CRYSTALLIZATION. POLARIZATION. 321 



decided form (Fig. 496) ; but when the slip of glass is cool, the crytals 

 are exceedingly minute. It would seem as if these last results were due 

 to interruptions in the formative process at certain points, consequent 

 upon the hardening influence of cold, and the starting of a fresh forma- 

 tion at those points. 1 A curious example of the like kind in the crystal- 

 lization of Sulphate of Copper to which a small quantity of Sulphate of 

 Magnesia has been added, is shown in Fig. 497. The same principle has 

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

 alone, by Mr. K. Thomas, 2 who has succeeded, by keeping the slide at a 

 temperature of from 80 Q to 90, in obtaining most singular and beautiful 

 forms of spiral crystallization, such as that represented in Fig. 498. 

 Mr. Slack has shown that a great variety of spiral and curved forms can 

 be obtained by dissolving metallic salts, or Salicine, Santonine, etc., in 

 water containing 3 or 4 per cent of colloid Silica. The nature of the 

 action that takes place may be understood by allowing a drop of the 

 Silica-solution to dry upon a slide; the result of which will be the pro- 



Fio. 498. 



Spiral Crystallization of Sulphate of Copper. 



duction of a complicated series of cracks, many of them curvilinear. 

 When a group of crystals in formation tend to radiate from a centre, the 

 contractions of the Silica will often give them a trangential pull. 

 Another action of the Silica is to introduce a very slight curling with 

 just enough elevation above the slide to exhibit fragments of Newton's 

 rings, when it is illuminated with Powell and Lealand's modification of 

 Prof. Smith's dark-ground illuminator for high powers, and viewed with 

 a l-8th Objective. With crystalline bodies, these actions add to the 

 variety of colors to be obtained with the Polariscope, the best slides 



1 See Davies on Crystallization and the Microscope,' in " Quart. Journ. of 

 Microsc. Sci.," Vol. iv., p. 251. 



2 See his paper ' On the Crystallization at various Temperatures of the Double 

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

 Sci.," 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. 



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