SURFACE TENSION 345 



also for the non-filterability of ionic protein and the opalescence 

 of its solutions. Between these alternatives, if indeed they are 

 alternatives, our data do not, as yet, suffice for us to decide. 



The opalescence of protein suspensions has been made by Kober 

 (52) the basis of a method of determining proteins quantitatively 

 (Cf. Chap. III). 



9. The Surface Tension of Protein Solutions. — The influence 

 of dissolved protein upon the tension of air-water surfaces has been 

 investigated by Quincke (85) (86), Zlobicki (134), Frei (31), Buglia 

 (19), Iscovesco (59), Shorter (115), Bottazzi (13) (15), and Berc- 

 zeller (9). Gelatin, egg-globulin and haemoglobin diminish the 

 air-water tension.* The diminution is greater the higher the 

 temperature (between and 24.4 degrees, Zlobicki). The surface- 

 tension of gelatin solutions and blood serum is increased by the 

 addition of small quantities of alkali, and diminished by the 

 addition of small quantities of acid (Buglia, Frei). 



According to Bottazzi (13) (15) the surface tension of serum 

 albumin solutions is at a maximum when their ionization is at a 

 minimum and at a minimum when their ionization is at a maxi- 

 mum. The maximum depression of the surface tension of water 

 by serum albumin occurs at a reaction just on the acid side of 

 neutrality. Undissolved protein does not affect the surface ten- 

 sion of water when shaken up with it (9). 



According to Berczeller (9), the surface tension of a protein 

 solution which is so electrol>i;e-free as not to coagulate on heating 

 nevertheless diminishes on heating. This reduction of surface 

 tension is reversible, for on standing for some time at ordinary 

 temperature the solution regains its original surface tension. 



The surface tension of protein solutions diminishes during 

 digestion (9). 



10. The Formation of Surface-films by Dissolved Proteins. — 

 Proteins, when dissolved in water, would appear, as a rule, to 

 diminish the surface tension at surfaces bounded by substances 

 other than air, since they tend to become concentrated at such 

 surfaces and form films of insoluble protein there, and, as Willard 

 Gibbs pointed out in his classic memoir on equilibrium in hetero- 

 geneous systems, such a tendency is indicative of a diminution in 

 the free energy of the surface at which such concentration occurs. 



* According to Iscovesco, however, pure egg-albumin raises the air-water 

 tension. 



