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THE POPULAR SCIENCE MONTHLY. 



energetic and sure remedies are generally employed. Even before 

 iodine was discovered, more than a hundred years ago, Russel had re- 

 marked 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 den- 

 sity 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 con- 

 tact 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 clos- 

 ing of the vessels. 



Salt water is denser than fresh, because of the gravity of the dis- 

 solved 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 Spitzbcrgen 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 con- 

 centrated 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 den- 

 sity 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 coimtries, 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 



