COMPOSITION OF SEA WATER THE SALTS OF THE SEA Water, chemically considered, is composed of two gases— hydrogen and oxygen, combined in the proportion of eight parts of oxygen to one of hydrogen by weight, or of two parts of hydrogen and one of oxygen by volume; or, more accurately, 100 parts by weight of water consists of 88.89 parts of oxygen and 11.11 parts of hydrogen. This, of course, is the chemical composition of perfectly pure water. But water is never naturally found absolutely pure, always containing greater or less quantities of certain substances, chemically dissolved, or mechanically suspended, in it. Sea water mainly differs from the so-called fresh water in being more or less salt. This saltness of sea water is due to the presence of certain saline matters held in chemical solution in it—the amount of solid matters dissolved in, and separable from, sea water, ranging from an average of 3.5 to an average of, in extreme cases, 4.5 per cent. The following is the mean of several analyses of sea water As a fair average, then, we may say that 100 lb& of sea water consists of 96i lb& of water and 3i lb& of solid matters in solution. And when we consider the vast extent of the ocean, the total quantity of solid matters dissolved in its waters must be enormous. In fact, it has been computed that it would cover an area of 7,000,000 square miles to a depth of fully one mile, and this estimate is most probably below the actual amount In nearly every part of the open ocean, the proportion of saline ingredients in solution is, generally speaking, the same. This remarkable uniformity in composition is no doubt the result of the continuous movements and consequent co-mingling of the waters of the ocean. There is certainly a slight difference in various parts ; but even this is attributable to local or tem porary causes, and scarcely affects the general statement that the sea, as regards saltness, is everywhere the same. The main causes of the slight differences are the varying degree of evaporation, and the supply of fresh water by rains and rivers. For instance, the seas of the Southern Hemisphere are slightly salter than those of the Northern Hemisphere—a result, evidently, of excessive evaporation in the former, and excessive precipitation in the latter. The areas of maximum saltness' are in the North and South Atlantic, between 20° and W. long. on either side of the Tropics of Cancer and Capricorn respectively. Towards the poles there is a gradual but very
slight diminution in the degree of salinity.
On the other hand, the difference is most marked in certain limited or peculiarly situated areas, whose waters are respectively fresher or salter than those of the contiguous open sea. Thus inland seas subject to minimum evaporation are generally fresher than those subject to maximum evaporation, although the supply of fresh river water to the latter may actually exceed that to the former ; for instance, the Baltic is considerably fresher than the Caspian, although the latter teeeives a vastly greater quantity of fresh water—the Volga alone being estimated to discharge as much fresh water as the whole of the rivers of Southern Europe taken together. But while evaporation is languid in the Baltic, it is exceedingly active in the Caspian.
The saltness is still further increased if an inland sea be subject to active evaporation and yet receive no considerable quantity of fresh river water. A notable instance of this is the Red Sea. I Closed in on all sides by arid deserts, it is subject to excessive evaporation—so much so, indeed, that were the Straits of Bab-el-mandeb permanently closed, and the com pensating inflow from the Indian Ocean stopped, its level would be lowered about eight feet annually, and in a few years at most its bed would be but a vast " Wady," nearly, if not entirely, destitute of water. The inflow, as we have said, from the Indian Ocean prevents the lowering of the level, by supplying an amount equivalent to that evaporated. No rivers enter this arm of the sea, and the whole region is nearly rainless, so that in the Red Sea we have maximum evaporation, and mini mum freshwater supply, resulting in a degree of salinity ex ceeding that of the open ocean. In the Mediterranean we have an example, again, of active evaporation, but combined with a considerable supply of fresh water by the Nile, Rhone, and other large rivers ; still the amount evaporated is in excess of the fresh water supply, and consequently the waters of the great inland sea are salter than those of the Atlantic. Instances of excessive evaporation and limited supply of fresh water, together with total absence of compensating supply by inflow from the adjoining ocean, are the Dead Sea, and the Sea of Aral, both, especially the former, excessively salt.