54 



SIkiIhs on Soil J^rotor.oit 



distinction niifjlit ho due to the adsorption of sonio of the potasli by 

 the soil. It \v;is found, however, b\- titration of tlie remaining alkali 

 with acid that, under the couditions of the above described experiments, 

 only 10-15 "o of the potash was put out of action, -a tjuantity quite 

 insufficient to account for the difference in the results obtained. 



Table 4. 



Dilutions 



From these experiments, therefore, it seems highly probable that 

 heating to •'58° C. kills a considerable number of the encysted protozoa. 

 But it has been shown that heating to .58° C. is absolutely necessary 

 if one wishes to make sure of killing off all active forms (particularly 

 ciliates). From what has been said it is evident that it is impossible 

 to fix upon a temperature which will destroy all active protozoa in soils 

 and leave the cysts perfectly uninjured. This was only to be expected. 

 In the case of the bacteria the power of resistance to heat of the active 

 forms varies enormously and sometimes even surpasses that of the 

 spores of less resistant species. The same remark would appear to 

 apply to the protozoa. Further, it must be remembered that during 

 a period of excystation or encystation of a particular species it is quite 

 impossible to draw a hard and fast distinction between cyst and active 

 form. And it is obvious that the various transition forms encountered 

 in such cultures must have very varied powers of resistance to heat. ■ 

 Any temperature selected for the purpose of distinguishing active 

 protozoa from cysts must therefore be of an arbitrary nature. And 

 as it is better to select a temperature which will kill all active forms 

 even if it does injure some of the cysts, rather than one which will leave 

 the cysts unharmed and also probably some of the active forms alive, 

 the continued use of 58° C. seems to be justified. This ^new is supported 

 by the results of experiments which will presently be discussed. It has 

 been found that the numbers obtained by the dilution method after 

 heating to 58° C. (referred to, later, as "Cysts") show variations corre- 

 sponding with variations in the treatment of the material. The method, 



