CH. VII.] MILK. 167 



molecular weight of 4450. The'calcium salts of paracasein 

 are very insoluble in solutions which have a reaction between 

 P H = 4 and PH = 7. It therefore follows that if casein be 

 hydrolysed to paracasein in the presence of soluble calcium 

 salts and the reaction be between the stated limits, then the 

 paracasein will be precipitated as an insoluble calcium salt. 

 This is the probable explanation of the well-known phe- 

 nomenon of the clotting of milk. Casein is not coagulated 

 on boiling. But when milk is boiled a skin forms on the 

 surface. A similar skin forms whenever a protein solution 

 mixed with an emulsion of a fat is heated. The skin con- 

 tains protein mixed with fat. If it be removed, another 

 skin immediately forms. 



183. Examine a drop of fresh cow's milk under the micro- 

 scope, using a high power. Notice the highly-refractive fat globules 

 of varying size, the smallest exhibiting the peculiar vibration known 

 as Brownian movement. 



184. Take the specific gravity of milk with a lactometer. 

 It varies between 1028 and 1034. 



NOTE. When the milk is skimmed the specific gravity rises to 1037, 

 owing to the removal of the fat which has a low specific gravity. Dilution 

 with water lowers the specific gravity. 



185. Take the reaction of milk by placing drops on pieces 

 of red and blue litmus paper and then washing off with distilled 

 water. The blue paper is usually turned red and the red paper 

 blue, i.e. the milk is amphoteric in reaction. 



Casein. 



186. Take 5 cc. of milk in a test-tube and dilute with distilled 

 water until the tube is nearly full. Add three drops of strong acetic 

 acid and mix thoroughly. A flocculent precipitate of casein is 

 formed, which mechanically carries the fat down with it. 



187. Repeat the above experiment but add 5 cc. of the strong 

 acetic acid. Usually no precipitate or only a slight one is obtained, 

 the casein being soluble in the excess of acid. 



