OF VITAL PHENOMENA 77 



H ions) in the solution the less will be the OH ion dissociation 

 by the protein. Therefore, at a certain H ion concentration of 

 the solvent the H* and OH' dissociation of the protein will be 

 equal, and the protein will migrate neither to the cathode nor 

 to the anode in an electric field. This is called the isoelectric 

 point of the protein, at which the protein can be most easily 

 precipitated (by alcohol, for instance). At this point the vis- 

 cosity of the sol is at its minimum. 



The albumins, globulins and some other proteins are more 

 acid than basic. The isoelectric points of some of the proteins 

 as determined by Michaelis (1912) are as follows: 



Protein PH 



Serum albumin 4.7 



Serum globulin 5.4 



Casein 4.7 



Gliadin 9.3 



Edestin 7. 



Oxyhemoglobin 6.74 



Pancreasnucleoprotein (trypsin) 3.52 



From typhus bacilli 4.4 



From paratyphus bacilli 4. 



The isoelectric point of sols is also the point of least hydration, 

 viscosity and stability, and of greatest surface tension. The size 

 of the colloid particles is probably largest at this point. 



In the precipitation of emulsoids as well as suspensoids, mono- 

 valent and bivalent cations are antagonistic to one another. 



Emulsoids are precipitated by suspensoids, but if too much 

 of the emulsoid is present for precipitation it forms coatings on 

 the suspensoid particles and prevents their being precipitated 

 by small amounts of electrolytes. In order to obtain complete 

 precipitation, the suspensoid must be of opposite electric charge, 

 to the emulsoid. 



Emulsoids may precipitate one another even when of the same 

 sign electrically. Thus nucleic acid precipitates other proteins 

 and specific precipitins precipitate the precipitable substances, 

 both being of the same sign (cf. Teague and Buxton, 1907). 

 According to Michaelis, the ratio between precipitin and pre- 

 cipitable substance in the precipitate is not constant, and is not 

 therefore a chemical reaction, but most probably an adsorption 

 phenomenon. 



The time element in colloid phenomena is little understood. A 



