Size in Relation to Body Surface 167 



medium is proportional not only to the density difference, 

 but also to some function of the surface. 



Surface tension and size. It was pointed out by 

 Thompson ('17, p. 34) that micro-organisms differ 

 most essentially from large organisms because the 

 forces which are important to them are surface forces, 

 or forces on a molecular scale, instead of the forces 

 such as gravity which act upon bulk. This fact ex- 

 presses itself in the prevalence of spherical forms, 

 which are particularly common among unicellular 

 species. The force which prevails in the attainment of 

 such a body form is, of course, surface tension. 



Surface tension may be said to be effective over dis- 

 tances of about 0.05 micra (Thompson, '17, p. 43), so 

 that an organism with this radius is all surface. Dif- 

 ferentiation of structure within such an organism 

 would require tremendous forces of other sorts, to 

 combat the intermolecular tensions in the regulation of 

 the distribution of substances. The equilibrium of 

 forces which accompanies a given set of bodily dimen- 

 sions may be regarded as the resultant of surface ten- 

 sions opposed to internal or massive forces. Some of 

 the massive forces are measurable as cohesion, elastic- 

 ity, and viscosity. 



Viscosity. In general, the larger the departure 

 from the spherical form, the greater are the massive 

 forces prevailing in a liquid or partially liquid mass. 

 The absolute viscosity of one species of unicellular or- 

 ganism has been measured; Paramecium was found 

 by Fetter ('26) to have a value 8000 times water. It 

 is a stiff gel instead of a true liquid. But Jacobs ('22) 

 showed that the viscosity of Paramecium and of Col- 

 pidium was readily modified; the production of a 

 higher intracellular acidity by the administration of 

 carbon dioxide caused at first a decrease and later an 

 increase of viscosity. It is not recorded whether sig- 



