HOSTETTER: apparatus for growing CRYSTAlvS 91 



and becoming saturated. Before the solution returns to the 

 crystalhzer it must pass down between more crystals into the 

 platinum gauze filter at (F) ; thence to the crystalhzer. En 

 route it is cooled slightl/ in the water- jacketed tube before enter- 

 ing the crystalhzer. The supersaturated current now passes 

 over the nuclei at (N) and excess material deposits thereon. 

 The cooled solution returns through (R) to the saturator. The 

 currents caused by the auxiliary stirring (A and A]) in the thermo- 

 stats are such as to assist the main circulation between the two 

 thermostats. 



Instead of depending upon chance for the formation of nuclei 

 it is far better to introduce small well-formed crystals which 

 have been developed elsewhere. If a warm saturated solution 

 is allowed to cool overnight there will usually be some small 

 crystals that are suitable as nuclei. Where these crystals have 

 been in contact with the bottom of the vessel there is under 

 each one a small terraced cavity. This has been brought about 

 partly by the fact that mother-liquor did not have access to the 

 bottom surface during growth. While growing in this manner 

 the original nucleus may be lifted several millimeters. When 

 these crystals are placed in the crystalhzer they should be oriented 

 with the cavity on top. New growth will soon build up the top 

 to a plane surface. Each day the crystal should be turned 

 over so as to avoid the development of deep hollows under- 

 neath. The crystals should likewise rest on plane glass and not 

 on the irregular bottom of the battery jar. Before introducing 

 crystals into the crystalhzer it is advisable to dip them into 

 a saturated solution (as in the saturator) and wash off loose 

 particles. 



It has been mentioned that the rate of growth is also influenced 

 by the number of nuclei. With a temperature difference of 

 0.3-0.5° and 5 nuclei present the rate of growth for potash 

 alum was about i.o mg. per hour per sq. cm. of crystal surface 

 exposed to the solution. 



The surface of the solution in the saturator was uncovered 

 and evaporation allowed to take place — thus helping in the 



