86 hostettsr: apparatus for growing crystals 



temperature and evaporation — must be under definite control. 

 Of lesser importance — but nevertheless, essential — ^are the di- 

 rection of concentration currents, and the number of cr3^stals 

 which serve as nuclei for growth. When these variables are 

 controlled it is not a difficult task to grow very perfect crystals 

 of large size. 



In this note we will confine ourselves to the growth of soluble 

 substances from solution, and merely mention the growth of 

 very slightly soluble substances in crystal form, which has been 

 developed very thoroughly,'- and the growth of crystals from 

 melts. ^ 



Neglecting the effects of hydrostatic pressure, it may be said 

 in general that there are four methods of producing super- 

 saturation in a saturated solution and, hence, growth of a crystal 

 immersed therein. In a solution saturated with respect to a 

 certain crystal phase at a definite temperature we may produce 

 supersaturation by (i) lowering (or, in rare cases, raising), 

 the temperature, (2) allowing the solution to evaporate, (3) 

 dissolving in this solution held at constant temperature, ex- 

 tremely finely divided particles of the crystalline phase, or (4) 

 adding another solvent in slight amount.^ It is quite evident 

 that a crystal-growing apparatus based on any, or a combination 

 of all, of these principles will be satisfactory if the variables are 

 properly controlled, but in most of the crystal-growing devices 

 described in the literature, the governing of some of the essential 

 conditions has been left entirely to chance. Practically, de- 

 vices based on temperature change are easier to control and, 

 probably, the most easily constructed. The apparatus finally 

 developed and described below is based on this principle. 



^ Almost all methods for the growth of "insoluble" precipitates in crystal form 

 are based on diffusion processes. For a discussion of the essential conditions see 

 Johnston. Journ. Amer. Chem. Soc. 36: 16. 1914. 



3 R. Nacken has recently described apparatus for growing crystals from melts; 

 but the crystal nucleus is supported on a wire which becomes enclosed by the crystal 

 as growth proceeds. (Neues Jahrb., 1915, II, 145.) Such crystals are, for this 

 reason, not suitable for pressure studies. 



^ As alcohol to an aqueous solution of a sulfate. 



