. 250. CHARACTERISTIC. 385 



It is not difficult to arrive in this way at the determina- 

 tion of an individual, provided those properties can be as- 

 certained, which the complete determination requires. The 

 determination will be defective only in consequence of the 

 impossibility of observing at all, or at least with sufficient 

 accuracy, one or more of the characteristic marks in the 

 mineral. 



In illustration of this, let us take the following example. 

 Let the form of the mineral which is to be determined, be a 

 combination of a scalene eight-sided pyramid, of an isosceles 

 four-sided pyramid, and of a rectangular four-sided prism ; 

 the cleavage parallel to the faces of two rectangular four- 

 sided prisms, in diagonal position to each other ; form and 

 cleavage therefore pyramidal, or belonging to the pyramidal 

 system. Let Hardness be = 6-5 ; Specific Gravity = 6*9. 



In this case, both hardness and specific gravity are pro- 

 minent characters, and exclude the individual at once from 

 the first and third, but not from the second class : with 

 the characters of this class, its other properties also perfectly 

 agree. Hence the individual belongs to the second class. 



Comparing the properties of the individual with the cha- 

 racters of the orders in the second class ; hardness and spe- 

 cific gravity will be found too great for the order Haloide ; 

 hardness too great for the orders Baryte and Kerate ; both 

 of them too great for the orders Malachite and Mica; 

 and specific gravity too great for the orders Spar and Gem. 

 But in the character of the order Ore, both hardness and 

 specific gravity fall between the fixed limits, and cannot 

 exclude the individual from this order. The other parts of 

 this character are now to be taken into consideration. If the 

 appearance of the individual be metallic, its colour must 

 be black, otherwise it cannot belong to the order Ore. But 

 the appearance is not metallic ; therefore the colour of the 

 individual is quite indifferent ; that is, this conditional cha- 

 racteristic mark does not affect the individual, and conse- 

 quently cannot decide. Since the appearance is not me- 

 tallic, the individual must exhibit adamantine or imperfect 

 metallic lustre. The first will be found, particularly in the 



VOL. I. 2 B 



