﻿Calcium Carbonate in Water. 971 



for calcite and arragonite ; those at the boiling-point are 

 identical with the value for arragonite at the boiling-point. 



There is still a difference of 1 to 2 per cent, in the values 

 obtained by the two methods; this must be attributed to a 

 minute trace of carbon dioxide, not removed by the washing. 

 It is probably the carbon dioxide that remains, as it were, 

 adsorbed on the surface of the crystals that causes the 

 different behaviour with cobalt nitrate solution. 



From the above it will be evident that the accuracy of the 

 method depends entirely upon the purity of the water used for 

 experiment, a trace of carbon dioxide being sufficient to raise 

 the solubility of the carbonate very appreciably. In all expe- 

 riments the water used was freshly distilled into one of the 

 siclia flasks, yet it would always contain a small amount of 

 carbon dioxide, and small differences in this amount would 

 be sufficient to affect the constancy of the results. 



Amorphous Calcium Carbonate. 



The conditions for the precipitation of calcium carbonate 

 in its amorphous form have already been discussed. In the 

 dilute solutions employed for the experiments with bicar- 

 bonate solutions, one or other of the crystalline forms was 

 always obtained ; only in the preparation of the solutions 

 was the amorphous form obtained as a cloudy precipitate, 

 which immediately, unless dissolved, transformed into a 

 crystalline form, calcite. 



The amorphous form is the most unstable of the three 

 varieties of the carbonate, and its solubility is necessarily 

 higher than those of the other forms. Nevertheless, the 

 figures given by Kohlrausch (Zeit. phys. Chem. xii. p. 234 

 (1893)) are identical with those for calcite. It is evident 

 that transformation of the amorphous form to calcite had 

 occurred in the course of the experiment. 



Similar results were obtained in the direct determinations 

 given below. Experiments were carried out both with 

 natural and prepared forms. The former consisted of 

 ordinary soft chalk ; the latter was obtained by heating 

 finely-powdered calcite to lime in a platinum crucible, and 

 reformings the carbonate by means of a current of carbon 

 dioxide passed over the slowly-cooled oxide. The experimen t 

 was carried out quantitatively to ensure the absence of un- 

 changed calcite — the heating being continued until the loss 

 of weight was equal to the theoretical. The cooling in a 

 current of carbon dioxide was also repeated until the original 

 weight was restored, in order to remove all traces of quick- 

 lime. The experiments were carried out exactly as before. 



