218 J. A. POLLOCK. 



becomes, as a necessary consequence, less than that of the 

 surrounding parts. The conditions here are not isothermal 

 ones, and the lowering of the tension may be greater than 

 that mentioned above. The newly formed bubble is thus, 

 with reference to its surface tension, for a time at least, 

 essentially uneven in texture, and to this fact, no doubt, 

 the disruption of the surface is mainly due. 



The characteristic small bubbles of the froth are only 

 formed when the mass of incoming gas possesses consider- 

 able downward momentum. In this case the pressure on 

 the base of the main bubble is high, and as the conditions 

 are anything but those of equilibrium, the liquid underneath 

 is being strongly accelerated. The consequent disruption 

 of the surface, as shown in the photographs, is so complete, 

 that its development cannot be followed in detail, but a 

 not improbable idea of the production of the small bubbles 

 may be formed by considering that the patches of extra 

 concentration on the base of the main bubble, as they offer, 

 at the moment of their appearance, slightly less resistance 

 than the rest of the surface, yield more to the impact of 

 the gas; in the general violent movement the patches may 

 thus get blown out from the bubble before other adjust- 

 ment of the surface takes place, and appearing first as 

 cylindrical protuberances, finally become completely 

 detached, owing to the well known instability of cylinders, 

 in this connection, when their length exceeds the circum- 

 ference. 



In the case of oil-contaminated water, the first result of 

 agitation will be the dissemination of minute drops of oil 

 throughout the liquid. On coming into contact with large 

 bubbles, these drops will spread out on the surfaces into 

 patches, thus establishing that weakening of the surface 

 tension in spots, which, according to the description just 

 given, is, under highly kinetic conditions, but the pre- 



