10 PHYSIOGRAPHY. . 256. 



dral JLime-haloide, accidentally inclines to several others, 

 none of their different shades possessing any degree of 

 brightness. Co 1 ours produced by occasional admixtures 

 or' minerals foreign to the species, do not properly belong 

 to the collective description ; because they are not mem- 

 bers of the series of colours of the species described. Yet 

 they are indicated by themselves, at least the common 

 shades, in order to exclude them from those, with which 

 they are not connected by transitions within the same 

 series. 



It is important to indicate correctly the colour of the 

 powder, or the streak. 



The indication of the limits will suffice for the degrees of 

 transparency. In general, refraction is simple in all those 

 species whose forms belong to the tessular system : it is 

 double in all those of the other systems. There is but one 

 optical axis in the rhombohedral and pyramidal systems, 

 which coincides with the crystallographic principal axis. 

 In the prismatic forms, there exist two optical axes, situ- 

 ated in planes which pass through the axis and one of the 

 diagonals. The direction of these axes, in respect to the 

 crystallographic axes and diagonals, in most cases has not 

 been sufficiently ascertained. The characters of the mine- 

 rals derived from their action upon light, will no doubt 

 form an essential part in future of the general descriptions. 



Lastly, the general descriptions contain the indication of 

 the form of aggregation, of hardness, specific gravity, and 

 other characteristic marks derived from, or respecting, the 

 substance of minerals, as odour, taste, &c., which may be 

 useful in the description of varieties ; all of them express- 

 ed with the brevity essential to the characters which com- 

 pose the Characteristic. It is necessary to observe here, 

 that the unity of comparison for the specific gravity of ex- 

 pansible fluids, is that of atmospheric air ; in the same 

 way in which that for liquid or solid bodies is the specific 

 gravity of distilled water. The specific gravities given 

 without adding any authority, have been newly deter- 

 mined and reduced to a temperature of 15 centigr. (59 

 Fahrenh.). 



