A small drop of the fat which has been previously melted and heated to several degrees above its melting-point, but has been allowed to cool again to near its setting point, is put on the surface of the cold mercury. This is best done by means of a thin glass rod about one eighth of an inch in diameter, the end of which has been rounded off in the blow-pipe flame.
It is important that the drop should be very small, and its temperature when placed on the mercury not much above its melting point, for if it be too hot it will spread over the surface of the mercury, which is not desirable.
If the rounded end of the rod be slightly dipped into the melted fat, and then brought to the surface of the mercury, a small hemispherical particle will attach itself there and speedily congeal, becoming more or less opaque in doing so. The weight of one of these hemispherical masses, which should not be more than the eighth of an inch in diameter, will be from 1⁄50 to 1⁄10 of a grain. Having placed the drop of fat upon the mercury, the bulb of a thermometer, with sufficiently minute graduations, is introduced into the mercury and hot water poured into the basin. The heat is thus communicated to the contents of the small beaker slowly through the water in the larger beaker, and the rise of temperature in the mercury may be easily regulated, and should take place at the rate of about one degree per minute.
The mercury, by virtue of its comparatively good conducting power, acquires a uniform temperature throughout, which is indicated by the thermometer, and at the same time communicated to the fat. The fat when the temperature approaches its melting point becomes partially transparent, and if the stem or elongated bulb of the thermometer be now brought up against it, the moment fusion takes place the liquid fat will run into the channel formed by the repulsion of the mercury and the outside of the thermometer tube. This process presents the following advantages:—
1. The heat-conducting power of the mercury, on which the fat is placed, ensures the equalisation of the temperature as indicated by the thermometer, and at the same time communicated to the fat.
2. The direct contact of the fat with the mercury, without the intervention of a bad conducting medium, such as glass, ensures a more immediate and correct indication of the temperature at which liquefaction takes place than would otherwise occur.
3. The minuteness of the quantity of fat operated upon reduces to a minimum the time occupied in its melting, and thus facilitates the determination with exactness of its melting point.
4. The time occupied in preparing small tubes and charging them with the fat is saved, and several experiments in succession may be easily and rapidly made with the same apparatus. The author observed that in butter as
well as other fats, such as tallows, there were at least two melting points, dependent upon the way in which the fat had been previously subjected to the action of heat, and that they may differ in butter to the extent of 3° or 4° F.; the low melting point being that of the fat after it has been heated to several degrees above its first melting point, and the higher melting point being that of fat which has been previously melted to the lowest possible temperature, and then immediately allowed to congeal.
5. Professor Wanklyn carefully weighs one gram of butter, and heats it in a platinum dish of the size shown in the accompanying figure, from four to six hours or even more—in short, until it ceases to lose weight. The loss of weight is the water, which should be calculated and expressed in per-centages.