218 BACTERIOLOGY. 
A litre of bouillon becomes on the addition of 1 per 
cent. of peptone more alkaline to litmus, but decidedly 
more acid to phenolphthalein. We ‘have, therefore, 
especially with the latter substance, to find by growing 
the bacteria just what reaction we want, and then test 
the fluid with phenolphthalein as the indicator. With 
exactly similar materials we can exactly reproduce at 
any time in the future the same reaction, but with dif- 
ferent materials this would be impossible. A bou- 
illon which contains 1 per cent. of peptone and reacts 
neutral to litmus is about 15 points acid to phenolph- 
thalein—that is, 15 c.c. of normal soda solution must 
be added per litre to make the bouillon neutral. 
When phenolphthalein is used we must have accu- 
rately standardized solutions of caustic soda and hydro- 
chloric acid. The test is carried out as follows: To 
10 c.c. of the hot nutrient bouillon add one drop of a 
1 : 300 solution in alcohol of phenolphthalein; into this 
is dropped slowly a 4 per cent. solution of caustic soda 
until a faint rose-tint appears. This indicates the be- 
ginning of an alkaline reaction. To make a litre neu- 
tral we would add 100 times as much of the decinormal 
solution of caustic soda as was required to make 10 c.c. 
neutral. As a rule, we use 1 per cent. peptone bou- 
illon of such an acidity that 15 cc. of normal soda 
solution must be added to each litre to make it neutral. 
The Sterilization of Different Media. 
Flasks and tubes of nutrient broth and agar are 
easily sterilized by placing them in an Arnold steam 
sterilizer (Fig. 20) for from fifteen minutes to one hour, 
according to the bulk of the fluid, upon two or three 
consecutive days. They can also be even more cer- 
