14 THE PROPERTIES OF SOLUTIONS. [cH. I. 



It will be noted that the electro-negative colloid is most readily 

 precipitated by BaCl 2 , which contains a di-valent positive ion. The 

 electro-positive colloid is most readily precipitated by K 2 SO 4 , which 

 contains a di-valent negative ion. 



The tubes may now be warmed to 60 C., and the further effect 

 noted. 



(d) By the addition of compounds ivith complex ions. 



It is found that colloids that carry a positive charge are 

 often readily precipitated by compounds with a complex 

 negative ion. Thus, proteins in acid solution generally 

 carry a positive charge, and they are precipitated by 

 phosphotungstic, phosphomolybdic, tannic, or ferrocyanic 

 acids. Probably the complex ions are more readily 

 adsorbed by the positively charged colloid than are the 

 simple ions. The charge of the colloid thus being neutra- 

 lised, precipitation of the complex takes place. 



If the colloid carries a negative charge it is often readily 

 precipitated by compounds with a complex positive ion, 

 such as the hydrochlorides of the alkaloids, aromatic 

 bases, etc. 



G. The Concentration of Hydrogen ions. 



The only satisfactory method of expressing the "re- 

 action" of a fluid is in terms of the concentration of hydro- 

 gen ion% per litre of the fluid. This concentration is so 

 important as a factor in the physiological properties of 

 fluids that the theory of the matter should be grasped by 

 students at an early stage of their physiological studies. 



Pure distilled water is very slightly ionised into hydro- 

 gen ions or hydrions and hydroxyl ions or hydroxidions. 



H 2 O ~ > H + OH. 



This dissociation proceeds to an equilibrium, in which, 

 according to the laws of mass action, 



(H) x (OH) 

 (H 2 0) 2 - 



