6o PHYSIOGRAPHY. [chap. 



same salt, gradually grows larger by deposition of new alum 

 in a solid form from the surrounding medium. This method 

 of growth is therefore entirely different from that by which 

 the growth of a living body is effected. It is as though a 

 man could actually grow bigger by putting on coat after 

 coat, instead of growing by the ordinary process of nutrition 

 from within. 



Just as there is nothing distinctive about the size of an 

 individual crystal, so there is nothing distinctive about the 

 size of the several faces of the crystal. A six-sided spire or 

 pyramid of rock-crystal may have one face very large, and 

 the next face so small as to be little more than a mere line. 

 Size of crystal and size of face thus count for nothing, but 

 what does tell for something in studying crystals is the slope 

 or inclination which one face has to another ; in other words, 

 the angle made by two neighbouring faces. A set of faces 

 symmetrically related, such as the six faces of the prism of 

 rock-crystal, is called technically a form ; and the faces of 

 any given form, however irregular in size and shape, are 

 always inclined to one another at the same angle. 



Although it is not every substance that can assume these 

 regular forms, yet by far the larger number of bodies, in- 

 cluding water, are capable of crystallization. When water 

 solidifies, by reduction of temperature, the particles group 

 themselves in definite directions, and thus produce regularly- 

 shaped solids, closely related to those of the rock-crysi-al. 

 In fact the forms of ice and the forms of rock-crystal are 

 characterised by the same kind of symmetry ; a symmetry 

 which is such that each crystal may be divided into six 

 similar parts. The best examples of this hexagonal sym- 

 metry in solid water is furnished by crystals of snow. 



If the air during a snow-storm be still, each flake that 

 falls will be found to exhibit a regular shape. A perfectly- 



