488 PROTOPLASM 



The oxygen atom of water, with its double charge concentrated 

 in one atom, is electrically stronger than the two combined 

 hydrogen atoms, with their two positive charges distributed in 

 two atoms (Fig. 138). This fact explains why cations, such as 

 Na+ (Fig. Ill A), or proteins with positively charged surfaces 

 are generally more strongly hydrated than are anions or nega- 

 tively charged proteins. 



A return to experimental facts will add conviction to the 

 essential points, in case hypotheses tend to obscure them. 

 Albumin from blood serum, after the removal of electrolytes by 

 dialysis^, moves slowly in an electrical field on prolonged applica- 



l!^ \^%/^ 



SM^ (Na) ®*-© 



^^ttff m 



A B 



Fig. 177. — A. A hydrated cation. B. A hydrated positive collodion particle. 



(From W. Pauli.) 



tion. The direction is to the anode, owing to a slight initial 

 negative charge of the protein. Pauli has shown that pure 

 horse serum, dialyzed for seven weeks, migrates to the anode 

 after twenty-four hours of electrophoresis under 250 volts and 

 2 X 10~^ amp. The addition of alkali or alkaline salt causes 

 a pronounced wandering to the positive pole, while acid or acid 

 salts promote a movement toward the negative pole. Most of 

 the natural proteins (albumin, globulin, glutin, casein, hemo- 

 globin) show a stronger acid than basic character. It frequently 

 happens that at the isoelectric point there is movement in both 

 directions over a short range of hydrogen-ion concentration. 



The Isoelectric Point. — Reference to the isoelectric 'point has 

 been made (page 375). It is the point of zero potential on 

 colloidal particles and therefore the point of no migration in an 

 electrical field. It was discovered by William B. Hardy in 

 his work on proteins. Its discovery represents, in a sense, the 

 beginning of the physicochemistry of proteins. Hardy, in 1899, 



