REACTIONS OF ALBUMINOUS SUBSTANCES. 37 



give a blue or purple color and upon boiling with an excess 

 of nitric acid a yellow, which becomes more reddish upon 

 being rendered alkaline. Millon's reagent, which gives a 

 red with all compounds containing a benzene nucleus 

 united with an hydroxyl group, produces the same color 

 with albuminous compounds, whence it is believed that 

 the above complex is contained in albumins. The xan- 

 thoproteic reaction is attributed to the same or the indol 

 group. Similarly each of the other tests appears to be 

 produced by some definite constituent of the protein mole- 

 cule. 



GENERAL REACTIONS OF THE ALBUMINOUS SUBSTANCES. 



The tests may be made upon any albuminous com- 

 pound ; for. example, egg-albumin. 



76. Make the solution alkaline with sodium hydrate 

 and add a few drops of a dilute cupric sulphate solution. 

 A blue or purple color results. An excess of the copper 

 solution must be carefully avoided, as it may produce a 

 blue color when no protein compound is present. (Biuret 

 test.) 



77. Add a small quantity of concentrated nitric acid 

 to the albumin solution and heat to boiling. A yellow color 

 is produced which becomes orange red when the liquid is 

 made alkaline with sodium hydrate or ammonia. (Xantho- 

 proteic reaction.) 



78. Make the solution of albumin acid with acetic 

 acid, then add at least an equal volume of a saturated solu- 

 tion of ammonium sulphate, and heat to boiling. Most 

 albuminous compounds are thrown down as a white pre- 

 cipitate. 



