378 THEORETICAL RESPONSE OF CELLS TO CONTACT 



sticking as hard as it otherwise would, but it is always true that the 

 force which holds a cell to a glass slide, indeed which holds any liquid 

 to any solid surface, is the force of surface tension.^ 



In extending this discussion to cover cases where the surface of G is 

 curved and G becomes a small sphere, Tait makes a fundamental 

 error. He argues that a particle of G will be ingested by C if the 

 surface energy can thereby be decreased. In other wordSj, G will be 

 ingested if the decrease in energy, due to exchanging sgTp for sgTc 

 {-sm), more than compensates for the increase in energy due to the 

 enlargement of the cp interface (Axn) after ingestion of G. Thus 

 a particle will be ingested if -sm>Ax7t or if sm + Axn < 0. 



For the comparison with the condition necessary for the spreading 



of a cell on a flat surface, that m -\- n = or < 0, Tait puts this in- 



A^ . A^ 



equality into the form m + — n < and reasons that since — is 



5 s 



Ax 

 "as a rule"^ less than l,m + — n will be more hkely to be less than 



s 



than will m + w. Hence he predicts that if a cell ingests a small 



particle of G it will surely spread on a flat surface but that the reverse 



may or may not be true. 



This prediction is erroneous, because, as we shall attempt to show, 

 even though the surface energy may be less at complete ingestion than 

 before ingestion, it is always at a minimum {still less) at incomplete 

 ingestion, unless w + w = 0; i.e., unless the cell would spread to 

 infinity on a flat surface. This means that no particle of G can ever 

 be completely ingested by C unless C will spread to infinity on G. 



The truth of this statement becomes evident from considerations 

 of the contact angle between C and G. As G becomes more and more 

 nearly ingested the angle of contact approaches 0. It can never reach 



unless — - = 1 for from equation (9) 

 n 



3 Some writers (Mathews, A. P., Physiol. Rev., 1921, i, 553) would restrict the 

 term surface tension to the free energy of cohesion on liquid surfaces. It is 

 used here to denote the intensity factor of the free surface energy on either solid 

 or liquid surfaces regardless of the nature of the forces involved, 



^ Actually it is always less than 1. It approaches 1 as a limit as the diameter 

 of the ingested particle approaches infinity. 



