MOVEMENTS OF OIL- DROPS 63 



conflicting currents, there is formed only a narrow, axial, 

 relatively quiescent streak, which is continued forwards 

 through the entire axis of the drop, as far as the centre of the 

 stream lines. Such a median streak (m, see the Fig.) is 

 also, however, formed in the case when a more considerable 

 quiescent region is found posteriorly, and is then an imme- 

 diate continuation forwards of its forwardly-directed apex. 

 The conditions described become much more distinct by 

 addition of lamp-black to the oil. The further peculiar fact 

 is then shown, that the particles of lamp-black, which 

 were originally distributed evenly through the oil, gradually 

 vanish completely from the posterior quiescent region, as a 

 result of which the latter becomes transparent * and clear; 

 just in the same way the continuation of the portion x y 

 which we have termed the median streak m, is also quite 

 free from lamp-black. At times the lamp-black becomes 

 more especially collected on the border of the resting portion 

 x, so that two dark masses arise here. 



Simultaneously, however, with these streaming processes, 

 the drop also shows a forward movement, when the ex- 

 periment succeeds well. The drop moves with more or 

 less speed in the direction of the soap solution approaching 

 it, and frequently 'creeps along for a considerable distance in 

 this manner. Usually this forward movement commences 

 at once on the soap solution touching the edge of the drop, 

 and then the latter bulges out strongly and suddenly 

 towards the soap simultaneously with the commencement of 

 the superficial extension-current. During the continuance 

 of the movement, the drop, as a rule, gradually assumes an 

 oval shape, with a somewhat pointed (i.e. more strongly 

 curved) anterior end and a broader posterior end. 



At the same time that streamings are going on in the 

 drop, there are naturally others, also, in the surrounding 

 water, which can be observed in the plainest manner on 

 addition of Indian ink to the soap solution. It is then 

 seen that the soap solution flows backwards from the 

 point of contact with the oil-drop along the surface of the 

 latter, to the spot where the streaming in the oil-drop extends. 

 At this point the current bends outwards, so that a back 



