414 WRIGHT AND HOSTETTER - . CRYSTAL GROWTH 



path difference by the insertion of a sensitive tint plate. Under 

 these conditions the parts of the crystal under tension appeared 

 first order yellow, the neutral portions purple, and the parts un- 

 der compression second order blue to blue green. 



Deposition on each of the vertical crystal faces of the crystal 

 under load took place at the rate of 0.12 mm. per hour, i.e., 

 the crystal increased in thickness every half hour 0.12 mm. 

 (0.06 mm. on a side). The interference colors in the newly de- 

 posited layers were identical with the colors in the original crys- 

 tal and no line of demarcation was visible. At a point where 

 the edge of the original crystal exhibited a neutral or sensitive 

 violet color, there the layer in process of deposition on that point 

 showed the same interference color, and so on for yellow and 

 blue interference colors. The gradations from the zone of com- 

 pression to that of tension in so far as they extended to the 

 outer vertical surface under examination were conformable with 

 the gradations in the original crystal. In short there could be 

 no doubt that the material on deposition from the supersatu- 

 rated solution entered into the crystal state in the state of strain 

 of the crystal atoms to which it became affixed. So far as meas- 

 urements could be made, the change in path difference for a load 

 of 20 kg. per square centimeter is about 200 mm per 1 cm. of 

 material traversed. This value is not accurate but is only a 

 rough approximation. On release of the load the crystal re- 

 turned to its original condition. 



After one hour's growth under these conditions the load was 

 increased to 40 kg. per square centimeter; the path difference 

 increased to about double the former value and the zones of 

 compression and tension were more sharply marked, with a nar- 

 row zone of gradation between the two. As before, the freshly 

 deposited layers entered into the crystal state in the condition 

 of strain of the part of the crystal on which they were in contact. 

 The load was increased to 60 kg. but this exceeded the crushing 

 strength of the crystal, which broke into fine powder on yielding. 



Other experiments with other crystals yielded similar results 

 and need not be described in detail. 



