remains to be done is to apply the rule, divide the weight of the body by its loss of weight in water.

For example, 805.5 grains of copper lose by immersion in water 90 grains; then 805.5 divided by 90 gives 8.95, the specific gravity of the copper. See also Nicholson's hydrometer, p. 85.

6. With the Specific Gravity Bottle.-This instrument in its most usual form is a bottle having an elongated, narrow neck, fitted with a ground

FIG. 41.

glass stopper, and holding, when filled, exactly 1000 grains of pure water at a given temperature (see Fig. 42). The reason for selecting 1000 grains for the contents is to avoid the necessity of making a calculation to obtain the specific gravity of a liquid.1 To use the instrument for a solid

FIG. 42.

Taking the specific gravity of a solid.

1000-grain bottle.

substance, the previously-weighed body is dropped into the bottle, which is then filled with water at the temperature of 15° C. (59° F.), the bottle carefully dried, and, after the counterpoise (the exact weight of the empty bottle) has been placed upon the opposite scale-pan, it is weighed. To obtain the loss of weight in water of the substance, it is only necessary to deduct the weight of the contents of the bottle (i.e., that of the water and the immersed body) from the weight of the body in air, plus that of the water which the bottle holds when full,-i.e., 1000 grains; the rule is then to be applied, divide the weight of the body by its loss of weight in water.

Example.-A piece of aluminum wire weighs 100 grains; when dropped into a 1000-grain bottle, and the bottle filled with water at the proper temperature, the weight of both is 1062 grains. As the bottle when filled with water alone held 1000 grains, and as the weight of the aluminum in air is 100 grains, both together weigh 1100 grains; hence 1100 grains, less 1062 grains, gives 38 grains, the loss of weight of the

1 See specific gravity of liquids, p. 77.

aluminum in water. Apply the rule,

100
38

=2.63, sp. gr. The specific

gravity of any insoluble powder, like calomel, litharge, etc., may be taken in exactly the same way, but care must be observed to agitate the powder with a small quantity of water in the bottle, before adding the rest, to cause the bubbles of air to escape.

c. With the Graduated Tube.-A graduated tube is provided in which each space indicates a grain or a gramme (C.c.) of water (or better if graduated in smaller subdivisions); the zero mark should be somewhat above the bottom of the tube, as shown in Fig. 43. Now, if water be poured into the tube exactly up to the zero mark, and a weighed solid body dropped into it, the water will rise in the tube and indicate the weight of a bulk of water equal to that of the substance; this is equivalent to the loss in water: then apply the rule, divide the weight of the body by its loss of weight in water. It is evident that this method cannot be as accurate as either of those above mentioned, as small differences are more clearly indicated by a good balance than by tube-reading.

d. By immersing the solid in a transparent liquid of the same density.-This method may be applied where the body is small, is not very heavy specifically, and is insoluble in the liquid. A heavy liquid is chosen, like solution of mercuric nitrate; the solid is found to float on the surface of the liquid, and water is added until the solid neither rises nor sinks, but swims indifferently: the specific gravity of the solid will of course be that of the liquid, which may be ascertained by the specific-gravity bottle (see page 77).

2. To take the specific gravity of a solid soluble in, but heavier than water.

FIG. 43.

100

-90

38

-80

-70

-60

50

40

30

20

-101

Graduated specificgravity tube.

A liquid must be selected in which the solid is insoluble, like olive oil, almond oil, benzin, or oil of turpentine: the specific gravity of the oil having been ascertained, it is used just as if it were water, the object being to find out the loss of weight that the substance suffers when immersed in the oil; this having been obtained, a simple proportion must be made, as follows: as the specific gravity of the oil is to the specific gravity of water, so is the loss of weight in the oil to the loss of weight in water: then apply the rule, divide the weight of the body by its loss of weight in water.

Example.-200 grains of citric acid lose by immersion in oil 115 grains; then, as

It is obvious that either the balance, specific-gravity bottle, or gradu

ated tube can be used in this case; but it is possible in some cases to coat the soluble substance with varnish and treat it then as an insoluble substance, and thus avoid the use of an oily liquid. A pill of bluemass may be coated with shellac varnish, and then treated as an insoluble substance as in 1, b. The practical difficulty, however, is to secure a thin coating which shall be impervious to water.

3. To take the specific gravity of a solid insoluble in, but lighter than water.

The solution of this problem requires the aid of a heavy insoluble body, which is to be attached to the light body, so as to secure the immersion of both: it is plain that if the loss of weight in water of the heavy substance is deducted from the loss of weight in water of both the heavy and the light body, the result must give the loss of weight in water of the light body alone: then the rule must be applied, divide the weight of the body by its loss of weight in water.

Example.-A piece of paraffin weighs 174 grains, a piece of brass loses by immersion in water 6 grains; when the brass is attached to the paraffin, both together lose by immersion in water 206 grains; by deducting 6 grains (the loss in water of the brass) from 206 grains (the loss in water of both) the loss in water of the paraffin alone is found,-i.e., 200 grains; then =0.870, sp. gr. of paraffin. See also Nicholson's

174

200

hydrometer, p. 85.

