27.5

weight of the piece of metal would be the difference between 100 grammes and 44 grammes, which is 56 grammes. Thus far, we have ascertained the actual weight of the metal; it is next necessary for us to ascertain its loss in water, or the weight of an equal volume of water. With this object in view, we place it in the lower cone c, at m; the instrument does not sink to o, for the metal, by immersion, has lost a part of its weight equal to that of the water displaced. Weights are added to the upper pan until the hydrometer sinks again to o; this being 27.5 grammes, is the weight of water displaced; then on dividing its actual weight (in air), by its loss in water we obtain 2.03 as its specific gravity. Bodies lighter than water are placed in the covered cup, thus securing immersion.

FIG. 54.

56

50:5

Rousseau's Densimeter.-This instrument (Fig. 54) is of use in ascertaining the specific gravity of liquids where only a small amount is available. The cup above is graduated to hold 1 cubic centimeter. When empty, the instrument sinks in distilled water to the point в; when the cup is filled with distilled water (1 Cc.), the point to which it sinks is marked as 20; then, the interval between 0 and 20, on the stem, is divided into 20 equal parts, the graduation being continued to the top, each division representing 0.05 gramme. Example: Supposing that the cup is filled with oil of neroli, the densimeter will sink to the mark 17.5; hence the specific gravity of the oil of neroli would be 0.05 x 17.5 0.875. Neither of these instruments gives such accurate results as the specific gravity bottle.

Rousseau's Densimeter.

SCALE AREOMETERS OR HYDROMETERS OF VARIABLE IMMERSION, BUT CONSTANT WEIGHT.— These are divided into different classes, according to their uses, being designated as Alcoholometers, Salimeters, Lactometers, Acidometers, Saccharometers, Urinometers, etc. There are two systems of these; in the one, the specific gravity is indicated directly on the scale; in the other (that of Baumé, Cartier, Twaddell, Tralles, etc.) an empirical scale is employed which may be reduced to specific gravity by referring to tables constructed for this purpose. The most important of these instruments is that of Baumé, under which name two distinct instruments are used. One of these is used for determining the densities of liquids heavier than water, such as acids, alkaline solutions, syrups, glycerin, milk, etc.; the other for liquids lighter than water, as alcohols, ethers, most volatile oils, etc. In the former (Fig. 55), for liquids heavier than water, the zero mark is obtained by placing it in distilled water; another mark is made at the place to which it sinks by floating it in a 15 per cent. solution of common salt.

FIG. 56.

The space between these two marks is then divided into 15 equal parts, the graduation being continued to 60° below. For liquids lighter than water (Fig. 56), zero is the point designated to which it sinks when placed in FIG. 55. a 10 per cent. salt solution, and 10° the point to which it sinks in distilled water; the distance between these is divided into 10 equal parts, the scale being extended up to 60°.**

10..

60

50--

20..

40

80-

30

40 -

20.

Twaddell's areometer, for liquids heavier than water, is common in England. It is so graduated that the reading multiplied by 5 and added to 1000 gives the specific gravity: thus, 15° Twaddell is equal to specific gravity 50 1.075. Alcoholometers are hydrometers for ascertaining the percentage strength of alcoholic liquids. They are often so constructed as to combine a thermometer, the mercury in the lower bulb answering for the bulb of the thermometer.

60

10

Baumé's Hydrometers.

The Tralles hydrometer is an alcoholometer having a centesimal scale, each division corresponding to a certain per cent. of absolute alcohol, by volume. Tables giving the equivalents may be found in the Dispensatories and other works of reference.

Sikes' hydrometer is used in England by the Excise Revenue officers for ascertaining the percentage strength of spirituous liquors. The instrument is of brass, the stem being graduated from 0 to 10, and a set of nine weights to place beneath the bulb. On noting the temperature, the weight employed, and

n =

degree of Baumé.

Such scales are unsatisfactory, as their equivalents vary in different instruments and different countries.

the point to which it sinks on the stem, and referring to a series of tables belonging to the instrument, the specific gravity is obtained.

FIG. 57.

Saccharometers are intended to show the specific gravity of sugar solutions; the scale is so arranged as to indicate directly the percentage of sugar.

The Elæometer is an instrument for ascertaining the specific gravity of fixed oils.

The Lactometer is employed to ascertain the specific gravity of milk, the scale indicating the percentage of added water.

The Urinometer is a small areometer with its scale limited to the specific gravity of urine, usually from 1.000 to 1.060. Most of these instruments are usually very faulty and inaccurate. and should never be relied upon unless obtained from a reliable source, or verified. A form offered by Dr. Squibb (Fig. 57), is furnished with a special cylinder having its sides indented so as prevent the instrument from remaining in contact with the sides of the cylinder. Should this happen, the friction and cohesion with the sharp edge of one of these indentations will be reduced to a minimum. In using an areometer it must be observed that the instrument should float freely, and should not adhere to the sides of the jar, otherwise the cohesion between the instrument and sides of the jar would prevent a free movement up and down in the liquid, and thus give rise to false readings. The bulb of an areometer is best shaped after that of a double cone, as in Fig. 57, so that there is but a single point which may come in contact with the side of the jar, Urinometer with Cylinder. (Squibb.) thus reducing the friction and cohesion to a minimum.

(c) DETERMINATION OF SPECIFIC GRAVITY BY MEANS OF THE SPECIFIC GRAVITY BALANCE.-Westphal Specific Gravity Balance.The application of this depends upon the principle that any selected body, when immersed in different liquids, loses exactly the weight of its own volume of that liquid. If the same body be immersed in different liquids, it will be seen that its loss in weight will be in proportion to their specific gravities. The small thermometer

suspended from the arm at K is held in equilibrium by a counterbalancing weight at J. When placed in a liquid, the arm HK rises; riders are then placed upon the beam, in the notches indicated, until equilibrium is restored. The riders L or L are equal in weight to the loss sustained by the thermometer in distilled water (4° C.), the rider M = , the rider N, and OT of the weight of L or L. When L or L is placed on 0: = the hook at K it indicates the specific gravity of 1.000; when the

other L is placed at 3 it denotes 0.3. Hence, when one of these riders (L or L1) is placed at K, and the other at 3, and equilibrium is thus restored, the liquid has the specific gravity 1.300. The rider M the second, and N the third, and O furnishes the fourth decimal place. Fig. 59 illustrates the use of the apparatus sufficiently.

Fig. 60 illustrates a simpler form, known as Mohr's Specific Gravity Balance; it will be seen from this, that any balance can be used for this purpose by detaching a pan and suspending a short thermometer in its stead by means of a fine platinum wire, then counterpoising both arms by adding weights to the side requiring it. The further operation is the same as that with the Westphal balance.

Jolly's Spiral Balance.-This consists of a spiral vertical spring suspended from an upright beam; on the side of this