34 PROTOPLASM 



form. But since one is accustomed to see macroscopic 

 froths in very varied forms, this point will require still some 

 explanation. The spherical form of drops of fluid can be 

 interpreted as a consequence of the capillary pressure 

 produced by surface tension, which, as is well known, is 

 always directed towards the centre of the curvature of the 

 surface, and is inversely proportional to the radius of the 

 curvature. Hence, a freely suspended mass of fluid will 

 only attain to a condition of equilibrium when the surface 

 tension is equal at every point, a state of stability only 

 realised in the spherical form. If we consider our froths of 

 microscopic fineness, and very regular composition, their 

 surface cannot be taken as the surface of a regular sphere, 

 although this fact is not sharply recognisable ; but we must 

 certainly assume, as I have already shown, that each of the 

 superficial alveoli projects with a feeble convexity. As a 

 consequence, the capillary pressure must be the same over 

 the whole surface if equilibrium is to be established. A 

 curvature of the surface differing in strength or in direc- 

 tion must doubtless exert an influence on the capillary 

 pressure, even though not directly, as at the surface of a 

 homogeneous fluid, but by alteration of the curvatures of 

 the single components, i.e. of the convex bulging surfaces 

 of the superficial alveoli. A more accurate study shows 

 that the more curved is the general surface, the stronger 

 must be the convex curvature of each individual bulging 

 surface, and vice versa. But since the total surface pressure 

 represents the sum of the effects of the pressures of each 

 single bulging surface, and their pressure becomes greater 

 towards the interior in proportion as they are more convex, 

 it follows that such a froth behaves in general as an ordinary 

 fluid, the pressure of which increases and diminishes with 

 the curvature of its surface. If this is the case, then such 

 a froth must assume the spherical shape as the one form in 

 which equilibrium obtains, and only show other shapes under 

 the influence of special external and internal forces. 1 



1 The above remarks can be represented more accurately in the following 

 manner. If the surface of the froth is level, the radially-directed lamellae of 

 the most external layer of froth will be vertical to the outer surface, and 



