22 PHYSIOLOGY CHAP. 



so very much more slowly for they re-dissolve on removal of the 

 salts by which they were precipitated. 



All proteins give specific colour reactions. The best known 

 are the following : 



Millon's Reaction. On adding a solution of mercuric and 

 mercurous nitrate and nitrite in nitric acid (Millon's reagent) and 

 heating, the white precipitate first formed turns red. 



Xanthoproteic Reaction. On heating with nitric acid the 

 solution of protein turns yellow, and then, on the addition of 

 ammonia, orange. 



Molisch's Reaction. On adding a few drops of a-naphthol 

 and running in concentrated sulphuric acid, under the solution, a 

 violet ring appears at the junction of the two fluids. If alcohol, 

 ether, or potash be now added it turns yellow. The substitution 

 of thymol for a-naphthol gives a fine rose carmine, which gradu- 

 ally becomes green. 



Biuret Reaction. A few drops of 2 per cent copper sulphate 

 added to a solution of protein made alkaline with caustic potash 

 or soda, produces a clear violet colour in the cold. Proteoses and 

 peptones, which are the primary decomposition products of the 

 more complex proteins formed by the action of proteolytic fer- 

 ments (infra], give a pure pink colour, r 



Sulphur Reaction. On warming with potash and a little lead 

 acetate, the white precipitate which first appears (lead hydroxide) 

 turns brown and then black, owing to the formation of lead 

 sulphide. 



These colour tests for proteins are important, not merely as 

 showing the presence of protein, but because they prove the 

 existence in the complex molecule of certain definite chemical 

 compounds to which the several reactions are due. The sulphur 

 test, e.g., indicates the presence of cystine which contains this 

 element ; Millon's test, of the tyrosine group ; the xanthoproteic 

 test, of aromatic groups ; Molisch's reaction, of a carbohydrate : 

 and so on. In fact, these chemical aggregates respectively always 

 give these identical reactions, which are accordingly known as 

 " constitutional tests." The biuret reaction is the most general 

 test for proteins, since it is given by all the proteins and their 

 most immediate derivatives (the proteoses and peptones). It is 

 given by biuret and other compounds which contain CO.NH 

 groups. It is also given by some of the less complex derivatives 

 (polypeptides), but not by the ultimate products of their decomposi- 

 tion (amino -acids). 



VII. Owing to our inadequate knowledge of the exact chemical 

 constitution of the different proteins their classification is still 

 based principally upon their physical or physico-chemical 

 properties, e.g. solubility in water or in certain salt solutions, the 

 temperature at which they coagulate, etc. The Chemical and 



