PRECIPITATION AND COAGULATION 125 



The general thermodynamic conditions of equiUbrium in sys- 

 tems in which the protein or other colloid forms a region divided 

 from the remainder of the system by a boundary at which abrupt 

 change of properties takes place (as in a suspended coagulum 

 or a jelly) have been elaborated by Tolman (75). 



In his first communication (17) Galeotti arrived at the con- 

 clusion that proteins do not form unions of definite composition 

 with copper, since the copper can be partially and progressively 

 removed from the protein by washing. I have dwelt in a pre- 

 vious chapter (Chap. V) upon the fallibility of this reasoning. 

 In his second communication (18), however, Galeotti obtained 

 data which indicate that at the moment of precipitation of egg- 

 albumin by AgNOs, a compound is formed of perfectly definite 

 molecular weight and solubility in distilled water. These data 

 will be more fully discussed in the succeeding chapter. 



Keeping the quantities AgNOs and water constant and alter- 

 ing the concentration of protein (egg-albumin) Galeotti finds 

 that as protein is successively added a rapid diminution in the 

 number of silver ions occurs, but with the appearance of a pre- 

 cipitate the diminution proceeds more slowly and finally tends to a 

 constant maximum. 



6. The Chemical Mechanics of the Precipitation and Coagu- 

 lation of Proteins by Salts. — We have seen that in order that 

 precipitation of a protein by salts may occur the protein must 

 be ionized, but for coagulation this condition is not requisite. 

 In determining the rate of precipitation the valency of the pre- 

 cipitating ion is of prime importance, in determining the rate 

 of coagulation it is of comparatively subordinate importance. 

 For precipitation very low concentrations of the precipitating 

 salt suflBce, for coagulation high concentrations of the salt are 

 required. This latter fact, and the fact that the presence of 

 coagulating salts aids coagulation by alcohol and by heat suggests, 

 as it did to Hofmeister, that coagulation is dependent upon 

 dehydration of the protein. 



Starting from the observation of Jones and Ota (35) that 

 certain salts, when dissolved in water, produce an abnormal 

 depression of the freezing-point, Jones and his pupils have built 

 up a very large body of evidence for the existence of hydrates 

 (or "solvates") of substances in solution (32) (37). These 

 investigators find that both ions and undissociated molecules can 



