SURFACE TENSION THEORY 



175 



A wire frame is made, to one side of which is attached a silk 

 thread. Over the w^hole area is a (ilin of soap. The thread M 

 takes up an indifferent position as shown in {A) as the surface 

 tension at the interface between F and S is exactly balanced by 

 the internal energy of i^ = internal energy of S. If now the film 

 is broken inside F, say by pricking with a needle, M tends to 

 become a circle. That is, the internal energy of S is increased 

 relatively to that of F. Howev^er it is brought about, the result 

 is an increase in the surface tension at the interface F —S, i.e., the 

 thread. It is of value to note that it is not necessary for the film 

 to be broken. Theoretically, all that is necessary is a difference 

 in internal energies on the two effective sides of the thread, the 

 lower internal energy being inside the loop. Further, no matter 





n 



^S!-_--i- 



^rrrr^ 





A 



Fig. 42. — Frami? of wire enclosing a Soap Film. 

 In .-1 there is a loop of fine silk floating in the film. 

 In B the film enclosed by the loop has been broken. (After Van tier Mensbrugghe.) 



how slight the difference on the two sides of the thread the rnovement 

 would be maximal — the " all or none " principle. 



Muscle consists of a number of chains of long oval elements 

 immersed in sarcoplasm. The membranes (plasmahaut) of the 

 elements may be represented by the thread mentioned above, the 

 protoplasm of element and sarcoplasm as the soap film. Anything 

 which causes a difference of surface tension at the interface between 

 sarcous element and sarcoplasm will cause the element to become 

 spherical, i.e. its length would decrease without alteration in 

 volume. 



We have seen (Chap. VI.) that a substance cannot produce any 

 effect on a surface unless it can spread over that surface completely. 

 The thinnest layer possible would be one a single molecule deep. 

 Adam has found that the area occupied by a fatty acid molecule 

 forming part of a condensed film on the surface of water is 21 X 10 ""^ 

 sq, cm. Hill calculates that one molecule of lactic acid should 

 occupy a surface of 24 X 10"^*^. Giving the acid the benefit of the 



