plus the known weight of water which the bottle will hold, subtract the weight obtained in the second operation. The remainder represents the weight of water displaced by the powder. Then apply the rule:— Example: 10 grammes of a powder is taken, poured into a counterpoised specific gravity bottle of 100 grammes' capacity, which, when filled with water, weighs 108.8 Gm. + 100 grammes = 110.0 grammes. 10 grammes and powder, Difference or weight of water displaced Therefore, the weight in air (10 grammes) divided by the loss of weight in water (1.2 grammes), gives us 8.33 as the specific gravity of the powder. This method may be employed for solids, provided they are in pieces of such size as to admit of being placed in the specific gravity bottle. SPECIFIC GRAVITY OF SOLIDS INSOLUBLE IN, BUT LIGHTER THAN WATER.*-Inasmuch as such a body floats on the surface of water, it necessarily follows, that we must attach it to a heavier body in order to secure the submersion of both. The loss of weight of the heavy body in water must be known, and when this is deducted from the loss of weight of both together, the difference will give the amount of water displaced by the light body alone. The light body is first weighed in air, it is then attached to a sinker (a piece of lead or iron) and again weighed; then both are suspended by a thread under the surface of distilled water and their combined weight in water noted. The light body is detached and the weight of the sinker alone in water is ascertained. The loss of weight of the sinker alone in water is now deducted from the loss of both; this gives the loss of weight of the light body; then apply the rule. Example: A piece of brass weighs 5 grammes; in water it weighs 4.4 grammes. A piece of wax weighs in air 13.37 grammes; when attached to sinker, both weigh 3.88 grammes in water : * The method of Symons (Phar. Jour. Trans., (3) XIX, 206) may be employed. He fixes an inverted funnel under one scale-pan, attaches a ten-gramme weight to it, and restores equilibrium. The previously weighed light solid is now held under water until air-bubbles are removed, and then placed under the funnel; the weight lost will represent the volume of the solid, or, in other words, will be the weight of a volume of water equal to the volume of the solid. SPECIFIC GRAVITY OF SOLIDS SOLUBLE IN WATER.-We proceed exactly in the same manner as in the case of solids heavier but insoluble in water; but instead of water, we employ some liquid in which the body is not soluble. The liquids usually selected for this purpose are the oils of turpentine, olive, or almond. The specific gravity of the body having been obtained just as if water had been used, the result is multiplied by the known specific gravity of the oil employed. Example: A crystal of citric acid weighs 10.0 grammes; when immersed in Oil of Turpentine it weighed 4.8 grammes. Then : then dividing 10 by 5.2 we get 1.92, the specific gravity of citric acid referred to oil of turpentine as standard, and multiplying this by 0.870, the specific gravity of oil of turpentine, we obtain 1.67 the specific gravity of the sample of citric acid, as referred to the usual standard, viz.: water. SPECIFIC GRAVITY OF LIQUIDS.-METHODS OF DETERMINING. (a) By means of the Pycnometer. Hydrometer. Specific Gravity Balance. Lovi's Specific Gravity Beads. (a) DETERMINATION OF THE SPECIFIC GRAVITY OF LIQUIDS BY MEANS OF THE PYCNOMETER. For ascertaining the specific gravity of a fluid we employ the Pycnometer or specific gravity bottle, which, in its simplest form, is a small, light glass flask with a long FIG. 48. and narrow neck.* The flask, after being counterbalanced, is filled with any convenient amount of distilled water at the proper temperature, the height of the fluid marked, and its weight noted. These flasks are usually constructed to hold 25, 50, or 100 grammes, or 1000 grains of distilled water at 15.5° C. (60° F.). The bottle when emptied and dried, is filled to the mark with the liquid to be tested and weighed. The weight found bears a simple ratio to the specific gravity, Thus if the flask holds 100 grammes of distilled water at 15.5° C., when filled with ether, it will hold 72.5 grammes, and with glycerin, 125 grammes. The specific gravities of these two liquids are, then, 0.725 and 1.250, respectively, Pycnometer. Some of the older forms are quite short, as shown in figure 48. obtained by dividing 72.5 and 125 by 100. Any ordinary vial may be converted into a specific gravity bottle, by selecting one as light as possible, which will hold either 50 or 100 grammes of water. The distilled water is accurately weighed into the vial, and the exact height of the water is indicated by a mark (scratch) on the neck. An exact counterpoise is prepared for the empty flask. When used, the flask is filled to the mark with the liquid at the proper temperature, and its weight is divided by 50 or 100 as the case may be. Supposing, for example, that, when filled to the 50 cubic centimeter mark with sulphuric acid, it weighed 92.15 grammes, then 92.15 50.00 acid.