EXTENSION-CURRENTS 61 



pressed entirely, by the formation of a new centre of stream- 

 ing after a short time in their vicinity. 



Several makings and breakings in quick succession always 

 produce a fairly energetic bursting of froth vesicles in the 

 interior of the drops a phenomenon which naturally makes 

 itself still more obvious with the employment of the inter- 

 mittent current. By the action of the intermittent induction- 

 current, using non-polarisable brush electrodes, it could 

 be determined with certainty that, as a result, very violent 

 extension-currents appear at the two spots of the edge of the 

 drop, which face towards the poles. On interrupting the 

 current they soon die down, and on closing it they quickly 

 begin again. This phenomenon harmonises well with the 

 positive streaming observed with the constant current, 

 since in the intermittent induction-current the poles change 

 quickly, and hence the action of the positive pole must show 

 itself on each side. 



7. The probable Explanation of the Streaming Movements 

 exhibited by the Drops of Foam 



In order to arrive at an explanation of the peculiar and 

 long continuing phenomena of the streaming in the drops 

 of foam, we must first consider their structure again. As 

 has been remarked, they consist of a framework of very 

 minute lamella of oil, the meshes of which are filled by a 

 watery fluid. It follows from the method in which the 

 froths are formed, that this fluid must be a watery solution 

 of K 2 C0 3 and potash soap, formed by the action of 

 potash upon the free fatty acids of the oil, or it may be 

 upon the glycerides themselves. If the froth-drops have 

 been cleared up in glycerine, the alveoli then contain a 

 solution of glycerine containing soap and K 2 C0 3 . 



The streaming phenomena of the froth-drops take place 

 on the whole in the fashion of the so-called superficial 

 extension-currents (Quincke, 1888; emulsion movements, 

 Berthold, 1886 ; contact movements, Lehmann, 1888), which 

 arise regularly whenever the surface tension of a fluid (#), 

 placed in air, or in a second fluid (&), is locally diminished 



