Page:Popular Science Monthly Volume 26.djvu/556

540 that remains liquid. Under the influence of variations of temperature, all the chlorides in the block will gradually disappear: some go into the sea and are dissolved; while the rest appear on the surface and form hydrated crystals, or a kind of "salt-snow." The sulphates thus prevail exclusively in old ices, which, according to Mr. Petterssen, constitute mixtures of solidified water and a peculiar chemical compound, the criohydrate of sulphate of soda, a body which, containing five parts of soda to ninety-five of water, is decomposed at a little below the ordinary freezing-point.

By these phenomena of selection, ice, under atmospheric vicissitudes, approaches a limit when its composition would be fixed, without reaching it in reality. Usually, the expulsion of chlorides is not complete, and sudden changes of temperature may liquefy it at once. The Swedish observer compares the ice of salt-water with a kind of granite, each constituent of which should take its turn at decomposing under special circumstances. The warm waters farthest from the pole would bear only the stable constituents brought down by the Arctic current, so that, to continue our comparison, the river, which has gradually eaten away the granite block, finally transports the last remains of the rock in the form of sands and clays, which are destined to accumulate in the sedimentary deposits.

Over and above the substances that exist in considerable proportions, many rarer elements are found in the waters of the ocean; minerals, gases, and organic remains, difficult perhaps to recognize, sometimes impossible to estimate, but which nevertheless play an important part. The phenomena of accumulation which we have considered are absolutely insignificant in comparison with the absorbing power of some of the algæ. In them Courtois discovered iodine in 1812, and Malagutti, after laborious researches, detected copper, lead, silver, and iron, metals which he afterward found in sea-water itself. The quantity of iodine contained is so little appreciable that many doctors have denied the therapeutic virtues which others have attributed to this water. Nevertheless, absorbed and condensed by marine plants, it becomes abundant enough to be extracted with profit. It likewise accumulates in animal organisms; and cod-liver oil owes its beneficent properties to it. The silver was absurdly attributed by Proust to the treasures of shipwrecked vessels. But the quantity, though infinitesimal in a measured quantity of water, is in the aggregate immense. Malagutti more rationally refers its origin to the solution of the lead ores, very abundant all over the globe, with which sulphurets of silver and copper are combined. By the action of salt, the sulphurets are converted into chlorides. As to iron, it would be strange if so universal a substance were not found in the sea; and the same may be said of phosphoric acid.

The researches of M. Dieulefait into the presence of lithium in sea-water have shown that the Dead Sea is an independent body of