??2 FORMATION OF CRYSTALS. 



the starting of a fresh, formation at those points. * A curious 

 example of the like kind in the crystallization of Sulphate of 

 Copper to which a small quantity of Sulphate of Magnesia has 

 been added, is shown in Fig. 410. The same principle has been 

 carried out to a still greater extent in the case of Sulphate of 

 Copper alone, by Mr. 11. Thomas, f who has succeeded, by keeping 

 the slide at a temperature of from 80° to 90°, in obtaining most 

 singular and beautiful forms of spiral crystallization, such as that 

 represented in Fig. 411. — The following List specifies the Salts 



Fig. 411. 



Spiral Crystallization of Sulphate of Copper. 



and other substances whose crystalline forms are most interesting. 

 When these are viewed with Polarized light, some of them exhibit 

 a beautiful variety of colours of their own, whilst others require 

 the interposition of the Selenite plate for the development of 

 colour. The substances marked d are distinguished by the curious 

 property termed Dichroism, which was first noticed by Dr. Wol- 

 laston, but has been specially investigated by Sir D. Brewster.J 



* See Davies on 'Crystallization and the Microscope,' in "Quart. 

 Journ. of Microsc. Science," N.S., Vol. iv., p. 251. 



+ See his paper ' On the Crystallization at various Temperatures of tbe 

 Double Salt, Sulphate of Magnesia and Sulphate of Zinc,' in "Quart. 

 Journ. of Microsc. Science," N.S., Vol. vi., pp. 137, 177. See also 

 H. N. Draper on ' Crystals for the Micro-Polariscope,' in " Intellectual 

 Observer," Vol. vi. (1865), p. 437. 



t " Philosophical Transactions," 1819. 



