GASES OF MILK 239 



formed in and by the secreting cells of the mammary 

 gland and does not exist in the blood of the animal. 



(iv) The inorganic salts or mineral constituents of 

 milk are many and variable, the chief being calcium 

 phosphate. Without the presence of soluble calcium 

 compounds, rennet cannot precipitate a " curd." Phos- 

 phates of potassium and magnesium are also present, as 

 well as chlorides of sodium and potassium, iron, silica, 

 and traces of other materials. In fresh milk there are 

 considerable proportions of compounds of citric and other 

 organic acids. 



(v) Gases of Milk Milk drawn fresh from the cow 

 contains up to 5 or 6 per cent, of its volume of gases, 

 the chief of which are carbon dioxide, oxygen, and 

 nitrogen, the former being in largest amount. 



Ex. 111. (a) Place a drop of milk on a slide and cover with 

 a thin slip. 



Examine with a in., J in., and -fe in. objective. Note the form 

 and size of the fat globules. Measure (Ex. 10) and make drawings 

 of them. 



If possible examine and compare the fat globules of Jersey, 

 Shorthorn and Ayrshire or Dutch cows. 



(b) Add a drop of osmic acid solution to a drop of the milk 

 and examine again as above : note the black colour of the fat 

 globules. 



Ex. 112. Dip a piece of blue or neutral litmus paper into 

 fresh milk and notice reddening or acid reaction. Dip a piece 

 of turmeric paper into it and note. 



Take 50 c.c. of fresh milk and add to it 2 c,c. of a 2 per cent, 

 solution of phenol-phthalein in alcohol. Run in slowly from a 

 burette a decinormal solution of caustic soda ; stir and note how 

 many c.c. are needed before a permanent pink colour is produced. 



Ex. 113. (a) Add a few drops of dilute hydrochloric acid to 

 some fresh milk. 



