The operations of pharmacy may be arranged under two classes: -

I. Operations which are purely mechanical.

II. Operations which are performed by chymical powers and agents.

The first are intended for determining the weight and bulk of bodies, diminishing their cohesion, and separating their integral parts: the second are intended for separating the elements of bodies from one another, and for re-uniting these elements into new combinations.

I. Pharmaceutical Operations Purely Mechanical. A. Of The Means Of Determining The Weight And Bulk Of Bodies

In pharmaceutical processes it is essential that the quantities of the substances employed be accurately ascertained; and for this purpose beams with scales and measures must be provided. Several sets of beams and scales are necessary; one set for large weights, from one pound to one hundred weight or more; another for weights not exceeding five pounds; and a third for small weights under two drachms. A good beam should remain in equilibrium, both by itself, and when the scales are suspended to each extremity; the largest sets should be exact to within half a drachm; the second should be sensibly affected by two or three grains at most; and the smallest by the hundredth part of a grain. Apothecaries, however, seldom have beams of such accuracy, and, generally, those which they employ are much injured by exposure to acid fumes, and from want of cleanliness. To preserve the delicacy of beams, they should be kept in very close cases, and not left suspended longer than is absolutely necessary; nor should they be overloaded.

Drugs are bought in the gross by avoirdupois weight, which is the standard of most articles of merchandise; but, for the composition of medicines, troy weight is directed to be used by the British Colleges. The following Table exhibits the manner in which the pound is divided, and the signs used, in prescriptions, for denoting the different weights: -

The differences between the avoirdupois pound and the troy or apothecaries' pound, and their subdivisions, are exhibited in the following Tables :-

Troy Or Apothecaries' Weight

Avoirdupois Weight

The troy weight has also been adopted by the Edinburgh College for apportioning liquids; but the London and Dublin Colleges, with more propriety, order liquids to be measured: and for this purpose the London College employs measures derived from the wine gallon, which is subdivided for medical purposes, in the manner exhibited by the following Table, which shows also the symbols used for denoting the several measures :-

Table Of The Proportions Of The Wine Gallon

1 Tables of the method of reducing the subdivisions of the troy pound into decimals of the troy pound are given in the Appendix to Part I.

The London College has introduced the minim measure as a substitute for the drop, the inaccuracy of which had been long experienced; as the fluidity and specific gravity of the liquid, the thickness of the lip of the phial, and even its degree of inclination, were all liable to vary the size of the drop; but by dividing the fluid drachm into sixty equal parts, a measure of bulk is obtained, which is as constant and uniform as the grain weight employed for solids.

For measuring liquids, graduated glass measures of different sizes, a, b, are to be preferred; and for quantities under five minims a slender, graduated glass tube, c, open at both ends, or the measure, d, is to be employed. When the tube is used, the graduated end is to be inserted into the liquid to be measured down to the mark indicative of the quantity required; and the upper end being then closely covered by the finger, the tube retains the proper quantity of liquid, which again drops from it on raising the finger from the upper end: the small glass measure is now pretty generally used instead of the tube. In extemporaneous prescriptions, the measures of a table-spoonful, cochleare majus, and a tea-spoonful, cochleare minimum, are used, when great accuracy is not required; the former being supposed to be equal to half a fluid ounce, the latter to a fluid drachm.

Elastic fluids or gases are also measured in glass jars, or tubes hermetically closed at one extremity, and graduated by inches, with their decimals; but, in ascertaining the bulk of gases, the temperature of the atmosphere, and its density at the time, as indicated by the thermometer and the barometer, must be attended to; for if the former be above or below 60°, the mean heat of the air, or if the mercury in the barometrical tube be under or above 30 inches, corrections must be made by calculation relative to the degrees of temperature and pressure. For the former, the observed column of air must be divided by 80, and the quotient multiplied by the degree of temperature above or below 60°. This correction is negative when the actual temperature is above the standard, and positive when it is below. For making the corrections with regard to pressure, see the table in the Appendix.

The specific gravity of bodies is also necessary to be known in many pharmaceutical processes; and as the effects of acids and alcohol depend on the degree of their concentration, a knowledge of their gravity enables this to be correctly ascertained. The specific gravity of any substance is "the quotient of its absolute weight divided by its magnitude, or the weight of a determinate bulk of any body; and as a standard for this purpose, the weight of a determinate magnitude of distilled water has been generally assumed as unity.1 It is seldom necessary to determine the specific gravity of solids; but for ascertaining that of fluids various means may be employed. If a little ball of rock-crystal, for instance, suspended by a hair or a fine platinum wire, be weighed first in air, and afterwards in distilled water of the temperature of 60° Fahr., the weight lost by the ball is equal to the weight of an equal bulk of the liquid; so that, by repeating this operation in other fluids, and dividing its loss of weight in any other liquid by its loss of weight in water, the quotient is the specific gravity of the particular liquid.

The specific gravity of liquids, however, is more generally determined by hydrometers, of which Mr. Nicholson's is by far the most accurate.2

The specific gravity of liquids is also very easily determined by the following simple method :-Take a small light bottle which stands firmly, e, and holds about a fluid ounce or two fluid ounces of water, and stop its neck by a piece of barometer-tube very accurately ground; or a conical stopper with a notch in it, f. First weigh the empty bottle and tube, then fill it with distilled water at 60°, recently boiled, till the water rises a little into the bore of the tube, and weigh the whole, scratching the weight in grains on the bottle, and also the weight in grains of the empty bottle, and tube. For facilitating calculation, the water should be brought to that height in the tube, at which its weight will be 500 grains or 1000, or 1500, or 2000; and this height must be accurately marked on both sides of the tube with a file. A bottle of this kind is now sold under the name of "Thousand grain bottle" with a weight, which is an exact counterpoise to it, when it is full of distilled water at 60°. By filling this bottle with any other fluid, and weighing it, the specific gravity of that fluid is ascertained by only calculating how much lighter or heavier it is than the same bulk of water.3 Thus, if it be filled with sulphuric ether, instead of weighing 1000 grains, as when it is filled with water, it will require 270 grains to be put into the scale with the bottle to restore the equilibrium, and thence, its sp: gr. is 0.730: but, if sulphuric acid be used, 875 grains will be required to be added to the 1000 grains' weight in the opposite scale, and consequently its sp. gr. is 1.875.

1 Lavoisier's Elements of Chymistry.-Trans. 376.

2 Nicholson's Journal, 4to; 110.

3 Aikin's Dictionary of Chymistry, ii___Appendix.

In ascertaining specific gravity, the substances should be brought by calculation to the temperature of 60°, if the thermometer be above or below that point at the time of performing the experiments; and the gravities should always be expressed according to their relation to distilled water. Although this is the method generally employed in philosophical and pharmaceutical operations, yet it is necessary to observe, that the strength of spirits, according to the excise laws in this country, is estimated by the proportion they contain of a standard spirit, termed hydrometer proof, which consists of 40 parts of pure alcohol and 51 of water. Sikes's hydrometer is the one employed by the Excise. The strength of spirits stronger than proof, or over proof, is ascertained by the bulk of water required to reduce a given bulk of the spirits to the specific gravity denominated proof, on Sikes's hydrometer; and the strength of weaker spirits, or under proof is estimated by the quantity of water it would be necessary to abstract to bring the spirits up to proof.

Thus, if 20 gallons of the spirit require the addition of one gallon of water to bring it to proof, the spirit is said to be one to twenty over proof'; and if, from the same quantity of spirits, one gallon of water must be abstracted to bring it to proof, it is said to be one in twenty under proof; and so on.