E. J. Russell and H. B. Hutchinson 117 



§ 6. Examination of gelatine plates prepared by Koch's method 

 shows that the flora which establishes itself in the soil after heating is 

 altogether diftorcnt from that originally present, but, on the other hand, 

 the flora of the toluened soil did not appear to have markedly altered. 

 It is true that certain species were completely suppressed by toluene, 

 but their number was only small : indeed out of 27 found in the 

 untreated soil only three failed to appear in the toluened soil. Of 

 these the most striking is a fluorescent organism, which however did 

 not appear to influence the changes one way or the other. (§ 36.) 

 Further, of the two streptothrix varieties, the brown predominated in 

 the untreated soil and the white in the toluened, but their difference 

 does not appear to be significant. (§ 26.) The curves for ammonia 

 production in the heated and toluened soil (Curve 1) are very much 

 alike, whilst the bacterial flora is very different: the curves for ammonia 

 production in the toluened and untreated soil are fundamentally 

 different, whilst the bacterial flora is not. We cannot therefore 

 attribute the difference in the rate of ammonia production to a change 

 in the type of bacterial flora. 



Our experiments indicate that the increased ammonia production in 

 the partially sterilised soil is due to the increased numbers of the 

 bacteria. The problem reduces itself to finding out why the bacteria 

 can increase so much more rapidly in the partially sterilised, than in 

 the untreated soils. 



§ 7. Further evidence that the comparative inertness of the bacteria 

 in the untreated soil cannot be caused by any bacterial factor is afforded 

 by the following considerations : 



(a) If a filtered soil extract containing bacteria from an untreated 

 soil is added to a toluened soil there is an increase in the rate of 

 ammonia production, and also in the number of bacteria. 



(b) But if untreated soil is added to toluened soil there is no 

 increase in the rates of ammonia production or of bacterial multiplica- 

 tion, but, on the contrary, a reduction. These results are set out on 

 Curve 3, Table 13, § 36. 



(c) As pointed out above, an extract of toluened soil is more active 

 than an extract of untreated soil. 



(d) But when the extract of toluened soil is added to the untreated 

 soil there is no increase in ammonia production. 



The conclusion may be drawn that the untreated soil contains a factor, 

 not hactei^ial, limiting tJte development of bacteria, this factor being put out 

 of action by toluening or heating. 



