EXTENSION-CURRENTS 73 



in this current we are really dealing with a phenomenon of 

 extension, as appears to be the case, it would then of course 

 follow that the surface tension between water and oil when 

 warmed is greater than that between the two fluids when not 

 warmed. With regard to the influence of temperature upon 

 surface tension, it is known in general that the latter 

 diminishes with increase of temperature ; but these state- 

 ments only have reference to the surface tension with air. 

 That the process described should be produced secondarily 

 by streamings in the surrounding water I consider very 

 improbable. 



The duration of the extension-currents described is, for 

 the most part, relatively short, when the experiment has 

 been arranged as prescribed, although the streamings 

 which are produced by means of soap solution in drops 

 of olive or paraffin oil not infrequently persist for hours, 

 during which the drops carry out extensive migrations. 

 Since, however, under these conditions, it is frequently 

 rather a matter of uncertainty whether some other causes, 

 such as pressure and the 

 like, may not produce 

 streamings in the drops, I 

 arranged an experiment, 

 which proved quite suit- 

 able, in order to obtain 

 streamings of long dura- 

 tion, as follows. A fine 

 capillary tube, partly filled 

 with soap solution, and 

 then sealed up at one Fig> 13 . 



end, is pushed towards 



a drop of paraffin mounted under a cover glass, until 

 it touches the drop directly and makes an indentation 

 in its margin. The extension -current then begins at 

 once, and, as the accompanying Fig. 13 shows, it assumes 

 a direction corresponding very .well to the position of 

 the end of the tube. In a preparation of this kind I 

 followed the streaming from 1 P.M. to 7 P.M., and the next 

 morning at nine o'clock a slight streaming could still be 



