V] THE FORM OF AMOEBA 361 



on a plate which we have warmed at one side a drop of alcohol 

 runs towards the warm area, a drop of oil away from it; and a 

 drop of water on the glass plate exhibits lively movements when 

 we bring into its neighbourhood a heated wire, or a glass rod dipped 

 in ether*. The water-colour painter makes good use of the surface- 

 tension effect of the minutest trace of ox-gall. When a plasmodium 

 of Aethalium creeps towards a damp spot or a warm spot, or 

 towards substances which happen to be nutritious, and creeps 

 away from solutions of sugar or of salt, we are dealing with pheno- 

 mena too often ascribed to 'purposeful' action or adaptation, but 

 every one of which can be paralleled by ordinary phenomena of 

 surface-tensiont- The soap-bubble itself is never in equilibrium: 

 for the simple reason that its film, Hke the protoplasm of Amoeba 

 or Aethalium, is exceedingly heterogeneous. Its surface-energies 

 vary from point to point, and chemical changes and changes of 

 temperature increase and magnify the variation. The surface of 

 the bubble is in continual movement, as more concentrated portions 

 of the soapy fluid make their way outwards from the deeper layers ; 

 it thins and it thickens, its colours change, currents are set up in 

 it and little bubbles glide over it; it continues in this state of 

 restless movement as its parts strive one with another in their 

 interactioAs towards unattainable equilibrium J. On reaching a 

 certain tenuity the bubble bursts: as is bound to happen when 

 the attenuated film has no longer the properties of matter in mass. 



* 80 Bernstein shewed that a drop of mercury in nitric acid moves towards, or 

 is "attracted by," a crystal of potassium bichromate; Pfliiger's Archiv, lxxx, 

 p. 628, 1900. "^ 



t The surface-tension theory of protoplasmic movement has been denied by 

 many. Cf. (e.g.) H. S. Jennings, Contributions to the behaviour of the lower 

 organisms, Carnegie Instit. 1904, pp. 130-230; O. P. Dellinger, Locomotion of 

 Amoebae, etc., Journ. Exp. Zool. iii, pp. 337-3o7, 1906; also various papers by 

 Max Heidenhain, in Merkel u. Bonnet's Anatomische Hefte; etc. 



X These motions of a liquid surface, and other still more striking movements, 

 such as those of a piece of camphor floating on water, were at one time ascribed 

 by certain physicists to a peculiar force, ^sui generis, the force epipolique of 

 Dutrochet; until van der Mensbrugghe shewed that differences of surface-tension 

 were enough to account for this whole series of phenomena (Sur la tension super- 

 ficielle des liquides, consideree au point de vue de certains mouvements observes 

 a leur surface, Mem. Cour. Acad, de Belgique, xxxiv, 1869, Phil. Mag. Sept. 1867; 

 cf. Plateau, Statique des Liquides, p. 283). An interesting early paper is by Dr 

 G. Carradini of Pisa, DelF adesione o attrazione di superficie, Mem. di Matem. e 

 di Fisica d. Soc. Ital. d. Sci. (Modena), xi, p. 75, xii, p. 89, 1804-5. 



