122 THE THEORY OF IONS 



Just as there are colloids which carry an electro- 

 negative charge, so there are others which carry a 

 positive charge ; and each kind will be precipitated 

 by an oppositely charged ion or colloid. Oppositely 

 charged colloids will precipitate one another, just 

 as the ions of salts precipitate colloids ; and, owing 

 to the greater size of the meres or colloidal particles, 

 the conditions for the formation of large aggregates 

 are very favourable. In this again we see the im- 

 portance of the ionic theory. A study of the pheno- 

 mena exhibited by the extra-cellular colloids in the 

 light of this theory will, it is hoped, throw more 

 light upon the immunity reactions. How these are 

 brought about is by no means yet clear to us. But 

 colloids undergo changes, not only by reacting upon 

 one another, but by giving off ions, as we have 

 already seen. Indeed, colloidal particles become 

 remarkably changed by the loss of a side-chain or 

 a few atoms from the molecule. As the colloidal 

 particles grow smaller by this loss of side-chains or 

 ions, their electrical charge grows, and they approxi- 

 mate more and more to the nature of ions, until, 

 finally, the colloid may pass over by dissociation in 

 the solution into strongly charged ions.* The con- 

 verse of this is possible and probably takes place 

 in the reconstruction of the digested and assimilated 

 nutritive particles into the colloidal material of 

 proteids, carbohydrates and fats. 



An important point in connexion with the pre- 

 cipitins is that the precipitation of colloids is only 

 possible in the presence of salts. If for instance the 



* Pauli's " Physical Chemistry in the Service of Medicine," 

 p. 120. 



