74 PROTOPLASM 



observed. In such preparations it may be occasionally 

 observed also, that the oil-drop gets drawn deep into the 

 capillary tube by a decrease of temperature, without the 

 streaming suffering. In the thread of oil contained in 

 the capillary tube a forward current then runs axially 

 towards the soap, and a backward current runs superficially 

 on all sides out of the tube. 



Finally, I will notice here certain other experiments which 

 were instituted with the intention of producing purely 

 circulatory streamings by means of extension phenomena. 

 If a thin strip of glass is fixed on a slide with Canada 

 balsam, and then a cover glass on this in the same 

 manner, a very shallow chamber, open on three sides, is 

 obtained. Then if, by means of a capillary tube, a drop of 

 paraffin oil is placed in this chamber at the edge of the strip 



of glass, it spreads out here 

 to some extent between the 

 ' slide, cover glass, and strip of 

 i0w'-:''''-,',/^^^^ '^^ ^ "^ "" glass. The remammg space 



jfjliy^fef? . under the cover glass is filled 



*'|t?;rix^'' with water (see the accom- 



%fJ''" panyingFig. 14). If now soap 



Pig. 14. solution is cautiously allowed 



to flow along the edge of the 

 glass strip to the drop of paraffin oil, a one-sided extension- 

 current is set up in the latter, which, as the figure shows, 

 causes a simple self-repeating circulation. The explanation 

 of this phenomenon is naturally to be sought in the fact, 

 that the contact between the soap and the oil takes place 

 first at the point a, and since this point only presents a free 

 edge of the oil-drop on one side, there can only be a one- 

 sided current formed, flowing away backwards, which must 

 result in a simple circulatory movement. 



From the streaming processes hitherto described in the 

 oil-drops, it can be plainly recognised that the currents 

 in the drops of oil-froth spoken of earlier are of the same 

 kind, i.e. extension-currents, which are produced by local 

 diminution of the surface tension. As is evident from the 

 mode of preparing the foams, the substance which causes 



