OPTICAL CHARACTERS OF MINERALS. 113 



chroism. Tourmaline, lolite and Mica are among the most distinct 

 examples. Several varieties of the first are nearly opaque in the 

 direction of the axis, while they show different degrees of transpa- 

 rency, and various colors, as green, brown and blue, in a direction 

 perpendicular to it. Mica is often green in the direction of the axis 

 and brown perpendicular to this line. The application of this prop- 

 erty is greatly extended by examining minerals in polarized light, 

 where many minerals show dichroism, which exhibit in common 

 light, the same color in every direction. 



5. The Tarnish consists in the alteration of the color of a mineral 

 upon its surface. It is useful to attend to this peculiarity of miner- 

 als (common only to such as have a metallic lustre) in order to avoid 

 confounding it with their real colors, which are discoverable only 

 by effecting a fresh fracture. It is frequent in Copper Pyrites. 



6. Simple minerals very seldom present more than one color at a 

 time. Instances occur, however, where the same crystal exhibits 

 two or more, as the red and green in Tourmaline, and the white 

 and purple in Fluor. Compound minerals, on the contrary, are 

 frequently variegated, and the Delineation of Colors, refers to the 

 figures which the different colors produce. With regard to these, 

 it is unnecessary to enter into detail. With regard to the dendritic 

 delineations, it must be recollected, however, that they are real imi- 

 tative forms, (. 77 ) and that, therefore, they do not refer to the 

 mineral upon which they are found : they may be only superficial, 

 or be distributed throughout the whole mass of the specimen. 



. 95. THE STREAK. 



If we scratch a mineral with a sharp instrument, either a 

 powder will be produced, or the scratched place assumes a 

 higher degree of lustre. Both these phenomena are inclu- 

 ded under the term streak. 



The lustre is heightened by the streak in malleable minerals, as 

 also with clay and several other decomposed minerals. 



The best method for observing the color of the powder, is to rub 

 the mineral upon a plate of porcelain biscuit, or upon a file, until 

 the powder appears. In those minerals which are too hard for a 

 process of this kind, the streak is of little importance. 



Some minerals retain their color in the streak ; others change it* 

 the former are most of those minerals which possess a white 



jo* 



