V] OF LIQUID FILMS 365 



flattened cell. What we see in a many chambered foraminifer, such 

 as Glohigerina or Rotalia, is the same thing, save that the stages are 

 more separate and distinct, and the whole is carried out to greater 

 completeness and perfection. The little organism as a whole is not 

 a figure of equihbrium nor of minimal area; but each new bud or 

 separate chamber is such a figure, conditioned by the forces of 

 surface-tension, and superposed upon the complex aggregate of 

 similar bubbles after these latter have become consoHdated one by 

 one into a rigid system. 



Let us now make some enquiry into the forms which a fluid 

 surface can assume under the mere influence of surface-tension. 

 In doing so we are limited to conditions under which other forces 

 are relatively unimportant, that is to say where the surface energy 

 is a considerable fraction of the whole energy of the system; and 

 in general this will be the case when we are dealing with portions 

 of liquid so small that their dimensions come within or near to what 

 we have called the molecular range, or, more generally, in which 

 the "specific surface" is large. In other words it is the small or 

 minute organisms, or small cellular elements of larger organisms, 

 whose forms will be governed by surface-tension; while the forms 

 of the larger organisms are due to other and non-molecular forces. 

 A large surface of water sets itself level because here gravity is 

 predominant; but the surface of water in a narrow tube is curved, 

 for the reason that we are here deahng with particles which he within 

 the range of each other's molecular forces. The like is the case with 

 the cell-surfaces and cell-partitions which we are about to study, and 

 the effect of gravity will be especially counteracted and concealed 

 when the object is immersed in a hquid of nearly its own density. 



We have already learned, as a fundamental law of "capillarity," 

 that a liquid film in equilibrium assumes a form which gives it a 

 minimal area under the conditions to which it is subject. These 

 conditions include (1) the form of the boundary, if such exist, and 

 (2) the pressure, if any, to which the film is subject: which pressure 

 is closely related to the volume of air, or of liquid, that the film 

 (if it be a closed one) may have to contain. In the simplest of cases, 

 as when we take up a soap-film on a plane wire ring, the film is 

 exposed to equal atmospheric pressure on both side^, and it ob- 



