energetic and sure remedies are generally employed. Even before iodine was discovered, more than a hundred years ago, Russel had remarked the efficacy of calcined sponges and corals, and of the ashes of sea-weed, substances richer in iodine than sea-water itself.
A general study of the physical properties of sea-water would not be complete if it was limited to that at the surface. It is necessary to obtain specimens drawn from different depths, especially as the density and temperature vary with the depth. Various apparatus have been contrived for bringing to the surface a quantity of water drawn from any desired level. A long-known means, and at the same time a simple and practicable one, is to let down by a rope an empty bottle corked. The increasing pressure upon the bottle becomes strong enough at certain depths to push the cork in and fill the bottle. The rope is then drawn up, and the liquid inside the bottle coming in contact with less dense waters, pushes the cork back into the neck of the bottle and closes it. Thus the water from the deep keeps itself free from mixture with that of the superficial levels. Other more perfect apparatus have been invented, all dependent upon the automatic closing of the vessels.
Salt water is denser than fresh, because of the gravity of the dissolved salts. But wherever large rivers enter the sea, as in the Black Sea and the Baltic, and in cold climates where evaporation is slow, the superficial water is light and of inferior salinity. The water of the Norwegian fiords is brackish, and that of the Gulf of Bothnia, at the upper end of the Baltic, is, in an extremity, potable. The glaciers of Greenland and Spitzbergen pour out in the summer torrents of fresh water which tend to freshen the spaces around their mouths. There is likewise a deficiency of salt in the waters of the White Sea, the Kara Sea, and the Siberian Ocean. Inversely, the Mediterranean, which does not receive, in proportion to its extent, so many nor so large rivers, and is exposed to the ardors of a burning sun, would become indefinitely concentrated by evaporation, were it not that an under-current of less dense water was sent into it by the Atlantic Ocean through the Strait of Gibraltar. Copious rains may play some part in the matter, and that is another reason why Mediterranean waters should preserve their density. Evaporation is very great in the tropics, but the liquid concentrated by it is also expanded by the heat, so that the two effects partly balance one another.
In all the old books on the physics of the globe, and even in some recent ones, no difference was made as to the law of maximum density between salt water and fresh. The latter begins to expand by heat at 4° C. (39° Fahr.), but, between the freezing-point and that temperature, it contracts when it is warmed, so that at 39° Fahr. it is denser than at any other temperature. In temperate countries, the water of the bottom of deep lakes remains at nearly 39° Fahr. by means of its weight, which prevents it from rising to the surface and
