Andrew Cunningham 



51 



of view of regularity, therefore, the 30 days' is no better than the five 

 days' period, although it gives a slightly higher result. But 30 days 

 is as long an incubation period as could be conveniently adopted and 

 consequently it was not considered advisable to investigate the efEects 

 of a longer period. And, as five days' incubation gives as satisfactory 

 results as 30 days, the former has been adopted in subsequent work. 

 The question of the higher results obtained after 30 days need not 

 be considered in view of the much greater convenience of the shorter 

 period. In any case the dilution method, here as with bacteriological 

 work, gives only relative, not absolute results. The whole of the 

 protozoa in soils do not develop in soil extract. 



Table 1. 



With a view to obtaining more regularity in the results some slight 

 modifications of the method were tried. In the dilution method, as 

 used in bacteriological work, the addition of small quantities of sterile 

 soil to the medium is found to have a beneficial effect on the growth 

 of the soil bacteria. It was thought that the same might apply to the 

 protozoa but this has not proved to be the case. Indeed, the use of 

 sterile soil results in a much slower excystation of the protozoa and 

 no improvement as regards regularity. The retardation of excystation 

 is probably due to the extraordinarily beneficial effect which the soil 

 has on the growth of the protozoa-free culture before the inoculation 

 with protozoa. It has frecjuently been observed that the development 

 of protozoa in a medium containing exceptionally large numbers of 

 bacteria is considerably hindered. In a further experiment the inocu- 

 lation with protozoa-free culture was omitted, only sterile soil being 

 added. The results obtained were very low and quite as irregular as 

 in the previous case. 



The effect of the reaction of the medium was next tested. But 

 soil extract -|- chalk as well as soil extract -i- -01 % hydrochloric acid 

 and soil extract + "01 % caustic potash effected no improvement with 



