Simpler Methods in Crystallography. 



341 



may be distinguished as v., vr., vii., viii., beginning at the 

 bottom left hand, and going counter-clockwise. Next, we 

 have to take into account the fact that for crystals of each 

 species certain directions are determined by the behaviour of 

 the crystal in relation to heat, light, electricity, magnetism, 

 etc. — in other words, in relation to its molecular constitu- 

 tion, and that on this basis is decided what direction of the 

 crystal shall be referred to a, h, c respectively. Further, 

 that in each species of crystal the relative lengths of these 

 axes, based upon the forms observed, is defined for each 

 species. It is these relative lengths, b being taken as unity, 

 which form the axial elements of the crystal, to which, in 



r/c.z. 



the Monosymmetric and the Anorthic crystals, has to be 

 added the angular elements. Let us, as before, assume that 

 it is required to make a map of Barytes, as viewed from 

 above — in other words, a c projection. The relative lengths 

 of the axes are — a, '8152; b,l; c, 1*31359, and the angular 

 elements 90° in all three directions. Therefore, on ox, 

 •815 of the radius is carefully measured on either side of 

 0, and a line is drawn from h, the radius on OY, through a 

 to the opposite part of the primitive, and a perpendicular 

 erected on that through o. It is well to do this in quadrant 

 V. only. The line, as a whole, may be signed M, and the 

 point where it cuts the primitive m (110). Set off the 



