Chemistry of the Sea
The specific gravity of Sea-water varies of course with the proportion of salts and the degree of heat it receives from the sun, or by the intermixture of currents of various temperatures; but in our own latitudes it is about 1·028—that is, a given volume of pure distilled water weighing 1000 grains, the same volume of sea-water weighs 1028 grains. Many useful substances are daily extracted from the sea for the use of man, among which we may mention pure water for the use of ships, salt, iodine, bromine, &c. Many attempts have been made to purity sea-water in order to render it potable, not only for supplying ships, but for the use of maritime towns and villages, where pump-water is often brackish, and where the inhabitants are frequently obliged to have recourse to rain-water. Now, when sea-water is submitted to congelation, it abandons its salt almost completely—a fact which appears to have been discovered many years ago by Chevalier Lorgna, who found that a mixture of three parts of pounded ice and two parts of common salt produced a cold of about 4° below the zero of a Fahrenheit thermometer, and that such a mixture caused sea-water to freeze rapidly. A mixture of various chemical salts in proper proportions produces a similar degree of cold. Lately, the cold produced by the evaporation of ether has been proposed for the same purpose. The purification is complete if the ice thus formed be melted and frozen again. In the Polar regions the ice formed from salt-water is more or less opaque, except it be in very small pieces, when it transmits light of a bluish green shade. When melted, it produces sometimes perfectly fresh water, and at other times water slightly brackish. The fresh-water ice resulting from rain or melted snow, as seen floating in the Arctic seas, is distinguished from the salt-water ice by its black appearance, especially when in small pieces, and by its transparency when removed from the water into the air. Its transparency is so great, when compared with sea-ice, that Dr. Scoresby used to amuse his sailors by cutting large lenses out of this fresh-water ice, and using them as burning-glasses to light the men’s pipes. Their astonishment was increased by observing that the ice did not melt, while the solar rays emerging from it were so hot that the hand could not be kept more than a second or two at the focus.—Macmillan’s Magazine.