[Illustration: FIG. 85.—APPARATUS FOR DETERMINING CRYSTALLIZING POINT.] The melted fat or fat acid is placed in a test tube contained in a large bottle, which serves as a jacket to protect the tube from sudden or violent changes of temperature. The efficiency of the jacket may be increased by exhausting the air therefrom, as in the apparatus for determining the heat of bromination, hereafter described. A very delicate thermometer, graduated in tenths of a degree, and having a long bulb, is employed. By means of the reading glass, the reading can be made in twentieths of a degree. The arrangement of the apparatus is shown in Fig. 85. The test tube is nearly filled with the melted matter. The bottom of the jacket should be gently warmed to prevent a too rapid congelation in the bottom of the test tube containing the melted fat, and the tube is to be so placed as to leave an air space between it and the bottom of the bottle. The thermometer is suspended in such a manner as to have the bulb as nearly as possible in the center of the melted fat. The thermometer should be protected from air currents and should be kept perfectly still. In case the congealing point is lower than room temperature the jacket may be immersed in a cooling mixture, the temperature of which is only slightly below the freezing point of the fatty mass. When crystals of fat begin to form, the descent of the mercury in the stem of the thermometer will become very slow and finally reach a minimum, which should be noted. As the crystallization extends inwards and approaches the bulb of the thermometer a point will be reached when the mercury begins to rise. At this time the partially crystallized mass should be vigorously stirred with the thermometer and again left at rest in as nearly, the original position as possible. By this operation the mercury will be made to rise and its maximum position should be noted as the true crystallizing point of the whole mass. In comparing different samples, it is important that the elements of time in which the first crystallization takes place should be kept, as nearly as possible, the same. A unit of one hour in cooling the mixture from a temperature just above its point of fusion until the incipient crystallization is noticed, is a convenient one for glycerids and for fat acids. =294. Determination of Refractive Power.=—The property of refracting light is possessed by fats in different degrees and these differences are of great help in analytical work. The examination may be made by the simple refractometer of Abbe or Bertrand, or by the more elaborate apparatus of Pulfrich. The comparative refractive power of fats can also be observed by means of the oleorefractometer of Amagat-Jean or the differential refractometer of Zune.[247]