AND CALCIUM CARBONATE, ETC., OF WATER SOLUTIONS. 255 



-table 5, in columns 3 and 5, depending on the partial pressure of the atmo- 

 spheric carbon dioxide. When the solution becomes saturated with gypsum 

 this will, by continued evaporation, crystallize out, but no matter whether 

 it is deposited in the same locahty as, or in some other locality^ than, the 

 first great deposit of calcium carbonate, the gypsum must inevitably be 

 continuously contaminated with some calcium carbonate, varying from 0.3 

 to 2.85 per cent, according to the partial pressure of the carbon dioxide in 

 the atmosphere within the limits given. Perfectly pure gypsum would not 

 be formed under such conditions. Vice versa, a very exact determination 

 of the amount of calcium carbonate present in gypsum prepared under 

 such ideal conditions could be used as a criterion of the carbon dioxide con- 

 tent of the atmosphere under which the gypsum was formed. 



Obviously such a hypothetical ideal condition as to constancy of tem- 

 perature and purity of the solutions never existed on the earth, and the 

 presence of other salts, notably of sulphates even in smaller quantities and 

 of chlorides in larger amounts, modifies decidedly the numerical values deter- 

 mined above: the foundation for the study of the relations of gypsum and 

 calcium carbonate having been laid for this ideal condition, it seemed desir- 

 able to pursue the inquiry to ascertain, at least roughly, the influence the 

 presence of other salts would have, especially the sulphates of magnesium, 

 potassium, and sodium, and the chloride of sodium. In the following pages 

 an attempt has been made to estimate only roughly the influence of the 

 presence of other sulphates. 



CALCIUM SULPHATE, CARBONATE, AND BICARBONATE IN THE 



PRESENCE OF SULPHATES. 



The solubihty of a difficultly soluble salt like calcium sulphate depending 

 on its solubility product 



Cca X Cs04 = KcaS04 



the presence of other sulphates in solutions that are not too concentrated 

 to interfere with the application of the laws of solution would, through the 

 increase in the concentration of the sulphate ions, have as its chief effect a 

 decrease in the concentration of the calcium ions, according to the equation 

 just given. Since about half of the gypsum is ionized in its saturated solu- 

 tion, a decrease in the concentration of its calcium ions would imply a 

 decrease in the solubility of gypsum. The decreased solubility of the gyp- 

 sum or the decrease in the concentration of calcium ions would, vice versa, 

 increase the solubiUty of calcium carbonate and bicarbonate; so, from both 

 causes, gypsum crystalHzing from a not too concentrated sulphate solution 

 under the conditions we are studying might be expected to be more contami- 

 nated with calcium carbonate than was found for aqueous solutions. 



In order to study these effects, it was decided to examine from this 

 point of view the probable effect of the sulphates of magnesium, potassium, 

 and sodium in the proportion in which they are found with calcium sul- 

 phate in the present sea-water, determining also the effect of varying partial 

 pressures of atmospheric carbon dioxide, as was done before. 



» Such a change of locality was suggested to the author by Dr. O. Willcox. 



