466 M. Gustave Rose on the Fat-mation 



of the solution*. These latter are always the most distinct, and 

 become, if the carbonic acid is allowed to escape slowly by 

 covering the vessel with a glass plate, at times so large, that 

 their forms may not only be easily recognised by the naked 

 eye, but even their angles may be measured by means of the 

 reflective goniometer. The crystals are calc spar, and have 

 always the form of the primary rhombohedron of calc spar ; 

 they are generally obtuse at their ends from the superposed 

 terminal surfaces. 



In the same way we also obtain calc spar if we mix a so- 

 lution of chloride of calcium with carbonate of ammonia or 

 with other alkaline carbonates at the common temperature of 

 the air. The precipitate thus caused is in the beginning very 

 voluminous and flocculent, and even retains this property if it 

 is filtered soon after precipitation, washed and dried : if how- 

 ever it is allowed to stand still for some time, it gradually 

 falls together and becomes granular. If we observe the loose 

 precipitate under the microscope, we only perceive small 

 opake grains, which on being highly magnified often appear 

 exactly similar to those rings which Ehrenberg has described 

 in chalk t; on the other hand, the precipitate which has be- 

 come granular appears to consist of small sharply defined 

 diaphanous rhombohedrons, perfectly distinct, and which, like 

 the crystals obtained by evaporation, are evidently primary 

 rhombohedrons of calc spar. 



The specific gravity of the flocky precipitate I found to 

 be 2*716, that of chalk 2*720, that of the granular precipitate 

 2*719j, The specific gravity of the flocky precipitate is 

 according to this somewhat less than that of the granular 

 precipitate and of chalk ; this circumstance may however only 

 arise from the specific gravity of the flocky precipitate having 

 been taken after it had already been dried, and it may in this 

 way have come out rather too low §. At all events, the diver- 



* As it would be difficult without very large apparatus to procure such a 

 solution of carbonate of lime, I applied to M. Soltmann, who with his usual 

 kindness caused to be prepared for me in his manufactory of artificial mineral 

 waters such a solution, and placed at my disposal several bottles of it, with 

 which the following experiments have been made. 



t PoggendorfF's Annalen, vol. xxxix. p. 105, and Phil. Mag. vol. x. p. 318. 



j Beudant fixes the specific gravity of the pulverulent calc spar rather 

 higher, namely 2'723. (Poggendorf!"s Annalen, vol. xiv. p. 485.) 



§ The method which I employed in the weighing of these, as also of other 

 pulverulent bodies which are mentioned in this memoir, is the following : 

 The powder was first mixed with water in a large goblet, and then poured 

 into a small cylindrical glass of about an inch in size, which has reverted 

 borders at the top,so that it could be suspended by means of a hair to a hook 

 situated under the scale. The glass with powder was now weighed in water, 

 then i)laced in a water bath, and the powder evaporated to dryness, and 



