STREAMING OF FOAM-DROPS EXPLAINED 77 



between the two drops will produce in the quiescent drop, when 

 they have come close enough together, a centre of stream lines 

 opposite to that of the approaching drop. I have often observed 

 this phenomenon, and am convinced that it is to be explained in 

 this manner, and not in any way by the friction of the streaming 

 fluid on the quiescent drop ; for the opposing centre of stream 

 lines in the quiescent drop makes its appearance when there is 

 a pretty considerable interval between the drops, and moreover, 

 the strength of its streaming is too great even at quite an early 

 period to be capable of interpretation simply as a phenomenon of 

 friction. 



The equal action of two drops upon one another is 

 shown, of course, still more intensely when they advance upon 

 one another with their marginal centres of stream lines foremost. 

 For then the more highly concentrated soap solution, which is 

 present at the centre of stream lines of each drop, comes together 

 from the two sides, thus causing a still stronger concentration 

 between the two drops. As has been already described, it can 

 then be plainly observed how the streaming of two drops 

 moving towards one another is gradually strengthened. All 

 these circumstances bring about the result, that two drops which 

 have come fairly close to one another, as a rule soon flow to- 

 gether, if their centres of stream lines do not lie exactly upon 

 opposite sides. 



It was not quite clear to me why the formation of 

 marginal centres of stream lines, in connection with forward 

 movement and amoaboid change of shape, did not usually 

 take place distinctly until the drops were somewhat strongly 

 pressed. The streaming is seen to continue without pressing 

 in the way already described, yet occasionally small drops 

 occur which, without the help of any pressure, develop a 

 centre of extension at their margin, and move about in a 

 lively manner. After being pressed, one or more marginal 

 centres of extension make their appearance, which gradually 

 entirely overcome the first mentioned streamings, and then 

 cause the phenomena already described. It is probable that 

 in drops which are not pressed the causes which give rise to 

 the two polar extension-currents have a much stronger effect 

 than in those which are strongly pressed. In the latter the 

 conditions causing a difference of surface tension at the poles 

 and at the equator become less and less. When, therefore, 



