On Crystallography. 483 



In this sign the quantity ^G' indicates two distinct faces, 

 which are furnieci on both sides of each ridge G (fia;. 56). 

 But it is not necessary to place two letters under this sign, 

 because, all the faces situated in the same manner beino- de- 

 signaled by the same letter on the /igurc, it is sufficient that 

 the sign ^G'" refers to the marked faces of the letter 0, which 

 requires only that this letter should be written once under 

 the sign. 



We shall easily conclude from the same principles, that 

 the dodecahedron with rhombic planes originatino- from 



the cube (fig. 55) is expressed by this single letter B B, that 



the octahedron originating from the same nucleus has for 



its sign A 'A', Sec. 



The rhomboid, by supposing it placed under the most 

 natural aspect, i. e. in sucli a manner that the two solid 

 angles, composed of three equal plane angles, are on one and 

 the san)e vertical axis, has not properly bases, but merely 

 two sumnnts, which are the cxlreniities of the axis. We 

 shall designate its angles and ridges as in fig. G2. The let- 

 ter e makes known that the angle which bears it is similar to 

 that marked wi;h the larger letter; so that if all the lateral 

 angles had their indications expressed, the three nearest the 

 upper suuimits would bear the letter E; and the three which 

 adjoin the lower summits, and which are diametrically op- 

 posite to ihe first, would have e as their indicative letter. 



As the rliomhoid has its six faces equal and similar, it is 

 cnly necessary to consider the decrenients relative to one 

 of the laces, as that which on the figure bears the letter P, 

 because all the rest are merely the returns of this. This 

 being done: 1st, the decrements which set out from the 

 upper angle A or from the upper edge B will have their in- 

 dicating cypher placed below the letter A or B; e<l, those. 

 vhieh set out from the lateral angles E will hn.ve their cy- 

 ]iher situated on one side, towards the top of the same let- 

 ter; 3d, with regard to tliose which set out from the lower 

 angle e, or from the lower edge D, the cypher destined to 

 express them will be placed above tiie letter eor D. 



I^t us suppose, for example, that fig. 63, represents ana- 

 logicul carbonated lime already mentioned : we shall have 



the following sign e D B, the interpretation of which is 



r r ( 



tasy, from comhining the letters wliich iiulicate the faces 

 with those v. hich express the decrements of which these 

 same faces are the result. 



Whaf 



