﻿Calcium Carbonate in Water. 973 



The method of estimation employed was that of Petten- 

 kofer, as improved by Walker (J. C. S. lxxvii. p. 1110 (1900)). 

 By its use the proportion of carbon dioxide in the atmosphere 

 could be determined accurately within *1 part in 10,000 ; 

 the determinations actually made varied from 3'56 to 3*88 

 parts in 10,000, with an average value of 3'69 parts in 

 10,000. 



This value can be taken as the conditions for all experi- 

 ments given below, for by observation it was found that the 

 final equilibrium of the solution wa? established so slowly 

 that constant results w T ere obtained in spite of slight changes 

 in the carbon dioxide values from day to day. For the same 

 reason small changes in the barometric pressure had no effect 

 on the solubility values obtained; the mean barometric height 

 was 754 mm. 



The experiments were performed at 25° 0. and 50° C. as 

 before ; powdered calcite was taken in the silica flasks with 

 freshly distilled water, and atmospheric air led through the 

 solution, after being passed through a long tube filled with 

 cotton-wool to remove all dust. The results are given below, 

 in grammes per litre, and are also shown in graphic form upon 

 the diagram on page 965. 



lo C Mean value. 



3 days (-04351)1 



5 „ (-04545) | 



18 „ -04607) 



19 „ -04592 I -04608 



21 „ -04625 J 



50° C. 



4 days (-02911) 



7- „ , -029301 -02925 



9 „ -02935/ 



The solubility at 25° C. is hence seen to be more than 

 three times as great as that in pure air ('04608 and '01433 gr. 

 per litre respectively) ; that at 50° O. is nearly twice as great 

 (•02925 and '01504 gr. per litre respectively). At 100° C. 

 the two values should be identical ('01779 gr. per litre). A 

 reference to the diagram (page 965) will show clearly the 

 remarkable difference in the characters of the two solubility 

 curves. 



At the ordinary temperatures a bicarbonate solution would 

 give the same values as calcite, since the precipitation takes 

 place in that form. Mineral arragonite would be, as in pure 

 air, slightly more soluble, but the exact determination of the 

 value is of little importance, the results given above being all 

 that are required for practical use. 



