IONS, NOT THE SALTS, OF IMPORTANCE 91 



due to the affinity between the salt and the protein, 

 and is of such " a character that the metallic ion 

 and the acid ion of a salt may unite with different 

 asymmetric parts of the protein molecule."* Thus 

 the solubility of egg-globulin depends upon the 

 presence of ionisable compounds, for egg-globulin 

 is precipitated by solutions which do not ionise as 

 sugar and urea, just as they are by non-ionised 

 water. All the globulins are kept in solution by 

 such combinations. These facts throw a new light 

 upon the importance of the mineral constituents of 

 the organism. " The ion-protein compounds are of 

 importance in the animal body through their ability 

 to decrease the sensitiveness to change," especially 

 to changes in concentration, alkalinity, and tempera- 

 ture, t 



When proteins are subjected to heat they undergo 

 a change in state called coagulation. The degree of 

 temperature at which this change takes place is 

 known as the coagulation-point. It has been shown 

 by Pauli and others that various salts have the 

 effect of raising or lowering this coagulation-point, 

 and that the effect is due to the combination of ions 

 of neutral salts with the colloid. 



Colloids unite with water and with salt in about 

 the same way ; and the presence of water and salt 

 in an organism mutually affect each other. If a 

 swollen colloid that is, one which has taken up 

 water is submitted to dry air it will lose water ; 

 but there is the greatest difficulty in removing the 



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

 p. 13.' 



t Ibid., p. 14. 



