80H 



Professor Sir James Dewar 



[Jan. 21, 



For a more extended study longer films must be used. These 

 <ri-aded blacks appear to be rather unstable. At times they become 

 grained and indefinite under slight stimulus of vibration or change 

 of temperatu]-e. A very striking effect is produced by cautiously 

 flowing a Httle solution 'through the lower part of the graded film ; 

 the various grades then become mottled, and coalesce with rapid 

 aggregation to the stable silvery boundary. This effect sometimes 

 occurs^ apparently spontaneously. With solutions of ammonium 

 oleate, slight excess of tension of ammonia seems to be a predisposing 

 cause, easily brought about by a small elevation of temperature. 



The form of graph observed in such a case is next shown. The 

 film started with development of three grades of lilack, which sud- 

 denly coalesced to the common single stage, which then extended 

 at a fairly uniform rate. 



Rate before coalescence 



Rate after coalescence 



Fig. 14. 



