380 



THEORETICAL RESPONSE OF CELLS TO CONTACT 



= 1. On each curve the point corresponding to the value of cos 

 A (Table I) used in calculating m for that curve is marked by a circle.' 

 The fact that the surface energy is always at a minimum at the marked 

 point proves that, when the forces of surface tension are in equilibrium 

 at the contact angle, the surface energy of the whole system is also at 

 a minimum. That part of each curve beyond the minimum is dotted 

 to indicate that it is imaginary, because the cell would continue to 

 ingest a particle only as long as the surface energy was decreasing. 



Fig. 3. Diagram showing geometrical construction used in calculating surface 

 areas involved, as a solid spherical particle of radius g is ingested by a liquid 

 sphere (shown in part) of radius ig or r. A is angle of contact. 



The error of Tait's statement and of the predictions based upon it 

 is evident from the figure. The minimum surface energy is not 



"^ By simple geometrical and trigonometrical consideration of Fig. 3 it is found 



that 



. (r + Ar)2 + g^ - {r + Ar + g ~ z ~ yy 



cos A = 



2g{r + Ar) 



This equation was used in calculating cos A as given in Table I. 



