V] OF FIGURES OF EQUILIBRIUM 225 



of our nodoid curve in Fig. 66 from o, p, the surface concerned 

 in the former case, to M, N, that concerned in the present, we shall 

 see that in the two experiments the surface of the hquid is not 

 homologous, but hes on the positive side of the curve in the one 

 case and on the negative side in the other. 



Of all the surfaces which we have been describing, the sphere 

 is the only one which can enclose space ; the others can only help 

 to do so, in combination with one another or with the sphere itself. 

 Thus we have seen that, in normal equiUbrium, the cylindrical 

 vesicle is closed at either end by a portion of a sphere, and so on. 

 Moreover the sphere is not only the only one of our figures which 

 can enclose a finite space ; it is also, of all possible figures, that 

 which encloses the greatest volume with the least area of surface ; 

 it is strictly and absolutely the surface of minimal area, and it 

 is therefore the form which will be naturally assumed by a uni- 

 cellular organism (just as by a raindrop), when it is practically 

 homogeneous and when, like Orbulina floating in the ocean, its 

 surroundings are likewise practically homogeneous and sym- 

 metrical. It is only relatively speaking that all the rest are 

 surfaces minimae areae ; they are so, that is to say, under the 

 given conditions, which involve various forms of pressure or 

 restraint. Such restraints are imposed, for instance, by the 

 pipes or annuli with the help of which we draw out our cylindrical 

 or unduloid oil-globule or soap-bubble ; and in the case of the 

 organic cell, similar restraints are constantly suppUed by solidifica- 

 tion, partial or complete, local or general, of the cell-wall. 



Before we pass to biological illustrations of our surface-tension 

 figures, we have still another preliminary matter to deal with. 

 We have seen from our description of two of Plateau's classical 

 experiments, that at some particular point one type of surface 

 gives place to another; and again, we know that, when we draw 

 out our soap-bubble into and then beyond a cylinder, there comes 

 a certain definite point at which our bubble breaks in two, and 

 leaves us with two bubbles of which each is a sphere, or a portion 

 of a sphere. In short there are certain definite limits to the 

 dimensions of our figures, within which limits equilibrium is 

 stable but at which it becomes unstable, and above which it 



T. G. 15 



