ALBUMINOIDS. 17 



This fact has little practical interest, since in most cases 

 the analyst has to deal only with cows' milk. 



II. ALBUMINOIDS. 



For the examination of milk for the characteristic al- 

 buminoids, dilute the sample with an equal volume of 

 water and add hydrochloric acid to a strong acid reaction 

 and boil. In this way a curdy precipitate of casein and fat 

 is quickly obtained. This is filtered off and washed on the 

 filter with water until all the acid is eliminated. Then 

 to remove the fats wash thoroughly with alcohol and 

 finally with ether. Subject this material to the follow- 

 ing reactions which characterize casein. 



1. It is readily soluble in very dilute alkali and by 

 careful addition of acid it is reprecipitated. 



2. By treatment with acids in excess, solution takes 

 place. It is precipitated from the acid solution by rennet. 



3. By boiling the acid or alkali solutions there will 

 be no precipitation. 



4. Oxalates of the alkali metals dissolve it pro- 

 ducing a milky opalescent fluid. By adding solid mag- 

 nesium sulphate to the solution a precipitate is formed; 

 simple dilution of the solution does not effect this pre- 

 cipitation. 



5. Sodium fluoride dissolves casein, especially when 

 heated. By the introduction of carbon dioxide precipita- 

 tion takes place. 



6. If a sample be placed in a porcelain crucible and 

 fused with a mixture of sodium carbonate and potassium 

 nitrate and then cooled and dissolved in dilute nitric acid, 

 the characteristic reactions of phosphoric acid may be 

 obtained. (Magnesium mixture, ammonium molybdate.) 

 (See page 30). 



The positive response to the above described reactions 

 shows conclusively the presence of casein. 



