Andrew Cunningham 



53 



22" C, A for nine days, R for J 6 days. Protozoa counts were then 

 made as above described. Table 2 shows the results. 



In these cases as a result of drying one would naturally expect at 

 least a certain number of the protozoa to encyst and the total numbers 

 to be equal to those obtained after heating to 58° C. provided the latter 

 treatment had no injurious action on the cysts. But in both cases 

 the heating appears to have destroyed a number of the encysted 

 organisms. In the first case the distinction is small but in the second 

 it is considerable. 



In order to obtain further evidence on this point the effect of treat- 

 ment with caustic potash on cysts and active forms was examined. As 

 a preliminary experiment soil extract cultures of protozoa showing 

 numerous active and encysted forms were treated for varying lengths 

 of time with equal quantities of a -5 % caustic potash solution, so that 

 the concentration of the alkali in the cultures was -25 % . At the end 

 of the period of treatment a drop of phenolphthalein solution was 

 added to each culture and the potash neutralised with dilute lactic 

 acid. The cultures were allowed to settle for half-an-honr and then 

 examined microscopically. After five days' incubation at 22° C. they 

 were examined once more. The results are given in Table 3. + indi- 

 cates the presence of active organisms. 



Table 3. 



The treatment with potash killed all active protozoa but left the 

 encysted uninjured and the latter were able to excyst within five days. 



The treatment with -25 % potash for one hour was applied in the 

 dilution method. Dilutions were made from a sample of soil, in the 

 one case after the soil had been heated to 58° C. in the 100 dilution, 

 in the other after it had been treated with -25 % potash for one hour, 

 also in the 100 dilution. Table 4 shows the results. 



Here it will be observed that the heat has had a more drastic action 

 on the cysts than has the potash. It appeared possible that this 



