172 



Mr. T. Goodey. A Contribution to our 



[May 19, 



cathode. On these lines it seemed possible to devise a method of applying 

 the electric stimulus to the soil protozoa, and cause them to leave the soil 

 and come to a point convenient for observation, assuming, of course, that 

 they were present in a free -living condition. 



In preliminary experiments, the current was passed into the culture liquid 

 through two non-polarisable electrodes (see fig.) similar to those described 

 by Verworn.* Each consisted of a glass tube about 8 cm. long by 6 mm. in 

 diameter, one end of which was plugged with fresh moist clay,f and into 

 this a tip of porous clay was inserted. 



A concentrated solution of zinc sulphate was placed inside the tube and 



into this an amalgamated zinc rod dipped, kept in position by means of 

 a rubber band surrounding it and fitting the mouth of the tube. The upper 

 end of each rod was fitted with a screw so as to serve as a terminal. By 

 inserting a milliammeter in the circuit the current passing could be read off. 



Actively swimming ciliated protozoa in culture liquid quickly responded 

 to the current and collected at the cathode. The presence of soil-particles, 

 however, in any considerable quantity prevented the ciliates from assembling 



* Loc. cit., p. 455. 



t Care must be taken to renew the moist clay plugs each day during their use, and to 

 keep the porous clay tips clean and free from ZnS0 4 which gradually soaks into them. 



Non-polarisable electrode. 



a. Rubber band. 



b. Amalgamated zinc rod. 



c. Concentrated zinc-sulphate solution. 



d. Moist clay plug. 



e. Tip of porous clay. 



d 



