164 COMPOUNDS OF THE PROTEINS 



course of time." Under these conditions, what is actually measured is 

 the concentration of the precipitating-agent which is requisite to bring 

 about a given degree of change (visible precipitation) within a given 

 brief period, that is to say, a velocity, and not an equilibrium. 



That protein, when it is precipitated by inorganic salts, actually 

 enters into combination with them and carries down a portion of the 

 precipitating salt, has been shown in a variety of instances. The most 

 exhaustive investigations of this character have been those of Galeotti 

 who has employed electrochemical methods of measuring the concen- 

 tration of individual ions in the solutions. By these means he has been 

 able to show, for example, that when Egg-albumin is precipitated by 

 silver nitrate, Ag + and NOj ions are removed from the solution and 

 precipitated with the protein in equivalent proportions; in other words 

 the protein combines with the whole molecule of silver nitrate to form 

 an insoluble compound. It has also been shown that copper sulphate 

 combines as a whole with egg-albumin to form .an insoluble compound, 

 but in alkaline solutions in the presence of an excess of the salt this 

 precipitate redissolves and the albumin is now found to have combined 

 with an excess of the copper, forming a soluble compound, while the 

 alkali takes up the excess of sulphuric acid thus set free. 



The very important observation has been made by Pauli that abso- 

 lutely Electrolyte-free Egg-albumin, prepared by prolonged dialysis, 

 is not ionic (i. e. } does not drift in an electric field) and that under these 

 conditions it is not precipitable even by heavy metals. It is, however, 

 coagulated by highly concentrated salts. In correspondence with this 

 it has been shown by Rohmann and Hirschstein that the amount of 

 silver nitrate which will combine with Casein to form an insoluble 

 compound is exactly equivalent to the amount of base (NaOH) 

 previously combined with the casein, i. e., to the number of COHN 

 -linkages that have been opened up and ionized by union with an inor- 

 ganic base. 



When we now turn from the phenomenon of Precipitation to that of 

 Coagulation we meet with quite a different series of relationships. 

 Instead of the Coagulative-power of salts being determined primarily 

 by valency, we find that specific ions of varying valencies have high 

 coagulative-powers, while others have low coagulative-powers, and 

 these specific relationships are rather constant for a wide series of 

 proteins and of other colloids. In coagulation, also, both ions of the 

 coagulating-salt participate in determining coagulative-power, although 

 they act in opposite senses, the cations coagulating and the anions 

 inhibiting coagulation, or else vice versa. Thus, Pauli found that in 

 egg-white (in which the protein is electronegative) the cations of added 

 electrolytes are the active agents in inducing coagulation, while the 

 anions inhibit coagulation. In the following table of Pauli's the 

 cations are arranged in ascending order of precipitating-power from 

 left to right, while the anions are arranged vertically, the weakest 



