PRESIDENTIAL ADDRESS. 



51 



The plant stimulation hypothesis presupposes that some 

 of the disinfectant is retained by the soil or at least, that , 

 some of the decomposition products remain. So far as 

 carbon disulphide is concerned, and this is the volatile 

 disinfectant upon which most work was done in the early 

 stages, distinct traces of the decomposition product, sul- 

 phuric acid, were found up to three months after treat- 

 ment. 1 As it could not be detected five months after, it 

 was concluded that it had been washed into the subsoil. 

 The presence of the sulphuric acid led to the idea that the 

 mineral constituents of the soil might have been made more 

 soluble, but upon investigating the phosphoric acid, neither 

 water-soluble nor citrate-soluble phosphate could be de- 

 tected. A treated soil, which was moist when stored, but 

 which slowly dried, contained traces of sulphuric acid, two 

 years after treatment. A watery extract of it was made, 

 and this was found to contain a greater amount of substance 

 precipitable by alcohol than that from a similar control soil. 

 The lime, magnesia, potash, ammonia and sulphuric acid 

 were found in greater amount than in the control, while 

 the phosphoric acid was unaltered and the nitrates were 

 reduced. From this experiment, Heinze considered that 

 the disinfectant acted partly by decomposing to sulphuric 

 acid which acted as a mineral solvent. The disinfectant 

 itself was found as traces in soil, a month after treatment, 

 and the greater the quantity used the greater were the 

 traces. 2 



By 1908 it was known that not only the volatile disin- 

 fectants, including such diverse substances as carbon 

 d isulphide, chloroform, carbon tetrachloride, ether, hydrogen 

 peroxide, benzene and toluene, but also the non-volatile 

 poisons, such as arsenious oxide and some metallic poisons, 



1 Moritz and Scherpe, Cent. Bakt. 2 te., 13, 573. 

 * Heinze (1907), ibid., 18, 56. 



