630 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



rents pass in all directions, while an interior current passes 

 toward the region of least tension. The reason for these cur- 

 rents may be seen by imagining that the drop is covered with a 

 stretched India rubber membrane in place of the surface film. 

 If this stretched membrane is weakened or cut at a certain 

 point the remainder of the membrane will pull away from this 

 point, simulating the surface current. At the same time fluid 

 from within will be pressed out at the weakened point,— thus 

 simulating the central current toward the point of least tension. 



The characteristic currents may be seen in Biitschli's experi- 

 ments or in those with the drops of clove oil, if some soot or 

 India ink has been mixed with the oil. Such currents are repre- 

 sented in Fig. I, taken from Butschli. If the axial current 

 carries forward more fluid than the superficial currents carry 

 backward, the drop may elongate in the direction of the axial 

 current and move as a whole in the .same direction. This often 

 occurs. 



Such currents as are 



feature of 



tension. Indeed, these ( 



of fluids du 



Figure i are an invariable 

 e to local decrease in surface 

 rents are the characteristic phenom- 

 ena ; they may be the only movements that occur. 



