SALINITY OF THE SEA The two characteristics of the oceans that distinguish them from bodies of fresh water and have always aroused men’s curiosity are their saltness and their tides. The _De imagine mundi_ gives a popular etymology of the word _mare_ from _amarum_, meaning bitter or salty.[855] Though there is no attempt in this book to show reasons for the salinity of the sea, the author followed Isidore and Bede in the opinion that the water at great depths is more bitter and salt than near the surface and that evaporation draws off the fresh water only and leaves the bitter and dense elements behind; similarly, that part of the sea water makes its way back to the sources of the springs, deposits its salt in the land, and bursts forth fresh and purified from its passage through the earth. In the _Image du monde_, on the other hand, there is a naïve explanation of why the sea is salt.[856] Great saline mountains in the deeps are said to be constantly dissolving away and thereby imparting a peculiar character to the water. Adelard of Bath, Gervase of Tilbury, and William of Conches treated the subject a shade more rationally, perhaps, in attributing the saltness to the influence of heat. Adelard says,[857] “I consider the cause of the saltness of the sea to be the heat of the sun and planets. For, since the true ocean passes through the heart of the torrid zone and since the course of the planets runs through the same zone, though obliquely, the ocean must of necessity be heated by such a great heat of the heavenly bodies that it is thereby rendered salt.” This explanation, he adds, is even subject to proof: for along coasts nearest the ocean, sea water “when dried in the sun on the rocks” may readily be converted into salt without any artificial aid; in more distant seas the water must be boiled and reboiled before this effect is produced. Furthermore, in summer all sea water is salter than in winter. William of Conches[858] and Alexander Neckam[859] also followed Aristotle in believing that water in its purest form has an insipid taste but that it is thickened and rendered salt by the sun’s heat in the torrid ocean, whence it is distributed to the other seas by currents. Gervase of Tilbury tends to exaggerate this theory: we read in the _Otia imperialia_[860] of a lake in the County of Aix, near Arles, the waters of which are congealed into ice by the cold of winter and into salt by the heat of August. This led Gervase to conclude that it would be impossible to sail around the earth, because the all-encircling ocean would be frozen stiff in the north and thickened into solid salt in the south. TIDES