468 M. Gustave Rose on the Format im 



for from the middle of a columnar crystal two others di- 

 verge. 



According to eitlier method, however, it is difficult to obtain 

 the arragonite pure, for generally a greater or less quantity 

 of rhombohedrons of calc spar are found mingled with the 

 columnar crystals. This intermixture of calc spar occurs 

 especially when the arragonite is prepared by evaporation, 

 and in this case it is easily explained, for the carbonic acid 

 escapes even before the solution has attained that temperature 

 at which only arragonite can form, and the precipitate thus 

 originated must necessarily take the form of calc spar. And 

 it is for this reason also that the specific gravity of arragonite 

 so obtained is lower than that of the pure mineral. 1 eva- 

 porated twelve quarts o\' a solution of carbonate of lime to dry- 

 ness in a large water bath, neglecting, however, to skim off 

 at the beginning the saline scum which formed first, and pro- 

 bably consisted entirely of calc spar; but after all the liquid 

 had evaporated I scraped the whole of the deposit together 

 with a card from the sides of the vessel, by which it was all 

 mixed together. The specific gravity of this arragonite 

 amounted only to 2*803. In order to obtain the arragonite 

 pure, I took a solution of carbonate of lime, which had previ- 

 ously stood exposed to the air for some time, and in which, 

 therefore, a precipitate had been formed. The solution was 

 filtered, and then gi'adually poured in small quantities into a 

 vessel containing boiling water, and afterwards evaporated 

 to dryness. The specific gravity of this deposit amounted to 

 2*836. According to this, therefore, it was rather purer than 

 the former; however, it was still mixed with much calc spar. 



A similar mixture of arragonite and calc spar may also be 

 obtained by precipitating from a hot solution of chloride of 

 calcium with carbonate of ammonia, although in this case the 

 precipitate is generally purer arragonite than by the method 

 of evaporation. 1 have varied in several ways the prepara- 

 tion by precipitation ; 1 have taken more or less concentrated 

 or weak solutions, mixed larger or smaller quantities of car- 

 bonate of ammonia, or chloride of sodium. I also obtained se- 

 veral times in experiments with small quantities, a precipitate, 

 which under the microscope, consisted entirely of pure arra- 

 gonite ; if, however, I wished to prepare a greater quantity in 

 the same way in order to obtain a sufficient quantity to deter- 

 mine the specific gravity, I always found in this intermixtures 

 of calc s})ar. 



There is, however, a very simple method cjf obtaining ar- 

 rngonile quite ]mre, which 1 accidentally tried after having put 

 all the others aside ; and it consists in this ; not pierely, as I 



