1JJ03.] Stir face-layers of Solutions and " Suspensions" 159 



opalescent, hangs for some 5 or 10 seconds from the edge of the 

 supporting tube. 



9. The presence of such solid membranes on a bubble must con- 

 tribute greatly to its persistence. Further, the mere presence of 

 solid particles on the surfaces of a bubble has, in many cases, been 

 found to add greatly to its persistence^ even when, judging by the 

 absence of a special resistance to shear in the film of a free surface of 

 the solution, such particles are not appreciably coherent, but lie loosely 

 side by side (e.g., in suspensions of sulphur, picric acid, quinine 

 bisulphate, salicylic acid). 



In the cases quoted, a bubble of air can be seen to pick up the 

 particles in suspension as it passes through the liquid, and to retain 

 them obstinately when it reaches the surface and comes to rest, so that 

 the bubble becomes thickly coated with solid particles, although the 

 liquid contained only a small quantity of suspended solid, and this 

 solid is specifically heavier than the solution. 



10. Every solution capable of forming moderately persistent bubbles 

 which has hitherto been examined has yielded solid or highly viscous 

 mechanical surface aggregates. This very remarkable fact indicates 

 that the power of forming such bubbles is due to the presence of 

 matter which has accumulated at the free surfaces in a solid or highly 

 viscous condition. The cohesion of the matter, so as to form an 

 appreciably coherent membrane, is apparently not essential, but it 

 occurs in most cases where the bubble is very persistent. 



Plateau recognised this association of the power of forming persistent 

 bubbles with a special superficial viscosity and a diminished surface- 

 tension, but did not connect these phenomena with the formation 

 of a coating of matter derived from the solution and specially con- 

 centrated at a free surface.* 



On theoretical grounds the presence of a thin film of liquid, even of 

 low viscosity, on the free surfaces, should also be capable of increasing 

 the persistence of a bubble if the liquid be such as diminishes the 

 surface energy of the mother solution, but increased persistence 

 brought about in this way appears to be very slight compared with 

 that occurring when the coating consists of solid or highly viscous 

 particles. 



11. The effects which the presence of solid particles in the surface 

 layer exercises upon the persistence, size and other properties of 

 bubbles, depend on many factors. Many solutions in which surface 

 accumulation of solid undoubtedly takes place have been found in- 

 capable of forming large or persistent bubbles. The size of the 

 particles, their surface-tension relations, the rate of their accumulation r 

 the rate of their re-solution when forced by mechanical means into 

 local excess, and the elasticities and flexibility of any membrane 



* rifle ' Slatique lies Liquides,' pp. 69 71. 



