MINERALOGICAL ARRANGEMENT. 445 



demands great precision of hand and eye to obtain 

 the true measurements of angles to a minute only, 

 and we cannot say that a difference of species may 

 not exist, with a difference of only a few seconds 

 in the angles of inclination of their planes. 



We know that the greater angle of a right rhombic 

 prism must lie somewhere between 90 and 180. If 

 the angle were 90 , the prism would be square, and 

 180 would reduce the prism to a plane. But be- 

 tween 90 and 180, we do not know how many 

 different prisms may exist. 



If they differed by degrees only, their number 

 could not exceed 89. If the difference consisted of 

 minutes, there might be 5399 such prisms, all dis- 

 tinguishable by the goniometer ; but if the differences 

 consisted of seconds only, there might evidently be 

 323999 rhombic prisms, of which no more than 5399 

 could be distinguished by the ordinary goniometric 

 instruments. 



But with all the uncertainties and difficulties at- 

 tendant upon the crystallographical determination of 

 a mineral species, the goniometer is probably the 

 most accurate guide we at present possess to lead us 

 to that determination. And it is almost the only one 

 of whicfi the practical mineralogist can at all times 

 avail himself. 



It appears almost unnecessary to state, that where 

 a mineral is defective in crystalline character, or its 

 chemical composition is unknown, it must be pro- 

 visionally distinguished from other minerals by some 

 other of its physical characters, as its specific gravity, 

 hardness, fracture, &c. 



Instances have been already alluded to where 

 chemistry would separate minerals from each other, 

 which, crystallographically, belong to the same 

 species ; of which the Amphiboles, and the Pyrox- 



