94 PHYSIOLOGY 



(3) MILLON'S REACTION. Millon's reagent is a solution of mercuric 

 nitrate in water containing free nitrous acid. On adding a few drops of this 

 to a protein solution a white precipitate is produced which turns a brick-red 

 colour on boiling. It depends on the presence in the protein of a hydroxy- 

 derivative of benzene, and is determined in the protein by the tyrosine, which 

 is oxyphenylalanine. 



(4) SULPHUR REACTION. On warming a solution of protein with 

 caustic soda in the presence of lead acetate a black colour is produced owing 

 to the precipitation of lead sulphide. The depth of coloration gives a rough 

 indication of the amount of sulphur in the protein under investigation. 



(5) THE HOPKINS-ADAMKIEWICZ REACTION. It was stated by 

 Adamkiewicz that on the addition of acetic acid and concentrated sulphuric 

 acid to protein, a violet colour was produced. Hopkins and Cole showed 

 that the success of this reaction depended on the presence of glyoxylic acid 

 CHO.COOH as an impurity in the acetic acid used. The test is therefore 

 performed now as follows : 



Glyoxylic acid is prepared by the action of sodium amalgam on a solution 

 of oxalic acid. A few drops of this solution are added to the solution of 

 protein, and strong sulphuric acid poured down the side of the tube. A 

 bluish violet colour is produced at the junction of the two fluids. This re- 

 action is due to the presence in the protein of tryptophane. 



The so-called Liebermann's reaction has been shown by Cole to be essentially a 

 modification of the above, and is due also to the presence of tryptophane. In this 

 test the protein is precipitated by alcohol, washed with ether, and heated with con- 

 centrated hydrochloric acid, when a blue colour is produced, glyoxylic acid being 

 derived from the alcohol and ether. 



(6) REACTIONS INDICATING THE PRESENCE OF CARBOHYDRATES. 

 Molisch's test is applied as follows. A few drops of alcoholic solution of 

 a-naphthol and then strong sulphuric acid are added to a protein solution. 

 A violet colour is produced, which on addition of alcohol, ether, or 

 potash turns yellow. The reaction is determined by the presence, either 

 as an impurity or a constituent part of the molecule, of. a carbohydrate 

 radical which, under the influence of strong sulphuric acid, is converted into 

 furfurol. The furfurol gives the colour reaction with the a-naphthol. 



Another test for the carbohydrate radical is the orcin reaction. A small 

 quantity of the dried albumin is added to 5 c.c. of fuming hydrochloric acid, 

 and the mixture is then warmed. When the albumin is nearly all dissolved 

 a little solid orcin is added on the point of a knife, and then a drop of ferric 

 chloride solution. After warming this mixture for some minutes a green 

 colour is produced which is soluble in amyl alcohol and gives a definite 

 absorption spectrum. 



B. METALLIC SALTS 



The following metallic salts form double insoluble compounds with pro- 

 teins, and therefore cause a double precipitation when added to solutions of 

 these bodies : ferric chloride, copper sulphate, mercuric chloride, lead acetate, 

 zinc acetate. 



