H4 THE SPIRIT OF THE SOIL 



the Phosphotungstic acid fraction derived from 

 i gramme of the treated peat, while each flask of the 

 third received the Silver nitrate fraction from 

 i gramme of the treated peat. Incubation was 

 carried on at 26 C., and at the end of forty-eight 

 hours no scum had formed on the flasks which had 

 received no auximone, but nitrification had pro- 

 ceeded at a normal rate. Both sets of flasks that 

 had received auximones showed scum, but no 

 nitrification. Only two interpretations of these 

 results were possible: either the subculture fur- 

 nished a satisfactory test for auximones, or scum- 

 forming organisms had been introduced with the 

 auximones. To test this possibility a flask contain- 

 ing a subculture was incubated at 26 C. for four 

 days with no result. One half of this culture was 

 sterilized by heating in an autoclave for half an hour 

 at 160 C. To this and to the unsterilized half 

 the Phosphotungstic acid fraction derived from 

 i gramme of peat was added, and the two flasks 

 were incubated for three days at 26 C. The un- 

 sterilized flask contained a thick layer of scum, 

 while the sterilized flask had none. The explanation 

 was that the scum-forming organism is present in 

 soil cultures, but that the formation of the scum in 

 this medium is due to the presence of the auximone. 



This experiment was several times repeated, and 

 the scum organism was found in each of several soils 

 tested. Under the microscope it proved to consist of a 

 mixture of beaded rod forms and spindle-shaped forms 



Further experiments emphasized the amazing 

 sensitiveness of this bacterial test to the Silver 



