190 



IflW TAKIETIES OF CAEBOITATE OF LIME. 



Trihexaedral 

 carbonate «f 



on the varieties of carbonate of lime, of which I hate now 

 93 in toy collection. 1 shall describe some of these, which 

 realize the possibility of this double employ of the same 

 figure with two different structures. 



The trihexaedral carbonated lime, PI. VI, fig. ?*» a spe- 

 cimen of which was presented me by Mr. Hericart de 

 Thury, exhibits icself in the form of a regular hexaedral 

 prism C C, terminated by two right hexaedral pyramids 

 P e. Three fiices, P, of each pyramid, taken alternately, 

 are parallel to those of the nucleus. The other three, de- 

 signated by I, which arise from a decrement by two rows in 

 height on the lower angles of the nucleus, are inclined to 

 the adjacent sides at the same angle as the preceding, 

 namely 135°; so that the secondary rhomboid, which the 

 union of these faces would produce if they existed alone, 

 would be similar to the nucleus. 



This result, which I have demonstrated in the geometrical 

 part of my treatise, may be considered as the limit of all 

 those, to which the double solutions 1 have spoken of lead; 

 because it is that, in which, one of the two quantities ex- 

 pressing the decrement becoming 0, the solid answering to 

 this term is the nucleus itself. 



In the ambiguous carbonated lime, fig. 10, the dodecae* 

 dron ^ ^, which in this variety is combined with the inverse 

 rhomboid//, and the sides C C of the regular hexaedral 

 prism, is similar to the metastatic dodecaedron, vulgarly 

 dogtooth spar; but it depends on a different law of decre- 

 ment, of the kind of those I have called intermediate. 

 This result requires a certain explanation to be well under- 

 stood. 

 Common sie- I" ^^^^ common metastatic dodecaedron, fig. II, the least 

 tastatic dode- saliant edges answer to the faces of the nucleus, while the 

 most saliant are turned toward its edges. I had inquired* 

 when I wrote the geometrical part of my treatise, whether 

 there were not a law of decrement capable of producing a 

 secondary crystal similar to the metastatic, so that the edges 

 turned toward the faces of the nucleus should be, contrary 

 to it, the most saliant; and I found, that this result would 

 tjike place from the int^^rmediate decrement ^E ^B *D'» 



♦ Fig. 8 represents the primitive form. 



On 



Ambiguous 

 carbonate of 

 lime. 



saedron. 



