74 LAKES OF NORTH AMERICA. 



sodium sulphate and sodium carbonate are precipitated previous to the 

 crystallization of common salt. 



It lias been found on concentrating sea-Avater that calcium carbonate 

 is usually the first constituent to be precipitated. This salt is not always 

 found when the waters of the ocean are analyzed, but may usually be 

 detected in sam^jles taken near shore. The yast quantity deliyered to 

 the ocean by riyers is soon eliminated by plants and animals and secreted 

 in their tissues. 



The succession of chemical precipitates formed in sea-water has been 

 described Ijy ^NI. Dieulafait ^'as follows : 



" First a yery weak precipitation occurs of carbonate of lime (calcium 

 carlionate), with a trace of strontium, and of hydrated sesquioxide of iron, 

 mingled with a slight proportion of manganese. The Ayater then con- 

 tinues to evaporate, but remains perfectly lim])id, without forming any 

 other deposit than the one I have mentioned, till it has lost 80 per cent 

 of its original volume. It then l^egins to leave an abundant precipitate of 

 perfectly crystallized sulphate of lime Avith two' equivalents of water or 

 gypsum, identical in geometrical form and chemical composition Avith 

 that of the gypsum-beds. This deposit continues until the Avater has 

 lost 8 per cent more of its original A^olume ; then all precipitation ceases 

 till 2 per cent more of the original quantity of Avater has CA'aporated 

 aAvay. Then a neAv deposit begins, not of gypsum, but of chloride of 

 sodium, or sea salt. . . . The deposition of pure or commercial salt con- 

 tinues till the A'Olume of the water has been again reduced b}' one-half, 

 when a precipitation of sulj)hate of magnesium begins to take place AA'itli 

 it. This continues, the tAvo salts being deposited in equal quantities, till 

 only 3 per cent of the original quantity of Avater is left. Finally, Avhen 

 the Avater has been concentrated to 2 per cent, carnallite, or the di»ul)le 

 chloride of potassium and magnesium, is deposited. Spontaneous evapo- 

 ration cannot go much further. The residual mother-Avater Avill not dry 

 up at the ordinary temperature, cA^en in tlie hottest regions of the globe ; 

 its chief constituent is chloride of magnesium. A Ijody of sea-water 

 evaporated naturally Avill, then, leave a series of de[)0sits in AAdiich Ave 

 Avill find, as Aye dig down, the folloAving minerals in order: deliquescent 

 salts, including chiefly chloride of magnesium : carnallite. or double chlo- 

 ride of potassium and magnesium; mixed salts, including chloride of 

 sodium and sul})hate of magnesia ; sea-salt, mixed Avith sulphate of mag- 

 nesia ; pure sea-salt ; pure gypsum ; Aveak deposits of carbonate of lime 

 AA'ith sesquioxide of iron, etc." 



1 Popular Science Monthly, October, 1892. , ■ 



