6 4 



PROTOPLASM 



current is gradually formed on each side in the surrounding 

 water, just as in the oil-drop (see Fig. 9). The locality of 

 this back current is marked by an aggregation of Indian 

 ink. By degrees, the dark curved line of Indian ink reaches 

 back to the advancing soap solution (see Fig. 10). 



The physical explanation of the radiating currents de- 

 scribed here, which Quincke and others have studied under 



rather different con- 

 ditions, is, according to 

 Quincke's interpretation 

 (1888), somewhat as 

 follows. Since the sur- 

 face tension at the 

 boundary between the 

 olive oil and the soap 

 solution is less than that 

 at the boundary between 

 the olive oil and the 

 water, the condition of 

 equilibrium in the ten- 



? 

 \ 



\ 



f 



Pa 



af 



n 



O 



\ 



''V. 



.* 



r~V" 



Fig. 10. 



sion on the surface of the 

 oil-drop must be dis- 

 turbed by contact with the soap solution on one side. Let us 

 conceive the tension of the surface to be represented by a 

 stretched elastic membrane. Then that part of the surface of 

 the drop which is in contact with the water must shrink 

 together to a certain extent, since its tension is greater than 

 the tension in that portion of the surface which is covered 

 by soap solution. Let us now imagine that under these 

 conditions the limiting layer between the oil- drop and the 

 surrounding fluid becomes torn and carried towards the 

 hinder end of the drop, a violent movement which will 

 naturally affect both the mass of oil and the surrounding 

 water with equal force, and cause, to a certain depth in both, 

 a similarly directed system of currents, leading away from 

 the point of contact or centre of extension. But with this 

 tearing away of the limiting zone between the oil and the 

 surrounding fluid, a new surface of oil comes into contact 

 with the surroundings. Since the thin layer of soap solution, 



