AND CALCIUM CARBONATE, ETC., OF WATER SOLUTIONS. 



261 



We note, first, the interesting fact that the solubility of the calcium 

 bicarbonate is very much reduced by the presence of the gypsum, as required 

 by the theory developed above. Cameron did not report any experiments 

 on the effect of a change in the partial pressure of carbon dioxide, but it 

 will undoubtedly be entirely analogous to that discussed in the previous 

 parts of this paper; it is probable that the solubility will increase roughly 

 proportionately to the square root of the pressure of carbon dioxide.^ 



We may now raise the question whether, owing to the displacement of 

 the bend in the curve of the solubility of bicarbonate as compared with 

 that of gypsum in the presence of sodium chloride, by a process of concen- 

 tration gypsum would crystallize out of any of the above solutions free 

 from carbonate, and if not what the contamination would be. 



If we go from solution 2 to solution 3, the sodium chloride concentration 

 is increased from 0.0625 to 0.1979 mole, which would result if 100 liters of 

 solution 2 were concentrated to 31.54 liters of solution 3. Of the 272 grams 

 of calcium sulphate in 100 liters of solution 2, 108.7 grams would be retained 

 in solution and 163.3 grams deposited. At the same time, of the 4.5 grams 

 calcium carbonate in solution (as bicarbonate), 2.77 grams would be de- 

 posited, that is 1.7 per cent referred to pure calcium sulphate. 



Table 8 gives the results of the calculations of this effect of progressive 

 concentration from solution 2, through solutions 3, 4, etc., to solution 8. 

 In column 1 the number of the solution used is given, referred to the num- 

 bers in table 7. In column 2 the number of liters used for concentrating is 

 given, and in the next column we have the number of liters to which the 

 solution has been concentrated. Column 4 gives the weight in grams of 

 calcium sulphate deposited, and the last column gives the proportion of 

 calcium carbonate to anhydrous sulphate deposited. 



Table 8. 



We see from table 8 that the most favorable point for the deposit of 

 pure gypsum is in going from solution 5 to solution 6, which is the point 

 where the curve of solubility of gypsum bends, its solubility now decreas- 

 ing and a relatively large amount of it being deposited; the bend in the 

 curve of the solubility of the bicarbonate has already been passed and there 

 is now no corresponding increase in the formation of carbonate, this increase 

 having occurred before and produced the strong contamination in the 

 previous series. 



» See note 3, p. 260. 



<** 



