Graduates which have the same scale marked on both sides, or which are encircled by the markings of the scale, admit of more accurate measurements, and do not require that careful attention to levelling the graduate necessary with the plainer varieties.

Glass graduates are best cleaned by washing with a mop, using soap and water if necessary, rinsing with clear water and allowing the graduate to drain either on a perforated tray or by hanging in a rack; but never should a towel be used to dry the graduate, as it is apt to leave lint adhering to the glass.

Approximate Measurements.-Owing to the varied density

of liquids, the number of drops contained in a certain volume must vary greatly with different liquids; moreover, the size of a drop is influenced by the size and shape of the vessel from which the drop is allowed to fall-so that a drop is a very uncertain quantity in the division of doses of medicines. The variability of adhesion to glass exhibited by different liquids, and the rapidity with which liquids flow from the same vessel held at different angles of inclination, are other factors which determine the size of drops, as is shown in the case of chloroform.

Instead of being identical with the minim, drops may vary from one-fifth to one and one-fourth minim.

For the purpose of better illustration, the following short table has been inserted, showing the great variability in size of drops of different liquids:

TABLE SHOWING THE NUMBER OF DROPS TO A FLUIDRACHM.

For the administration of medicines certain familiar domestic measures are employed, which, although subject to considerable variations, are usually estimated as having the following capacity:

A teaspoonful, equal to one fluidrachm ;
A dessertspoonful, equal to two fluidrachms;
A tablespoonful, equal to one-half fluidounce;
A wineglassful, equal to two fluidounces;
A teacupful, equal to four fluidounces; and
A tumblerful, equal to eight fluidounces.

Figs. 26, 27, and 28 represent convenient medicine glasses, well adapted to family use.

These vessels are now obtainable accurately graduated and made to correspond to apothecaries' fluid measure; hence they are preferable to the variable tea-, dessert- and tablespoons generally met with, and should be employed altogether in the sick-room.

CHAPTER III.

SPECIFIC GRAVITY.

A KNOWLEDGE of the subject of specific gravity is of importance to the pharmacist, as it frequently enables him to detect impurities or to determine the identity and quality of the drugs he handles. Specific gravity does not indicate absolute weight, but merely a relative value, or the relation between the volume and weight of bodies as compared with a standard-the standard for liquids and solids being distilled water, while atmospheric air or hydrogen is used for gaseous bodies; in other words, specific gravity expresses the ratio between the weight of any gaseous, liquid, or solid body and that of an equal volume of the respective standard.

The terms specific gravity and density are frequently used synonymously in pharmacy and chemistry. In physics density is defined to be the mass, or quantity by weight, of a substance in a unit volume, the latter being either a cubic centimeter, as in the metric system, or a cubic foot, as in the English system. In the metric system, where density expresses the number of grammes in a cubic centimeter of a homogeneous substance, density is identical with specific gravity referred to water at 4° C. (39.2° F.), since the gramme is the mass or weight in a cubic centimeter of water at 4° C. (39.2° F.), and thus comparison with the accepted standard is established. This identity, however, vanishes if the specific gravity has been referred to water at a higher temperature; and although the difference between density and specific gravity at such higher temperatures may not be very great, it is sufficient to destroy identity, since 1 cubic centimeter of water above 4° C. weighs less than 1 gramme. In pharmacy and chemistry these slight differences are practically ignored, and hence the terms density and specific gravity are generally used interchangeably, namely, to express the ratio between the mass of a unit volume of water and the mass of a unit volume of the substance being tested. In the English system, where the cubic foot is taken as the unit of volume, density will be expressed by a number 62.4 times as great as the number indicating the specific gravity of a substance at 4° C. (39.2° F.), since a cubic foot of water at 4° C. (39.2° F.) weighs 62.4 pounds, or, in other words, contains 62.4 units of mass. The variations in weight of a cubic foot of water at temperatures above 4° C. (39.2° F.) would have the effect of increasing this ratio (62.4) between density and specific gravity.

As the volume of all bodies varies with temperature, it is essential that the comparison of weights be made at some fixed temperature.

On scientific principles and for the sake of uniformity it is desirable that specific gravity always be referred to water at 4° C. (39.2° F.). In some countries this temperature, at which pure water assumes its greatest density is taken for the comparison of weights, while in the United States and German Pharmacopoeias 15° C. (59° F.) has been fixed, with very few exceptions, as the normal temperature; the British Pharmacopoeia has selected 15.5° C. (60° F.). As the comparison of weight of equal volumes of bodies may be made at any temperature desired or convenient, and as the specific gravity will vary accordingly, it is necessary to state the temperature in connection with specific gravity; for instance, to say that a liquid has the specific gravity 1.42 would not indicate at what temperature the liquid had been weighed, nor would it indicate comparison with water at any fixed temperature. To say that a liquid has the specific gravity 1.42 at 15° C. would still leave a doubt as to the temperature at which an equal volume of pure water had been weighed for comparison, for it may have been 4° C., 12° C., or even 25° C'., and in either case the specific gravity named would not be correctly stated. To say, however, that a liquid has the specific gravity 1.42 at 15° C. as compared with water at the same temperature, leaves no room for doubt as to the true ratio existing between the liquid and water-it therefore expresses true specific gravity. The United States Pharmacoperia (1890) expressly states that all of its specific gravities are to be considered as taken at 15° C. and compared with water at the same temperature, whenever no special temperature is mentioned.

Generally it will be found more convenient to weigh substances at a temperature above 15° C. than to cool them down to and keep them at that point, and hence the average room-temperature, 22° C. (71.6° F.), or even 25° C. (77° F.), has been suggested as the standard temperature, and would, no doubt, as a rule be preferable. It is to be hoped that in the coming revision of the U. S. Pharmacopeia either a higher temperature will be adopted as standard or alternative specific gravities at 25° C. (77° F.) will be stated in addition to those at the lower standard.

Whenever a body has been weighed at a temperature different from that at which the standard volume of water has been fixed, it is customary to indicate this difference by writing both temperatures in the form of a fraction, the temperature at which the water was weighed being always written as the denominator and the temperature at which the body being tested is weighed as the numerator; thus, the expressions 0.927 at C., 1.250 at C., and 1.340 15° 25° 15°

at

15°
1°

15°

C., indicate specific gravities found at 15° C. and 25° C.

as compared with or referred to water at 4° C. and 15° C. respec

tively.