CHAPTER VIII 



ON THE MEASUREMENT OF CRYSTALS AND THE USE OF THE 



GONIOMETER 



IT is often necessary to measure the angles between two crystal 

 faces in order to identify the forms present on the crystal, partic- 

 ularly when the specimen is much distorted. . This may be true 

 even with such well-developed and easily recognized forms as the 

 prism and rhombohedron on quartz. The forms present in com- 

 plicated combinations must always be proven by the measurement 

 of the angles between the faces. The measurement of the angles 

 will also help in the identification of the mineral species ; and in 

 chemical compounds their variation from the theoretical value may 

 afford a means of estimating the purity of the compound, as chemi- 

 cally pure substances possess a constant and characteristic angle be- 

 tween crystal forms, though in isomorphous groups these angles are 

 very nearly equal ; yet when pure each member of the group will 

 possess an angle distinctly its own. 



The goniometer and the principles of the reflecting goniometer 

 have been described in Chapter I. 



For the identification of crystal forms when the crystals are not 

 too small, the Penfield card contact goniometer, model B, Fig. 

 11, is a very convenient and sufficiently accurate little instru- 

 ment, and it has the advantage of cheapness, so that each student 

 may be provided with or possess one. It answers also as a protrac- 

 tor and scale in the drawing of crystals. 



In using the instrument, the card with the scale is held at right 

 angles to the edge and one of the crystal faces, the angle between 

 which and the adjacent face it is wished to measure. The arm of 

 the instrument is then rotated until it is in contact with the second 

 crystal face ; the crystal with the goniometer in place is now held 

 up to the light, with the line of vision parallel to the edge between 

 the two faces; the arm and card are carefully adjusted to fit the 

 two faces and the angle between them, so that no light is seen be- 

 tween the instrument and the crystal faces. The instrument must 



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