graduate is produced in one operation. More popular, however, are those in which the graduating is done by hand, the quantity of liquid

being poured into the ungraduated vessel from a burette, and this quantity of liquid is carefully marked, the finishing of the graduate being done by the engraver.

Fig. 19.-Liter flask. Fig. 20.-Volu- Fig. 21.-Burette. Fig. 22.-Meniscus

metric pipette.

(Sollmann).

The subdivisions of graduations are into drachms and minims in ordinary graduates, and into cubic centimeters in the metric.

The minim graduate, however, is far from being reliable, as a large amount of the liquid is retained by capillarity of the glass vessel, and, therefore, for measuring minute quantities, a pipette should be used. Such pipettes, graduated to minims, were put on the market by Dr. E. R. Squibb (Fig. 18), and are more reliable not merely by reason of the dispensing of the full quantity of the liquid, but, being narrower than a graduate of equal capacity, there is less chance for error in pouring more or less of the liquid into the utensil; the narrower the graduate, the more accurate can the quantity desired be determined.

In this way, for delicate analytic work, a liter graduate is rarely used, there being substituted for same a flask of such size that when filled with a liter of water the water rises within the narrow neck of the flask. Such liter flask is used chiefly in volumetric analysis,

Fig. 23.-Pouring into graduate.

liquids being delivered and measured from same by means of pipettes (Fig. 20) and by burettes (Fig. 21).

In both cases the liquid is measured within a narrow tube, hence based on the same principle as the liter bottle (Fig. 19).

In placing liquids in a comparatively small utensil, like a burette, it will be observed that, through capillarity, the liquid is attracted to the sides of the vessel, thus making an edge which it is sometimes difficult to read; such an edge is called a "meniscus," and in reading the quantity of the liquid, the line representing the quantity should be exactly in the center of the meniscus (Fig. 22).

It is hardly necessary to state that in measuring liquids in a graduate the latter should be held perfectly level and the line representing the desired quantity even with the eye (Fig. 23).

CHAPTER III

SPECIFIC GRAVITY

IN Chapter II care was taken to mention that 455.7 grains was the weight of a fluid ounce of distilled water. Why distilled water? Would it not be the weight of a fluidounce of chloroform as well? Common sense teaches that "some liquids are heavier than others," as we put it that chloroform is heavier than water. A fluidounce of chloroform weighs 678.9 grains, against 455.7 grains, the weight of the same volume of water. Accordingly, we see that chloroform is nearly one and one

half times heavier than water. That gives the idea of specific gravity, the definition of which can be given as the relative weight of equal bulks of different bodies, adding thereto the provision that, for solids and liquids, the unit of specific gravity is distilled water.

Now, to go back to our chloroform example: the volume, the bulk, the capacity of that one-ounce graduate remains the same whether it contains water or chloroform. Therefore a fluidounce-be it of water or of chloroform-represents "equal bulk." The respective weights of a fluidounce of the two liquids is "relative weight," and, reducing to unity, the specific gravity is obtained.

Instead of the term specific gravity, the word "density" is the more happy choice, since the term "specific gravity" suggests the gravitating force of the earth, which has only indirect connection with the subject.

The estimation of the specific gravity of various substances is of value, first, as the indication of purity or strength of the substance, and, second, through the data thus afforded we can estimate the volume of the substance, while in a minor degree it is sometimes of service in the diagnosis of disease.

The first application of density given above can be best explained by a simple illustration, thus: The average 92 per cent. sulphuric acid has a specific gravity of 1.834; in other words, is 1.8 times heavier than water. If this acid is mixed with water, the density of the liquid will vary to somewhere between 1.834 and 1.000, and, indeed, varies in proportion to the amount of water added.

In the same way official 91 per cent. alcohol has a specific gravity of 0.820, and is, therefore, 0.8 times as heavy as water. If such alcohol be mixed with water, the specific gravity of the liquid will rise to some point between 0.820 and 1.000, and, indeed, in proportion to the amount of water added; hence by taking the specific gravity of such liquid, we can estimate the exact strength of the liquid. Tables of this character have been carefully worked out, and are found in the pharmacopœia.

The other two uses of specific gravity being of minor importance, will be discussed in the appropriate place.

Estimation of the density of a body differs according as the substance is solid or liquid, and since, by the explanation given above, the specific gravity of liquids becomes simple, we will first discuss the estimation of the specific gravity of liquids; afterward, that of solids.

There are several ways of estimation of the specific gravity of liquids: (a) By use of pyknometer or specific gravity bottle; (b) with the hydrometer; (c) with the Mohr-Westphal balance; (d) with Lovi's beads; (e) with the Jolly spiral balance.

