354 THE FORMS OF CELLS [ch. 



some objection) of a specific surface-energy. Surface-energy, and 

 the way it is increased and multiplied by the multiphcation of 

 surfaces due to the subdivision of the tissues into cells, is of the 

 highest interest tq the physiologist; and even the morphologist 

 cannot pass it by. For the one finds surface-energy present, often 

 perhaps paramount, in every cell of the body ; and the other may find, 

 if he will only look for it, the form of every solitary cell, hke that of 

 any other drop or bubble, related to if not controlled by capillarity. 

 The theory of "capillarity," or "surface-energy," has been set forth 

 with the utmost possible lucidity by Tait and by Clerk Maxwell, on 

 whom the following paragraphs are based: they having based their 

 teaching on that of Gauss*, who rested on Laplace. 



Let E be the whole potential energy of a mass M of hquid; let 

 Cq be the energy per unit mass of the interior hquid (we may call it 

 the internal energy); and let e be the energy per unit mass for a 

 layer of the skin, of surface S, of thickness t, and density p (e being 

 what we call the surface-energy). It is obvious that the total energy 

 consists of the internal plus the surface-energy, and that the former 

 is distributed through the whole mass, minus its surface layers. 

 That is to say, in mathematical language, 



E={M -S. l.tp) e^ + S . I^tpe. 



But this is equivalent to writing : 



= MeQ + S .I.tp{e- eo); 



and this is as much as to say that the total energy of the system 

 may be taken to consist of two portions, one uniform throughout 

 the whole mass, and another, which is proportional on the one hand 

 to the amount of surface, and on the other hand to the difference 

 between e and Cq, that is to say to the difference between the unit 

 values of the internal and the surface energy. 



It was Gauss who first shewed how, from the mutual attractions 

 between all the particles, we are led to an expression for what we 



* See Gauss's Principia generalia Theoriae Figurae Fluidorum in statu equilibriif 

 Gottingen, 1830. The historical student will not overlook the claims to priority 

 of Thomas Young, in his Essay on the cohesion of fluids, Phil. Trans. 1805; see 

 the account given in his Life by Dean Peacock, 18.55, pp. 199-210. 



