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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 as well as the rapidity with which liquids are allowed to flow from vessels, 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 ;
Figs. 24, 25, and 26 represent convenient medicine glasses, well adapted for 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.
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 means relative weight, 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 is the ratio between the weight of any gaseous, liquid, or solid body and that of an equal volume of the respective standard.
The principle of specific gravity was first announced by Archimedes, a Greek philosopher, who formulated the law that all bodies immersed in a liquid are buoyed up with a force equal to the weight of the liquid displaced by them; hence a piece of metal of the size of one cubic inch, when immersed in water, will exert as much less pressure on the bottom of the container as will equal the weight of one cubic inch of water-or a fraction over 252 grains. Floating bodies always displace their own weight of water, irrespective of their volume, while immersed bodies always displace their own volume of water, irrespective of their weight; hence all bodies whose volume weighs less than an equal volume of water are sure to float, only so much of the body being immersed as equals a like weight of water, while all bodies whose volume weighs more than an equal volume of water must sink and be completely immersed, as this downward pressure of the body exceeds the upward pressure or buoyant force of an equal volume of water.
As the volume of all bodies varies with temperature, it is essential that the comparison of weights be made at some fixed temperature and that equal volumes of the standard and body examined be weighed at the same temperature. In some countries the temperature of 4° C. (39.2° F.), at which pure water assumes its greatest density, is taken for the comparison of weights, while in the United States and German Pharmacopeias, 15° C. (59° F.) has been fixed, with very few exceptions, as the normal temperature; the British Pharmacopeia has selected 15.6° 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 the same temperature—hence the ratio would be an uncertain expression; 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 40 C., 12° C., or even 25° C., and, in either case, the specific gravity named would not be absolutely correct; 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 the true specific gravity. The United States Pharmacopoeia (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.
As it is frequently more convenient to weigh substances at a temperature above 15° C. than to cool the substance down and keep it at that point, the average room-temperature, 22° C. (71.6° F.), or even 25° C. (77° F.), has been suggested by some authorities, and will often be found preferable.
Barometric pressure is not without effect on the relation between the volume and weight of bodies, hence absolute specific gravity, like absolute weight, is only obtainable in vacuo ; for pharmaceutical purposes this difference is always ignored and the barometric pressure assumed to be normal, 700 Mm. or 30 inches.
The specific gravity of a solid or liquid is always expressed by a number which shows how often the weight of a certain volume of water is contained in the weight of the same volume of that solid or liquid; and the specific gravity of a gaseous body is expressed by a number which shows how often the weight of a certain volume of atmospheric air (or hydrogen) is contained in the weight of the same volume of that gaseous body. The specific gravity of water is therefore stated to be 1, and the specific gravity of air (or hydrogen) is likewise stated to be 1. The following simple rule may be given for finding the specific gravity of any liquid or solid substance by calculation : Divide the weight of a given volume of any liquid or solid by the weight of an equal volume of distilled water, both weighings having been made at the same temperature. The quotient expresses the specific gravity.
Specific Gravity of Liquids. The determination of the specific gravity of liquids is far more frequently required than that of solids. The different instruments employed for that purpose are specific gravity flasks or pyenometers, loaded glass cylinders, specific gravity beads, and specific gravity spindles or hydrometers. Any small task, of 25 or 50 Cc. capacity, with a long, narrow neck and made of thin glass, will answer as a
specific gravity bottle. Its weight, or tare, is first carefully ascertained and noted; pure water is then poured into the flask until it reaches a short distance up into the neck, when a mark should be made with a file at the upper and lower edge of the meniscus or concave surface; having noted the temperature of the water, the flask and contents are weighed, and from it the tare of the flask is deducted, the remainder being the weight of that particular volume of pure water at the given temperature. The tare, temperature and weight of water, are carefully etched on the side of the flask, which is now ready to be used for taking the specific gravity of any liquid, by filling it to the mark in the neck with the liquid to be tested, then weighing and dividing the net weight of the liquid by the weight of the water, the quotient being the specific gravity of the liquid. Suppose the flask weighs 324 grains and holds, up to the mark, 647 grains of water; filled to the mark with sulphuric acid, it weighs 1511.5 grains, which leaves 1511.5 — 324 = 1187.5 grains as the weight of the acid. Now applying the rule, to divide the weight of a given volume of a liquid by the weight of the same volume of water, the specific gravity is found to be 1187.5 = 647 = 1.835+.
Small glass-stoppered flasks, graduated to hold 100, 250, 500, or 1000 grains of distilled water at 15.6° C. (60° F.), are a more convenient form of pycnometer ; they come packed in tin cases and are accompanied by a metal counterpoise to balance the empty bottle (see Fig. 27). "In using these flasks it is necessary to fill them with the liquid to be tested, to a little above the mark in the neck to which the glass stopper reaches when inserted, so that the air and small excess of liquid shall be forced out through the capillary tube