68 PROTOPLASM 



to imagine that the oil-drop is suspended in a fluid of the 

 same specific gravity. Now since, as we have seen, the capil- 

 lary pressure of the oil-drop necessitates its form remaining 

 the same, the oil-drop itself will take part in the replacement 

 of the soap solution that is flowing away backwards ; since of 

 course the pressure, which causes this replacement, works 

 equally upon water, soap solution, and oil-drop. Hence the 

 latter will wander into the soap solution or be to some 

 extent attracted by it in the same measure as the soap 

 solution streams backwards. For the rest, I think that it 

 should also be possible to explain the phenomenon by a more 

 accurate investigation of the force exerted by the lateral 

 pressure of the currents which come into play. These forces 

 must undergo alteration corresponding to the law that the 

 lateral pressure in streaming fluids is diminished by an 

 amount which is proportional to the square of the rapidity 

 of the streaming. 



In agreement with the explanation here developed of 

 the progression forward of the drop, it may be noted that 

 this movement is in general not set up until the phenomena 

 of streaming reach a certain intensity. Slow or moderately 

 strong currents may continue a long time without the edge 

 of the drop moving forwards in the slightest. But when 

 the streaming attains to a certain intensity, the forward 

 movements become more and more distinct, since then the 

 forces are sufficient to overcome the friction that is always 

 present between the drop, the cover glass, and slide. 



The superficial extension-currents described in the foregoing 

 pages can be produced in just the same manner by allowing dilute 

 solutions of KHO, NaHO, NH 4 0, or Iv,CO, and Na 2 CO s , to 

 approach the drop of olive oil instead of soap solution. The 

 efficiency of these materials depends in part on the alteration of 

 the surface tension which they occasion directly, in part and prin- 

 cipally upon the formation of soap, set up by their action upon 

 the free fatty acids of the oil. The soap thus formed at the 

 point of contact, and partly dissolved in the surrounding water, 

 has naturally the same effect as soap solution added directly. 

 Since, however, bv the action of alkaline solutions on the oil- 



* ' v 



drops, granules of a scarcely soluble soap are also formed, which 

 become partly collected on the surface of the drop, partly 



