BY R. GREIG-SMITH. 843 



blue upon treatment with acetic acid and potassium ferricyanide. 

 Sulphide of iron was thus indicated. The presence of sulphide 

 and the absence of ciliates, and vice- versa, point to the possibility 

 of the sulphur-oxidising bacteria having been destroyed by the 

 treatment with toluene. The fact was recalled that, in some of 

 the tests, the amoebae moved sluggishly, and it appeared probable 

 that they had been partly poisoned by traces of sulphuretted 

 hydrogen in the culture-fluids. 



The following experiment was made to confirm the idea. 

 Garden-soil was suspended in Giltay's solution in a conical flask, 

 containing a test-tube holding potassium sulphide. A few drops 

 of dilute sulphuric acid were added to the sulphide, and the flask 

 was corked. Five days later, amoebae and monads were detected, 

 but no ciliates were found. The living protozoa moved slowly, 

 and were undoubtedly unhealthy. In another experiment, a 

 stream of sulphuretted hydrogen was passed through two bottles, 

 one containing a damp, the other a dry soil. After five minutes, 

 the bottles were corked. Next day, the soils were spread out 

 and aired. The dry soil, wlien sown in Giltay's solution, gave 

 rise to a mixed fauna; the damp soil was free from protozoa. 



It is clear that, if the conditions are such that a reduction of 

 sulphate is possible, as, for example, in soils containing much 

 organic matter, there is the possibility that the action of a 

 volatile disinfectant, by destroying some of the groups of the 

 sulphur-oxidising bacteria, may indirectly affect the growth of 

 the protozoa, and especially of the ciliates. This, however, is not 

 always the reason for the non-appearance of ciliates in cultures 

 from toluened damp soils, as will be shown subsequently. 



In view of the fact that toluene had been found to have no 

 effect upon protozoa in air-dried soil, so far as certain members 

 of each group were concerned, it was thought advisable to con- 

 firm the matter by examining a few more soils. Accordingly, 

 six fresh soils were obtained from the Hawkesbury Agricultural 

 College, through the kindness of the Principal, Mr. H. W. Potts. 

 The weather had been very dry for the preceding four months, 

 and the soils reached the laboratory in the air-dry condition. 

 The moisture ranged from 0-5 % in a gravelly soil(\V,H.C. = 16), 



