62 



PHYSICAL CHEMISTRY 



of cells. Quincke assumed that the surface tension of proto- 

 plasm is the difference between the surface tension of water and 

 that of a certain albumin solution. From this assumption the os- 

 motic pressure of Micrococcus progrediens (diameter = .16 mm) 

 would be 5.75 atmospheres greater than the medium, without 

 taking into account the increase in surface tension due to con- 

 vexity. This effect decreases rapidly with the diameter of the 

 cell, for one of .02 mm diameter being only .046 atmospheres. 



Surface Tension of Aqueous Solutions 

 As a rule, inorganic neutral salts, and many sugars, very 

 slightly raise the surface tension, whereas acids, bases and most 

 organic substances lower the surface tension. The lowering is 

 not proportional to the concentration, but when a small amount 

 of the substance lowers the tension a certain degree, each addi- 

 tion of the same amount has less and less effect, Fig. 24. 



05 imol ' 1 



Fig. 24. Surface tension-adsorption curve (adsorption isotherm) of 

 butyric acid (from a class experiment). The surface tension relative 

 to pure water is plotted on the ordinates and the mol concentration of 

 the butyric acid on the abscissae. 



A curve similar to Fig. 24, which is the typical surface tension 

 adsorption or adsorption isotherm curve was first figured by 

 Dupre (1866). The necessary consequence of the shape of this 

 curve is that the solute reducing the surface tension is adsorbed. 

 This relation, demonstrated by Dupre and formulated a few 

 years later by Gibbs, was first called to the attention of physiolo- 

 gists through the comparatively recent writings of Wilhelm 

 Ostwald. 



