It has been remarked that the sea is less salt towards the poles than at the equator; that the saltness increases, in general, with the distance from land, and the depth of the water; that the interior seas, such as the Baltic, the Black Sea, the White Sea, the Sea of Marmora, and the Yellow Sea, are less salt than the ocean. The Mediterranean is an exception to this last rule; it is, as we have seen, salter than the ocean. This difference is explained by the fact that the quantity of fresh water brought into it by rivers is less than that lost by evaporation. The Mediterranean must therefore grow salter with time, unless its water is discharged into the ocean by a counter current, which would run under the current coming from the Atlantic by the Straits of Gibraltar. The Black Sea, on the contrary, the water of which has a density of only 1.013, receives from rivers more fresh water than it loses by evaporation. The saltness of this interior sea is only half as intense as that of the ocean. The Sea of Azov and the Caspian Sea are still less salt than the Black Sea. The following table shows the relative composition of the water in these three interior seas:-- +--------------------------+--------------+--------------+--------------+ | | Black Sea. | Sea of Azov. | Caspian Sea. | | In 100 Gallons of Water. | Density | Density | Density | | | 1·013 | 1·009. | 1·005. | +--------------------------+--------------+--------------+--------------+ | Chloride of sodium | 14·0195 | 9·6583 | 3·6731 | | Chloride of potassium | 9·1892 | 0·1279 | 0·0761 | | Chloride of magnesium | 1·3045 | 0·8870 | 0·6324 | | Sulphate of magnesia | 1·4704 | 0·7642 | 1·2389 | | Sulphate of lime | 0·1047 | 0·2879 | 0·4903 | | Bicarbonate of magnesia | 0·2086 | 0·1286 | 0·0129 | | Bicarbonate of lime | 0·3646 | 0·0221 | 0·1705 | | Bromide of magnesium | 0·0052 | 0·0035 | traces | | +--------------+--------------+--------------+ | | 17·6663 | 11·8795 | 6·2942 | +--------------------------+--------------+--------------+--------------+ In lakes without any outlet, as the Dead Sea and the Lake of Ural, the degree of saltness is considerably augmented. Numerous experiments have proved that the water of the Dead Sea is six times salter than that of the ocean. MM. Boutron and O'Henry analysed, in April, 1850, after the rainy season, some water of the Dead Sea, taken at about two leagues from the mouth of the Jordan; its density was then 1·10.