Page:Popular Science Monthly Volume 21.djvu/848

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a slight proportion of manganese. The water then continues to evaporate, but remains perfectly limpid, without forming any other deposit than the one I have mentioned, till it has lost eighty per cent, of its original volume. It then begins to leave an abundant precipitate of perfectly crystallized sulphate of lime with two equivalents of water, or gypsum, identical in geometrical form and chemical composition with that of the gypsum-beds. This deposit continues till the water has lost eight per cent more of its original volume, then all precipitation ceases till two per cent more of the original quantity of water has evaporated away. Then a new deposition begins, not of gypsum, but of chloride of sodium, or sea-salt. The separation between the end of the deposition of gypsum and the beginning of that of chloride of sodium is so marked that it is utilized on a grand scale in the works of the salt-marshes. The salters allow all the gypsum to be deposited in the ordinary basins, and then run the water thus cleared of gypsum into special vessels, whereby they obtain a pure salt, which they can collect down to the very bottom of the basin after the last mother-waters have been drawn off. The deposition of pure or commercial salt continues till the volume of the water has been again reduced by one half, when a precipitation of sulphate of magnesia begins to take place with it. This continues, the two salts being deposited in equal quantities, till only three per cent of the original quantity of water is left. Finally, when the water has been concentrated to two per cent, carnallite, or the double chloride of potassium and magnesium, is deposited. Spontaneous evaporation can not go much further. The residual mother-water will not dry up at the ordinary temperature, even in the hottest regions of the globe; its chief constituent is chloride of magnesium. A body of sea-water, evaporated naturally, will then leave a series of deposits in which we will find as we dig down the following minerals in order:

Deliquescent salts, including chiefly chloride of magnesium.
Carnallite, or the double chloride of potassium and magnesium.
Mixed salts, including chloride of sodium and sulphate of magnesia.
Sea-salt, mixed with sulphate of magnesia.
Pure sea-salt.
Pure gypsum.
Weak deposits of carbonate of lime, with sesquioxide of iron, etc.

The examination of this list and the facts that have been expounded draw with them a large number of consequences; I will only call attention to two of them. The first is, that the different groups of substances named in the list should become more and more rare as we ascend from the base to the summit; for each of them corresponds with a more advanced period of evaporation, and the chances for its production become less and less favorable as we rise. The second consequence—and I regard it as a capital one—is, that, when we meet one of the superior groups, we should expect to find, below it,