4. To take the specific gravity of a solid soluble in, but lighter than water.

The use of the specific-gravity bottle is recommended in cases of this kind, and the process is the same as in 2: the selection of a suitable liquid lighter than the body, and in which it is insoluble, is, however, usually attended with difficulty. The proportion would be, as the specific gravity of the light liquid is to the specific gravity of water, so is the loss of weight in the light liquid to the loss of weight in water. Then the rule must be applied, divide the weight of the body by the loss of weight in water.

SPECIFIC GRAVITY OF LIQUIDS.

The specific-gravity bottle (pycnometer or pyknometer) is the most accurate instrument for taking the specific gravity of liquids. Fig. 42 shows an improved form : it is used as follows. The liquid to be tested is first brought to the proper temperature, 4° C. (39.2° F.), 15.6° C. (60° F.), or 15° C. (59° F.), according to the standard selected for the bottle; the bottle is filled with the liquid to the mark on the neck, dried carefully, and weighed accurately, using the counterpoise on the opposite pan. If the 1000-grain or 100-gramme bottle has been used, the weight of the liquid at once indicates the specific gravity: thus, the bottles would hold 1163 grains or 116.3 grammes of hydrochloric acid, 1250 grains or 125 grammes of glycerin, 725 grains or 72.5 grammes of ether, and 13,558 grains or 1355.8 grammes of mercury, and the specific gravity of each would be respectively 1.163, 1.250, 0.725, and 13.55, thus directly showing the relation to the specific gravity of water, 1. To

FIG. 44.

FIG. 45.

show the use of an ordinary prescription-vial in this process, one containing about a fluidounce may be taken; if it holds 455.7 grains of pure water to a mark upon the neck it will be convenient, because it will at the same time give the weight of a fluidounce of the liquid. It is evident that a bottle holding any moderate quantity may be used in the same way.

1.16

A fluidounce bottle would hold 529.9 grains of hydrochloric acid, 569.6 grains of glycerin, 330.3 grains of ether, and 6178.5 grains of mercury, and the specific gravity would be obtained by the following proportion: Example.-As 455.7, the number of grains of pure water that the bottle holds, is to 1.000, the specific gravity of water, so is 529.9, the number of grains of hydrochloric acid that it holds, to 1.163, the specific gravity of hydrochloric acid. 455.7: 1.000 :: 529.9 : 1.163.

Specific-gravity bottle.

Lovi's beads.

In practice, it is simply necessary to divide the number of grains of liquid that the bottle holds by 455.7 and adjust the decimal point, or multiply the weight of the liquid by 2.1945, the modulus of the bottle. The accuracy of these bottles depends entirely upon the care with which they are made and used, and it is better to scratch, with a file, two marks upon the neck of a long-necked flask, one showing the upper edge of the meniscus and the other marking the lowest point (see Fig. 44). In filling, it is a good practice, after bringing the liquid to the proper temperature, to exceed slightly the quantity indicated by the mark on the neck, and then to make a small roll of filtering-paper and neatly absorb the excess by inserting the roll in the neck so that it shall just touch the surface. The more expensive specific-gravity bottles have an accurately-fitted stopper made of thermometer-tube, and hold exactly 100 grammes, or 1000 grains, when the bottle, including the capillary tube of the stopper, is entirely full (see Fig. 42). They are not so convenient as a correctly-marked, narrow-necked bottle (see Figs. 44 and 46), nor are they practically more accurate. They have to be filled to the brim and the stopper then inserted; this causes an overflow, and the necessary wiping and the natural warmth of the hands usually expand the liquid by raising the temperature, and prevent accurate results. A more elaborate form of apparatus for taking specific gravity is needed for some purposes; for instance, where standards are required for the liquid preparations of the Pharmacopoeia or in investigations where great accuracy is necessary. Dr. Squibb has devised the pycnometer shown in Fig. 46, which is admirably adapted for taking specific gravity with the objects above mentioned.

A set of these bottles is shown in the illustration; the stopper is a tube lengthened out so that the central channel will permit the bottle to hold the volume of water at any temperature between 4° C. (39.2° F.) and 25° C. (77° F.), thus permitting any of the standards of tempera

ture to be used, the tube being graduated to half-millimetres, and at the top it is enlarged so as to allow room for the expansion of light liquids and to permit the bottle to be loosely closed with a cork whilst adjusting the temperature, the cork being removed during the weighing. These bottles are all used in a bath of water containing ice when the lower standard temperatures are used; a leaden collar to keep the bottle in position in the bath, a pipette for coarsely adjusting the volume of liquid, and a thermometer are also shown in Fig. 46.

It is necessary to verify the marks of the contents of the bottles from time to time, as the glass flask contracts for a year or two after it has been made. The liquid that it is designed to test is weighed into the bottle, as in the ordinary cases of taking specific gravity; the bottle is loosely corked, loaded with the leaden collar, and set in a bath of water which has been brought to the desired temperature. When the liquid