COLLOIDS. 25 



On comparing the concentration of various salts just sufficient for precipita- 

 tion, where at one time the same anion with different cations was tested and 

 another time the same cation with different anions, PAULI has arranged the cations 

 and anions in the following order in increasing precipitation ability: 



CNS <I <Br <N0 3 <C1 <OCO.CH 3 <HP0 4 <S0 4 

 NH 4 <K<Na<Li. 



The protein used in these experiments was white of egg. According to PAULI 

 certain ions have a precipitating action and others a solvent action. The action 

 of a salt corresponds to the algebraic sum of the action of the ions. 1 PAULI has 

 attempted to associate the precipitation ability of the salts in relation to their 

 action upon the coagulation temperature, but without any positive results. 2 



Nevertheless SPIRO 3 has shown that the kind of protein as well as its con- 

 centration are of importance for the precipitation action, and HOBEB 4 has recently 

 shown that the series I<Br<Cl<S0 4 and Li<Na<K<Rb<Cs is valid in 

 alkaline reaction, but that the series is reversed in acid reaction. In nearly 

 neutral reaction irregularities in the ion series occur which can be considered as 

 a transition series between the two just-mentioned series. That the reaction must 

 be of great importance in the precipitation of proteins seems very probable in 

 consideration of the fact that the proteins take a decided electric charge on the 

 addition of acid or alkali. In regard to the precipitation by salts of the heavy 

 metals, the hydrophile colloids do not seem to differ essentially from the suspension 

 colloids. 6 



On boiling a protein solution the protein suffers an irreversible change 

 and under certain circumstances flocks out. Boiled but not flocked 

 egg-white behaves with precipitating substances, like a suspension colloid. 6 

 In regard to the precipitation of proteins see Chapter II. 



Theories of Precipitation Phenomena. 



At least for the suspension colloids there is no question that they 

 are flocked out by ions which carry an electric charge opposite to the 

 colloid particles, and also by other colloids having an opposite charge. 

 This fact follows from HARDY'S theory, according to which the flocking 

 out is a neutralization process in which the charge of the colloid is 

 just neutralized and the colloid therefore precipitates. 7 The mixture 

 formed on precipitation has been shown to be electrically neutral (iso- 

 electric) as the precipitated particles show no cataphoresis. In this 

 manner it is easily understood that polyvalent ions have a stronger 

 precipitating action than monovalent, as the electrical charge in, for 



* Hofmeister's Beitrage, 3, 225 (1902). 



2 Pfliiger's Arch., 78, 315 (1899). 



3 Spiro, Hofmeister's Beitrage, 4, 300 (1903). 



4 Ibid., 11, 35 (1908). 



5 Pauli, Ibid., 6, 233 (1905). 



6 Hardy, Proc. Roy. Soc., 66, 110 (1900). 



7 Zeitschr. f. physik. Chem., 33, 385 (1900). 



