THE LINEAR FORCE OF GROWING CRYSTALS 319 



Inasmuch as a crystal lying upon the bottom of an evaporating 

 dish must rest upon one of its faces, this face will support a load 

 represented by the weight of the crystal less the buoyancy correc- 

 tion, and this load will impose limiting conditions upon the rate of 

 growth upon this face when compared with the neighboring faces, 

 as was pointed out in our paper in 1905, and by Bruhns and Meck- 

 lenburg in 1 9 13. A weight placed upon the crystal merely adds 

 something to the total load supported by this face (and perhaps 

 covers an additional portion of the crystal surface), but contributes 

 no new factor to the problem. If this superimposed weight is very 

 large, the distribution of the resulting strain may become important, 

 but these are questions of degree only. Limitations of circulation 

 or diffusion in the capillary liquid layer below the crystal are 

 affected by the amount and distribution of the aggregate load, and 

 not at all by its character (whether crystal substance or foreign 

 matter). This fact would hardly appear to require demonstration, 

 but has certainly caused some confusion, nevertheless. 



It is then clear that the exposed top and side faces (or the side 

 faces alone if the top is covered) may grow freely while the bottom 

 remains more or less undernourished, depending upon the load 

 which is supported there and the consequent impairment of circu- 

 lation. Nevertheless, if the degree of supersaturation and the 

 amount of material which is being furnished to the crystal through 

 evaporation and diffusion is sufficient in quantity and properly cir- 

 culated, the saturation concentration opposite that face also, that 

 is, in the thin layer of liquid upon which the crystal rests, may be 

 reached and the crystal may grow upon the bottom as well as upon 

 the sides. Failure of the circulation in this supporting layer may, 

 and in fact usually will, restrict the growth here to the periphery 

 of the supporting face, causing it eventually to rest upon a thin 

 outer rim 1 of new growth rather than upon its initial flat surface, 

 but growth will nevertheless take place here as elsewhere. The 



1 As was pointed out in our paper in 1905, these supporting rims are often so thin 

 as to debar the usual methods of area measurement. At that time an approximate 

 measurement was obtained by inking the crystal with an insoluble ink and printing 

 its impression upon a plane glass plate coated with white celluloid. The impressions 

 thus secured contain lines so fine as to defy reproduction by the usual means and 

 probably yield but a rough approximation of the surface area which supports the load. 



