VII] OF FLOATING DROPS 467 



The surface-energy of which we are speaking here is manifested 

 in that contractile force, or tension, of which we have had so much 

 to say*. In any part of the free water-surface, for instance, one 

 surface-particle attracts another surface-particle, and the multi- 

 tudinous attractions result in equilibrium. But a water-particle in 

 the immediate neighbourhood of the drop may be pulled outwards, 



Pig. 152. 



so to speak, by another water-particle, but find none on the other 

 side to furnish the counter-pull; the pull required for equiUbrium 

 must therefore be provided by tensions existing in the other two 

 surfaces of contact. In short, if we imagine a single particle placed 

 at the very point of contact, it will be drawn upon by three different 

 forces, whose directions lie in the three surface-planes and whose 



* It can easily be proved (by equating the increase of energy stored in an 

 increased surface with the work done in increasing that surface), that the tension 

 measured per unit breadth, T^^, is equal to the energy per unit area, EfO,- Surface- 

 tensions are very diverse in magnitude, -but all are positive; Clerk Maxwell 

 conceived the existence of negative surface-tensions, but could not point to any 

 certain instance. When blood-serum meets a solution of common salt, the two 

 fluids hasten to mix, long streamers of the one running into the other; this 

 remarkable phenomenon, first observed by»Almroth Wright {Proc. R.S. (B), 

 xcii, 1921) and called by him " pseudopodial intertraction," was described by 

 Schoneboom (ibid. (A), ci, 1922) as a case of negative surface-tension. But it 

 is a diffusion-phenomenon rather than a capillary one. 



