460 On Cry St allograph!/. 



ihat it takes place by one range on the angle marked h-.* 

 the same letter, and that it refers to the base A E O I 

 (fie- 48. )• D' F- indicate that for one single ridge of molc- 

 cuTes subtracted from the length of the side D, there are 

 two rtdges subtracted along the side F. 



It is useful to have a language for pointing out these 

 different signs, so that they can easily be written to dicta- 

 tion. We can announce the signs 0% 'O, by saying, 

 O two on the right, three on the left : in order to an- 



2 



nounce O O, we shall say O raider two, O above four : 



4 



1 



lastly, the sign (O D' F') will be thus announced, within 

 parenthesis, O undej- one, D one, F two. 



We shall give an example of the combination of these 

 different signs, in the expression of a compound crystalline 

 form. Bui we must previously determine the order ac- 

 cording to which the letters ought to be arranged whicb 

 concur in one and the same expression. Now it we adopt- 

 ed the order of the alphabet, a kind of confusion would 

 result in the table presented by the formula. It seems 

 more natural to conform to the order which would direct 

 an observer even in the description of the crystal, i. e. 

 to commence by the prism or by the middle part, and in- 

 dicate its different faces as they are successively presented 

 lo the eye, then passing to the faces of the summit or of 

 the pyramid. This will be elucidated by the various ex- 

 aiTiples to be cited in the course of this article. 



Let us now suppose that fig. 52 represents the variety of 

 feldspar, ca.\kc{l'il)inary , the priniiiive form of which is 

 seen in fig. 48. In this variety, Llic pane / (fig. 5-2.) results 

 from a decrement by two ranges on the ridge G (fig. -18.) 

 going towards H ; the pane M" (fig. 52.) answers to that 

 which is marked with the same letter (fig. 48), and which 

 is only partly concealed by the cffrct of the decrement. 

 The pane T (fig. 52.) is parallel to T (fig. 4 8.) ; the penta- 

 gon X (fig. 52) proceeds from a decrement by two ranges 

 on the angle 1 (fig. 48.) parallel to the diagonal which 

 goes fromA lo 0: lastly, as this decrement no longer at- 

 tains its limit, the summit bears a second pentagon P 

 (fio. 52.) parallel to the base P (fig. 48). All this descrip- 

 tion may be thus translated into five letters G- M T F P. 



I confined myself in the first place to give the pure and 

 simple expressions of the indicative signs, similar to those 

 which we have seen. But I afterwards perceived thai I 

 could not talce too many precautions, in order to free this 



language. 



