254 PHYSIOGRAPHY. 



. 



In order to express the series of colors, more room is required in 

 the general description than can well be allowed, since to do this 

 fully requires that all the single members be mentioned. As a sub- 

 stitute, however, an outline of these series is given, by indicating 

 their principal points. This mode of treating the subject does by 

 no means injure the use of the series of colors, nor diminish their 

 importance in enlivening the collective descriptions. Colors produ- 

 ced by occasional admixtures of minerals foreign to the species, do 

 not properly belong to the collective description ; because they are 

 not members of the series of colors 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 connect- 

 ed by transitions within the same series. 



The color of the powder, or streak, is next indicated. To which 

 follow the limits for the degrees of transparency, and notice of action 

 on light, (or refraction.) 



Lastly the general descriptions contain the indication of the form 

 of aggregation, of hardness, specific gravity, and other character- 

 istic marks, derived from, or respecting, the substance of minerals, 

 as odor, taste, &c., which may be useful in the description of varie- 

 ties ; all of them expressed with the utmost degree of brevity, 

 possible. 



A great number of the different varieties of certain species is 

 produced by the composition of their individuals. The collective 

 description of the simple varieties, having already been drawn up 

 with considerable minuteness, it will now be the easier to survey the 

 compound varieties. The general consideration of the twin crystals 

 (. 72 & 73), contains the principles of the method, according to 

 which, those belonging to any particular species, may be indicated 

 with precision and convenience. These compositions will also be 

 illustrated, where necessary, by diagrams. 



It will be sufficient only to mention the imitative forms, in order 

 to recal their properties to the memory, these being commonly so 

 much alike in every instance, that they allow of a general explanation, 

 which has been given in its proper place. The condition of their 

 surface, or of the faces of composition in their interior, the shape of 

 the particles of composition, and the mode of that composition itself, 

 may likewise be indicated. The same applies also to amorphous 

 compositions, or, as they are called, to the massive varieties. As to 

 these, the most important properties to be mentioned will be, the 

 shape of the component particles, their size, mode of aggregation, 

 and fracture. Thus we are capable of expressing in a few words> 



