Page:Physical Geography of the Sea and its Meteorology.djvu/248

222 442. Experiments on the freezing-point.—The dilatation of the glass tube is included in this table. To determine the freezing-point of average sea water I filled a glass jar 18 inches high, and 3 inches in diameter, with specimens of average sea water obtained in mid-ocean and near the equator. On the 12th of February, 1858, the thermometer in the shade being 23°, I exposed this jar of water, with a standard thermometer immersed, to the out-door temperature. When the thermometer in the jar reached 27°, small crystals of ice, like macles of snow, were observed to form near the bottom, to rise, and to increase as they rose. In truth, the phenomenon presented most beautifully in miniature a snow-storm reversed, for the flakes appeared literally to "fall upward;" and while it was "snowing up "in the jar, covering the top with ice, the water in it rose in temperature from 27°.2 to 28°, thus showing the maximum density of the water to be not above 27°.2. As soon, and invariably as soon, as the first crystals of ice began to appear, the water immediately rose to 28°, and there remained as long as the process of congelation was going on. In some instances the water was brought down, as in a confined vessel, to 18° before freezing; but as soon as freezing commenced, the thermometer would mount up to 28°. The same water was used for the following series of observations upon the thermal changes of the specific gravity of sea water, fresh water being the unit: