XVI.] . STOMACH. GASTRIC JUICE. MILK. 175 



drop of osmic acid ; in a short time the globules 

 become stained brown-black. 



2. Test the reaction of fresh cow's milk with litmus 

 paper. It will be found to be alkaline : oc- 

 casionally it is acid owing to the presence of 

 free lactic acidi 



3. Dilute a little milk five to ten times with 

 water; neutralize it with dilute acetic acid, no 

 precipitate will fall. Continue to add the acetic 

 acid drop by drop, a copious precipitate of casein 

 will occur carrying down with it nearly all the 

 fat. When there is a distinct flocky precipitate 

 no more acid should be added as casein is soluble 

 though not very readily in excess ; it is not 

 precipitated on merely neutralizing since alkaline 

 phosphates are present in milk (cp. Lesson ix. 

 § 16, c). To precipitate the whole of the casein 

 the milk must be much diluted. 



4. Filter off the precipitate. The filtrate should be 

 clear ; if it is not, either too little or too much 

 acetic acid has been added; in this case add 

 either a little more acetic acid or a little dilute 

 sodium carbonate and filter again. 



Boil the filtrate; a precipitate of albumin (with 



a little globulin) takes place. Filter, and to the 



filtrate 



Apply Trommer's test (Lesson XV. D, § 7), a yellow 



precipitate will be obtained showing the presence 



of milk-sugar. 



