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or 208.9 + 71.82 +5+111.72 = 397.44; official sodium carbonate is given as Na,CO, + 10H,0= 285.45, in which case the weight of all the atoms of the crystalline compound, including the water, is accounted for in the molecular weight, 285.45, as follows : 2 atoms of sodium= 23 x 2 or 46, 1 atom of carbon = 11.97, 3 atoms of oxygen = 15.96 X 3 or 47.88, ten times 2, or 20 atoms of hydrogen=1 X 20 or 20, and 10 further atoms of oxygen = 15.96 X 10 or 159.6, or 46 + 11.97 + 47.88 + 20 + 159.6 = 285.45.

The number following simple elements expresses only the weight of a single atom, as bromine, Br = 79.76, sulphur, S = 31.98, etc. Atomic and molecular weights are of value in the proper construction of equations for the purpose of demonstrating chemical reactions.

THE OFFICIAL DESCRIPTION. While the official definition is a brief but exact statement of the nature and source of drugs and of the composition of chemicals, the official description amplifies the definition by adding the physical characteristics of drugs, such as shape, size, odor, and taste, together with a statement of possible impurities and adulterations and means for their detection. For chemicals is added a clear account of their physical properties, their behavior toward different solvents, and such tests as shall enable the pharmacist to detect impurities and establish the fulfilment of pharmacopoeial requirements. The official description is always printed in small type, and forms a most valuable and important part of the Pharmacopoeia.

Dispensatories.

A dispensatory is a commentary on the Pharmacopæia, and, as such, has become indispensable to both physicians and pharmacists. While the text of the Pharmacopoeia is confined to the definition and description of drugs and chemicals as well as to the official tests and requirements and accepted formulas for numerous preparations, much valuable additional information is given in the dispensatories, such as historical data, action, and uses, as well as doses of medicines, together with comments on and explanations of pharmaceutical and chemical processes. Besides the official drugs and chemicals, a large number of unofficial remedies and formulas are also treated in detail. Two dispensatories are published in this country: the United States Dispensatory, established in 1833, by Wood and Bache, which has now reached its seventeenth edition, and the National Dispensatory, established in 1879, by Stillé and Maisch, of which five editions have thus far been published.

CHAPTER II.

WEIGHTS AND MEASURES.

METROLOGY (from the Greek word uitpov, measure, and moyos, a discourse) is a study of the art and science of measurements as applied to extension, volume, and weight of matter. Measure of extension may be either of length or of surface, while measure of volume or bulk applies to the cubic contents. Measure of weight is the determination of the gravitating force of bodies, that is, of their attraction by the earth toward its centre, such attraction bearing a direct relation to the quantity of matter contained in a body; hence weight is pressure exerted by a body upon a horizontal plane supporting it. True weight can only be obtained in vacuo, where the exact measurements of the force of gravitation cannot be interfered with by atmospheric pressure; all measurements of weight in any medium, such as air or water, must therefore give low results. Ordinary operations of weighing, being conducted in air, give apparent weight of the substance only.

Weighing and measuring being operations of daily occurrence in pharmacy which require care and exactness, a knowledge of the standards of weights and measures in use in this country and elsewhere is absolutely necessary. With more or less modification the standards at present in use in pharmacy in the United States and Great Britain are the same as those formerly employed by the Romans, and which in all probability were by them derived from the more ancient Greek nation. Three different systems of weights are at present employed in all English-speaking nations; namely, avoirdupois weight, apothecaries' weight and metric weight.

AVOIRDUPOIS weight, as its name would seem to indicate, is probably of French origin (avoir du poids, to have weight), and was no doubt introduced into Great Britain during the reign of the Norman dynasty ; it first appeared in the English statute-books in 1335. Avoirdupois weight is employed in the sale of all commodities except precious metals and precious stones, hence drugs are always bought and sold by pharmacists by this system. In Great Britain avoirdupois weight is also employed in the formulas of the British Pharmacopeia, and is now known there under the name of Imperial weight. In 1824, the value of an avoirdupois pound was defined by law in England to be 3 98of the old standard troy pound. The divisions of avoirdupois weight are the pound, ounce, drachm, and grain, which are symbolized by the following characters : lb, oz.,

drm., gr.; each pound contains 16 ounces and each ounce 16 drachms or 4371 grains. The term drachm is rarely employed, quantities less than an ounce being usually designated by common fractions, such as it oz., § oz., 1 oz., or in grains. The avoirdupois pound containing 7000 grains (4371 X 16) is the only pound used in the United States and Great Britain except at the mints; the standard pound is the equivalent in weight of 27.7015 cubic inches of distilled water at 62° Fahrenheit and normal barometric pressure.

APOTHECARIES' weight was probably derived from troy weight, which latter was introduced into Great Britain, by merchants from Lombardy, toward the close of the thirteenth century; it is employed altogether in the writing and compounding of physicians' prescriptions, and is divided into grains, scruples, drachms, and ounces, of which 20 grains are equal to 1 scruple, 3 scruples are equal to 1 drachm, and 8 drachms are equal to 1 ounce. The apothecaries' ounce is of the same value as the now obsolete English troy ounce. The following symbols are employed to designate the divisions of apothecaries' weight, and always precede the number indicating the quantity intended, which is expressed in Roman numericals, thus, gr. j, for one grain, Dij

