E. H. Richards 307 



difference was only 2-6 %, or just about the experimental error of the 

 method. 



Time taken for Nitrogen fixation. 



As will be seen subsequently the amount of nitrogen, 2-5 mgms., fixed 

 in 28 days corresponds fairly well with the 2-2 mgms. fixed by a mixed 

 culture of Azotobacter and B. lactis aerogenes acting on 0-5 gram of starch 

 for 25 days at 20° C. 



An attempt was made to cultivate the nitrogen fixing organisms 

 which had operated so energetically in the first comparative experiment 

 with horse and bullock faeces (p. 302) by inoculation from one tap- 

 water unit of each type. This was not done until the close of the experi- 

 ment, i.e. 134 days from the start, as the information afforded by the 

 last of the horse dung experiments was not then available. Repeated 

 trials with 2 % mannite-mineral salt solution gave negative results. 

 The inoculations should have been made from the 14th to the 28th 

 day during the period of active fixation. Examination of material 

 from the fermentation flasks revealed nothing like Azotobacter or 

 Clostridinm. Bacterial life of any kind appeared to be in abeyance but 

 time did not permit of any thorough examination of the types present. 



The brown film referred to previously seemed to be a mechanically 

 formed skin analogous to that forming on heated milk rather than a 

 biological scum or zoogloea. It puckered in just the same way as milk 

 and had a rather silky surface. There were no bubbles. 



Concurrently with the chemical examination of the final series of 

 horse dung experiments (p. 306), inoculations were made into both 

 mannite and dextrose media. To each 50 c.c. of 2 % solution 1 gram of 

 CaCOg was added and the flasks incubated at 25° C. The first inocula- 

 tion from the 14-day flask gave considerable growth in mannite and 

 still more in dextrose. The most conspicuous organism in both cases 



