CRYSTALLOGRAPHY 



9 



Fio. 7. Crystals of Silver 

 obtained by Electrolysis. 



crete cases. The metals which arc easily reduced rlc< trolytically 

 arc deposited on theHftutffin crystalline form ; these are, however, 

 always distorted and irregular throii.iih structural anouialie>. With 

 :i very weak current good crystals of copper, silver, or lead may \>v 

 obtained, Fig. 7. Crystallization is a 

 method employed in the separation and 

 purification of chemical compounds, and 

 especially is this so in the commercial 

 field, where efficiency and cheapness are 

 factors of such great importance. Gran- 

 ulated sugar, one of the few chemical 

 compounds produced in enormous quan- 

 tities in almost absolute purity, is sepa- 

 rated by crystallization. 



The purity of a crystalline compound 

 will depend upon the rate of separation, 



the viscosity of the mother liquid, and its solubility. If perfect 

 crystals are sought, great care must be exercised in the control of 

 the growth of the crystals, the deposition of molecules must go 

 on very slowly. If there is a sudden decrease in temperature of 

 the solution, a heavy shower of molecules upon the forming crys- 

 tals results; they will increase more rapidly along the edges at 

 the expense of the center of the faces, producing skeleton crystals. 

 The hollow faces may ultimately build out, leaving interior cavi- 

 ties filled with mother liquid. All foreign matter incorporated in 

 the body of a crystal, whether of liquid, gas, or solid, is known as 

 an inclusion. The purity of a crystalline salt is inversely propor- 

 tional to the rapidity of formation and to the size of the crystals. 

 When a pure salt is required, it is best to let the crystals form 

 slowly and remove them from the mother liquid while still small. 



Constancy of angles. The size of a crystal and the general 

 shape will depend to a large extent upon the conditions prevailing 

 at the time of its formation ; the question may be asked, if the size 

 and shape of a crystal is variable, is there anything that is constant 

 upon which the science of crystallography may be based? Nico- 

 laus Steno, a Danish geologist, in 1669, while cutting sections of 

 quartz crystals, noticed that, however variable the outline of the 

 sections may be, due to irregularities of growth and to the difference 

 in size of faces, whenever the sections from the various crystals 

 were cut in a parallel direction the corresponding angles were 

 always equal. The ordinary quartz crystal is terminated by six 



