VII] OK CELL-AGGREGATES 295 



The surface energy of which we have here spoken is manifested 

 in that contractile force, or "tension," of which we have already 

 had so much to say*. In any part of the free water surface, for 

 instance, one surface particle attracts another surface particle, and 

 the resultant of these multitudinous attractions is an equilibrium 

 of tension throughout this particular surface. In the case of our 

 three bodies in contact with one another, and within a small area 

 very near to the point of contact, a water particle (for instance) 

 will be pulled outwards by another water particle; but on the 

 opposite side, so to speak, there will be no water surface, and no 

 water particle, to furnish the counterbalancing pull ; this counter- 



Fig. 100. 



Fig. 101. 



pull, which is necessary for equilibrium, must therefore be provided 

 by the tensions existing in the other two surfaces of contact. In 

 short, if we could imagine a single particle placed at the very point 

 of contact, it would be drawn upon by three different forces, 

 whose directions would lie in the three surface planes, and whose 

 magnitude would be proportional to the specific tensions charac- 

 teristic of the two bodies which in each case combine to form the 

 "interfacial" surface. Now for three forces acting at a point to 

 be in equilibrium, they must be capable of representation, in 

 magnitude and direction, by the three sides of a triangle, taken in 

 order, in accordance with the elementary theorem of the Triangle 

 of Forces. So, if we know the form of our floating drop (Fig. 100), 

 then by drawing tangents from (the point of mutual contact), 



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

 increased surface to the work done \.\ increasing that surface), that the tension 

 measured per unit breadth, T„,,, is equal to the energy per unit area, ^n,,. 



