E. H. Richards :311 



occurs when the animals are fed with cake ; on grass alone no nitrogen 

 is fixed. 



5. The organisms concerned in the fixation of nitrogen are present 

 in garden soil. 



6. Evidence is adduced to show that fixation is brought about by 

 a mixed culture of Azotobacter and B. lactis aerogenes. Of these the 

 latter is normally present in faeces; Azotobacter is not, but readily 

 infects faeces. Both organisms are present in the soil used and will 

 fix nitrogen in raw faeces but not in sterile faeces. 



{Received October 2lst, 1910.) 



21—2 



