OnVnjstallography» 147 



Ittent descends* or ascends, setting out from the angle, or 

 from the t(\g'^ marked with this letter. If it be placed to-» 

 uards the top and to the right or left of the letter, it 

 designates a decrement which takes j)lace in the lateral di- 

 rection to the right or to the left of the angle which bears 

 the same letter. 



6. When a letter is found written twice successively with 

 the same cypher placed on two different sides, such as ^G 

 G% or G* «G, ^A A% or A^ ^A, the two edges or the 

 two angles which it designates, ought to be considered on 

 the figure according to the same relative positions, i, e.^ 

 for example, as in the sign ^G G% the quantity ^G indi- 

 cates the effect of the decrement on the edge G situated on 

 the left, and the quantity G^ the effect of the decrement 

 on the edge situated on the right. 



7. When a letter bears the same cypher repeated both on 

 the right and left, as ^G% it is applied indifferently to any 

 one of the ridges G which it designates. It is the same 

 \('ith the letters which belong to the angles. 



^ 8. The parenthesis in such as (OD' F^) designates an in- 



3 



termfidiary ,diecrement. The letter O expresses in the first 

 place, that the decrement takes place by three rows on the 

 angle O, and that its effect is ascending. JD ' F^ make known 

 that for one rid2;e of molecule subtracted along the side 

 marked D, there are two ridges subtracted along the side 

 marked F. 



9. Every small letter comprehended in the sign of a 

 crystal, indicates the angle or edge diametrically opposite to 

 that which bears the large letter of the same kind, or the 

 figure in which the small letter in question is omitted as su- 

 perfluous. We must except the letter e, which is always 

 employed on the figure of the rhomboid, and which indi- 

 cates, according to the principle, the angle opposite to that 

 which bears the letter E. 



10. When a sign contains two letters of the same kind, 

 the one large and the other small, with different cyphers, 

 the two opposite edges or the two opposite angles to which 

 these letters answ^, are considered as each undergoing ex- 

 clusively the law of decrement indicated by the cypher 

 added to the letter. 



* We only allude hereto thegdneral progress of decrements, to which the 

 particular cases refer that seem to form an exception. For instance, if the 

 decreaient be produced by one row on the anjjie at the summit of a rhom- 

 boid, then the face produced will he horizontal. But this, decrement com^s 

 within the description of those that are descending, and of which it is as it 

 were the limit. 



K 2 11. Every 



