D. W. Cutler 439 



affected, showing that the action between the surface particles and 

 protozoa is practically instantaneous. 



* In the above table F indicates that the suspension was filtered through the solid 

 material. 



Ejfect of filtering suspension through soil. 



The soil or sand was placed in the bulb of a 20 c.c. pipette, and 

 the suspension allowed to filter through. Examination of the filtrate 

 showed that the number of organisms retained by the solid matter was 

 the same as in the experiments detailed above. 



Further the results obtained by using a suspension of protozoa pre- 

 viously killed by heating at 80° C. for 5 minutes were identical with 

 those obtained when the organisms were alive: the action is therefore 

 physico-chemical and is not determined by any "vital factor." 



In Table V are given in tabulated form the results obtained when 

 the various factors described above are varied. 



Experiments with Ciliates. 



Both active and cystic forms of Colpoda cucullus were investigated. 

 The procedure was that employed for amoebae and flagellates, except 

 that the counting was always done by the 500 square method described 



