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 acid. 



o. 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 

 >f milk-sugar. 



