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! 
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. 
