JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. 9 FEBRUARY 19, 19 19 No. 4 



APPARATUS. — An apparatus for growing crystals under con- 

 trolled conditions. J. C. HosTETTER, Geophysical Labora- 

 tory, Carnegie Institution of Washington. 



Crystals, to be suitable for the study of the effects of pressure, 

 must be perfectly developed and of comparatively large size. 

 The criterion of perfect development, in this case, is not in the 

 possession by the crystals of those rare faces that so delight 

 the crystallographer, but in the possession of maximum strength. 

 In general, it may be said that the causes which diminish the 

 transparency of those crystals that are normally transparent, 

 also decrease the strength of the crystals. Perfect transparency 

 in a crystal results only when the rate of growth is small and 

 constant, or nearly so, throughout the entire growing period. 

 Nontransparency in a crystal (except that due to the inclusion 

 of foreign sohds), is usually indicative of suddenly increased 

 growth rate with the attendant development of cleavage planes 

 and, frequently, inclusion of mother-liquor. For growing crystals 

 that are suitable for pressure studies there is required, there- 

 fore, apparatus in which all variables affecting rate of growth 

 are under control. The degree of supersaturation in the mother 

 liquor at any time determines the increment of growth;' con- 

 sequently, the conditions affecting supersaturation — primarily, 



^ While this is generally true there are certain important exceptions which can- 

 not be adequately explained at the present time. An interesting case is that of 

 gypsum described by W. J. Fawcett, Proc. Roy. Soc. Canada 7: 218. 1913. 



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