=284. Estimation of Water.=—In the official method for butter fat, which may be applied to all kinds, about two grams are dried to constant weight, at the temperature of boiling water, in a dish with flat bottom, having a surface of at least twenty square centimeters. The use of clean dry sand or asbestos is admissible, and is necessary if a dish with round bottom be employed. In the method recommended by Benedikt, about five grams of the sampled fat are placed in a small flask or beaker and dried at 100° with occasional stirring to bring the water to the surface. According to the method of Sonnenschein, the sample is placed in a flask carrying a cork, with an arrangement of glass tubes, whereby a current of dry air may be aspirated over the fat during the process of drying. When the flask is properly fitted its weight is taken, the fat put in and reweighed to get the exact amount. The fat is better preserved by aspirating carbon dioxid instead of air.[234] The moisture may also be readily determined by drying on pumice stone, as described in paragraph =26=. In this case it is well to conduct the desiccation in vacuum or in an inert atmosphere to prevent oxidation. PHYSICAL PROPERTIES OF FATS. =285. Specific Gravity.=—The specific gravity of an oil is readily determined by a westphal balance (=53=), by a spindle, by a sprengel tube, or more accurately by a pyknometer. The general principles governing the conduct of the work have already been given (=48-59=). The methods described for determining the density of sugar solutions are essentially the same as those used for oils, but it is to be remembered that oils and fats are lighter than water and the graduation of the sinkers for the hydrostatic balance, and the spindles for direct determination must be for such lighter liquids. The necessity of determining the density of a fat at a temperature above its melting point is manifest, and for this reason the use of the pyknometer at a high temperature (40° to 100°) is to be preferred to all the other processes, in the case of fats which are solid at temperatures below 25°. [Illustration: FIG. 82.—BALANCE AND WESTPHAL SINKER.] When great delicacy of manipulation is desired, combined with rapid work, an analytical balance and westphal sinker may be used conjointly.[235] In this case it is well to have two or three sinkers graduated for 20°, 25°, and 40°, respectively. Nearly all fats, when melted and cooled to 40°, remain in a liquid state long enough to determine their density. The sinkers are provided with delicate thermometers, and the temperature, which at the beginning is a little above the degree at which the sinker is graduated, is allowed to fall to just that degree, when the equilibrium is secured in the usual manner. The sinker is conveniently made to displace just five grams of distilled water at the temperature of graduation, but it is evident that a round number is not necessary, but only convenient for calculation.