CONVERSION OF CHEMICAL ENERGY 83 



If one part of the surface of the sphere has a lower surface 

 tension than the rest of the surface a projection will occur at 

 that part because to balance the same pressure a lower 

 surface tension will require a smaller radius, or, what is the 

 same thing, a projection from the surface of the sphere. 



A local change in surface tension may be brought about 

 either by the action of some substance in the surrounding 

 medium or by some local chemical change inside the cell 

 itself. Such local chemical change may be a liberation of ions, 

 causing a change in electrical charge which as described on 

 p. 48 will produce a change of surface tension. 



Under the influence of surface tension any substance which 

 can lower the surface tension will accumulate at the surface of 

 separation between the two liquids, no matter whether it 

 comes from the cell contents or the surrounding medium. 

 Surface condensation may produce such a concentration that 

 precipitation may occur with the production of a surface 

 layer.* 



Salts in the surrounding medium may act on the colloids 

 at the surface producing gelation or the reverse. f Such 

 precipitation and resolution is quite a different matter from 

 the passive semi-permeable membrane that was previously 

 considered to be the distinguishing feature of cells because it is 

 instantly renewable and it is easily altered by cellular activity. 



The comparable effects on cells of similar osmotic con- 

 centrations can be explained by comparing the vapour 

 pressure of the two phases. A solution of water in phenol 

 has the same vapour pressure of water as the solution of 

 phenol in water in equilibrium with it. By adding salts to the 

 watery solution the vapour pressure of water is decreased, 

 hence water will pass from the phenol phase until the vapour 

 pressures are equal. Therefore the phenol phase will decrease 

 in volume and the change in volume will be in proportion to 

 the osmotic concentration of the surrounding salt solution. 



The cell colloids exert osmotic pressure and these colloids 

 are affected by various physico-chemical conditions ; thus it is 

 possible that they may have an influence in the water exchange 

 of ceUs.J | | 



Differences in concentration between the cell and its 

 surroundings might be accounted for by differences in solu- 

 bility in the two, but Kite has shown that dyes which do not 



* W. Ramsden, Proc. Roy. Soc., 1904, vol. 72, p. 156. 



f G. H. A. Clowes, Journ. Physik. Chem., 1916, vol. 20, p. 407. 



| B. Moore and H. E. Roaf, Biochem. Journ., 1907, vol. 2, p. 34, 



