318 GEORGE F. BECKER AND ARTHUR L. DAY 



saturated at the level of the upper crystal, it will be potentially 

 supersaturated at the level of the lower. In short, the rate of 

 growth of an isometric crystal depends altogether upon whether the 

 concentration of the layer of solution in contact with it is or is not 

 potentially supersaturated with respect to that particular crystal. 

 One serious result of this is that if diffusion toward a certain face is 

 obstructed (e.g., when that face lies against a glass plate), that face 

 will be unable to grow like the other faces. To this we shall 

 revert. 



Now it is the importance of this principle to the question under 

 consideration which has been overlooked by Bruhns and Mecklen- 

 burg. As soon as it is appropriately applied, their observations 

 correlate perfectly with ours. 



The effect upon the saturation concentration of differently 

 oriented faces of the same crystal in non-isometric systems is a 

 matter less well understood and is beyond the scope of this inquiry. 

 We may therefore omit further consideration of it in this connection. 



In plain terms and without taking account of unnecessary com- 

 plications, the situation in a saturated solution, under the condi- 

 tions now under consideration, may be described somewhat in this 

 way. Given a body of saturated solution of a salt in an open 

 vessel, the amount of the dissolved substance which can remain in 

 solution for a given temperature is limited, and it may begin to 

 separate out either when the temperature is changed or when con- 

 tinued evaporation from the free surface of the liquid has sufficiently 

 increased the concentration. Differences in the concentration due 

 to the slowness of diffusion will cause gravitative readjustments, 

 bringing the portions containing the maximum amount of dissolved 

 matter to the bottom. A single crystal of the salt exposed in the 

 bottom of the vessel will now grow upon its exposed faces, and if the 

 rate of evaporation is not too great, this growth may take care of 

 all of the excess of solute resulting from the evaporation process- 

 If the evaporation proceeds at a greater rate, other nuclei will form, 

 and if two or more crystals are feeding upon the product of the 

 evaporation their relative rate of growth (+ or — ) may depend 

 upon relative stability, position, size, or the amount and distribu- 

 tion of load. 



