Page:Polar Exploration - Bruce - 1911.djvu/75

Rh crystals formed in freezing a non-saturated saline solution are pure ice, the salt from which they cannot be freed belonging to the adhering brine, and that the freezing-point of water is lowered by the presence of salt or other foreign matter dissolved in it. Thus it may be said that, in nature, ice never melts and water never freezes at exactly 32° F. The melting-point depends on the medium and on the pressure to which the ice is subjected. If the pressure is constant it varies with the nature of the medium; if the nature of the medium is constant it varies with the pressure.

The reader is now able to distinguish the different species of ice met with in polar seas. The chief fact to be noted is that, in these seas, we meet with two kinds of ice, the one having its origin in the sea and the other on the land or in the air. The former has the lower melting-point of the two and melts first. While it is melting it takes all the heat available and so preserves the fresh-water ice, which melts after all the salt-water ice is gone. In old hummocky ice this process of purification has been going on intermittently whenever the weather was warm enough. I trust I have made these matters plain, but it is impossible by words to give a true idea of the marvellous colour and beauty of the ice in polar seas, or of its irresistible power when driven hither and thither