_Sea Water._—The nature of the substances permanently leached out is also seen by considering the composition of sea water, since the ocean is the final reservoir for all the leachings of the land. It might be objected that the ocean may have received its salts from other sources; but this objection is overborne by the fact that substantially the same salts are found in landlocked lakes, in which, as they have no outflow, the leachings of the adjacent regions are perforce, as a rule, the only possible source of the salts. It is true that the nature of the salts differs somewhat in different lakes, as might be expected; but a general statement of that nature will, after all, be the same as that made in regard to sea-water. The following table of the average composition of sea-water, according to Regnault, illustrates these facts. MEAN COMPOSITION OF SEA-WATER. Sodium Chlorid (common salt) 2.700 Potassium chlorid .070 Calcium sulfate (gypsum) .140 Magnesium sulfate (Epsom salt) .230 Magnesium chlorid (bittern) .360 Magnesium bromid .002 Calcium carbonate (limestone) .003 Water (and loss in analysis) 96.495 ------- 100.000 The average saline contents of sea-water would thus be 3.505 per cent In twenty-one determinations of the saline contents of the Atlantic Ocean, the percentage ranged from 3.506 to 3.710 per cent Of this mineral residue, common salt constitutes from about 75 to over 80 per cent. We see that most prominent among the ingredients mentioned here is common salt (sodium chlorid), which forms nearly four-fifths of the total solid contents. Next in quantity are the compounds of magnesium, viz. Epsom salt and bittern, with a very small amount of the bromin compound. Next come the compounds of calcium (lime), of which gypsum is the more abundant, while the carbonate, so abundant on the land surface in the various forms of limestone, is present in minute amounts only, yet enough to supply the substance needed for the shells of shellfish, corals, etc. Least in amount of the metallic elements mentioned is potassium. Calculating the total amounts of chlorin, we find that it exceeds in weight any one other element present in the salts of sea-water, being two-sevenths of the whole solids.