I 



of Calc Spar and Arragotiite, 469 



had previously done, pouring the hot solution of carbonate of 

 ammonia into the hot sohition of chloride of calcium, but the 

 leverse, by pouring the latter into the first. In this manner 

 a completely loose precipitate is obtained, which, observed 

 under the microscope, consists of still smaller crystals than 

 those obtained by former precipitations, but entirely free from 

 any intermixture of rhombohedrons of calc spar. This is also 

 proved by its higher specific gravity, which, according to many 

 experiments I performed, amounts to 2*94-9. I found the spe- 

 cific gravity of a single transparent crystal of arragonite from 

 Bilin in Bohemia to be 2-94'5*. Beudant gives that of pul- 

 verulent arragonite at 2*9466. 



In order to preserve unchanged the arragonite obtained 

 by precipitation, it is necessary to wash and dry it imme- 

 diately. If it is allowed to stand after precipitation for any 

 length of time in tlie fluid, it is in a very curious manner gra- 

 dually converted into calc spar. Eight days are quite suffi- 

 cient for that purpose. Minute rhombohedrons are formed, 

 which under the microscope are perfectly distinct, and are 

 often so grouped in series, that their aggregation forms columns 

 similar to those from which they originated. But this change 

 also takes place, although much slowe. , »« fht newly precipi- 

 tated crystals are kept immersed in pure water. 1 observed 

 this in the precipitate of entirely pure arragonite, the specific 

 gravity of which had previously been ascertained. 1 had pre- 

 served the remainder in a vessel, into which I had conveyed 

 it by a jet immediately off the filter on which it had been 

 washed, with the intention of again determining the specific 

 gravity of a portion of it, but which accidentally could not be 

 done till after the lapse of eight days. The specific gravity 

 which I now found amounted however to only 2*909. As I 

 presumed that in this weighing some error might probably 

 have occurred, I determined on the following day the specific 

 gravity of a second quantity, and on the third day that of a 

 third ; but I now found still lower numbers, namely, on the 

 second day 2*883, and on the third 2*881. Having then ex- 

 amined the arragonite under the microscope, I found perfect 

 4'hombohedrons among the minute crystals of arragonite, by 

 which the cause of the lower specific gravity was now ex- 

 plained. 



Although this metamorphosis of arragonite and calc spar 

 takes place so easily, yet it only happens when the arragonite 

 is newly precipitated. If it has been well dried first, it re- 



• Breithaupt CVolIstandige Charakteristik des Mineralsysteras, p. 65.) 

 gives the specific gravity of a similar crystal of arragonite at from 2937 to 

 2'.038, which is evidently too low. 



