66 PROTOPLASM 



tact with the soap solution must become more strongly 

 curved, i.e. arched forwards or bulged out, in order to main- 

 tain the equilibrium with the unaltered pressure of the 

 surface in contact with water. Since as a matter of fact 

 the internally-directed pressure is inversely proportional to 

 the radius of the curvature of the surface, this difference of 

 pressure will become equalised by stronger curvature of the 

 surface of the drop. We find then, both in Quincke's ex- 

 periments and in mine, that the drop actually assumes 

 the form corresponding. Now by the phenomenon of 

 extension the oil in the superficial region of the drop is 

 caused to stream backwards, and at the same time the 

 difference in tension at the surface of the drop continues 

 to exist. The consequence of this would be, that, in order 

 to equalise the constant diminution of the bulging out at the 

 centre of extension which is produced by the down currents, 

 the surface would continually bulge forwards, so that the 

 shape of equilibrium is maintained. This bulging forwards 

 demands, however, an afflux from within, which is supplied 

 by the axial current, and which strengthens the latter to a 

 certain extent. 



Finally, there is the question whether the forward move- 

 ment of the drop in the direction of the soap, i.e. in the direc- 

 tion of the axial stream, can also be explained as a simple 

 result of the differences in the surface tensions of the oil- 

 drop. Quincke affirms it to be the case, since he supposes 

 that the capillary pressure, which is, as described, 

 stronger behind, drives the drop forwards in the direction 

 of the centre of the extension-currents, where the pressure 

 is least. I cannot agree with this view, since I cannot see 

 how this difference of pressure could produce more than the 

 alteration in the form of the drop which has just been 

 described. A continuous forward movement of the drop 

 could ultimately only be explained in this manner on the 

 assumption that the differences in surface tension became 

 continually greater. Lehmann, on the other hand (1889, 

 Bd. ii. p. 499), tries to refer the forward movement to the 

 friction between the superficial currents of the drop and the 

 surrounding fluid, as a result of which the freely suspended drop 



