CHARACTERISTIC. . 



it will be a useful rule for beginners to occupy themselves 

 at first with the determination of such individuals as pre- 

 sent properties, which may be easily and fully investigated. 

 The rest will come of itself, when their knowledge of the 

 Mineral Kingdom, and particularly of the properties of 

 minerals, increases, and when they have, by experience, 

 acquired the skill to judge properly of form and cleavage, 

 at least so far as is necessary for the determination of the 

 system of crystallisation, even in those cases where form 

 and cleavage are somewhat difficult to be observed. This 

 exercise is particularly recommendable to every person 

 who intends to acquire a satisfactory knowledge of mine- 

 rals, with the help of the Characteristic. 



. 251. IMMEDIATE AND MEDIATE DETERMINA- 

 TION. EXAMPLE. 



If a mineral can be determined without the help 

 or intervention of one or of several other minerals, 

 the determination is said to be an immediate one. 

 If, on the contrary, we must employ one or several 

 other minerals for this purpose, we only obtain a 

 mediate determination. 



The immediate determination has been explained and 

 illustrated by an example in . 250. An example will 

 likewise be useful in the mediate determination. 



The variety of hemi-prismatic Augite-spar, which has 

 received the name of Amiantus, occurs in such very delicate 

 crystals, that even supposing they should be regular, their 

 form could not be observed, even through the most power- 

 ful magnifying instruments : it is the same with cleavage. 

 The crystals are flexible, like fibres of flax, their hardness 

 accordingly not to be ascertained. Their surface is so large 

 compared to their bulk, that wherever they may be placed in 

 water, or in another liquid, they neither sink nor rise : 

 although their specific gravity is not inconsiderable ; but 

 we halve no means to ascertain it. However, there are va- 



