192 Prof. J. A. Pollock on the 



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 con- 

 centration 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 adjustment of the 

 surface takes place, and appearing first as cylindrical pro- 

 tuberances, finally become completely detached, owing to the 

 well-known instability of cylinders, in this connexion, when 

 their length exceeds the circumference. 



Tn 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 preliminary 

 to the disruption of the surfaces. Such an origin would 

 ensure to the small bubbles that layer of contamination 

 essential for their stability, and the explanation of their 

 production here suggested would thus account for their 

 uniform durability. 



Owing to the circumstances of the development, the exact 

 manner of the creation of these small bubbles must remain a 

 matter of some conjecture, and the foregoing description is 

 put forward in no other sense. The surprising feature, 

 which requires explanation, is the fact that the addition to a 

 solvent, which alters the effect of its agitation from a few 

 evanescent bubbles to a. dense lasting froth, may involve, 

 in the value of the surface tension, a diminution, under 

 isothermal conditions, of only about one per cent., an amount 

 altogether too small, if taken by itself, to be considered as 

 changing the conditions. 



The small bubbles, here under review, are characteristic of 



