Origin of the Small Bubbles of Froth* 195 



III. Summary. 



The object of the investigation was to find the origin of 

 the small bubbles which constitute the characteristic feature 

 of the froth which is produced by violent agitation. The 

 froth is readily created when gas under pressure is forced 

 into a liquid through the orifice of a vertical pipe ; and in 

 order to study the conditions of its formation, instantaneous 

 shadow photographs have been taken of the jet of gas in 

 various solutions. It is seen from an inspection of the 

 photographs, that the bubbles which result from the breaking 

 up of the jet may be separated, with respect to the exact 

 manner of their production, into three classes. 



1. Large bubbles are created by the partition of the main 

 bubble which is being continuously formed -on the end of the 

 pipe, considerable portions of the gas being " pinched off," 

 as it were, by the action of currents in the liquid, or, having 

 lost their downward momentum, becoming separated from 

 the mass of incoming gas owing to their buoyancy. 



2. The photographs accompanying the paper show small 

 bubbles which are clearly the result of the detachment of 

 cylindrical protuberances formed on the surfaces of the larger 

 bubbles, the protuberances having lengthened until the state 

 of instability has „been reached. These bubbles are seen in 

 various stages of their attachment to the parent surfaces, and 

 are shown to occur in water, and in a non-frothing as well as 

 in a frothing solution. It is considered 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 entering 

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

 cent, aqueous solution of acetic acid, being seen to be extra- 

 ordinarily great compared with the slight breaking up of the 

 bubble, under similar conditions, in water. The surprising- 

 feature in connexion 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 



02 



