﻿Calcium Carbonate in Water. 969 



This is in agreement with the results of Rose — that, at 

 25° C, calcite is obtained ; at 100° C. arragonite, slowly 

 transforming into calcite ; at 50° C. a mixture of the two 

 forms. 



Nevertheless the previous solubility values were not con- 

 firmed. At 25° C. and 50° 0., equilibrium was established 

 so slowly that the final value could not be determined, the 

 fall in concentration being extremely slow ; at 100° C, a 

 fairly constant value was obtained; this, however, was much 

 higher than the value found for arragonite at 100° C. 

 Examples of the results obtained, with the times of experi- 

 ment, are given below, expressed as before in grammes per 

 litre. 



25° a 50° c. 



3 days -034441 -03097? -028611 — 



5 , -03421/ — y -02635/ -026921 



9 „ — -02962 1 — -02480/ 



100° C. 



40mins -02511 — 



80 „ -02331 -02329 



120 , -02346 -02333 



180 „ -02309 — 



The explanation of the results at 100° C. was at first 

 thought to lie in the fact that the needle-shaped crystals 

 precipitated were not, as has been held, identical with 

 mineral arragonite, but were another modification of the 

 carbonate (compare Meigen on the formation of " Vaterite,'* 

 Cliemiker-Zeitung, xxxiv. p> 1015 (1910))» This view seemed 

 to be confirmed by the action of cobalt nitrate upon the 

 precipitate. 



The differences of behaviour of the various forms of 

 calcium carbonate with boiling cobalt nitrate solution have 

 been examined, qualitatively and quantitatively, by Meigen 

 (Central-Matt, ii. p. 1128(1901); Chemiktr-Zeitung 3 xxxiv. 

 p. 1015 (1910)). When finely-powdered arragonite is 

 boiled with a dilute solution of cobalt nitrate, a lilac-coloured 

 precipitate is quickly produced ; in the case of calcite, the 

 action is much {•lower, and the precipitate is tinted blue. 

 Presence of other metallic salts in the solution confuses 

 those colour reactions. 



The precipitated crystals gave, when the test was applied, 

 an immediate flocculent lilac-coloured precipitate. Mineral 

 calcite and arragonite, powdered to the same degree of 

 fineness (as observed by the microscope), were only slightly 

 tinted after boiling for sotne minutes, and no noticeable 

 precipitate was formed. 



