II] THE COLLOIDAL STATE 17 



Expt. 8. Precipitation of suspensoid sols by electrolytes. The sols of gold, silver 

 and arsenious sulphide carry an electro-negative charge : hence they are most readily 

 precipitated by di- or tri-valent positive ions, such as Ba" or Al'". Add a few drops 

 of barium chloride solution to the three sols (Expt. 3) respectively, and note that 

 they are precipitated, though some time may elapse before the precipitation is 

 complete. The ferric hydroxide sol, on the contrary, carries a positive charge. Hence 

 it is most readily precipitated by a sulphate or phosphate. If a drop of sodium 

 sulphate solution is added while the sol is hot, it is immediately precipitated. 



Expt. 9. Precipitation of emulsoid sols by electrolytes. Saturate the starch, protein 

 and soap solutions prepared in Expt. 4 with solid ammonium sulphate. Precipitation 

 takes place in each case, and it is seen how large a quantity of electrolyte is needed 

 for the "salting out" of emulsoid sols. Filter off the protein precipitate and suspend 

 in distilled water. It will go into solution again, showing that the reaction is 

 reversible. 



REFERENCES 



1. Bayliss, W. M. Principles of General Physiology. London, 1920. 3rd ed. 



2. Burton, E. P. The Physical Properties of Colloidal Solutions. London, 

 1916. 



3. Hatschek, B. An Introduction to the Physics and Chemistry of Colloids. 

 London, 1919. 3rd ed. 



4 Philip, J. C. Physical Chemistry: its Bearing on Biology and Medicine. 

 London, 1913. 2nd ed. 



5. Taylor, W. W. The Chemistry of Colloids. London, 1915. 



o. 



