86 COLLOIDS IN BIOLOGY AND MEDICINE 



Peptones have no protective action at all [Peptones may cause 

 precipitation. E. ZUNZ, Bull. Soc. Roy. des Sc., Med., et Nat., June 

 11, 1906. Tr.], whereas some of the albumoses, especially sodium 

 lysalbinate and sodium protalbinate have a very powerful protective 

 action which C. PAAL utilized in preparing a large number of inor- 

 ganic colloids. 



Sensitiveness to flocculation may vary considerably with the 

 nature of the protective colloid. 



The flocculation of hydrosols may be brought about not only by 

 electrolytes but also by hydrosols, providing they have an opposite 

 electric charge, as has been shown by W. BILTZ. Thus, for instance, 

 arsenic trisulphid, gold and platinum hydrosols, etc., are flocculated 

 by ironoxid, aluminiumoxid, chromiumoxid hydrosols, etc. A proper 

 relative mixture is required, which means the charge of the positive 

 sol must be counterbalanced by the charge of the negative sol. If 

 a sol is in excess, no flocculation occurs, and the entire complex con- 

 sisting of both colloids migrates, when placed between two elec- 

 trodes, in the direction of the sol (J. BILLITER*) which is in excess. 

 This explains why protective colloids may under some circumstances 

 produce flocculation instead of protection, namely, when they are 

 added in minimal quantities. Thus, for instance, mastic emulsions 

 are flocculated by 0.0003 to 0.0001 per cent of gelatin (H. BECHHOLD 

 and also M. NEISSER and U. FRIEDEMANN). Hydrochloric acid in a 

 dilution incapable of producing flocculation by itself can coagulate 

 gold hydrosol, mastic or oil emulsion in the presence of one part of 

 gelatin per million. 



As has already been mentioned we must not confuse the salting 

 out of hydrophile colloids with flocculation, though there are border- 

 line cases which complicate the phenomenon. If we add for instance 

 a salt of a heavy metal to a dilute albumin solution, a more or less 

 irreversible albumin-metal compound will form, dependent on the 

 nature of the salt upon which the excess of albumin acts as a pro- 

 tective colloid. Acids may cause a loss of charge and the metal salt 

 may then exhibit some flocculating and some salting out action. 

 Such a case might occur, for instance, on adding zinc sulphate to 

 albumin (studied by Wo. PAULI). The process is as follows: 



ZnSO 4 + Albumin. 



0.05 n maximum flocculation (irreversible till 1 n). 



In precipitate disappears. 



2 n precipitate reappears (reversible). 



4 n maximum precipitate (reversible). 



There are transitions between hydrophobe and hydrophile col- 

 loids which are responsible for the transitions between flocculation 



