Protozoa in Relation to Soil Bacteria. 



299 



filtered culture liquid. A ridge of plasticine was placed along the top of 

 the glass partition in order to prevent the contents of each half mixing 



Pig. 1. — Apparatus used for separating protozoa from bacteria ; for description, 



see text. 



together. When sufficient filtered liquid had been added the plasticine was 

 gently removed and there was a slight flow from the filtered liquid into 

 the side containing the protozoa. An electric current was now passed 

 through the liquid in the trough by means of non-polarisable electrodes. 

 The latter was constructed in exactly the same way as those described and 

 figured in an earlier paper.* The current used was obtained from the 

 220 volts electric-light circuit and served the purposes of the experiment 

 excellently. A large dissecting microscope was used for observing the 

 protozoa in the trough. It was arranged that the current should flow 

 through the liquid in the trough from the mixed culture to the filtered 

 liquid, and when the current was switched on the ciliated protozoa imme- 

 diately began to swim with the current over the glass partition into the 

 filtered liquid, where they soon became so numerous that they could be 

 seen with the naked eye as motile white particles. After allowing the 

 current to flow for about five minutes the vast majority of the ciliates had 

 been driven over in the desired manner. The current ,was then switched 

 off and the plasticine ridge replaced on top of the glass partition. The liquid 

 originally containing the mixed culture of ciliates, etc., was then siphoned 

 off and the trough was carefully wiped out with a wad of cotton-wool, which 

 * Goodey, 'Boy. Sdc. Proc.,' B, vol. 84, p. 172 (1911). 



