VII.] 



CRYSTALLIZATION. 7D 



the position of the optic axes is different for different 

 coloi;rs. 



In the oblique systems, the parameters are unequal, and 

 so are the axes of elasticity.^ 



In the singly oblique system, two of the axes are in- Singly 

 clined to each other {i.e. are not at right angles); the gygj^^f^ 

 third is at right angles to the other two. To this third, or 

 rectangular crystallographic axis, one of the axes of elas- 

 ticity is parallel ; but the other two axes of elasticity are 

 not parallel to the remaining crystallographic axes, and 

 are different in position for light of different colours : 

 and, of course, the optic axes are also different in position 

 for different colours. 



In the doubly oblique system,^ one of the axes is at Doubly 

 right angles to the second, and inclined to the third ; the °ystem, 

 second and third are inclined to each other. Its optical 

 properties, so far as I know, have not been described, but 

 they are no doubt similar to those of the system to be 

 next described. 



In the anorthic system, none of the crystallographic AnortMc 

 axes are at right angles to each other : the axes of elas- ' 

 ticity are not parallel to them, and are different for rays 

 of different colours, as are also the optic axes. 



Crystals are classified on much the same principles as 

 organisms. Among both, the species group themselves 

 into genera, and the genera into classes or systems ; 

 among both there are varieties, which sometimes form a 

 gradation between species : and among both, species or 

 genera are found that form a gradation between classes. 

 As already stated,^ crystalline varieties are due to slight Varieties, 

 differences in chemical composition : those which form a 

 gradation from one species to another are due to the 

 presence of plesiomorphous substances. 



1 In strictness of language, the rhombohedral system is an oblique one ; 

 but it is not usual to call it so, as. in its general properties it has more 

 resemblance to the rectangular systems. 



2 Only one species of this system has been described : it has been found 

 by Naumann as the crystallizing form of an artificial salt. (Dana's 

 Mineralogy. ) 



3 P. 69. 



