ORIGIN OF THE SMALL BUBBLES OF FROTH. 223 



that the typical protuberances, from which the bubbles 

 develop, owe their origin to variations in the external 

 pressure on the parent surfaces, due to the motion of the 

 liquid relative to those parts of the larger bubbles. 



3. Finally, there are bubbles, on the whole smaller still, 

 which occur only in cases of the violent agitation of oil- 

 contaminated liquids, or of solutions capable of frothing. 

 These are, indeed, the characteristic bubbles of the froth 

 which is produced in these circumstances. With the special 

 method of agitation which was used in this investigation, 

 these bubbles are shown to be due to the disruption of the 

 lower surface of the main bubble formed by the gas enter- 

 ing the liquid, the disintegration of the surface, in a 0*1 

 per cent, aqueous solution of acetic acid, being seen to 

 be extraordinarily great compared to the slight breaking 

 up of the bubble, under similar conditions, in water. The 

 surprising feature in connection with the occurrence of 

 these bubbles is the fact that the addition to a solvent, 

 which changes 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. The 

 suggestion is made that the disruption of the surface, , 

 which results in the small bubbles, is due to a high gas 

 pressure associated with a weakening of the surface ten- 

 sion in small patches, which may occur in a newly formed 

 surface while the surface concentration is acquiring its 

 full value, or in the case of an oil-contaminated liquid, 

 may be caused by the appearance of small oil spots on 

 the surfaces of large bubbles, resulting from the contact 

 of the bubbles with minute drops of oil disseminated, by the 

 agitation, throughout the liquid. 



In the earlier part of the paper attention is again directed 

 to the fact that the surface energy of a liquid, per unit 



