76 



PROTOPLASM 



(D) or the slide (0) (see Fig. 15) than at the equator of the 

 drop. Now, since soap solution of high concentration 

 diminishes the surface tension more strongly than that which 

 is more dilute, two centres of extension-currents will arise 

 at the two poles o and u, since here the surface tension is 

 lower that is to say, the superficial layer of the drop will 

 flow from here towards the equatorial zone a. At the latter 

 region the streams meet, and thence turn horizontally 

 inwards towards the centre of the drop, where they naturally 



rr 



'a 



J 



a 



Fig. 15. 



bend round into the two axial currents caused by the 

 centres of extension o and u. Since the conditions re- 

 main constantly the same, as long as there is the same 

 difference in the concentration of the soap solution on the 

 surface of the drop, and since this is provided for by the 

 constant exudation of new soap solution and its continual 

 diffusion into the water, it is conceivable that the stream- 

 ing phenomena here described may endure for a very long 

 time. 



On the same cause also depends essentially the pheno- 

 menon described above, namely, that a drop which is streaming 

 forwards with a marginal centre of stream lines, gradually falls 

 into more lively streaming, if it approaches one of the glass 

 strips which support the cover glass. In this case the strip of 

 glass lying in the way offers a hindrance to the diffusional 

 distribution of the soap solution, which produces the centre of 

 stream lines at the anterior end of the drop. Hence a relative 

 concentration of the soap solution is brought about, accompanied 

 by a strengthening both of the streaming and of the forward 

 progress. The same thing, however, will also take place, if one 

 drop approaches the surface of another, which then furnishes a 

 similar hindrance to the diffusional distribution of the soap solu- 

 tion as does the strip of glass in the former case. But under these 

 conditions the more concentrated soap solution which is formed 



