chloride, ascertained by calculation, is the amount of silver in the quantity of substance on which the operation was conducted; a rule-of-three sum gives the quantity per cent.-the form in which the results of quantitative analysis are usually stated. Occasionally a constituent of a substance admits of being isolated and weighed in the uncombined state. Thus the amount of mercury in a substance may be determined by separating and weighing the mercury in the metallic condition; if occurring as calomel (Hg,Cl) or corrosive sublimate (HgCl), the proportion of chlorine may then be ascertained by calculation (Hg=200; Cl=35.5).

So, then, a body may be isolated and weighed alone in a balance and its quantity thus ascertained; or it may be separated and weighed in combination with another body whose combining proportion is well known; this is quantitative analysis by the gravimetric method.

Quantitative analysis by the volumetric method consists in noting the volume of a liquid required to be added to the substance under examination before a given effect is produced. Thus, for instance, a solution of nitrate of silver of known strength may be used in experimentally ascertaining an unknown amount of a chloride in any substance. The silver solution is added to a solution of a definite quantity of the substance until flocks of chloride of silver cease to be precipitated: every 108 parts of silver added (or 170 of nitrate of silver: Ag=108, N= 14, 0,=48; total 170) indicate the presence of 35.5 of chlorine, or an equivalent quantity of any chloride. The preparation of standard solutions, such as that of the nitrate of silver, to which allusion is here made, requires considerable care; but when made, certain analyses can be executed with far more rapidity and ease than by gravimetric processes.

The quantitative analysis of solids and liquids often involves determinations of temperature and specific gravity. These processes will now be explained, after which an outline of volumetric and gravimetric quantitative analysis will be given. The scope of this work precludes any attempt to describe all the little mechanical details observed by quantitative analysts; essential operations, however, are so fully treated that expert manipulators will meet with little difficulty.

The analysis of gases and vapours also involves determinations of the varying pressure of the atmosphere, as indicated by the barometer (from Bapos, baros, weight, and μérpov, metron, measure), a glass tube 33 or 34 inches long, closed at one end, filled with mercury, and inverted in a cup of mercury. The mercury remains in the tube owing to the weight or pressure of the atmosphere on the exposed surface of the liquid, the average height of the column being nearly 30 inches. In the popular form of the instrument, the wheel-barometer, the cistern is formed by a recurvature of the tube; on the exposed surface of the mercury a float is placed, from which a thread passes over a pulley and moves an index whenever the column of mercury rises or falls. For further information concerning the influence of pressure on the volume of a gas or vapour, and for descriptions of the methods of analyzing gases, refer to Ganot's Physics' (translated by Atkinson), Miller's Chemical Physics,' and "Analysis of Gases" in Watts's 'Dictionary of Chemistry.'

MEASUREMENT OF TEMPERATURE.

As a rule, all bodies expand on the addition, and contract on the abstraction of heat, the alteration in volume being constant and regular for equal increments or decrements of temperature. The extent of this alteration in a given substance, expressed in parts or degrees, constitutes the usual method of intelligibly stating, with accuracy, precision, and minuteness, a particular condition of warmth or temperature-that is, of sensible heat. The substance commonly employed for this purpose is mercury, the chief advantages of which are that it will bear a high temperature without boiling, a low temperature without freezing, does not adhere to glass to a sufficient extent to "wet" the sides of any tube in which it may be enclosed, and from its good conducting-power for heat responds rapidly to changes of temperature. Platinum, earthenware, alcohol, and air, are also occasionally used for thermometric purposes.

The construction of an accurate thermometer is a matter of great difficulty; but the following are the leading steps in the operation. Select a piece of glass tubing having a fine capillary bore, and about a foot long; heat one extremity in the blowpipeflame until the orifice closes, and the glass is sufficiently soft to admit of a bulb being blown; heat the bulb to expel air, imme