ESTIMATION OF SPECIFIC GRAVITY OF LIQUIDS

With the Pyknometer.-As mentioned above, the estimation of the specific gravity of a liquid is rendered simple because of the ease with which the weight of a bulk of given liquid can be compared to the weight of an equal bulk of water. Thus, in the simplest methods of estimation all that is necessary is to estimate the weight of a fluidounce of water in a graduate, then pour out the water, dry the graduate,

pouring in the same measure of the liquid, the specific gravity of which is desired, weighing same, and then comparing the weight of the fluidounce of water with the weight of the fluidounce of liquid. As many times heavier as is the weight of the fluidounce of the liquid than the weight of the fluidounce of water, so many times heavier is that liquid than water, which figure represents the specific gravity. The inaccuracies of this method of estimation are due to the same causes as any inaccuracy in the measurement of liquids by means of the graduate, as mentioned in the preceding

chapter, viz., the uncertainty in reading an exact volume of a liquid when placed in as wide a vessel as a graduate; hence, in order to obtain accurate results, we generally weigh the liquid, the weight of which is to be compared to the weight of an equal

100

Fig. 24.-Pyknometer.

bulk of water, in a flask called a pyknometer, two modifications of which are shown in Figs. 24 and 25.

The only advantage a pyknometer (or specific gravity bottle) has over the graduate method just given is its greater accuracy. By tilting a graduate in measuring it is very easy to get a little more or a little less than the quantity desired, and every grain counts in estimating specific gravity.

In glancing at the narrow neck of the pyknometer it can be seen how the danger of incorrect readings is lessened. Then, too, the pyknometers usually hold 1000 grains or 100 grammes, which renders long division unnecessary. If a 1000-grain bottle holds 1500 grains of a certain liquid, it is obvious that the specific gravity of that liquid is 1.50.

In both cases the possible variation in the volume of the liquid when manipulated by a person of ordinary intelligence is practically nil.

The method of estimating specific gravity by means of the pyknometer is by all odds the most accurate means at our disposal, but the process is one requiring considerable time. To facilitate more rapid reading other instruments have been devised, most useful of which is the hydrometer.

The Hydrometer.-As to the principle of a hydrometer, if asked in which fluid one could float better, in fresh or in salt water, the instant answer would be in salt water. Why? Simply because salt water, being heavier, buoys one up more. And, following the rule, the heavier the liquid, the more it will buoy a body up, and, indeed, proportionally. To prove this may be cited that while a piece of iron will sink in water, on mercury it will float around like a chip in water. Why? Simply

because while iron is eight times heavier than water, mercury is thirteen times heavier, and the lighter iron will float on the heavier mercury.

The hydrometer consists of a glass tube at the base of which two bulbs are blown, one containing enough shot or mercury to enable it to obtain an upright position when immersed in water, and the other and upper one filled with air (Fig. 26). According to the theorem of Archimedes, a body when immersed in water loses as much weight as the weight of the water it displaces, that is, a body displaces its own weight of a liquid and, accordingly as the liquid is lighter or heavier than water, so does the hydrometer sink or rise when immersed in same.

There are two classes of hydrometers, in one of which the weight of the hydrometer remains the same, whereas the point at which it floats varies accordingly as the liquid is lighter or heavier, and the other form, which is designed to float always at the same point in a liquid, which feat is accomplished by weighting down the hydrometer when immersed in a heavy liquid, i. e., by meeting the buoyancy of the heavy liquid by adding weights in a pan at the top of a floating instrument.

Of the first class, we have such well-known hydrometers as Baumé's and Daniel's, while on the principle of the second type are based the Nicholson, Fahrenheit, and the Mohr specific gravity balance.

Baume's Hydrometer.-The sketch and explanation on preceding page explain the construction of the abovenamed hydrometer, except details as to its graduation.

In constructing the hydrometer, it was early noted that such an instrument designed to measure accurately the specific gravity of the commonly known liquids, ranging, say, from ether (0.720) to bromine (3.000), would of necessity be so long as to be inconvenient.

Fig. 26.Hydrometer (schematic).

Baumé, at the outset, made two forms of this hydrometer, one for liquids heavier than water, and one for liquids lighter than water. The one for heavier liquids is graduated by weighting same with sufficient shot so that it would be almost completely immersed in water, and the point which was in contact with the surface of the water was marked or denominated zero. He then immersed this instrument in a 15 per cent. solution of common salt, whereupon, by reason of the superior buoyant force of this liquid over water, the hydrometer was pushed up and the point of contact of the hydrometer with the surface of the salt solution was marked off and denominated 15, and the space between the zero mark and 15 accurately divided into 15 spaces (provided always that the circumference of the tube was the same all along its length).

The remainder of the tube was divided into spaces equal to the 15 spaces between the water contact and the solution of salt contact, and this marking was arbitrarily denominated "degrees Baumé heavy."

In a similar manner was the hydrometer for lighter liquids constructed, this hydrometer being weighted so as to float in a 10 per cent. solution of salt at a point very near the base of the tube, and this point was marked zero.