222 PHYSICAL GEOGRAPHY OF THE SEA, AXD ITS METEOROLOGY. 



442. Exjperiments 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 ex- 

 posed this jar of water, with a standard thermometer immersed, 

 to the out-door temperature. \Vhen the thermometer in the jar 

 reached 27^, small crystals of ice, like macles of snow, were ob- 

 served 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 congela- 

 tion 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 : 



between 18° and I90 Fahr., and found that it decreased in bulk till it reached 

 22°, after which it expanded a little, and continued to do so tUl the fluid was 

 reduced to between I90 and I80, when it suddenly expanded, and became ice 

 with a temperature of 280. It should always be recollected that a saturated 

 solution of common salt does not become solid, or converted into ice, at a less 

 temperature than 40 Fahr. ; and, therefore, if the sea should he, as is sometimes 

 supposed, more saline at great deptlis, and as it appears to be in the IMediterra- 

 nean from the experiments of Dr. Wollaston, ice could not be formed there at 

 tlie same temperature as it could nearer the surface.— (F/cZe M. de la Beche, 

 Manual Geology, p. 22.) 

 * Specific gravity at 2000=0-9908. 



