PHAGOCYTOSIS 293 



uninfluenced by it, mobile and free. The molecules on the surface 

 are in a different condition, however. They are subjected to the 

 force of cohesion from within, but not from without, and are there- 

 fore drawn strongly toward the center. The result is the same as 

 though the drop were subjected to pressure from without and the 

 surface layers were in a state of compression. There is in conse- 

 quence a constant tendency of all the surface molecules to be drawn 

 toward the center and a resulting tendency to a diminution of the 

 surface area. It is as though the surface of such a drop were a thin, 

 elastic membrane which tended to contract and diminish in size and 

 surface. The force with which this takes place is spoken of as sur- 

 face tension, 43 and the energy underlying it is called, by Ostwald, 

 surface energy. Since a drop of one fluid suspended in another with 

 which it cannot mix is relieved of the disturbing factor of gravitation, 

 its surface tension has the effect of contracting the small mass into a 

 form which, for the given volume, will expose the smallest possible 

 surface, and this is, of course, the sphere. It is for this reason that, if 

 we shake up such systems as water and chloroform, or oil and water, 

 the chloroform or the oil will be distributed through the water as 

 small droplets. The degree of surface tension of any fluid is meas- 

 urable by a number of reasonably accurate methods which may be 

 found in any text-book of physics and which we need not consider 

 here. It is of course dependent in each case upon the nature of the 

 surrounding medium. We have taken into consideration above only 

 the force which is exerted within the drop by the cohesion, that is, 

 the attraction toward the center. This would be uninfluenced from 

 without only in a vacuum. In nature the surface molecules, though 

 forcibly drawn toward the center, are also affected from without by 

 the attraction exerted by the molecules of the substances surrounding 

 the drop. There is a constant balance, therefore, at any part of the 

 surface of a drop of fluid between the cohesion tension from within 

 and attractions from without. The resultant of the two forces de- 

 termines the surface tension, which will be greater or less in inverse 

 ratio to the attraction from without for any given drop, and a varia- 

 tion of the external attraction at different points on the periphery of 

 the drop will naturally influence' the shape of the drop. For a relief 

 of attraction at one point would tend to permit that part of the sur- 

 face to retract, and an increase in this attraction would tend to 

 allow it to bulge, with the formation of a sort of pseudopod. 



In studying the importance of surface tension 44 in determining 

 the motions of unicellular organisms a number of important attempts 

 have been made to imitate cell motion by means of the suspension of 

 various substances of strong cohesive properties in liquid media. The 



43 Michaelis. "Dynamik der Oberflachen," Steinkopf, Dresden, 1909. 



44 For a thorough discussion of this phenomenon see also Gideon Wells, 

 "Chemical Pathology," Saunders, 1911. 



