PRESIDENTIAL ADDRESS. 



77 



nutritive, and this effect is at a maximum when the ratio 

 of soil to water is about two to one, that is when the soil 

 is little more than saturated. 



With regard to the toxicity of the teachings of grassed 

 soils, Miller 1 remarked that it was strange that they should 

 be toxic, while the same water in contact with the soil in 

 a pot should be nutritive. It is strange, but the fact 

 remains that when the soil is watered, a toxicity does not 

 make itself palpably evident. If the same soil is leached r 

 the toxicity can be clearly shown. It is the same with the 

 action of the soil extracts upon the growth of bacteria, no 

 palpable or direct evidence can be shown until the soil- 

 water ratio is the same as would be used in leaching a soil. 

 There is thus a parallel between my results with bacterio- 

 toxins and those of the Duke of Bedford and Pickering with 

 plant toxins. There may be more than a parallel, for as 

 the authors consider that the plant toxins are derived from 

 the decay of root debris by bacteria, there is the probability 

 that the bacteriotoxins and plant toxins are one and the 

 same. 



Toxins only act as poisons when they are in solution in 

 sufficient concentration to enable their toxic character to 

 become evident. In smaller amount their action is reversed,, 

 and they act as stimulants. This applies to metallic 

 poisons and to disinfectants, and I have shown that it also 

 applies to the bacteriotoxins. 2 When they are diluted by 

 a half or even more, the solution instead of being toxic 

 becomes nutritive, in some cases extremely so. Consider- 

 able dilution however weakens the concentration of the 

 nutrients in the extract, and the bacterial increase due to 

 stimulation by the weakened toxin becomes non-evident. 



The soil toxins may be chemical entities, but they are 

 different from such toxic substances as coumarin, vanillin^ 



1 Chem. Soc. Ann. Eept., 1914, 232. 

 a Proc. Linn. Soc. New South Wales, 1913, 725. 



