88 THE ACTION OF ENZYMES 



pour off the clear liquid and label it Melon's reagent. Add a 

 drop or two of fresh white of egg to 5 c.c of Millon's reagent, 

 and boil ; a brick red precipitate is obtained. 



(c) Xantho-proteic reaction. Add i c.c. of the egg-white 

 solution to 5 c.c. of concentrated nitric acid, and boil ; a yellow 

 precipitate is obtained. Pour off the excess of nitric acid and 

 add ammonia, the precipitate is coloured an orange or brown 

 tint. 



(d\ Hopkins and Cole's reaction. Add about | a c.c. of the 

 egg-white solution to 3 to 5 c.c. of glacial acetic acid ; pour in care- 

 fully some strong sulphuric acid ; if no deep purple colour is 

 obtained use a dilute solution of glyoxylic acid instead of the 

 acetic acid. 



Ex. 46. Pour some undiluted white of egg into a test-tube, 

 and immerse the tube in a beaker of boiling water for a minute 

 or two ; notice the coagulation. 



Try the same experiment with white of egg diluted and shaken 

 with twenty times its volume of tap water. Is the protein 

 coagulated ? 



Ex. 47. Break up a piece of ripe Stilton or Camembert 

 cheese in a mortar with a little water. 



Wash out the whole into a beaker with distilled water and 

 filter. 



Try the colour tests for proteins in this extract by methods 

 described in Ex. 45. 



In what respects do the colour reactions differ from those 

 given with egg-white ? 



Ex. 48. Dissolve a gram or two of gelatine in 50 c.c. of 

 boiling water ; cool and try the protein colour tests with small 

 portions of this solution. 



Compare results with those obtained with egg-white and 

 cheese extract. 



Ex. 49. Try the colour tests with a solution of Witte's 

 peptone and compare results with those obtained with other 

 substances used in previous examples. 



