CRITICISM OF BERTHOLD 297 



increase in the marginal angle, a condition of equilibrium 

 would be established. The retreat or progression of the 

 drop must therefore have another cause, which has, in fact, 

 long been known. Since the alcohol greatly lowers the 

 surface tension of the water drop towards the air, a violent 

 extension-current is of course set up at once on the surface 

 of the drop, which has, as its consequence, a transfer of 

 fluid of the drop from the alcohol edge to the opposite one. 

 Here the alcohol rapidly evaporates again, for which reason 

 the extension-current persists, in the same way as we have 

 already seen similar currents persist for a long time. By 

 this continual flowing away of liquid from the edge of the 

 drop nearest the alcohol, it is caused to shrink away before 

 the alcohol.i 



Berthold has paid no attention to these necessary and 

 so often described currents, and hence has also not noticed 

 that they are direct evidence against his explanation of 

 amoeboid movement. Por instance, if we wish to apply the 

 phenomenon of the retreating drop to an Amoeba, we should 

 be obliged, on the contrary, to imagine that we had an 

 adhering drop of alcohol or some other fluid of relatively 



^ A drop of this kind adhering to glass and retreating before alcohol, moves 

 therefore in exactly the opposite manner to that in which a drop that is sus- 

 pended in a second fluid, or at least is only very slightly adherent, would move 

 forwards under the same conditions ; for we know of old that the lowering of 

 the surface tension at the free surface of such a drop produces a forward 

 movement in the direction from the centre of the drop towards this point of 

 the surface. That this phenomenon is not exhibited in an adhering drop of 

 water is in any case a consequence of adhesion and of its being in air. 

 Even when the drop of water is in a fluid, and adheres strongly, the relations 

 may -take a similar course. In my experiments with drops of oil and oil- 

 foam, I sometimes made the strange observation that in spite of the existence 

 of an energetically streaming centre of extension, they did not, as usual, 

 move forward in its direction, but in precisely the opposite direction. 

 Hence the course of events was exactly the same as for the drops of water, on 

 the supposition, that is to say, that the oil streaming away from the centre 

 of extension was not sufficiently replaced by the current streaming towards 

 it ; and in this way a continual abstraction from the drop at the extension 

 centre, and a continual addition to it at the opposite margin, was going on, 

 which produced a progression of the entire drop away from the centre of 

 extension. Formerly I was unable to explain this phenomenon satisfactorily ; 

 I now think that I was dealing with drops which adhered strongly, and for 

 which the relations were similar to those for water drops on a glass plate 

 when approached by alcohol. 



