378 CHARACTERISTIC. . 46. 



ed above by = 120 0', under which angle they meet, 



appear in several varieties as faces of cleavage. This rela- 

 tion, and all others of the same kind, are expressed by signs 

 analogous to those used in the forms of crystallisation. In 

 general, every face of cleavage is expressed by the sign of 

 the face of crystallisation to which it corresponds. The 

 degree of perfection of the faces of cleavage has likewise 

 been indicated, and needs no farther explanation. The 

 student, however, who intends to employ the Characteris- 

 tic for the determination of occurring individuals, has par- 

 ticularly to attend to those faces of cleavage, which are 

 most apparent. 



The degrees of hardness, which are in general expressed by 

 H., and those of specific gravity, expressed by G., are given 

 in the Characteristic with their limits, or those points be- 

 tween which the hardness and the specific gravity of the 

 varieties are found to be contained ; observation will very 

 seldom yield these limits themselves; and only in such 

 cases, where it cannot have any prejudicial influence upon 

 the determinative process itself. Evidently this must apply 

 in a still higher degree to the characters of the orders and the 

 genera. Those who make use of the Characteristic, must 

 not therefore compare one character with another* but they 

 have to compare occurring individuals of the species con- 

 tained in it with these characters. 



Besides these three characteristic terms, the characters 

 of some species contain also the indication of several occur- 

 rences of colour, more particularly the streak ; also the 

 lustre, or the general aspect ; sometimes also the state of 

 aggregation, taste, &c. In most cases they would have 

 been superfluous, had a more accurate knowledge of the 

 forms existed. In this particular, we have reason to expect 

 a great deal from future and accurate observations, which 

 will enable us to keep the characters free from all such 

 marks, as do not allow of a perfectly strict definition. 



The specific characters of Jluid minerals require another 



