r>0 LABOUATOEY BACTERIOLOGY. 



EXEECISE XXXVIII. 



THE EFFECT OF VARYING REACTION OF THE MEDIUM ON 

 NITROGEN FIXATION BY AlZOTOBACTER. 



1. Make up 1200 cc. of "mannite" solution as described 

 except that it is not neutralized. 



2. Divide into twelve 100 cc. portions in 300 cc. 



3. Neutralize thus : 



1 & 2 — i of the acidity neutralized 

 3 & 4 — i of the acidity neutralized } Az. vinelandii. 

 5 & 6 — all of the acidity neutralized 

 7 & S — i of the acidity neutralized 

 9 & 10 — i of the acidity neutralized \- Az. chroococcum. 

 11 & 12 — all of the acidity neutralized 



4. Plug and sterilize in the autoclave as usual. 



6. Inoculate with 1 cc. of vigorous cultures of Az. vine- 

 landii and Az. chroococcum. 



6. Incubate 6-7 days at 30° C. 



7. Add 0.7 gm. Hg. and 20 cc. H2SO4 and determine the 

 nitrogen present by the regular Kjeldahl method. 



8. Determine thus the effect of varying reaction of the 

 medium on the nitrogen fixed by the pure cultures. 



