REACTIONS IN HETEROGENEOUS SYSTEMS 55 



The isoelectric point is also the point of minimum viscosity* 

 and of the minimum osmotic pressure, f both of which probably 

 depend upon the least amount of ionically dissociated protein. 



The classification of colloids and their properties are sum- 

 marised in the following table. 



TABLE IX 



EMULSOID. SUSPENSOID. 



Like emulsions these consist of Like suspensions these consist 



liquid particles in the disperse of so lid particles in the disperse 



phase. phase. 



They are lyophile, that is, they j They are lyophobe, that is, they 



have an affinity for the solvent. [ do not seem to have an affinity 



The solution is less than the com- ; for the solvent. 



bined volume of solute and j The solution is the sum of the 



solvent, that is, contraction | volumes of the solute and 



takes place. solvent. 



Surface tension is less than that j Surface tension practically the 



of the solvent. same as that of the solvent. 



The viscosity is greater than that 



of the solvent. 

 The solutions give good osmotic 



pressures. 

 They are in many ways like true 



solutions, being, for instance, 



less readily precipitated by 



salts. 



The viscosity is almost the same 

 as that of the solvent. 



The solutions give very little or 

 no osmotic pressure. 



They are much less like true 

 solutions, being, for instance, 

 very easily precipitated by 

 salts. 



PROTECTIVE EFFECT OF EMULSOID COLLOIDS ON 

 PRECIPITATION OF SUSPENSOID COLLOIDS 



Emulsoid colloids are not only less easily precipitated by 

 salts but they hinder the precipitation of suspensoid colloids. 

 This protective influence is measured by its influence in pre- 

 venting the precipitation of a gold suspensoid and is expressed 

 as the gold value .J 



SIZE OF COLLOIDAL PARTICLES 



The size of colloidal particles has been determined by 

 centrif ugalisation and by ultrariltratioir.j The latter con- 

 sists of forcing the solution through niters of different degrees 

 of fineness so that only particles less than a certain size can 

 pass through the filter. The colloidal particles vary in size 

 from 6 w to 250 w or 6 to 250 mm. Xio~ 6 . 



* W. Pauli and H. Handovsky, loc. cit. 



f L. Adamson and H. E. Roaf, B'iochem. Journ., 1908, vol. 3, p. 422. 



% R. Zsigmondy, Zeit. f. Anal. Chem., 1902, vol. 40, p. 697. 



J. Perrin, Compt. Rend., 1908, vol. 146, p. 967. 



|| H. Bechold, Biochem. Zeit., 1907, vol. 6, p. 379. 



