FIG. 39.

MEASURING FLUIDS.

For the measurement of fluids, the apothecary employs almost altogether the glass graduate. This has entirely supplanted the old form of porcelain measuring cup, which being graduated inside, made it very difficult to measure off fluids with any degree of accuracy. Glass graduates are now furnished in various forms and designs; the tumbler-shaped is generally preferred, because of the ease with which it is cleaned. These are recommended for measuring quantities over one fluidounce (30 Cc.). Because of its smaller diameter near the bottom, the conical form admits the measuring of the smaller quantities with more accuracy; however, some forms of these are not as easily cleansed as those of wider bottom. Lately, an improvement has been made by graduating measures doubly; upon one side, the U. S. fluid measures are marked, while on the other, measures of the metric system are indicated. For the accurate measurement of smaller volumes, small graduated cylinders are preferably employed. These are graduated into fluidrachms and minims, or cubic centimeters and fractions thereof.

Porcelain Measuring
Cup.

Great care should be exercised in the selection of graduates, and only measures of guaranteed accuracy, or such which have been verified, should be accepted. It is advisable to select measures in which the graduating mark passes entirely around the graduate, which assists in securing the level of the fluid with

more accuracy.

We cannot expect to measure accurately, if the graduate be

held at an angle, instead of perpendicularly, as it should be. The measure should be grasped just above the base, between the thumb and forefinger, the weight being supported upon the other three fingers; and it should be held so that the rings of graduation are in a perfectly horizontal plane.

In order to test the accuracy of a graduate or measuring cylinder, it should be carefully counterbalanced on a pair of sensitive scales, then the requisite weight of distilled water corresponding to a given volume at 60° F. poured in, for which purpose the fluidounce may be reckoned equal to 455.69 grains, or the cubic centimeter equal to 1 gramme. Then the measure should be

placed on a perfectly level surface, and the height of the water as compared with the marks of the measure carefully noted.

For measuring very small volumes, such as 20 minims (or 1 Cc.) or less, the minim graduate should not be used, since there is a considerable loss of fluid in draining off, which adheres to the side of the measure. If the graduate can be rinsed out with other fluids subsequently entering the same mixture, the loss is avoided. Where such is not the case, recourse is had to the measuring pipette (page 165).

These pipettes are constructed of various capacities; the larger sizes, which usually have a bulb, hold 5, 10, 20, and 50 cubic centimeters respectively; the smaller, which are plain, graduated tubes, hold one fluidrachm, graduated into fractions and minims, or one cubic centimeter divided into tenths

CHAPTER II.

SPECIFIC GRAVITY. DENSITY.*

Specific Gravity may be generally defined, as the weight of a body compared with the weight of an equal volume of some standard substance. For liquids and solids, the standard chosen is distilled water at 4° C. (39° F.). Specific gravity expresses a relative, and not an absolute value. It states how many times. lighter or heavier the known volume of a body is, than the same volume of another, which is taken as a standard. When we say that the specific gravity of mercury is 13.5, we mean that 1 cubic centimeter, or 1 fluid ounce, or 1 liter, etc., of it is 13.5 times as heavy as the same volume of water. A knowledge of specific gravity often assists qualitative tests in the identification of minerals, metals, salts, organic substances, fluids, etc. It often enables us to ascertain their purity, as the presence of foreign bodies exerts an influence upon their specific gravities, causing them to be raised or lowered. It enables us to tell at once what any given volume of a liquid should weigh, or conversely, what volume will be required to contain any given weight. From the specific gravity we can readily calculate the percentage strength of many fluids, such as acids, alkaline solutions, solutions of salts, etc. From the specific gravity of urine, the physician diagnoses certain diseases.

Since all bodies expand or contract by change of temperature, it is necessary carefully to observe this when determining specific gravities, for 1 Cc. of water at the temperature of 4° C. or 39° F. (its greatest density) weighs 1 gramme, while the same volume at a temperature of 15.5° C. or 60° F. weighs 0.999 Gm. The standard volume of water for unity (1000) used for comparison, varies in different nations, being on the Continent of Europe generally taken at its maximum density (4° C.), in Great Britain at 16.6° C., and in the United States at 15.5° C., together with the Pharmacopoeia standard of 15° C.

