EXPLANATION OF THE MOVEMENTS 65 



carried backwards by the so-called extension-current, im- 

 mediately dissolves, or at least becomes very quickly diluted 

 in the water surrounding the hinder region of the drop, the 

 condition of equilibrium immediately becomes disturbed again, 

 and in this manner a continuous extension-current results, 

 which flows backwards on all sides from the point of contact. 

 In such a manner the continued duration of such cur- 

 rents under the conditions given is well explained. On the 

 other hand, this mode of consideration does not seem to me 

 to account completely for the axial current, and still less for 

 the forward movement of the drop in the direction of the 

 axial current. Mensbrugghe (1890-91) also has already 

 expressed the strongest doubts as to whether the axial 

 forward current as well as the forward movement can be 

 effects of the surface tension, and looks upon them rather 

 as a result of chemical attraction between the soap 

 and the oil. It seems clear, in the first place, that 

 the axial stream is a simple consequence of the superficial 

 system of currents, which radiate out equally on both 

 sides from the centre of extension, and are continued 

 into the depth of the oil-drop. These two streams in their 

 passage backwards must produce, as the result of friction, 

 an eddy on each side within the oil mass, as is plainly 

 shown in Fig. 9, on p. 62, and the combined effect of these 

 two eddies appears as the axial current. Moreover, there 

 is an additional factor which aids in producing the axial 

 stream. When the oil-drop is in water, it will necessarily 

 assume a spherical form in consequence of the surface tension 

 being equal on all sides. Only under these conditions is there 

 equality on all sides of the internally -directed capillary 

 pressure. This pressure is to be regarded as the result of the 

 surface tension, taking effect in the normal to the curv- 

 ature of the surface, and inversely proportional to the 

 radius of curvature. If then by contact of the edge of the 

 drop with the soap solution the surface tension (i.e. the 

 tension acting on the surface) is lowered, this lowering neces- 

 sarily influences the shape of the drop. Since by diminution 

 of the tension the internally -directed pressure is also 

 lowered, the spot at the surface of the drop which is in con- 



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