EXPLANATION OF THE MOVEMENTS 67 



is driven forwards. I think that the inadmissibility of this 

 view is fairly clear. If the forces which are the cause of the 

 streaming movements had their seat in the interior of the 

 drop, then such friction at its surface would be quite pos- 

 sible. As a matter of fact, however, this is by no means 

 the case, but the active forces take origin at the limiting 

 surface between the drop and the surrounding water, and 

 produce in the latter exactly the same streaming as they 

 also impart to the surface of the drop. Under these 

 circumstances, therefore, the possibility of friction between 

 the streaming surface of the drop and the surrounding water 

 appears excluded. Mensbrugghe believes, as has been re- 

 marked, that the forward movement, as well as l the whole 

 phenomenon in general, depends on chemical attraction, which 

 produces, in opposition to the surface tension, forces of pres- 

 sure directed outwards towards the approaching soap. Un- 

 fortunately I am as little able to agree with this opinion, 

 since I convinced myself by numerous experiments that the 

 whole complex of phenomena can also be called forth in 

 the very same manner with a body so chemically unalter- 

 able as paraffin oil. Commercial paraffin oil was treated 

 with concentrated sulphuric acid at 100 C., and well 

 washed out, and then also showed just the same phenomena. 

 N"ow since it cannot be well assumed that paraffin oil 

 and dilute soap solution have any appreciable chemical 

 action one upon another, it seems beyond doubt that the 

 processes described must depend only upon purely physical 

 causes. 



The explanation of the forward movement of the oil- 

 drop towards the soap solution may be sought, according to 

 my idea, in something of the following kind. By the 

 action of the extension-current some of the soap solution is 

 being continually drawn away from its point of contact with 

 the drop, and carried off to the hinder end of the drop. 

 For this soap solution abstracted in front a compensation 

 must be made by the neighbouring fluids, and this will of 

 course be effected by the general pressure within the fluid, 

 which works equally upon the soap solution, the water, and 

 the oil. The easiest way of representing such a condition is 



