PRESIDENTIAL ADDRESS. 65 



shown 1 that the maximum activity of the protozoa and of 

 the bacteria correspond very closely. 



Cunningham, 2 by sterilising soils with formalin and decom- 

 posing the residual disinfectant with lime, claims to have 

 shown that the introduction of protozoa does cause a diminu- 

 tion in the bacterial counts after 25 days. We do not know 

 how the bacteria, introduced with the protozoa, increased 

 during the first 25 days, but it is probable that there was an 

 enormous increase followed by a decrease by the twenty- 

 fifth day. I was not alone in showing that the direct intro- 

 duction of so-called pure cultures of protozoa into disin- 

 fected soil did not result in a diminution of bacteria, for 

 Russell and Hutchinson had previously obtained similar 

 results, while Lipman 3 found that it made no difference in 

 the bacterial counts whether one added suspensions of soil, 

 freed from protozoa by filtration, or unfiltered suspensions 

 to an already sterilised soil. This has also been recently 

 confirmed by Goodey 4 who worked with a soil that had 

 been stored for seventy years, and was quite free from 

 protozoa and contained few bacteria. The addition of fresh 

 or foreign bacteria caused at first an enormous increase, 

 which was followed by a decrease when the numbers 

 remained at a level below that of the moistened soil. It 

 is a general rule that, when a foreign flora is added to a 

 soil, the numbers of bacteria increase greatly within the first 

 twenty days and then decrease. The same phenomenon 

 occurs with a soil sterilised by heat and simply moistened. 

 On the other hand, a soil sterilised by a volatile disinfectant 

 shows a gradual rise to a new bacterial level. 5 This makes 

 one believe that the action of heat is largely to destroy 



1 Cunningham and Lohnis, Cent. Bakt. 2 te, 39, 596. 



2 Journ. Agric. Sci., 7, 49. 



3 Bull. N. J. Expt. Stn. No. 248. 

 * Proc. Eoy. Soc, B 606, 88, 437. 



5 Proc. Linn. Soc. New South Wales, 1913, 671. 



E— May 3, 1916. 



