MILK AS A FOOD 31 



however, are to be found only in the milk of certain indi- 

 vidual cows. The specific gravity of the mixed milk from 

 a herd falls somewhere between 1.028 and 1.034. If the 

 specific gravity is either above or below these limits, it is 

 always suspicious. If the specific gravity is high, it sug- 

 gests skimming; if low, watering. Skim milk has a specific 

 gravity of from 1.033 to 1.037. 



Milk freezes at a temperature somewhat lower than the 

 freezing-point of water ( 0.55 C.). 



On account of the presence of dissolved salts of various 

 kinds milk conducts an electric current. Koeppe, who in- 

 vestigated this question, concluded that in cow's milk 

 0.072, and in human milk 0.04 grammolecules (Molen) 

 exist in the ionic condition. In other words, 58 per cent of 

 the molecules in cow's milk are dissociated, while only 26 

 per cent of the molecules in human milk are dissociated. 



The specific heat of milk has been determined by Fleish- 

 mann. For milk containing 3.17 per cent of fat he finds 

 the specific heat to be 0.9457. This same author also de- 

 termined the coefficient of expansion of milk by heat and 

 found it to be greater than that of water between 5 and 

 15 C. According to Fleishmann milk shows no maximum 

 of density above 1 C. 



Milk is much more viscid than water. Thus at C. the 

 ratio of the intervals required for the delivery of the same 

 volume of water and milk is 100 : 221.1; at 15 C. the 

 ratio is 100 : 188.7; and at 30 C. the ratio is 100 : 169.0; 

 that is, milk becomes less viscid as its temperature rises. 



Perfectly fresh milk of carnivorous animals is, as a rule, 

 acid in reaction. This is probably due to the carbon dioxide 

 and phosphates. Human milk and that of herbivora is 

 usually slightly alkaline when freshly drawn. Cow's milk 

 is described as amphoteric; that is, it turns red litmus blue, 

 and blue litmus red. On standing exposed to the air for 

 some time, milk almost always becomes more or less acid 

 in reaction in consequence of the conversion of the milk 



