CH. VII.] MILK. 169 



precautions. This process must be repeated until the solution can 

 be heated till sulphuric fumes are found without the solution charr- 

 ing. If too much of the substance has been taken and a black pasty 

 mass is formed, it is necessary to add a few more drops of sulphuric 

 acid. Allow the yellow solution to cool, and then add about 5 cc. of 

 distilled water. Add strong ammonia, drop by drop, until the 

 reaction is just alkaline, cooling under the tap during the addition. 

 An alkaline reaction is generally indicated by the sudden increase 

 in the colour of the solution. Add a single drop of nitric acid to 

 make the solution acid. Add about a half volume of ammonium 

 molybdate solution and boil. A yellow precipitate of ammonium 

 phospho-molybdate indicates the presence of phosphorus in the 

 casein. This is originally present in organic combination, but has 

 been oxidised by the above method (Neumann's) into inorganic 

 phosphoric acid. 



194. Treat 5 cc. of milk with 5 cc. of saturated ammonium 

 sulphate solution. The casein is precipitated, entangling the fat 

 with it. Filter, labelling the nitrate B. Treat the precipitate with 

 water. The casein dissolves. Treat this cloudy solution with 

 acetic acid. The casein is precipitated. 



NOTE. The casein dissolves in water because it is precipitated as a salt 

 by ammonium sulphate. 





Fats. 



195. Transfer the remainder of the precipitate obtained in 

 Ex. 188 to a dry test-tube. Shake it vigorously with 5 cc. of ether. 

 Pipette off the ethereal solution. Heat an evaporating basin by 

 placing it on a boiling water bath. Turn out the flame and then add 

 the ethereal solution to the dish. The ether evaporates rapidly and 

 leaves a small amount of fatty residue. Wipe the dish round with a 

 piece of writing-paper. A translucent grease spot is formed. 



196. Estimation of fat in milk by Meig's method. 



To 10 cc. of milk in a 100 cc. glass-stoppered cylinder add 20 cc. 

 of distilled water and 20 cc. of ether. Shake for 5 minutes. Add 

 20 cc. of 95 to 98 per cent, alcohol and shake again for 5 minutes. 



