410 On the Formation of Bottom Ice. 



cast a piece of ice into a glass vessel filled with water below 

 freezing point. This phenomenon in the sea is, therefore, 

 according to onr ideas, analogous with the formation of bottom 

 ice in rivers. In them the cold water must be carried by the 

 stream under the free surface of the water. But this is not 

 required in the sea, and the difference between salt and fresh 

 water lies in this, namely, that salt water has its maximum of 

 weight or solidity below and not above freezing point. The 

 salt water upon which Despretz made his experiment froze to 

 ice, when it was shaken, at — 2° 55 Cent., but its greatest 

 maximum of weight was at 3° 67. But the freezing point, as 

 well as the maximum of weight, depends naturally on the salt- 

 ness of the water. In a mixture of sea water brought from 

 Trieste, Genoa, and Heligoland, Neumann found that the freez- 

 ing point was — 2° 6, and the maximum of weight at — 4° 74 

 Cent. The cooling of sea water down to freezing point takes 

 place in the same manner as the cooling of fresh water to 

 4° of cold. The colder water, in consequence of its being heavier 

 than that which is warmer, sinks down to the bottom. The 

 sea can, therefore, freeze without the water being agitated, and 

 this can take place at a considerable depth below the surface. 

 But the depth, of course, depends on the intensity of the cold. 

 That the sea water, when it is at a temperature below the freez- 

 ing point, can sink to a considerable depth has without doubt a 

 great influence on many of the phenomena which are closely 

 connected with its freezing. And it is in a great measure 

 owing to this that the icebergs which float about the Polar Seas 

 keep themselves in a vertical position of often 1500 feet high. 

 In the water of fresh lakes, which is always coldest on the sur- 

 face, a piece of ice which reached some feet under the water, 

 could only increase in size through the freezing of the water 

 on the water line, and, consequently, could only increase in 

 circumference, but not in thickness. In the sea, however, on 

 the contrary, we see often blocks of ice with the coldest water 

 below them, and they therefore increase in size on account of 

 a superficial freezing which takes place in a vertical position. 



The reasons why both fresh and salt water should attain a 

 degree of temperature below the common freezing point, with- 

 out congealing into a solid form, are of the greatest weight in 

 accounting for the above-mentioned phenomena, and should bo 

 deeply studied. Above all, the observations by our thermometers 

 on the actual degree of warmth in the water are of great con- 

 sequence. But these observations must be conducted with 

 the greatest care. If a thermometer in the usual manner is 

 lowered into the water which is below freezing point, an ice 

 formation will soon encircle it, which raises it to freezing 

 point, and we cannot, therefore, by this means declare what 



