40 THE PROPERTIES OF PROTEINS 



maximum of precipitation by alcohol. Further, the P H 

 corresponding to these points agrees satisfactorily with the 

 iso-electric point determined by electrophoresis. 



Finally, the osmotic pressure of a protein in the presence of a 

 buffer solution shows a clear and easily reproducible minimum 

 at the iso-electric point. As the ionic dissociation of the 

 protein particles produces an increase of the osmotically 

 effective concentration under the ordinary conditions of experi- 

 ment, so the formation of neutral particles, at the expense of 

 the ionised particles, causes a diminution in the observed 

 osmotic pressure. As the protein is ionised with liberation of 

 H-, metal, or acid ions these kinds of ions are retained by the 

 electrostatic attraction of the colloidal protein ions ; the latter 

 cannot penetrate the osmotic membrane. The imbibition of 

 glutin discs is analogous to the osmotic behaviour. 



Table 9. 

 Glutin 0-5 per cent., N/ioo Acetate. 



Imbibition of 5 per cent, gelatin. 



Acetic acid : acetate. 



Dialysed for three days. 

 Per cent, water taken up. 



