MOVEMENTS OF ADHERENT DROPS 303 



eddies, colouring particles usually collect in them, especially 

 the coarser ones, which form a dark streak in the middle of 

 the band ; sometimes also these particles may be distributed 

 at quite regular intervals in the dark band. The peculiar 

 transverse line (Z) between the two eddies, which is in 

 continuation laterally with the secondary eddies (B), can 

 also be explained at once from the nature of the streaming 

 in the median longitudinal section. As we found in the 

 drops of paraffin oil, so also in the streaming drops of water 

 the black mixed with them for the most part gradually 

 collects in the posterior quiescent region, and from here 

 enters the forward current again. 



Now if the ether be so closely approximated to the drop 

 as to cause its edge to shrink away, so that it finally flees 

 from it which cannot be done so well with drops of water 

 mixed with black as with pure drops the following process 

 is in the main exhibited. By the edge shrinking back 

 and becoming straighter and therefore broader, the median 

 forward current increases in breadth, and the two eddies (A) 

 become quite separated from one another. It then appears 

 as if there was only one very violent eddy in motion at the 

 edge, passing above from the margin to the opposite edge 

 of the drop, but soon descending downwards and rushing 

 towards the edge in proximity to the ether again. This is 

 the vortex current, which we have already become acquainted 

 with in the form of the dark transverse band. Only at the 

 most external portions of the edge of the drop are the eddy 

 (A) and the lateral back currents into the forward current 

 still to be seen. It can be easily explained how these 

 currents owe their origin to the fact that the ether now 

 exerts its action violently upon a broader portion of the 

 margin. 



I will take this opportunity of remarking that, in the 

 well-known experiment, so frequently performed, of ap- 

 proximating a drop of ether to the surface of some water, 

 whereby a noticeable depression is produced in it under 

 the ether, or if the layer of water be very shallow, the-^ 

 bottom of the vessel is even laid bare, the same vortex 

 movements can easily be demonstrated if ivory black be 



