CASEIN. 55 



solving in dilute ammonia and reprecipitating with 

 acid. 



128. Test the casein for nitrogen and sulphur in the 

 same manner as albumin was tested (Experiments 72 and 

 73). 



129. Test it also for phosphorus by mixing about 

 a gramme of the dry substance with equal parts of sodium 

 carbonate and potassium nitrate and fusing in a porcelain 

 crucible. After cooling, dissolve the mass in water, acidify 

 strongly with nitric acid, and add ammonium molybdate. 

 A yellow precipitate, at once or after warming, shows the 

 presence of phosphoric acid. 



130. Try the solubility of casein. It is insoluble in 

 water, but soluble in alkalies. Its alkaline solution is not 

 coagulated by heat. With lime water it forms a milky 

 solution. Its solutions give the biuret reaction and Millon's 

 reaction. 



131. Demonstrate the acid nature of freshly precipi- 

 tated casein by dissolving it in a very dilute solution of 

 sodium carbonate. If not quite enough of the latter is used 

 to produce a complete solution and if the mixture is then 

 filtered, the nitrate will have an acid reaction showing 

 that the sodium casein which it contains has some of the 

 properties of an acid salt. Boiling causes no coagulation, 

 but the casein can be set free as a precipitate if hydro- 

 chloric acid is added to strong acid reaction. (Compare 

 with results of acidifying solutions of soap or of soluble 

 carbonates.) 



132. If the rennin ferment is at hand or can be pre- 

 pared, test with it a solution of casein in lime-water. The 

 casein is changed into paracasein calcium (cheese) which 

 is only slightly soluble in water. 



