92 hostetter: apparatus for growing crystals 



maintenance of saturation. In the crystallizer, however, the 

 only supersaturation desired is that under control and conse- 

 quently evaporation was prevented by a layer of kerosene 

 floating on the solution. Crystals introduced into the crystallizer 

 were lowered beneath the oil in a bottle. 



It is of great advantage to have large crystals or aggregates of 

 crystals for use in the saturator to serve as "raw material" 

 from which to grow well developed crystals. In the case of the 

 alums, for instance, large-sized material is commercially available, 

 and such is the case with numerous other salts. Where material 

 can not be secured in large particles the finely-divided salt must 

 be used and as this can not be piled up in the saturator like 

 large-sized material, recourse is had to the use of small bags 

 suspended in the solution and refilled from time to time with the 

 fine crystalline material. Under the influence of the oscillating 

 temperature obtaining in the solution in the saturator such 

 finely divided material soon coalesces into aggregates which are 

 entirely suitable for this purpose. 



If the presence of fine thread or of wire is not objectionable 

 in the final crystal the nucleus may be suspended in the solution 

 by such means. This avoids the necessity of turning the crystal 

 each day and also gives a more symmetrical crystal. Crystals 

 grown around such suspensions are usable for a great variety 

 of purposes but as pointed out previously such crystals are not 

 suitable for pressure investigations. 



Even though the crystals grown under thoroughly controlled 

 conditions appear beautifully clear and perfect to the unaided 

 eye, microscopic examination^ frequently reveals the presence 

 of minute inclusions, the causes of which require further study. 

 Also, it may be mentioned here that "isotropic" crystals grown 

 under these conditions often show zones of local strain when 

 examined in polarized light. ^ 



An observation may be recorded here regarding the development 

 of faces on alum crystals. As normally grown such crystals 



* Very kindly made by my colleague Dr. H. E. Merwin. 



" Wright, F, B., and Hostetter, J. C. Journ. Wash. Acad. Sci. 7: 415. 1917. 



