660 CONTRIBUTIONS TO OUR KNOWLEDGE OF SOlL-FBRTILITY,vi., 



toluene to kill olf all protozoa, but, in the later part of their paper, 

 the statement occurs that toluene does not kill oft' all the larger 

 oi'ganisms, one, at least, a small ciliated protozoon being left; and 

 this is probably concerned with the diminution of the activity of 

 the treated soil, after a long period, as, for example, in the second 

 crop. The impression is left, however, that the disinfectant kills 

 oft' living and encysted forms of Colpuda cucullus, the chief food 

 of which is said to be bacteria. 



In my experiments, I found tiiat Colpoda cucullus was, of all the 

 soil-protozoa, least affected by disinfectants. It occurred in infu- 

 sions seeded with soils which had been treated for three days with 

 20 % of toluene (Kahlbaum), or with 10 % of chloroform 

 (Schering). 



With regard to its food, the partiality for bacteria is open to 

 question. Fi'om observation, they appear to feed upon organic 

 debris of any kind, and any bacteria that they consume are drawn 

 in accidentally. They are specially fond of the slimy matter 

 exuded by the encysting cell. It should not be forgotten that the 

 digestion of the organic debris will give rise to waste products con- 

 taining nutrients available for bacteria, thus augmenting the food 

 at the disposal of the remaining microbes, winch will respond by 

 growing more quickly. The bacterial increase should not, for this 

 reason, be lessened by the presence of Colpoda. It appears that, 

 if any real phagocytic effect in reducing the bacterial numbers is 

 to be ascribed to any protozoa, it should be to the amoebae rather 

 than to the ciliates. The ama?ba) are destroyed by comparatively 

 small amounts of disinfectants; they were detected in infusions 

 seeded with soils which had been treated with 1% of chloroform, 

 but not with 2%. 



It is unfortunate that Kussell and Ilutcliinson did not use 

 enough disinfectant to ensure the comi)lete destruction of all the 

 protozoa in their experimental work, as tliere is the doul)t raised 

 that, so far as the protozoa are concerned, their disinfection had 

 been abortive. And yet the point claimed by these authors is, that 

 tlie protozoa, and especially Colpoda, had been destroyed, and, in 

 consequence, the bacteria had increased. Miglit not the proportion 



