PRESIDENTIAL ADDRESS. 43 



are not attacked by micro-organisms directly or indirectly; 

 the mineral fragments are slowly dissolved, and even the 

 hitherto supposedly immune paraffin can be utilised by 

 certain bacteria as a source of carbon. * I have shown that 

 even the familiar Bac. prodigiosus is capable of decompos- 

 ing it. 2 



The flora and fauna of the soil are liable to fluctuation. 

 The seasons have a certain influence on account of the 

 variations in the temperature; some groups of bacteria 

 have a maximum efficiency in winter, others in summer. 

 The repeated wetting and drying of the soil has an influence 

 in destroying the more delicate kinds. The changes that 

 are made in the number of the bacteria by treatment of the 

 soil in different ways have only recently been determined, 

 and this has been largely due to the fact that an increase 

 in the fertility can be obtained through an alteration in the 

 nature and extent of the flora, by means of heat or by dis- 

 infectants. The most important work has been done with 

 the volatile disinfectants, as, having performed their work r 

 they simplified matters by disappearing from the soil, leav- 

 ing behind a state of affairs which brought about an increase 

 in the crop subsequently grown in the soil. 



Although the beneficial effect that follows the treatment 

 of soil with a disinfectant had been known for a long time, 

 the reason for the increase of the crop had not been inves- 

 tigated. This was specially seen in the treatment of vines 

 or potatoes with the copper-lime spray. In the absence of 

 disease the crop was generally increased by the treatment. 

 We now know that traces of the poisonous copper compound^ 

 either directly sprayed upon the soil or washed down by 

 rain from the leaves, acted upon the soil flora. The study 

 of the action of disinfectants really began in 1894, when 



1 Eahn, Cent. Bakt., 2te, 16, 362; Sohngen, Ibid., 37, 595. 



2 Proc. Linn. Soc. New South Wale=, 1914, 538. 



