830 THE POPULAR SCIENCE MONTHLY. 



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 precipi- 

 tate of perfectly crystallized sulphate of lime with two equivalents of 

 water, or gypsum, identical in geometrical form and chemical com- 

 position 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 be- 

 tween 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 ex- 

 pounded 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 corre- 

 sponds 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, 



