672 CONTRIBUTIONS TO OUR KNOWLEDGE OP SOIL-FERTILITT, vi. 



decreasing the numbers of bacteria, an experiment was made to test 

 the matter. The same alluvial soil, after chloroforming, received 

 four e.c. of water, and of porcelain-filtered extracts of the strength 

 used, viz., 100 grm. to 500 c.c. After filtering, a portion was boiled 

 for an hour under an aerial condenser, and cooled. The moisture- 

 content of the tests was 19'1%. 



Experiment vii.— The Effect of Porcelain-filtered Extracts. 



The unheated filtered extract had, at first, a toxic action when the 

 numbers were lower than the control. But, as the added toxin 

 decayed, the numbers rose. The heated extract had a pronounced 

 nutritive efL'ect. The differences are not great, but they indicate 

 that the soluble substances in the extracts have a certain, though 

 small, influence upon the growth of bacteria. 



From the foregoing, it will have been seen that the larger ciliates, 

 as Colpoda cucullus, are not destroyed when comparatively large 

 amounts of volatile disinfectant are added to soil. Upon adding 

 suspensions of protozoa, there was no evidence of any limitation 

 in the numbers of the soil-bacteria. Any enhanced effect was due 

 to the addition of the bacteria contained in the suspensions. The 

 filtration of a soil-extract had no influence, beyond that of remov- 

 ing some of the bacteria in the suspension. Any phagocytic ten- 

 dencies that the soil-protozoa possess, have no influence in limiting 

 the numbers of bacteria in the soil. So far as the growth of bac- 

 teria is concerned, the effect of heat is of a different character 

 from that of a volatile disinfectant. Inferentially, the toxins and 

 nutrients of the soil are alone concerned with the changes that occur 

 wlien soils are heated, or treated with volatile disinfectants. 



