PRECIPITATION OF PROTEINS BY INORGANIC SALTS 159 



ions present in the mixture, and that the relative precipitating-powers 

 of the sulphates of univalent, divalent and trivalent metals can be 

 expressed by the ratios 1 : 35 : 1923. 



In all of these cases the colloid employed was Electronegative ; that is, 

 in electrolysis it was precipitated at the anode, the colloid behaving 

 like the acid radical of a salt. The experiments which we have cited 

 show that in such instances the ion of the added electrolyte which is 

 effective in bringing about precipitation is the cation, since the valency 

 of the cation determined the precipitating-power of the salt, while the 

 valency of the anion was immaterial to the result. If, however, we 

 dilute the white of a a egg with ten times its volume of distilled water, 

 filter off the flakes of Globulin which are precipitated, and then heat the 

 solution to boiling-point, we obtain an opalescent solution or suspension 

 of Egg-albumin which is very easily coagulated by traces of various 

 salts. If a trace of acid be added to. the solution and an electrical 

 current passed through it, the protein is precipitated at the cathode, 

 while if, instead of acid, a trace of alkali be added, the protein is 

 precipitated, not at the cathode but at the anode. In the former case 

 the protein behaves like a cation or the basic radical of a salt, and is 

 said to be "Electropositive," while in the latter case the protein behaves 

 like an anion, or the acid radical of a salt, and it is said to be "Electro- 

 negative." It is probable that in both these instances the protein 

 migrates to both electrodes, but the protein ion which carries the 

 inorganic radical with it, in the first case the acid and in the second the 

 alkali, is held in solution by the acid or alkali it bears after it has given 

 up its electrical charge to the electrode. 



Now in alkaline solutions of this heat-modified egg-albumin it is 

 found that the cations of added salts are the active agents in precipitat- 

 ing the protein, just as in the case of the sulphides of antimony, arsenic 

 or cadmium, but in acid solutions of the heat-modified egg-albumin 

 these relationships are entirely reversed, the valency of the cation of 

 the added salt becomes immaterial and the precipitating-power of the 

 salt is determined by its anion. The following results, obtained by 

 W. B. Hardy, illustrate this inversion of the precipitating ion when the 

 protein, from functioning as an acid, comes to function as a base: 



PROTEIN IN PRESENCE OF A TRACE OF ALKALI; ELECTRONEGATIVE. 

 Temperature 16 degrees. Coagulating salt 1 gram-mol. in 80.000 c.c. 



Coagulated at once. On slightly warming. Did not coagulate. 

 A1 2 (SO 4 ) 3 MgSO 4 Na 2 SO 4 



Cd(NO 3 ) 2 BaCl 2 K 2 SO 4 



CuSO 4 CaCl 2 NaCl 



CuCl 2 



PROTEIN IN PRESENCE OF A TRACE OF ACID; ELECTROPOSITIVE. 



Coagulated instantly. No effect. 

 A1 2 (SO 4 ) 3 CuCl 2 



CaSO 4 Cd(NO 3 ) 2 



K 2 SO 4 BaCl 2 



Na 2 SO 4 NaCl 



MgSO 4 



