334 LABORATORY WORK 
The method for using this indicator is as follows: 
Use only a few drops of the indicator in the 10 c.c. of medium. The mixture 
remains colorless as long as it is acid to phenolphthalein. Add }{9 normal NaOH 
as above until it becomes a faint but permanent pink. Then compute, as above, 
the number of cubic centimeters;of normal NaOH needed to bring the whole medium 
to the neutral point of the indicator. The neutral point of phenolphthalein, how- 
ever, is considerably more alkaline than that of brom thymol blue; hence add ro 
c.c. less per liter of normal NaOH than necessary to bring it to the neutral point. 
Fic. §7.—Showing method of filtering culture media. 
The phenolphthalein method is not as accurate as the brom thymol blue method, 
and cannot be applied to all culture media. For media of the above composition, 
however, both methods give approximately the same results. 
(c) Boil vigorously for fifteen minutes and then restore the original weight by 
adding water. Test the reaction again as before to see if itis correct. If the reac- 
tion is right, cool to about 60°F. and add the white of an egg which has been mixed 
with a little water, adding slowly with stirring. Heat slowly and allow to simmer 
until the egg has coagulated and the liquid is clear. This may take half an hour or 
more. Add water to bring to the original weight again, plus the weight of the added 
egg, and then filter as follows: 
(d) Place a considerable quantity of absorbent cotton in a large funnel (Fig. 
s7). Over the top of a second funnel above the first, place some cheese-cloth. 
Pour the hot agar medium into the cheese-cloth. It will run through rapidly; then 
