PROCEEDINGS OF GEOLOGICAL SOCIETIES. 



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nate of lime in excess of carbonic acid is thus left exposed, you will get, 

 not arragonite, but crystals of calcite. This is a remarkable fact. By 

 heating a common solution of carbonate of lime — that is, a solution con- t 

 taining the ordinary amount of carbonate of lime dissolved by virtue of the 

 presence of carbonic acid — you get arragonite, and not calcite. An ordi- 

 nary solution of carbonate of lime, or — what is equivalent — a much stronger 

 one, gives calcite by exposure at the ordinary temperature ; and this same 

 solution, when heated, deposits arragonite. If such a solution be evapo- 

 rated in a platinum vessel, the residue contains carbonate of lime in all 

 three forms, namely, the amorphous or chalk, arragonite, and calcspar. 

 This is an important point. 



Having obtained these results, Rose then went on to investigate the pre- 

 cise changes corresponding to different degrees of temperature. These are 

 his chief results : — At 100 Centigrade — that is, the boiling-point of water — 

 the greatest part of the residue was arragonite in characteristic small 

 prisms. At 90° the most arragonite was formed, and the crystals were 

 larger than at other temperatures. At 70° the rhombic crystals of calc- 

 spar predominated, and were accompanied by hexagonal plates and small 

 stars of calcspar, and the arragonite crystals were small. At 50° there was 

 more calcspar, and the proportion of plates and stars, as compared with 

 the rhombs of calcspar, increased. The arragonite crystals were thicker, 

 and often bent or curved. At 30° no arragonite whatever was formed. 

 The rhombic crystals of calcspar were comparatively large, and there were 

 still some plates and stars. It is very remarkable that he should get these 

 varieties of deposit under these different degrees of temperature and solu- 

 tion. All the arragonite occurred at a higher temperature than 30°, and 

 the hexagonal plates of calcspar were formed at a lower temperature than 

 70°. Calcspar is always formed in solutions containing carbonate of lime, 

 when carbonic acid is set free. By exposing in a warm place a well-stop- 

 pered vessel containing a concentrated solution of carbonate of lime in ex- 

 cess of carbonic acid, crystals of calcspar were deposited. 



Experiments were also made by Hose, with especial regard to the influ- 

 ence of the degree of dilution, and so forth, upon the result. He took two 

 ordinary small flasks — one containing a very dilute solution of carbonate 

 of soda, and the other containing a dilute solution of chloride of calcium ; 

 he tied them together, and then immersed them in a cylindrical vessel of 

 water. He left them under the water, and, in consequence of diffusion, a 

 mixture took place with extreme slowness, the chloride of calcium mixing 

 with the carbonate of soda. In this case, the result was the formation of 

 arragonite ; and if the solutions were a little stronger, he then got calcspar. 

 The conclusion, therefore is, that, under special conditions of dilution, ar- 

 ragonite may be formed even at the ordinary temperature. 



In native arragonite a little carbonate of strontia is found frequently, but 

 not always. Its maximum amount, taking a goodly number of analyses 

 recorded, is stated at 2^- per cent. It ranges from a trace to 2| per cent. 

 At one time it was supposed that the carbonate of strontia was a universal 

 and essential constituent of all arragonite, and that this determined its 

 crystalline form. That opinion, however, has been shown to be an error, be- 

 cause we not unfrequently find arragonite free from strontia. A little water 

 is also generally present in arragonite. The extremes given by a number 

 of analyses are 0"17 per cent, and 0'41 per cent. Water is given as a con- 

 stituent in all of them. Arragonite is deposited from hot springs, as at 

 Carlsbad, where there is a well-known spring. It occurs also in gypsum 

 at Molina, in Arragon, and at Dax, in the Landes. It is also found in ba- 



