178 



NORMAL MILK : ITS ADULTERATIONS, ETC. 



organisms in milk, and thus make it keep for a longer time tlian 

 it otherwise would, preservatives are frequently added. The 

 most common additions for this purpose are boric acid and its 

 sodium salt, borax ; salicylic acid, either alone or mixed with 

 borax and boric acid, and sometimes in alcohol or glycerol 

 solution ; fluorides, such as sodium fluoride or potassium acid 

 fluoride ; fluosilicates and fluoborates ; ;S-naphthol and salts 

 of the /8-naphthol-sulphonic acids (abrastol), formaldehyde, and 

 benzoic acid. Potassium nitrate has also been recommended, 

 but it is a comparatively weak antiseptic, and is little, if at all, 

 used. Hydrogen peroxide is also used. 



Fig. 16.— Blood in Milk. 



Effect of Preservatives.— The effect of boric acid preser- 

 vative and formaldehyde has been studied by the author and 

 Harrison; taking milk which just curdles on boiling as the 

 limit of fresh milk, the following are the extra times that the 

 preservatives will keep milk fresh : — 