* = 1.843, the specific gravity of the Pycnometers are either sold with a brass counterpoise or their exact weight is etched upon the bottle. Before the pycnometer is used, it should be first well rinsed with water, then with alcohol, and dried. After bringing the liquid to the proper temperature, by placing the vessel containing it in a bath of known temperature, the flask is filled nearly to the top of the neck, and the stopper is inserted, care being taken to avoid the retention of air bubbles. The super fluous fluid displaced by the stopper is then removed, and the flask then wiped perfectly dry and clean, and weighed. This form of specific gravity bottle has the objection that the temperature cannot be carefully observed, which renders it unsuitable for very accurate work. A better form of specific gravity bottle (Fig. 49) is that in which the stopper consists of a thermometer. The liquid FIG. 49. 30 25 20 10 Pycnometer with Thermometer. is introduced in the side tubulure until it reaches the mark at m. Care should be taken that no air bubbles be allowed to collect around the neck at h. It should be borne in mind that the temperature of observation should always be stated, and also that at which the standard was determined. To illustrate the importance of this Dr. Wright (Jour. Soc. Chem. Ind., XI, -298) cites the following:-"A sample of oil at 20° C. is stated by one observer to have the specific gravity of 0.92475 referred to water at 4° C., another the specific gravity of 0.92560 referred to water at 15.5° C., and the third the specific gravity of 0.92635 referred to water at 20° C. :" from appearance, these figures do not seem concordant, but if we remember that when reduced to "weight per cubic centimeter," it will be seen that they are identical. Taking the specific gravity of water at 4° C. as 1.000, then at 15.5° C. it is 0.99908, and at 20° C. it is 0.99827, hence For the various corrections to secure accuracy in taking specific gravities, see the above-cited reference. Another form is that devised by Dr. Squibb (Fig. 50), in which the neck of the bottle is lengthened, so that it will permit the bottle to hold the volume of water at any temperature between 4° C. and 25° C., thus adapting it to all standards in use. The capacity and tare is indicated on each bottle. A leaden collar is placed over the neck to keep it in position in the bath in which it is placed. (b) DETERMINATION OF THE SPECIFIC GRAVITY OF LIQUIDS BY MEANS OF THE HYDROMETER OR AREOMETER.-This instrument depends for its use on the fact that, when a solid is immersed in a liquid specifically heavier, it will sink until it reaches a point where the weight of the liquid displaced is equal to the weight of the floating body. Hydrometers are long glass tubes with two bulbs blown at one end. The lower (smaller) bulb is weighted with sufficient mercury or shot to cause the tube to float upright, the upper (larger) bulb is to impart buoyancy. Hydrometers are divided into two classes, the "Weighing Hydrometers" and "Scale Hydrometers." FIGS. 51, 52. THE WEIGHING AREOMETER OR HYDROMETER OF CONSTANT IMMERSION BUT VARIA BLE WEIGHT is so called because it is always immersed to the same mark, but requires different weights to effect this. The first of these was constructed by Fahrenheit; the same principle, however, is followed in a Hydrometers. b a. Weighing Hydrometer. b. Scale Hydrometer. Nicholson's Hydrometer. that of Nicholson's hydrometer (Fig. 53), which consists of a hollow metal cylinder B with a cone c, weighted so that the cylinder may float vertical. At the top is a stem terminated by a pan, in which the substance whose specific gravity is to be determined is placed. When floated, the apparatus stands partly out of the liquid, hence, the first step would be to ascertain what weight is necessary to cause it to sink to the standard point o; let this weight be 100 grammes. Let us assume that we desire to ascertain the specific gravity of a piece of metal; the weights are removed from the pan, and replaced by the sample; weights are now added, until the instrument again sinks to o; in order to accomplish this, we have added 44 grammes; then the actual |