The weights and measures signs employed in prescriptions are:

The last four signs are rarely employed in prescriptions.

Probably owing to the fact that the retailing and dispensing of drugs go on simultaneously in the same pharmacies, also on account of the fact that few dispensing chemists have sets of apothecaries'weights over the 3ij., while all have complete sets of avoirdupois or Imperial weights, doubt is frequently expressed as to what the signs in prescriptions imply. Once upon a time apothecaries'weights alone were employed in compounding drugs, but the British Pharmacopoeia adopted the Imperial system, and in 1877 the late Dr. Redwood, then an editor of that Pharmacopoeia, gave an authoritative and reasonable explanation of what is to be understood by the various signs for weights and measures. He pointed out that the British Pharmacopoeia adopts the avoirdupois weights, and the signs employed-lb., oz., gr.-apply to the weights of this system. The other signs-Э, 3, - however, mean what they always meant- namely, the scruple, drachm, ounce, and pound of the apothecaries' system. Difficulties can only occur in the ounce and pound. Strictly speaking, therefore, it should be understood that means an apothecaries' or Troy ounce of 480 grains, while 'oz.' means an avoirdupois ounce of 437 1/2 grains. So 'lb' is the Troy pound of 12 Troy ounces, and ' lb.' is the avoirdupois pound of 16 avoirdupois ounces. These signs were recognised by the British Pharmacopoeias until 1914, when the Imperial weights and measures gave way to the metric system except in statements of doses. In the preface to the British Pharmacopoeia, 1864, the following statement was made:

All who prescribe and dispense medicines are recommended to discontinue henceforth the use of the drachm and scruple weights.

No reference was made to the signs, but this was done in the preface to the 1867 edition, thus:

It is strongly urged upon all medical men to avoid the use of the terms ounce and pound with reference to any other than the avoirdupois or Imperial standard weight; but it will be optional with the physican in prescribing to use the symbols Э and 3, the former representing 20 and the latter 60 grains [if such should be found to conduce to accuracy or convenience]. In the measurement of liquids the Imperial measure is used for the higher denominations, and the fluid ounce and its sub-divisions into fluid drachms and minims for the lower denominations, of volume. [These measures are convenient, and have become familiar, having been used throughout the United Kingdom for many years.]

The 1867 preface was reprinted in the 1885 edition, and in the 1898 one the remarks quoted were also reprinted (without the two sentences in brackets) as being 'still applicable.' At the same time the alternative employment of metric weights and measures was 'extended to every official paragraph which makes reference to the usual Imperial weights and measures.' The preface to the 1914 edition states:

In this Pharmacopoeia the Centigrade thermometric scale and the metric system of weights and measures are used for all pharmaceutical and analytical computations. The metric system has also been employed for the specification of doses, in the expectation that in the near future the system will be generally adopted by British prescribers. ... As a transitional provision doses have also been expressed in terms of the Imperial system. ... In prescriptions the symbol 3J is often used to represent 60 grains, and also to represent 1 fluid drachm; and the symbol to represent sometimes 480 grains, sometimes 437.5 grains, and also to represent 1 fluid ounce. As these symbols are apt to be misread, it is ecommended that prescribers should cease to employ them.

The Pharmacopoeia of 1885 introduced for the first time he fluid grain, which is the volume of 1 grain of water at the normal conditions of temperature and atmospheric pressure. The metric system is exclusively employed on the Continent, and both liquids and solids are weighed except in France, where the rule 'solids by weight, liquids by measure' is followed. The use of the system is legal in the United Kingdom, and since its recognition by the British Pharmacopoeia, 1914, sets of metric weights and measures are requisite for the equipment of every dispensing counter. It is always preferable, in dispensing foreign prescriptions, to weigh all the ingredients as would be done on the Continent. The tare of the bottle is taken with a quantity of small shot contained in a chip box.

In weighing fluids for a mixture, the German rule is to put the smallest quantity ordered in the bottle first, then the next larger quantity, and so on. The reason is that the delicacy of the scales diminishes with the increased weight, and as the medicines ordered in small quantities are generally the most powerful, they need to be dispensed with the greatest degree of accuracy. When so many drops of a fluid are ordered it is usual to put the drops in first, so that if a few drops too many fall in they can be returned. Fluids up to 1 gramme are generally dropped, and it is reckoned that of the fatty oils, the heavier essential oils, and tinctures, 20 drops = 1 gramme; of other essential oils, chloroform, acetic ether and spirit of ether, and aqueous fluids, 25 drops = 1 gramme; of ether, 50 drops = 1 gramme. These calculations are not quite accurate, but they accord with the Prussian medicinal tariff, which is understood by the prescriber and dispenser to be a common basis of reckoning.

The rule brings out prominently the superiority of the English system of measuring, and the immense advantage of so small a unit of measure as the minim. The principle of the rule (taking the smallest quantities first) has its application, however, to our method, the exceptions being where following the rule would give a result not desired. The smaller quantities and thinner fluids should invariably be measured first, or, if this does not quite suit a special method of compounding, they should be measured with a fresh measure-glass. To measure, for example, 1 drachm of hydrocyanic acid in a measure which has just been used for measuring 1/2 ounce or 1 ounce of glycerin or syrup of squill clearly courts error. Equally incorrect would it be to start with a 4-ounce measure-glass, however correctly graduated or however suitable for measuring the other ingredients of a prescription, to measure a small quantity of a powerful remedy, such as hydrocyanic acid. Oily liquids- e.g., ext. filicis liq. - should be poured into some of the aqueous ingredient of the prescription contained in the measure.

How To Measure.

Grasp the measure with the thumb and forefinger of the left hand, and bring the second finger under the measure so as to keep it level. The other fingers thus remain free to grasp the bottle-stopper.

It is much the best plan to make it the practice to use a separate measure for all potent preparations, such as arsenical solutions, solution of strychnine, hydrocyanic acid, and the like, and, if the nature of the mixture permit it, measure such preparations last- that is, after the mixture is made up with water or other diluent, leaving room for the potent preparation.

The most useful measures for the dispensing-counter are 2-ounce conical, 4-drachm conical (graduated throughout for each 20 minims), and 20-minim pipettes (graduated for each minim). The conical measures should be selected of rather elongated shape, the inner sides forming a cone having straight sides (no bulging). Generally speaking, the 2-ounce measure should not be used for measuring quantities under 2 drachms, nor the 4-drachm measure for quantities under 20 minims.