a teat de 
= “ 
me 
* 
7 
QUALITATIVE ANALYSIS OF MARKET MILK. o35° 
The solution thus neutralized is too strongly acid for bacteria 
growth. The grade of acidity which we have chosen for our 
experiments (see below) is 1.5 per cent. acid to phenol-phtha- 
lein, and the litmus neutral point of the litmus solution is 
found to be about 2.5 per cent.; z. ¢., it requires 25 cubic cen- 
timeters of normal NaOH per liter to bring it to the neutral 
point of phenol-phthalein. In order to bring the litmus to the 
grade of acidity desired, namely, 1.5 per cent., there evidently 
must be added to the neutral litmus solution 10 cubic centime- 
ters NaOH for each liter, thus bringing the acidity from 2.5 
per cent. down to 1.5 per cent. This correction is therefore 
made. It is evident that the litmus solution thus obtained 
may be mixed with a culture media having the same de- 
Pree! OL acidity without any change in acidity of the mixture. 
After bringing the litmus solution to the desired acidity, the 
whole is placed in a sterilized flask plugged with cotton, and 
sterilized with steam upon three successive days in the same 
manner as ordinary liquid media. Under these circumstances 
. the sterilization does not decolorize the litmus solution, which 
id e 
retains its deep blue color through the successive sterilizations. 
This litmus solution is then set aside to be mixed with the 
culture media at the time of using. It may be kept indefin- 
itely, care being taken to sterilize it after it has been opened for 
any purpose. 3 
The litmus obtained in commerce has a varying. strength and 
therefore cannot be relied upon always to give the same results 
where treated in the same way. Our method of meeting this 
difficulty is to buy litmus in large quantities and then by test- 
ing a sample of the material to determine the amount of the 
litmus solution that is needed to give the desired depth of blue 
in the gelatin plates. Having once determined this quantity, 
the solutions of this lot of litmus may be used in the same pro- 
portions until it is exhausted. A new lot of litmus requires a 
new test and a new proportion. By acting according to this 
simple means it is always possible to have the desired amount 
of blue color in the litmus. We have found usually that 2 
cubic centimeters of this litmus solution are required for 8 
cubic centimeters of culture medium. , 
