122 PHYSICAL CHEMISTRY IN MEDICINE. 



related to them. With this we come to the question of 

 the colloidal properties of the proteins. 



If an electric current is carefully sent through a solution 

 of egg albumin poor in salts, the protein migrates, as 

 shown by chemical analysis, to the positive pole. This 

 migration is very slight, and, since the protein particles 

 have been by optical means proved to be very small, 

 must be attributed to a weak electronegative charge 

 which they carry. A current of 250 volts for twenty- four 

 hours is required to render evident this migration of the 

 protein. The slight charge and the minuteness of the 

 particles explain the very considerable stability of the 

 protein toward precipitating ions. While the more 

 strongly charged colloidal metals and the majority of 

 the inorganic colloids are precipitated in weak salt solu- 

 tions, this is not true of protein. Through this fact is 

 rendered possible the vitally important existence of salts 

 and protein side by side. If the protein particles are 

 given a greater charge than they possess normally, 

 they are readily precipitable. In the presence of acids, 

 for example, the proteins assume a strong electropositive 

 charge, as evidenced by their very considerable migration 

 toward the negative pole, and are now readily precipitable 

 through electronegative colloids. We make clinical use 

 of this procedure daily when we first give dilute protein 

 solutions a positive charge through the addition of acetic 

 acid, after which, upon the addition of potassium ferro- 

 cyanide, they produce" the well-known precipitate with 

 the colloidal negative ferrocyanic acid. In fact, in the 

 majority of the sensitive reactions for albumin, we have 

 to do with the effect of an oppositely charged colloid 

 upon a suitably electrified albumin. 



