THE GONIOMETER. 



431 



we have three axes differing in length, and all forms which can be arranged 

 about these unequal and oblique axes. Sulphate of copper will be found 

 in this group. The system has been called anorthic, or 

 triclinic, because the axes are .unequal and inclined, as in 

 the oblique prism based upon an obliqued angled parallelo- 

 gram. Axinite crystal, as annexed, will show one form 

 in this system. 



As may be gathered from the foregoing, it is not easy 

 to determine a crystalline form with certainty, a great part 

 of the crystal may be invisible. A crystalline mass is a 

 mineral, which consists of an arrangement of crystals heaped 

 together. If it do not possess these the mineral is 

 amorphous, or shapeless. We will now endeavour to describe some of the 

 physical characteristics of minerals. 



Fig. 454- Sixth system. 



Fig. 455. Wollaston's Goniometer, an instrument fo: 1 measuring the angles of crystals. 



The GONIOMETER (sec fig. 455) is the instrument used for measuring 

 the angles of crystals. Wollaston's reflecting instrument is most generally 

 used. It consists of a divided circle, graduated to degrees, and subdivided 

 with the vernier. The manner of working is easy, though apparently com- 

 plicated. The vernier is brought to zero, when an object is reflected in one 

 face of the crystal. The crystal is turned till the same object is viewed from 

 another face. The angle of reflection is then measured, and can be read off 

 on the circle. 



We have already referred to the physical characteristics of the minerals, 

 and one of these attributes is co/iesion. When we find a substance is difficult 

 to break, we say it is " hard." This means that the cohesion of the different 

 particles is very great. Minerals vary in hardness ; some are extremely 

 difficult to act upon by force, and a file appears useless. At the other side 

 we find some which can be pricked or scratched with a pin ; and these 



