ON THE ELECTRICAL CHARGE OF PROTEIN. 139 



cells in consequence of the frequently recognizable paral- 

 lelism between changes in state in colloids and changes 

 in physiological function. This is no doubt dependent 

 upon the fact that the colloidal constituents of living 

 matter show, at least in part, a physico-chemical identity 

 with the properties of isolated proteins. 



It is our purpose to-day to give as far as possible a 

 survey, based on the personal investigations of many years, 

 of the more important physico-chemical properties of the 

 proteins, and to point out, at least in a cursory way, the 

 relation between these properties and many biological 

 phenomena. 



II. 



As in the case of crystalloids, so both the behavior in 

 solution and the behavior in the solid precipitate serve 

 to characterize the colloids. An accurate knowledge of 

 the conditions which determine their precipitation has 

 recently assumed great importance in the study of the 

 colloids. 



We can to-day regard it as settled that between a true 

 suspension and a colloidal solution , there exists only a 

 difference in the size of the suspended particles. In a 

 colloidal solution, they are always so small that through 

 their friction upon each other they are kept in suspension. 

 The colloidal particles seem, therefore, to be no longer 

 affected by the force of gravity, just as is the case with 

 those smallest dust particles in the air that become visible 

 only in the sunlight. The colloidal particles can also be 

 rendered visible in many cases by utilizing 'intense illu- 

 mination methods. Even though gravity is unable to 

 cause a clumping of the colloidal particles, other forces 