For general purposes, we express the value of any specific

* On account of its more general use the term specific gravity is retained, although "density" is more accurate. The former involves the consideration of the action of the earth's attraction on the body to be examined. This attraction is not uniform over the whole of the earth's surface, being 1 to 2 per cent. less at the Equator than at the Poles. Practically, no difficulty arises from this fact, for we generally take the specific gravity or density of a body by means of a balance, in which case the relationship between object weighed and the weights, remains unchanged. When we employ the Jolly spring balance (Fig. 61), the above difficulty arises, in which the object is less attracted at the Equator, than at other parts of the earth's surface.

gravity to three places of decimals, which usually involves more or less error in the third place; however, where greater accuracy is required, it is expressed to four, five, or more decimal places, the necessary corrections being applied to eliminate errors as far as possible.

SPECIFIC GRAVITY OF SOLIDS.

The methods for the determination of the specific gravity of solids depend on the principle discovered by Archimedes, that when a solid is immersed in water, it loses in weight an amount. equal to the volume of water it displaces. The solid whose specific gravity is to be taken-assuming it to be insoluble in water-is well cleansed and freed from adhering moisture, then accurately weighed (denote this weight by x); then, by means of a horse-hair, silk thread, or fine platinum wire, it is suspended

FIG. 46.

SUEEN

Hydrostatic Balance.

FIG. 47.

Immersion of Solid.

from one arm or pan of a balance, so that it is entirely submerged, and does not come in contact with the sides of the vessel holding the water (Fig. 47); its weight is then noted (denote by y). A thermometer placed in the water indicates the temperature (t). The specific gravity is then found by the rule (1):— "Divide the weight of the body in air by its loss of weight in water: the quotient will be the specific gravity." That is, the specific gravity at the temperature t° is equal to x-y.

r

This determination may be carried out more expeditiously by means of the hydrostatic balance (Fig. 46), which consists of two counterbalanced pans (hung at unequal heights); under the shorter one is placed a hook, from which the body is suspended. The latter is first weighed alone in the usual manner in air, then immersed in the vessel of water at the proper temperature; this causes the short pan to become lighter. Weights are then added until the balance gains its equilibrium; the weight thus placed in the short pan to accomplish this, constitutes the loss of weight in water.

DISPLACEMENT IN GRADUATED CYLINDER.-Into a cylinder graduated in cubic centimeters, or better, fractions thereof, water of the proper temperature is poured, until it reaches some definite mark on the graduated scale. The body previously weighed in air is now dropped into it, which causes the level of water to rise, and thereby shows the exact bulk of water displaced by the solid; this is equivalent to the loss of weight in water, since each cubic centimeter of water weighs one gramme. Then apply the rule:—

Separation of Different Insoluble Bodies by Means of their Specific Gravities. This method is employed mainly by mineralogists for separating various minerals from crushed rock ore. Dense liquids of known specific gravity are employed, in which these solids will just float. For substances lighter than water, dilute spirits may be used; for those heavier than water, solutions of common salt, solution of mercuric nitrate, solution of potassio-mercuric iodide (3.196 sp. gr.), ethylene bromide, etc., are used. All that is necessary, is that the density of the liquid be known, it being presumed that the solid is not acted upon by the fluid.

SPECIFIC GRAVITY OF INSOLUBLE POWDERS HEAVIER THAN WATER.-Weigh off a portion of the powder, then introduce it into a counterpoised specific gravity bottle, which is so constructed as to hold a known weight (1000 grains, etc.,) of distilled water. Some water is poured in, and the contents gently rotated so as to remove any air bubbles that may adhere to the powder, then the flask is filled to the mark indicating its capacity, and weighed. From the weight of the powder in the air,