472 M. Gustave Rose on the Formation 



transparency and form. Notwithstanding this, the mass has 

 become completely converted into calc spar, as the examina- 

 tion of the specific gravity proves. The mixture of arragonite 

 and calc spar obtained by evaporation on the water-bath, the 

 specific gravity of which 1 had found by experiment to be 2-803, 

 was, in order to be certain that all became changed (since it is 

 in this case impossible to see the change as in the large arrago- 

 nite crystals,) exposed in a platina crucible to a very high 

 temperature. By this a part of the carbonate of lime becomes 

 already caustic : this however may immediately be removed 

 by dissolving it in a large quantity of water, decanting the 

 water, and washing the powder. Examined under the micro- 

 scope the larger arragonite crystals had become full of flaws, 

 in the manner above indicated. I found the specific gravity 

 of the mass thus treated to amount to 2-700, like that of the 

 heated arragonite. This circumstance proves, that in the 

 small crystals of arragonite the smallest parts are capable of 

 extending and reverting themselves, without the form of the 

 crystal being in any degree lost; they are the most perfect 

 pseudomorphous crystals of calc spar in the form of arra- 

 gonite. It is probably not impossible to convert larger crystals 

 into such pseudomorphous crystals, if the crystals of arrago- 

 nite be exposed to a slowly increasing heat. 



The results of the experiments above related are as fol- 

 lows: 



1. That in the humid way both calc spar and arragonite are 



formed, the first at a lower, the latter at a higher tem- 

 perature ; but in the dry way calc spar alone is formed. 



2. That carbonate of lime immediately after precipitation 



from a cold solution is in an indistinct crystalline state, 

 which agrees with chalk, from which subsequently the 

 distinct crystalline state proceeds. 



3. That arragonite changes y&ry easily into calc spar, in the 



humid way, if the arragonite obtained by precipitation is 

 allowed to stand under water or in a solution of carbo- 

 nate of ammonia ; in the dry way, if the arragonite is ex- 

 posed to a low red heat, at which the large crystals fall 

 into a coarse powder, but small crystals retain at the same 

 time their form, and produce pseudomorphous crystals. 

 A further consequence is that the origin of arragonite can 

 no longer be ascribed, as has frequently been the case, to the 

 small quantity of strontia, which natural arragonite generally 

 contains. This follows indeed, even from the circumstance, 

 that there is arragonite which does not contain any carbonate 

 of strontia ; but the case is now completely proved by the fact 

 that arragonite may be prepared artificially with ease without 



