IN PROTEIN SOLUTIONS 13 



point of coagulation, when the colloid is subjected to 

 electrical radiations. For instance, as can be fore- 

 seen, the /3-rays of radium precipitate a positive 

 colloid, e.g., ferric hydroxide (W. B. Hardy, V. Henry 

 and A. Mayer, A. Fernau and Wo. Pauli *). 



Let us next consider the connection between these phenomena 

 and those displayed by a typical true solution. Here also is a 

 stability dependent on charged particles, the ions. We know 

 no means of causing a notable separation of the ions from 

 solutions ; but neutral particles can be deposited from the 

 solution in the form of a precipitate. The property discussed 

 above gives a suitable analogy to the behaviour of normal 

 electrolytes, if we so choose the latter that the limit of solubility 

 of the neutral particles formed is exceeded. The precipitation 

 of barium as sulphate, the electrolytic deposition of a metal 

 from its salt, and similar reactions can be expressed in a form 

 analogous to the precipitation of colloids. 



We can state, in general, that whenever the neutral particles 

 of a colloid are not stable in solution it is precipitated by 

 removal of the electric charge. 



There are, however, colloids in which the neutral particles 

 are stable,, and thus electrical neutralisation does not lead to 

 precipitation. This observation was first made with natural 

 albumin (Wo. Pauli), and it holds also for starch, gelatin, agar, 

 gum arabic, some forms of silicic acid, etc. 



The first group of colloids, with neutral particles, which are 

 unstable in solution, includes the hydro- (or lyo-) phobe, or 

 anhydro-colloids (the suspensoids of Wo. Ostwald and P. P. 

 von Weimarn), while the second group, with stable neutral 

 particles, includes without exception hydrated or solvated 

 colloids. 



We must now consider a more important factor. For a 

 colloid to be stable in an electrically neutral state, it is necessary 

 for the discharged particles formed to be sufficiently hydrated. 

 As we shall see, many proteins (casein, globulin, boiled albumin, 

 etc.) form heavily hydrated particles in solution when electri- 



* Biochem. Zeitsch., 1915, 70, 426 ; Kolloid Zeitsch., 1917, 20, 20, for 

 the literature. 



