D. W. Cutler 435 



(c) Soil from Broadbalk wheat field. 



(d) Partially sterile soil from the Broadbalk field treated for one 



hour with steam. 



(e) Ignited soil. 



(/) Eothamsted clay. 



To 1 gram of each of these substances was added 2 c.c. of a suspension 

 containing 1,645,000 amoebae and flagellate cysts per cubic centimetre. 

 The mixtures were then gently agitated for 10 minutes, after which the 

 soUd particles were allowed to settle at the bottom of the tube, and the 

 number of protozoa per c.c. of the supernatant fluid estimated by one 

 of the direct methods described in Part I of this paper. 



In all cases a control tube, containing the suspension but no solid 

 matter, was tested at the end of the experiment to see whether many 

 protozoa had sunk to the bottom of the tube : in no case was the rate of 

 sinking sufficient to affect the experiment. As a further test, after each 

 class of material had been investigated, the tube was vigorously shaken 

 and another count made. In no case was there any evidence of sedimenta- 

 tion of the cysts apart from absorption by the sohd matter. 



Coarse Sand. The total number of cysts per c.c. in the supernatant 

 fluid over the sand particles was 1,500,000: the suspension added con- 

 tained 1,645,000 cysts per c.c: the number taken up by the sand was 

 therefore 145,000 cysts per c.c. of fluid. 



Fine Sand. Under the same conditions the supernatant fluid con- 

 tained 5.50,000 organisiiis per c.c. : the fine sand was therefore capable 

 of withdi-awing from the suspension 999,000 cysts per c.c. 



Soil and partially sterilised soil. These two substances gave identical 

 results; in each case 1,643,250 cysts per c.c. were taken out from the 

 suspension. 



Ignited soil. This was tried to ascertain whether the colloids of the 

 soil were concerned in the withdrawal of protozoa from the suspension. 

 If they are, ignition which destroys some of the colloid properties might 

 be expected considerably to reduce the number of cysts taken up from 

 the suspension. This actually happened, but the reduction in efl'ective- 

 ness was much smaller than was anticipated, for the ignited soil took 

 up 1,501,250 organisms per c.c, or 142,000 per c.c. less than the partially 

 sterilised or untreated soils. 



Clay. In this case microscopic examination was rendered diflicult 

 by the non-settlement of the clay particles, but the estimation could 

 still be made : 1 gram of clay withdrew from the suspension all the pro- 

 tozoa. A later experiment, however (Table IV), demonstrated that 1 gm. 



