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 c.c. 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- 



