THE ACTION OP IONS ON COLLOIDS 89 



between the changes which go on in colloids outside 

 the body and the changes that go on inside the body 

 is a condition which cannot be denied. Many of 

 such changes are brought about by the interaction 

 of ions of substances upon the meres or particles of 

 the colloidal bodies. The reactions observable 

 between ions and proteins (the chief colloidal sub- 

 stances of living organisms) become therefore of 

 importance in the investigations of bio-chemistry. A 

 glance at some of the results obtained by Pauli, 

 Hoffmeister and others, as given in Pauli 's admirable 

 work called " Physical Chemistry in the Service of 

 Medicine," may therefore be useful. 



All crystalloids modify the gelation of colloids, but 

 only the electrolytes or ionisable substances have the 

 power of precipitating them. The non-electrolytes 

 have no power to precipitate them in any concen- 

 tration ; but they can influence the property of gela- 

 tion just as the electrolytes and they act in both 

 directions : urea inhibits while sugar favours gela- 

 tion. 



The precipitating power of the electrolytes also 

 varies. A table given by Pauli* shows that the 

 cations Mg, NH 4 , K, Na and Li increase in power of 

 precipitation in the order given ; but the anions 

 sulphate, phosphate, citrate, tartrate, chloride, 

 bromide, iodide and sulphocyanate inhibit the 

 action of the metallic ions, and the power to prevent 

 the precipitation of proteins also increases in the order 

 given. Thus the sulphates increase in precipitating 

 power from Mg to Li ; and, on the other hand, the 



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

 p. 73. 



