of Calc Spar and Arragonite. 467 



sity is too small to be explained by a difference in the chemical 

 composition, which, however, has been supposed, in this flocky 

 precipitate. We may infer from this, that the specific gravity 

 of all these bodies, and consequently also their chemical com- 

 position, is the same. They differ only in this respect, that 

 the carbonate of lime in the flocky precipitate, as in chalk, 

 with which compared under the microscope it exactly agrees, 

 is in a very imperfect crystalline state, while the granular pre- 

 cipitate, which is subsequently formed from the imperfect cry- 

 stalline, is in an obviously crystalline state. That chalk is 

 in the same condition as the carbonate of lime immediately 

 after precipitation, is a result which is probably not without 

 interest in a geological point of view. 



According to the above method, we therefore obtain only 

 calc spar; if, on the contrary, the solution of carbonate of lime 

 in carbonated water is evaporated in a water bath to dryness, 

 a loose crystalline powder is obtained, which under the mi- 

 croscope appears for the greatest part as an aggregation of 

 distinct crystals, which manifestly have the form of arragonite. 

 They generally appear as six-sided columns somewhat dilated, 

 or as very acute six-sided double pyramids, like many sap- 

 phire crystals ; at times, however, they appear as single py- 

 ramids, so that they are therefore crystallized differently at 

 the two ends. 



In the same manner we may also obtain arragonite by pre- 

 cipitating a boiling solution of chloride of calcium with hot 

 carbonate of ammonia. The crystals, of-which such a pre- 

 cipitate consists, examined under the microscope, are smaller 

 than those obtained by evaporation ; they are very distinct, 

 however. In this case they very often occur as triad-crystals ; 



then weighed again. In this manner nothing is lost of the powder, while 

 on the other hand it would be difficult to avoid a loss if the experiment 

 had been made in a reverse manner, the powder being first weighed dry 

 and then in water. Yet notwithstanding this, I have constantly found the 

 specific gravity of the precipitate which was immediately carried, after being 

 precipitated and washed, to the water, rather higher than when the pre- 

 cipitate was first dried and then mixed with water, even when I boiled it in 

 the water, which probably arises from the powder in the latter case, on 

 being mixed with the water, not being freed from small air-bubbles. The 

 specific gravity of the immediate precipitate I have always found to coin- 

 cide with that which is obtained if we make use of small crystals for the 

 determination ; hence it appears necessary always to determine the specific 

 gravity of pulverulent bodies in this manner if it be possible. Beudant also 

 found (in the above-mentioned memoir) the specific gravity of a body in a 

 pulverulent state always rather lower than in small crystals, which probably 

 is to be ascribed to the above-mentioned circumstance : however Beudant 

 has not stated in what manner he determined the specific gravity of the 

 pulverulent body. 



12 R 2 



