ORIGIN OF THE SMALL BUBBLES OF FROTH. 221 



acetic acid solution has a surface tension which decreases 

 as the concentration increases, so the photographs of Plate 

 X may be taken as exhibiting the bubbles in three typical 

 liquids, and, by showing the similarity of the detail in the 

 three cases, proving a common method of production. 



Here the development, previously suggested as that of 

 the small bubbles of froth, is clearly apparent ; cylindrical 

 protuberances are formed on the larger surfaces which 

 lengthen until the state of instability is reached, they then 

 become detached as separate bubbles. 



In seeking for an explanation to account for these pro- 

 trusions into the liquid of the surfaces of the gas bubbles, 

 any consideration of a local weakening of the surface 

 tension seems here out of the question. Firstly, a cause 

 is not apparent for an accidental diminution of the surface 

 tension in somewhat large patches, confined within narrow 

 limits as to size ; secondly, the effect is shown both with 

 tap and distilled water, used with every precaution to avoid 

 contamination, and no difference occurs when the gas is 

 carefully filtered; moreover, bubbles of the class under 

 consideration, produced in water and in non-frothing solu- 

 tions, show no special durability. That the phenomenon 

 is not directly dependent on any directed momentum of the 

 gas is proved by the fact that in the photographs the pro- 

 jections are shown on the surface of closed bubbles, in 

 which the gas can have little, if any, general momentum 

 relative to any part of the surface. 



The outline in the photographs is fairly sharp as the 

 exposure was of the order of a millionth of a second, but 

 in each case the liquid was in a state of considerable turbu- 

 lence. With this fact in view, it is suggested that the 

 bubbles, whose development is shown, owe their origin to 

 variations in the external pressure on the parent surfaces 

 due to the motion of the liquid relative to these parts of 



