CLASSIFICATION OF BACTERIA 289 



acid; a pink color shows it to be alkaline. If acid (as is usual), run 

 in from the burette, drop by drop, stirring the mixture constantly, 

 N/ 20 NaOH until a faint, permanent, pink develops. The amount 

 of N/ 20 NaOH used to produce this color if read in c.c.s and frac- 

 tions, gives directly the percentage of normal alkali theoretically 

 needed to neutralize the medium ; the same figure indicates the 

 percentage acidity (in normal acid) of the medium as it is at the 

 moment. The difference between the existing percentage acidity of 

 the medium and the percentage acidity desired, read in c.c.s and 

 fractions, indicates the amount of normal alkali or acid per ioo> 

 c.c.s of media which should (theoretically) be added, to attain the 

 desired reaction. 



Examples of Titration of Media. — 1. If the filtrate (Step 3 

 schedule given above for making the principal artificial media) be 

 titrated it will prove to be about 2.5% acid. The steps are: 



(a) Remove 5 c.c.s to the evaporating dish; add 45 c.c.s 

 neutral water. (The water is added merely to give enough bulk 

 to boil, without too much evaporation or burning of the media,, 

 as might occur if 5 c.c.s of media, undiluted, were used.) The 

 absolute quantity of water added is of no moment, but it is im- 

 portant that the same quantity be used in each titration, since the 

 color developed when the phenolphthalein turns pink will vary in 

 depth if the amounts of water added vary because of the variations 

 in dilution thus introduced.) 



(b) Boil 1 minute; the albumens coagulate and the reddish 

 color of the meat infusion diluted by the added water largely 

 disappears. 



(c) Add 1 c.c. phenolphthalein solution. 



(d) To the still simmering mixture add N/ 20 NaOH as de- 

 scribed above. 



(e) If 2.5 c.c.s of N/ 20 NaOH are necessary to neutralize the 

 |5 c.c.s of meat infusion, 2.5 c.c.s of normal NaOH (twenty times. 



