THE SPECIFIC GRAVITY OF THE SEA, ETC. 

 Thermal Dilatation of Sea Water* 



213 



442. The dilatation of the glass tube is included in this table. 

 Experiments on the To determine the freezing-point of average sea water 

 freezing-point. J ^j^gj ^ gkss jar 18 iuches 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 tempera- 

 ture. When the thermometer in the jar reached 27°, small crys- 

 tals of ice, like macles of snow, were observed to form near the 

 bottom, to rise, and to increase as they rose. In truth, the phe- 

 nomenon 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 in- 

 stances the water was brought down, as in a confined vessel, 



* This agrees more nearly with Despretz Tp. 245) than with Dr. Marcet. The 

 latter states that sea water decreases in weight to the freezing-point until actually 

 concealed. In four experiments Dr. Marcet cooled sea water down to between ]8 ' 

 and 19^ Fahr., and found that it decreased in bulk till it reached 22^, after w^iich 

 it expanded a little, and continued to do so till the fluid was reduced to between 

 19^ and 18°, when it suddenly expanded, and became ice with a temperature ol* 

 28^. 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 4^ Fahr. ; 

 and, therefore, if the sea should be, as is sometimes supposed, more saline at great 

 depths, and as it appears to be in the Mediterranean from the experiments of 

 Dr. Wollaston, ice could not be formed there at the same temperature as it could 

 nearer the smface. — (Vide M. de la Beclie, Manual Geology, p. 22.) 



