CULTURE MEDIA. 49 
The cylinder of potato rests upon the constricted portion of the tube, 
as shown in Fig. 21. 
Sometimes a potato paste is employed. The potatoes are boiled 
for an hour and the skins removed, after which they are mashed 
with a little sterilized water, placed in suitable plates, and sterilized 
by exposure for half an hour on three successive days in the steam 
sterilizer. Bread paste may be made in the same way, and is a very 
favorable medium for the growth of certain bacteria and also for the 
common moulds. 
e eT 
‘titi iil i= 
rar i 
i 
Fie. 20, Fie. 21. Fie. 22. 
Neutralization of Culture Media. — For ordinary purposes 
neutralization of acid culture media is accomplished by the use of a 
saturated solution of sodium carbonate, the reaction being tested with 
strips of blue and red litmus paper. But for certain investigations 
it is essential that a more sensitive and reliable indicator should be 
used, and that an exact method of titration be employed. Schultz 
(1891) recommends the use of phenolphthalein as an indicator and 
titration with a solution of caustic soda (four-per-cent stock solution, 
to be diluted to 0.4 per cent for use). One drop of phenolphthalein 
solution, containing one gramme to three hundred cubic centimetres 
