INTRODUCTION. 



Milk, which is a secretion of the lacteals of all female 

 mammalia, is especially characterized by the presence of 

 three elements common to it : casein, butter fat and milk 

 sugar. Besides these there have been found a large num- 

 ber of substances of more or less importance in different 

 kinds of milk. 



The mineral substances, phosphoric acid and calcium 

 salts are of importance in aiding the digestibility of milk. 



In close relation to the above is the ever present citric 

 acid in cow's milk. In normal milk there was found 

 among other substances, alcohol, acetic acid (Bechamp), 

 ammonia (Latschenberger), milk sugar, urinous sub- 

 stances (Bouchardat, Quevenne, Morin, Picard, Lefort), 

 lecithin, cholesterin, hypoxanthin (Tolmatchefif, Schmidt, 

 Muhlheim), fiourine (Wilson), sulpho-cyauic acid (Mus- 

 so), and kreatinin (Weyl). 



The milk of different species is distinguished not only 

 by its physical characteristics and distinct quantities of 

 substances present, but also by the fact that they show 

 themselves different toward various chemical reactions. 

 For instance, the casein of human milk is a different sub- 

 stance from the casein of cow's milk (Biedertfi, Camerer, 

 Lehman). The fat of various kinds of milk presents dif- 

 ferent chemical reactions, whereas the milk sugar in all 

 kinds is the same (Deniges). 



Examination of milk for estimating its value, embraces 

 the quantitative determination of the specific gravity, dry 

 matter, albumin, fat, sugar, ash, phosphoric acid in the 

 ash, and dirt, and the detection of adulterations and pre- 

 servatives. 



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