1 64 Separation of Solids in Surface-layers of Solution x. [, 1 1 1 1 . 



(1) yielding "mechanical surface aggregates;" (2) intense special 

 superficial viscosity ; or (3) forming persistent bubbles. 



It will be seen that there is a very considerable parallelism in the three- 

 phenomena. Exact parallelism could not be expected, since the different 

 physical properties of the surface accumulations must necessarily affect 

 the phenomena in different and highly complex ways. Special super- 

 ficial viscosity has only been recorded when very intense, and all minor 

 degrees have been ignored, since the presence of dust, etc., in minute 

 quantity may, as has been shown by Lord Rayleigh,* produce some 

 " superficial viscosity," and it has been practically impossible entirely 

 to exclude such contamination. Power to form persistent bubbles has 

 been shown by either : 



i. The possibility of blowing 2-inch bubbles, 

 ii. The formation of a froth. 



iii. The formation of small bubbles which last in closed vessels at 

 least 30 minutes. 



In all cases the substances employed have been of the greatest 

 attainable purity, the de-soluted solids have been shown to consist of 

 the same material as that in solution or suspension, and numerous 

 control experiments have been made. 



* ' Boy. Soc. Proi-.,' vol. 48, pp. 127110, 1890. 



