352 THE FORMS OF CELLS [ch. 



A liquid in the mass is devoid of structure ; it is homogeneous, and 

 without direction or polarity. But the very concept of surface- 

 tension forbids this to be true of the surface-layer of a body of liquid, 

 or of the "interphase" between two liquids, or of any film, bubble, 

 drop, or capillary jet or stream. In all these cases, and more empha- 

 tically in the case of a "monolayer," even the liquid has a structure 

 of its own ; and we are reminded once again of how largely the living 

 organism, whether high or low, is composed of colloid matter in 

 precisely such forms and structural conditions. 



Surface-tension is due to molecular force * : to force, that is to say, 

 arising from the action of one molecule upon another; and since 

 we can only ascribe a small "sphere of action" to each several 

 molecule, this force is manifested only within a narrow range. 

 Within the interior of the liquid mass we imagine that such molecular 

 interactions negative one another ; but at and near the free surface, 

 within a layer or film approximately equal to the range of the 

 molecular force — or to the radius of the aforesaid " sphere of action " 

 — there is a lack of equilibrium and a consequent manifestation of 

 force. 



The action of the molecular forces has been variously explained. 

 But one simple explanation (or mode of statement) is that the 

 molecules of the surface-layer are being constantly attracted into 

 the interior by such as are just a little more deeply situated; the 

 surface shrinks as molecules keep quitting it for the interior, and 

 this surface-shrinkage exhibits itself as a surface-tension. The process 

 continues till it can go no farther, that is to say until the surface 

 itself becomes a "minimal areaf." This is a sufficient description 

 of the phenomenon in cases where a portion of liquid is subject to 

 no other than its own molecular forces, and (since the sphere has, 



* While we explain certain phenomena of the organism by reference to atomic 

 or molecular forces, the following words of Du Bois Reymond's seem worth 

 recalling: ' Naturerkennen ist Zuriickfiihren der Veranderungen in der Korperwelt 

 auf Bewegung von Atomen, die durch deren von der Zeit unabhangige Centralkrafte 

 bewirkt werden, oder Auflosung der Naturkrafte in Mechanik der Atome. Es ist 

 eine psychologische Erfahrungstatsache dass, wo solche Auflosung gelangt, unser 

 Causalbediirfniss vorlaiifig sioh befriedigt fiihlt" (Ueber die Grenzen des Natnr- 

 erkennens, Leipzig, 1873). 



t There must obviously be a certain kinetic energy in the molecules within 

 the drop, to balance the forces which are trying to contract and diminish the 

 surface. 



