262 EQUILIBRIUM BETWEEN CARBON DIOXIDE OF ATMOSPHERE 



Nothing is stated in Cameron's paper whether the air used was "labo- 

 ratory air" with an excess of carbon dioxide in it or pure country air.' If 

 laboratory air was used, the amount of calcium bicarbonate obtained in 

 solution and subsequently precipitated would be excessive and the results 

 in the last column of table 8 might be materially lower for air with a carbon- 

 dioxide content of 0.0003 atmosphere. 



It is interesting to note that according to Usiglio's ^ work on Mediter- 

 ranean water, calcium sulphate began to be deposited when the water 

 reached a density of 1.13. This corresponds to a chloride content of about 

 17 per cent, or about 3 gram molecules of sodium chloride per liter, a con- 

 centration reached in Cameron's experiments for solution 7, from which, 

 going to solution 8, gypsum would be obtained with about 0.8 per cent car- 

 bonate. At this concentration, the concentration of other sulphates is 

 still so small that they would tend to increase the contamination with 

 carbonate, as described above, rendering the calcium sulphate less soluble 

 and the bicarbonate more so. 



At a lower partial pressure of carbon dioxide than 0.0003 atmosphere, 

 the proportion of carbonate would be reduced approximately in the ratio of 

 the square roots of the ratios of the partial pressures,^ i.e., a partial pressure 

 of only 0.00003 atmosphere would reduce the carbonate to about og parts of 

 the values given in the last column of table 8, or to still less on the prob- 

 able assumption that the experiments on which the table is based were not 

 carried out with pure air, and would produce a very pure deposit. 



THE EFFECT OF TEMPERATURE CHANGES. 



Reliable data on the solubility of calcium carbonate at temperatures 

 other than 16°, the temperature at which Schloesing's experiments were 

 carried out, are not at hand, and so the effect of changes of temperature on 

 the conditions we are studying can not be estimated. It may be pointed out, 

 however, that aside from a probable increase in the solubility of calcium 

 carbonate, a higher temperature would affect chiefly the solubility coeffi- 

 cient of carbon dioxide, and through it would reduce the formation of 

 calcium bicarbonate. The solubility * of gypsum is about the same at 50° 

 to 65° as at 18° and ionization constants are usually not changed greatly 

 by changes of temperature; but the coefficient of absorption^ for carbon 

 dioxide at 65.5° is just about one-ninth as large as the coefficient at 18°. 

 This would result, according to equation (42), in reducing the formation of 



calcium bicarbonate to -J ^ or one-third of the value found for 16°. In other 

 words, a rise of temperature of some 50° would probably have about the 

 same effect on the solubility of calcium bicarbonate as a decrease in the 



* Results obtained by McCoy (American Chemical Journal, 29,461 (1903), in repeating 

 other work of Cameron on conditions of equilibrium involving the carbon dioxide of the 

 air and showing decided discrepancies, form a very strong indication that pure air was not 

 used by Cameron. 



' Encyclopaedia Britannica. 

 ^ See note 3, p. 260. 



* Comey, Dictionary of Solubilities, p. 422. 



* Dammer, Handbuch der Anorganischen Chemie, ii, 1, p. 371. 



