SURFACE-TENSION THEORY 115 



variable, but usually small distance, from the non-moving ectoplasm, beyond 

 which point the decrease to nil is very rapid. This affords conclusive 

 proof that the force inducing movement does not act at the boundary 

 of the endoplasm and cell-sap, but throughout the whole substance of 

 the former. By this distribution of velocity the friction between the 

 successive layers is in most cases reduced to a minimum, for when a 

 liquid flows under uniform pressure around a curved path, the outer layers 

 must move more rapidly than the inner if tangential friction is to be avoided. 

 The inwardly directed pressure exerted by the surface-tension films 

 on the outer surfaces of the cell-sap and ectoplasm, and on the inner 

 surface of the endoplasm, is greatest per unit area at the corners of the 

 cell, owing to the convex curvatures at these points, and least at the sides 

 where the membranes are flat. The actual surface-tension in each film 

 is, however, the same throughout, and therefore no special influence would 

 be exerted on streaming at the angles of the cell. Even in small cells 

 the pressure due to surface-tension is never more than a small fraction 

 of an atmosphere, and hence no appreciable diminution of the internal 

 osmotic pressure is produced by it. When, however, rectangular cells 

 with rounded angles and with uniformly permeable walls are placed in 

 weak homogeneous plasmolyzing solutions, retraction of the protoplast 

 almost always occurs first at one or other of the angles of the cell, owing 

 to the influence of the convexity of the surface-tension films at these points 

 in increasing the inwardly directed pressure due to surface-tension. 



- SECTION 52. Electro- chemical Surface-tension Theory. 



If the wire from the positive terminal of an electrical battery is 

 connected with a drop of mercury lying in ^dilute sulphuric acid, on 

 connecting the negative terminal with the acid the mercury moves away 

 from the positive pole. By making and breaking the circuit the mercury 

 can be caused to creep to and fro. This phenomenon was first observed 

 by Ermann'in 1809, and it is due to the production by electro-chemical 

 action of a difference in the surface-tensions on the two sides of the drop 

 of mercury, the latter moving towards the side of least surface-tension. 

 It is this principle which is utilized in Lippmann's capillary electrometer, 

 and a feeble current will cause a relatively large amount of movement. 

 Hence the propulsive force for streaming movements could easily be 

 generated in some more or less closely analogous manner, although the 

 precise character of the mechanism is impossible to postulate. 



If the lowering of surface-tension took place on the same side of all 

 the particles in an emulsion, they would move in this direction, for 

 the inwardly directed pressure due to the surface-tension on the other 



I 2 



