260 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



chloroform, which is then evaporated spontaneously. The 

 vessel containing the residue is placed in melting ice, 2 c.c. 

 of sulphuric acid added, and stirred until the residue is 

 dissolved. Barium dioxide is dusted into the mass, with 

 constant stirring, until the liquid begins to foam, when 

 3 c.c. of hydrogen dioxide (3 per cent.) are added drop 

 by drop. The dish is then removed from the cold bath, 

 the contents diluted with water to convenient bulk, and 

 filtered. The acid filtrate is extracted with chloroform. 

 The benzoic acid will have been converted into salicylic 

 acid by the process and the latter may be detected by 

 dilute solution of ferric chloride or ammonio-ferric sul- 

 phate.'' 



Alkalies. Alkalies delay the curdling of milk by 

 neutralizing the acid formed by bacteria. The substances 

 which have been used for this purpose are the carbonate 

 and bicarbonate of soda, chalk, and potash. When mar- 

 ket milk gives an alkaline reaction to litmus or has a low 

 acidity, as determined by Mann's test, it should be tested 

 for alkalies. This test is made as follows: 



Ten c.c. of milk are mixed in a test tube with an 

 equal quantity of 95 per cent, alcohol, a few drops of a 

 1 per cent, solution of rosolic acid are added and mixed 

 with the other fluid by shaking. If an alkali is present 

 the mixture will assume a rose-red color. If no alkali 

 is present a brownish-yellow color will appear. 



STANDARD METHODS OF COUNTING BACTERIA 1 



Collection of Samples. The milk should be mixed 

 thoroughly before the sample is taken and not less than 

 10 c.c. should be collected for examination. Bottled 



1 From the report of the Laboratory Section of the Ameri- 

 can Public Health Association, Oct. 24, 1916. 