, for two scruples, 3iij, for three drachms, ziv, for four ounces. As far back as 1266, during the reign of Henry III., a statute was enacted in England which provided that an English silver penny, called a sterling, round and without clipping, should equal in weight 32 wheat-grains, well dried and taken from the centre of the ear, and that of such pence 20 should make 1 ounce, and 12 ounces 1 pound. About 1497, in the time of Henry VII., the weight of the silver penny, however, was changed to the equivalent of 24 wheat-grains. These statutes clearly indicate the origin of the pennyweight and the troy system, from which the apothecaries' weight, still in use at the present day, was subsequently derived. The choice of wheat-grains from the centre of the ear arose from a desire for uniformity in size and weight, as did likewise the directions to employ the grain well dried. The adoption of troy weight by physicians and pharmacists dates back to 1618, when the first London Pharmacopæia was compiled. In 1826, Imperial measures and standards were legalized in England, and in 1827 exact copies of these standards were furnished the minister of the United States Government at London ; namely, the standard yard, a bronze bar of 36 inches length, a brass troy-pound weight of 5760 grains, and a brass avoirdupois-pound weight of 7000 grains; copies of these standards were supplied to the different States in 1836 by Act of Congress. The length of the standard yard is determined by comparison with a pendulum beating seconds of mean time, in a vacuum, at the temperature of 62° Fahrenheit, at the level of the sea, in the latitude of London ; the length of such a pendulum was found to be 39.13929 inches.

From what has been said above it is clear that every troy or apothecaries' ounce is heavier than the avoirdupois ounce by 42

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192) of the number, thus 26.33

ounces

grains; hence to find the corresponding value in avoirdupois ounces
of any given number of troy or apothecaries' ounces, add to the
423

85
latter

17

of that number, thus Zxxiv = 24
4371 875 175

17
avoirdupois ounces plus of 24, which is 24 + 2.33, or 26.33

175
ounces ; if, on the other hand, avoirdupois weight is to be converted
into apothecaries' or troy weight, subtract from the number of ounces
421

85
given
480

192

17
= 26.33 of 26.33, which is equal to 26.33 — 2.33, or 24

192
apothecaries' or troy ounces.

While apothecaries' weight is employed in compounding prescrip-
tions both in this country and Great Britain, it is not used in either
the United States or British Pharmacopoeias, and will no doubt be
entirely abolished in the course of time, when a uniform international
system of weights shall have been adopted by the medical and phar-
maceutical professions of both countries. The grain is the connect-
ing link between avoirdupois, troy, apothecaries' and Imperial weight,
being the same in all.

The fluid measure used by pharmacists of the United States is derived from the old wine measure of England (now extinct), which allowed to each wine gallon the volume of 231 cubic inches, or 58340.011 grains of distilled water at 15° C. (59° F.); the Imperial gallon of Great Britain contains 277.273 cubic inches, or 70,000 grains of distilled water at 62° Fahr. In both cases the gallon is divided into 8 pints, but the pint of wine measure contains 16 fluidounces, while the Imperial pint contains 20 fluidounces. The United States fluid measure has the following units : the minim, the fluidrachm, and the fluidounce, which are represented by the following signs: m, f3, f3; in addition, the pint and gallon are sometimes employed in commercial transactions, being designated by the abbreviations O, from Octarius, for pint, Cong., from Congius, for gallon. The units of Imperial fluid measure bear the same names as those employed for United States fluid measure, but differ from them in value; thus, while the Imperial minim of water weighs 0.91 (0.9114583) grain, the United States minim of water weighs 0.95 (0.9493) grain, and, since both fluidounces contain 480 minims, the Imperial fluidounce of water weighs 437.5 grains, but the United States fluidounce 455.70 grains, at 15.6° C. (60° F.). Each fluidounce is divided into 8 fluidrachms and each fluidrachm into 60 minims.

It must not be overlooked that many liquids, although dispensed and sold by the apothecary by Auid measure, are purchased from the manufacturer by weight, and whenever the specific gravity of the liquid differs materially from that of water there must be also a

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marked difference in the relative volume; thus glycerin, syrups, chloroform, ethers, acids, essential oils, and many chemical solutions, are always purchased by weight. The following list shows the number of Auidounces in one pound of the respective liquids, of pharmacopoeial quality: One Pound of Sulphuric Acid measures about

8} fluidounces. Monsel's Solution measures about

10 Chloroform

101 Syrup

111 Glycerin

12 Goulard's Extract

123 Ammonia Water

16 Stronger Ammonia Water measures about 17 Spirit of Nitrous Ether

18 Essential Oil measures from

13 to 18 Ether measures about

217 The METRIC or DECIMAL system of weights and measures, which is the only official system of the present United States Pharmacopoeia, is supposed to have originated in the fertile mind of the French statesman, Prince de Talleyrand, toward the close of the last century, and was en forced in France by law in December, 1799. It has already become the legal standard in all civilized countries except the United States and Great Britain, and is destined to become the universal standard for commercial transactions, as it is already for strictly scientific work, the world over.

The use of metric weights and measures was legalized in the United States and Great Britain in 1866, but neither country has as yet officially adopted them, although the prospects for such desirable action are brightening. In 1878 the use of the metric system was made obligatory in the purchase of medical supplies for the United States Marine-Hospital service. Since the introduction of a new system of weights and measures must, no doubt, for a time create some confusion, a careful study of the same is required of pharmacists and physicians. The principles upon which the metric system was founded are as follows: The reduction of all weights and measures to one uniform standard of linear measure; the use of an aliquot part of the earth's circumference as such standard; the application of the unit of linear measure to matter in its three modes of extension-length, breadth, and thickness-as a standard of all measures of length, surface, and solidity; the cubic contents of linear measure in distilled water at the temperature of its greatest density to furnish at once the standard measure of weight and of capacity ; everything susceptible of being weighed or measured to have only one measure of weight, one measure of length, and one measure of capacity, with their multiples and subdivisions exclusively in decimal proportions, and every weight and every measure to be designated by an appropriate significant characteristic name applied exclusively to itself.

As a basis, the authors of the metric system adopted a quadrant

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