270 PHYSIOLOGICAL CHEMISTRY 



CONH 2 . NH . CONH 2 , a substance obtained by heating urea to 

 180 C. 



Eepeat the test but first add (NH 4 ) 2 S0 4 . 



2. Milton's Reaction. To a few cubic centimeters of albumin 

 solution add 2-3 (no more) drops of Millon's reagent (1 pt. by 

 weight of Hg. dissolved in 2 pts. of cone. HN0 3 and diluted with 

 two volumes of water). A yellowish or white precipitate forms. 

 Heat the solution carefully. The precipitate will turn pink or 

 red. Eepeat the test, first adding sodium chloride solution to the 

 albumin solution. This result should be borne in mind in testing 

 for protein in a liquid containing sodium chloride. Shake up a 

 small amount of dry casein with water and apply the Millon test. 

 Perform the test on a dilute solution of phenol. A beautiful 

 red color results. The reaction is given by substances containing 

 a hydroxyphenyl group C 6 H 4 . OH. Most proteins contain 

 tyrosine, a substance possessing this grouping, and it is because 

 of the presence of this substance that the proteins give the 

 Millon reaction. 



3. Xanthoproteic Reaction. To a few cubic centimeters of 

 egg albumin solution add concentrated nitric acid. Warm the 

 mixture until the whitish precipitate has dissolved. The solu- 

 tion is yellow. Cautiously add ammonia until the reaction is 

 alkaline and observe the deepening of the color to orange. Re- 

 peat the test using a small portion of dry casein. Invert the 

 stoppered bottle of concentrated nitric acid slightly so as to get 

 a drop on the glass stopper. Carefully apply the stopper mois- 

 tened with nitric acid to a small area on the palm of the hand. 

 In a moment pour on the yellow spot a drop or so of ammonia. 

 Observe the orange spot. Rinse off the hand carefully under the 

 tap. The test is given by substances containing a benzene ring 

 and is due to the formation of certain nitro-compounds. Most 

 proteins contain amino acids in which there is a benzene ring, 

 and thus will respond to this test. This is also true of the pro- 

 teins of the skin. 



4. Hopkins-Cole Reaction. Mix 2-3 c.c. of albumin solution 

 with an equal volume of glyoxylic acid solution. Add an equal 

 volume of concentrated sulphuric acid, pouring it down the 



