436 Observations on Soil Protozoa 



of clay was capable of taking out. of a suspension about 2,500,000 

 organisms per c.c. 



It was often possible by careful focussing to see the cysts closely 

 applied to the surface of the solid particles of matter. This was especially 

 true of sand, but a similar result is obtained, though less frequently, 

 with the varieties of soil employed. 



It may be objected that during the course of these experiments 

 many of the protozoa excysted and so caused an inaccuracy in the results. 

 This is of course possible, and in order to test it counts were again made 

 in the original suspension at the end of the experiment. In every case 

 the second count was comparable with the first, the difference between 

 the two being too small to affect the results. The following are typical 

 of the difference in numbers obtained at these two counts: the second 

 set of numbers is not always lower than the first, as would be the case 

 had excystation occurred to any marked extent; the variations are 

 within the experimental error. 



At the beginning of At the end of the 



the experiment experiment 



550,000 560,000 



885,000 880,000 



1,645,000 1,650,000 



1,980,000 1,990,000 



2,800,000 2,775,000 



In the next series of experiments the strength of the suspension was 

 varied through wide limits. The results given in Table IV and fig. 1 show 

 that however many flagellate and amoebic cysts are present in the sus- 

 pension the number taken up by each substance is a constant, variations 

 in different experiments being so small that they may be legitimately 

 attributed to experimental error, and not to any variation in the power 

 of the substances themselves. Sharp lines of demarcation exist between 

 the various substances as regards their capacity for withdrawing protozoa 

 from a suspensioH. 



Experiments tvith Active Flagellates and Amoebae. 



These experiments were carried out in nearl)' the same manner as 

 the preceding, except that for greater accuracy the animals in the super- 

 natant fluid were first killed by osmic vapour. 



The results are given in Table IV: the number of active forms with- 

 drawn from 1 c.c. of the fluid by the solid particles is similar to the 

 number of cysts taken by the same substance. 



