374 THEORETICAL RESPONSE OF CELLS TO CONTACT 



To begin with a simple case, we may consider the various positions 

 which a perfectly fluid hypothetical cell would assume on a flat sur- 

 face of glass in terms of the surface tensions between the cell and the 

 plasma, the plasma and the glass, and the glass and the cell. In 

 Fig. 1 let a represent the cell suspended in plasma, P, before coming 

 into contact with the glass, G, and let h represent the same cell in an 

 equilibrium position with respect to G. In taking this position or any 

 other position in contact with any solid body, G, the cp interface, 

 X, has been increased (or decreased), and an area, s, of the gp inter- 



^cTp 



sgTp 



sgTc 



Fig. 1. Diagram of a liquid sphere, C, representing a cell suspended in plasma 

 P, before (a) and after {b) coming into contact with a glass surface, G. x is the 

 area of the cp interface; cTp, the surface tension of the cp interface; s, the area 

 of the gc interface at (b) and the corresponding and equal gp interface at (a). 



face has been exchanged for an equal area of the gc interface. The 

 surface energy, E, at h is expressed by the equation 



E = xcTp + SgTc - sgTp (1) 



where cTp, gTc, and gTp represent the surface tensions of the cp, 



gc, and gp interfaces respectively. 



The problem is to calculate the height, h, of the cell above the glass 



at equilibrium in terms of gTc, gTp, and cTp, the volume of the cell 



remaining constant. Now by definition the surface energy, E, at 



•Ti • 1 • . , dE ^ _, - , dE , 



eqmubnum must be at a mimmum, and ^;- = 0. To find — - let us 



ah an 



