THE PROTEIN SUBSTANCES. 69 



other reagents employed in the detection of alkaloids. The precipitates are 

 voluminous, and in most cases complete in presence of sufficient acid. 

 White of egg, much diluted, may be used in illustration. 



The above reactions may be explained on the assumption 

 that the proteins here act as pseudo-acids or pseudo-bases. 

 In perfectly pure solution they are neutral and indifferent to 

 some indicators, but the addition of a mineral acid imparts 

 to them the character of pseudo-ammonium bases which yield 

 precipitates as the alkaloids do under like circumstances. On 

 the other hand, with salt solutions they become pseudo-acids 

 and form now insoluble precipitates of complex salts. But 

 it has been shown that while some of the proteins may be neu- 

 tral to litmus they may at the same time be quite distinctly 

 acid to phenol phthalein, and require a decided amount of 

 decinormal alkali solution to give a reaction by displacing the 

 acid in combination with them. This is taken to indicate that 

 they should be looked upon as true bases, rather than as 

 pseudo-bases. The amount of acid which may unite with 

 certain proteins has recently been found with considerable 

 accuracy, and becomes in some degree a measure of the 

 basicity. In other cases it may be shown that they have a true 

 rather than a pseudo-acid character and unite with alkali to 

 form real salts. 



Behavior with Millon's Reagent. In this we have one 

 of the most characteristic reactions of the protein bodies. 

 Millon's reagent is made by dissolving mercury in twice its 

 weight of strong nitric acid, completing the reaction by heat. 

 The solution obtained is diluted with twice its volume of 

 water. It contains a little nitrous acid. When warmed with 

 \vhite of egg and other proteins it imparts a deep red color 

 to the coagulum produced and often to the containing solu- 

 tion. The reaction is common to benzene derivatives which 

 have a hydroxyl group attached to the nucleus, and is given 

 by phenol for example. The reaction in the protein substances 

 is due to the presence of the tyrosine group in the complex 

 molecule. This group seems to be present in all protein bodies 



