96 THE PROTEIN SUBSTANCES. 



favorable conditions, for instance in the presence of neutral salts, most 

 proteids can in this way be precipitated in a solid form as coagulated 

 proteid. The hydrosol is converted into hydrogel, but as a modification 

 takes place, this process is irreversible. The various temperatures at 

 which coagulation of different proteids occurs in neutral solutions con- 

 taining salt have in many cases given us good means for detecting and 

 separating proteids. The views in regard to the use of these means are 

 somewhat divided. 1 



A modification can be brought about also by the action of acids, 

 alkalies, or salts of the heavy metals, in certain cases by water alone, 

 and also by the action of alcohol, chloroform, 2 and ether, by violent 

 shaking (RAMSDEN S ), etc. 



An adsorption of proteids by a suspension colloid such as silicic acid, 

 colloidal ferric hydroxide and kaolin, can easily take place, and indeed the 

 proteid of a solution can be removed by the use of colloidal ferric hydrox- 

 ide or shaking with kaolin (RoNA and MICHAELIS 4 ) . That the proteids 

 can serve as preventives in the precipitation of suspension colloids has 

 been mentioned in Chapter II in speaking of the gold equivalent. In 

 the same manner a mastic suspension is protected from the precipitating 

 action of an electrolyte by an excess of a proteid solution, while the reverse 

 may be brought about, namely, a proteid solution can be precipitated by 

 a large quantity of mastic emulsion in the presence of a proportionately 

 small amount of electrolyte. The method for the removal of proteid 

 from solutions, as suggested by MICHAELIS and RONA, S is based upon 

 this behavior. 



Proteid solutions free from electrolytes have, according to PAULI, & 

 no electric charge, hence they do not migrate in an electric field. On the 

 addition of a trace of acid the proteid becomes electro-positive and moves 

 toward the cathode, while on the addition of alkali it becomes electro- 

 negative and wanders in the electric current to the anode. In the natural 



1 See Halliburton, Journ. of PhysioL, 5 and 11; Corin and Berard, Bull, de 1'Acad. 

 roy. de Belg., 15; Haycraft and Duggan, Brit. Med. Journ., 1890, and Proc. Roy. 

 Soc. Edin., 1889; Corin and Ansiaux, Bull, de 1'Acad. roy. de Belg., 21; L. Fredericq, 

 Centralbl. f. PhysioL, 3; Haycraft, ibid., 4; Hewlett, Journ. of PhysioL, 13; Duclaux, 

 Annal. Institut Pasteur, 7. In regard to the relationship of the neutral salts to the 

 heat coagulation of albumins see also Starke, Sitzungsber. d. Gesellsch. f. Morph. u. 

 PhysioL in Miinchen, 1897; Pauli, Pfliiger's Arch., 78. 



2 See Salkowski, Zeitschr. f. physiol. Chem., 31; Fr. Kriiger, Zeitschr. f. Biologic, 

 41; Loew and Aso, Bull. Coll. Agric., Tokio, 4. 



3 Ramsden, Zeitschr. f. physik. Chem., 47 and Arch. f. (anat. u.) PhysioL,. 1894. 



4 Biochem. Zeitschr., 5. 



5 Biochem. Zeitschr., 2, 3 and 4. 



6 Hofmeister's Beitrage, 7. 



