184 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



the boiling with caustic soda is prolonged, a rose-pink colour is 

 obtained on addition of copper sulphate, i.e., the biuret re- 

 action. This has been already observed, as a characteristic 

 reaction for enzymes, when examining the properties of 

 amylase. The formation of this colour is due to the production 

 of biuret or an allied substance ; the biuret group, it will be 

 noted, therefore is characteristic of the decomposition pro- 

 ducts of albumin, and the biuret reaction is a useful indicator 

 of the extent to which decomposition has taken place. 



Experiment. The Xanthoproteic reaction, i.e., the orange 

 colour obtained on warming with strong nitric acid and subse- 

 quently adding ammonia, is a general reaction for albumins as 

 they are broken up by the strong nitric acid. 



Experiment. Milton's reagent will be found to give a brick- 

 red precipitate on boiling with the albumin solution. 



It will already have been noticed, and it is of course common 

 knowledge, that egg-albumin is coagulated on heating; this 

 would suggest that egg-albumin belongs to the class of sub- 

 stances known as colloids. This can be demonstrated by 

 enclosing a solution of albumin in a parchment cylinder, 

 adding a little thymol to prevent putrefaction, and immersing 

 the cylinder in water. On boiling a portion of the external 

 water at intervals no coagulation will take place ; the albumin, 

 will be found still present in the interior of the parchment/ 

 cylinder, and capable of coagulation. 



Like other colloids, egg-albumin can be precipitated by* 

 the addition of certain salts. Thus ordinary sodium chloride,, 

 magnesium sulphate, zinc acetate, may be employed for this-; 

 purpose, and especially ammonium sulphate. 



The chief methods in use for the separation of albumin 

 substances consist in fractional precipitation by means of 

 certain salts. 



It is possible to obtain certain albumins in an approxi- 

 mately crystalline state. Egg-albumin may be taken as an 

 example. The following description of the method of pre- 



