VIII.] CHEMISTRY OF DOLOMITES AND GYPSUMS. 91 



SUPPLEMENT. 



[In reference to the formation of dolomite, as indicated above, 

 in 3, it was shown in the continuation of these researches, pub- 

 lished, as already mentioned, in 1866, that the result was best 

 attained when a mixture of the two carbonates, precipitated 

 together and still amorphous, was gradually heated with water, 

 under pressure, to a temperature of 1 20 C. ; and the question 

 was raised, whether all the deposits of dolomite in nature have 

 been thus heated, or "whether there are yet unknown con- 

 ditions under which the double carbonate, dolomite, can be 

 formed at lower temperatures." 



As to the reaction noticed in 2, it was found that the partial 

 loss of carbonic acid during the spontaneous evaporation of 

 solutions holding gypsum and bicarbonate of magnesia often 

 renders the results of experiments very imperfect, but that 

 the conditions of complete success in the separation of gypsum 

 from such solutions are attained when the evaporation is con- 

 ducted in an atmosphere containing a large proportion of car- 

 bonic-acid gas (ante, page 43). This result was first described 

 in a note to the American Association for the Advancement of 

 Science, in August, 1866. (Canadian Naturalist, III. 123.) 



I have found in solutions prepared with sulphate of mag- 

 nesia and bicarbonate of lime, as in 2, the proportion of 

 sulphate of lime to the water as 1 : 404 and 1 : 413 ; while a 

 saturated solution of gypsum in pure water at 16 C. con- 

 tained 1 : 372, which nearly agrees with Giese's determination 

 of 1 : 380. Taking, as a mean of these, 1 : 400, we have for a 

 litre of water 2.50 grammes of anhydrous sulphate of lime, 

 equal to 3.16 of gypsum. From a solution prepared as above 

 I have, in a successful experiment, separated by evaporation 

 in the open air 1.18 grammes of gypsum to the litre, leaving 

 in solution an equivalent of magnesian bicarbonate. Of the 

 latter compound, Bineau obtained solutions in which 11.2 

 grammes of magnesia (equal to 23.5 of monocarbonate) were 

 dissolved in a litre of water with very nearly two equiva- 

 lents of carbonic acid; and I have found it easy to prepare 



