62 



PROTOPLASM 



by bringing a spot in the surface of (a) into contact with a 

 third fluid (c), with which (a) possesses a lower surface 

 tension than with (&). This case arises, for example, if we 

 allow a weak solution of soap to approach one side of a drop 

 of oil, which is placed in water under a cover glass on a 

 slide. 



This experiment is best performed by the method of 

 mixing lamp-black with the oil-drop and Indian ink with 

 thesoap solution, or by colouring the latter strongly with 

 an aniline dye. It is then seen that at the edge of the 

 drop, shortly before the soap solution touches it, an energetic 

 system of radiating currents is set up, a system consisting of 

 an axial stream from the interior of the oil-drop, which 

 reaches the surface, and flows away on either side. In 

 their course backwards that is to say, towards the spot at 

 the margin which lies diametrically opposite to the point of 

 contact between the soap solution and the edge of the oil 

 drop the two down currents become continually slower. 



Finally, they meet at the 

 hinder pole of the drop, to 

 gradually turn and pass 

 into the forward axial 

 current. Closely studied, 

 the relations of the currents 

 are somewhat as seen in the 

 accompanying Fig. 9, in 

 which the rapidity of the 

 local currents is roughly in- 

 dicated by the length of the 

 arrows. As the figure teaches us, at the hinder edge of the 

 drop there is a region x of almost complete quiescence, which 

 is roughly triangular in form, with the apex turned towards 

 the so-called centre of extension. The extent of this 

 quiescent posterior portion depends on the intensity of the 

 streaming ; the stronger are the two streams rushing back- 

 wards on each side, the farther they reach towards the hinder 

 pole, and hence the more limited is the quiescent region x. 

 Intense currents reach finally even to the hinder pole itself, 

 and there meet. Then by the mutual interaction of the 



Fig. 9. 



