298. Melting Point Determined by the Spheroidal State.—The method described by the author, depending on the assumption of the spheroidal state of a particle of liquid removed from all external stress, has been found quite satisfactory in this laboratory, and has been adopted by the official chemists.[245] In the preparation of the apparatus there are required:
(a) a piece of ice floating in distilled water that has been recently boiled, and (b) a mixture of alcohol and water of the same specific gravity as the fat to be examined. This is prepared by boiling distilled water and ninety-five per cent alcohol for a few minutes to remove the gases which they may hold in solution. While still hot, the water is poured into the test tube described below until it is nearly half full. The test tube is then nearly filled with the hot alcohol, which is carefully poured down the side of the inclined tube to avoid too much mixing. If the alcohol is not added until the water has cooled, the mixture will contain so many air bubbles as to be unfit for use. These bubbles will gather on the disk of fat as the temperature rises and finally force it to the top.
Fig. 84.—Apparatus for the
Determination of Melting point.
The apparatus for determining the melting point is shown in [Fig. 84], and consists of (a) an accurate thermometer reading easily tenths of a degree; (b) a cathetometer for reading the thermometer (but this may be done with an eye-glass if held steadily and properly adjusted); (c) a thermometer; (d) a tall beaker, thirty-five centimeters high and ten in diameter; (e) a test tube thirty centimeters long and three and a half in diameter; (f) a stand for supporting the apparatus; (g) some method of stirring the water in the beaker (for example, a blowing bulb of rubber, and a bent glass tube extending to near the bottom of the beaker).
The disks of fat are prepared as follows: The melted and filtered fat is allowed to fall from a dropping tube from a height of about twenty cubic centimeters on a smooth piece of ice floating in recently boiled distilled water. The disks thus formed are from one to one and a half centimeters in diameter and weigh about 200 milligrams. By pressing the ice under the water the disks are made to float on the surface, whence they are easily removed with a steel spatula, which should be cooled in the ice water before using. They should be prepared a day or at least a few hours before using.
The test tube containing the alcohol and water is placed in a tall beaker, containing water and ice, until cold. The disk of fat is then dropped into the tube from the spatula, and at once sinks until it reaches a part of the tube where the density of the alcohol-water is exactly equivalent to its own. Here it remains at rest and free from the action of any force save that inherent in its own molecules.
The delicate thermometer is placed in the test tube and lowered until the bulb is just above the disk. In order to secure an even temperature in all parts of the alcohol mixture in the vicinity of the disk, the thermometer is gently moved from time to time in a circularly pendulous manner.
The disk having been placed in position, the water in the beaker is slowly heated, and kept constantly stirred by means of the blowing apparatus already described.
When the temperature of the alcohol-water mixture rises to about 6° below the melting point, the disk of fat begins to shrivel, and gradually rolls up into an irregular mass.