PROTEINS. 627 



of mercury in the same weight of pure nitric acid, and adding to the 

 cool solution 2 volumes of water.) This reaction is given by tyrosine. 



3. Biuret reaction. Boil a small quantity of protein with 5 c.c. 

 of solution of sodium hydroxide, and after cooling add one or two 

 drops of dilute solution of cupric sulphate : a violet to pink color is 

 obtained, according to the amount of copper solution and the nature 

 of the protein. (Plate VIII., 2.) (It may be necessary to heat the 

 solution before a distinct color appears.) 



This reaction is given by biuret, hence its name. 



Boil a small quantity of protein in a test-tube with absolute alcohol ; filter, 

 wash with absolute alcohol, then with ether, and use the dry material for the 

 following tests. 



4. Adamkieivicz's reaction. Dissolve a small quantity of the pro- 

 tein by boiling with glacial acetic acid. Allow to cool, and, holding 

 the test-tube in an inclined position, let 2 c.c. of concentrated sul- 

 phuric acid flow down the side of the tube. A violet or purple 

 color develops where the liquids meet. 



This reaction is due to tryptophane, and is produced by an impurity 

 in the acetic acid used, i. e., glyoxylic acid, CHO COOH (Hopkins- 

 Cole). A few drops of a dilute solution of glyoxylic acid can be 

 added when the acetic acid used fails to give a positive test. 



5. Lieberman's reaction. To some of the dry protein add concen- 

 trated hydrochloric acid : the protein turns deep blue to violet. On 

 standing, the color fades. Probably due to tryptophane. 



Many of the above color-reactions will be given by the cleavage-products 

 of proteins, and by various other substances, but the proteins alone will 

 respond to all of the five tests. 



(a) Albumins. These substances are soluble in water and are pre- 

 cipitated from their aqueous solution by large quantities of mineral 

 acids and by saturation of their solution with ammonium sulphate. 

 In a solution containing 1 per cent, of neutral salt they are coagulated 

 between 60 and 75 C. (140 and 167 F.). 



They include ovalbumin (white of egg) ; serum-albumin of blood- 

 serum and serous fluids ; lactalbumin of milk ; and vegetable albumins. 



(b) Globulins. These compounds are insoluble in water, but dis- 

 solve in water containing from 0.5 to 1 per cent, of some neutral salt. 

 The solution coagulates on heating, is precipitated by saturation with 

 magnesium sulphate or sodium chloride, and by the addition of an 

 equal volume of saturated solution of ammonium sulphate. Globulins 

 are precipitated if the salt be removed from their solution by dialysis. 



