THE COMPOSITION OF MILK 21 



detected only a trace, or none at all; moreover, milk 

 always contains considerable carbonic acid and a slight 

 quantity of nitrogen. Upon standing, tbis relation 

 changes, for carbonic acid escapes and oxygen is 

 absorbed.* 



III. THE COMPOSITION OF MILK OF DIFFERENT 

 SPECIES OF ANIMALS 



It has been stated that the milks of different species 

 of animals consist of essentially the same materials, but 

 that in their quantitative composition they show quite a 

 little difference. As will be considered in detail later, 

 there are great variations with individual animals and, 

 therefore, the figures given are to be regarded as aver- 

 ages and they are to be accepted with some reserve, for 

 not all are the result of a sufficiently great number of 

 analyses; still, they give quite a good picture of the 

 striking peculiarities of the different kinds of milk. 



Cow's milk is white, opaque, has a slightly sweetish 

 taste and a very slight odor. It shows amphoteric reac- 

 tion to litmus paper, e.g., it colors red litmus paper blu- 

 ish and blue reddish. To other indicators, milk some- 

 times shows itself to be acid, sometimes alkaline. Ac- 

 cording to Courant, it can be demonstrated that 100 c.c. 

 of fresh cow's milk shows the same amount of alkalinity 

 toward blue litmus as 41 c.c. of a one-tenth normal solu- 

 tion of sodium hydrate and the amount of acidity, as 

 measured by phenolphthalein, is equivalent to that of 

 19.5 c.c. of a one-tenth normal solution of sulphuric 

 acid. However, milk does not always react the same, 

 it changes especially during the course of the period 

 of lactation. The casein of cow's milk is precipitated 



* Richmond says : "As the milk is kept the amount of oxygen 

 decreases and that of carbon dioxide increases; this is probably 

 due to aerobic microorganisms, which absorb the oxygen and give 

 out carbon dioxide." [L. P.] 



