24l* CONTUIHUTIONS T(t (»UR KNoWLKDnK OF SOIL-FKR TILITV. V., 



fatty matters of the sewage-sick soil are translated with the 

 solvent, and deposited near the surface. It is clear that the 

 treatment of the sewage-sick soil with volatile disinfectant does 

 more than destroy the so-called phagocytic protozoa. 



Tn dealing with the action of the fat-solvents upon soils, it is a 

 comparatively easy matter to eliminate the protozoa by heating 

 the soil at 60° for 10 minutes.* WheJi this is done, the action 

 of the solvent upon the agricere becomes clearly demonstrated. 

 In the following experiment, a portion of the air dried, sewage- 

 sick soil was heated at 62° for 10 minutes. Part of this heated 

 soil was treated with chloroform, and upon the evaporation of 

 the solvent, the soil was thoroughly mixed. Portions, each 

 weighing lOgrm., of the not heated, heated, and heated and chloro- 

 formed soils were put into small bottles, and moistened with tlu"ee 

 c.c. of sterile water. On the twelfth day, a further two c.c. of 

 water were added to each of the remaining bottles, which wei'e 

 then covered with a small bell-jar to minimise evaporation. 



Soir^-BACTKRIA IN OUOOl GRM. 



Sewage-sick Soil. 



I At start. I 4 days. 



Untreated soil • 52 



Soil heated at 62° j 16 



Soil healed at 62' and treated 



with chloroform 13 



680 

 15,800 



24,600 



12 days. I 25 days. 



2,700 



11,>500 



4,300 

 9,000 



45,400 j 41,600 



39 days. 



5,400 

 8,000 



90,000 



It may be that the heating at 62°, by killing oft' tlie protozoa, 

 has enabled the bacteria to grow more freely during the first four 

 daj^s, but the subsequent decrease rather points to the heat 



* After noting that the harniful protozoa, etc., are destroyed at 60°, 

 Russell and (folding heated the sewage-sick soils to 98° for tiirce hours. 

 For the purpose of their argument, this excessive liealing was unnecessary. 

 The effect is to destroy the natural toxins, and, as has been pointed out 

 by Pickering, Russell and Hutchinson, and myself, to develop heat-toxius. 

 The presence of natural .soil-toxins is denied by Russell and Gelding, who 

 say that there is no satisfactory evidence of any toxic substance in soils 

 that are not acid. Since their paper was published, I have shown that 

 tap water can extract from ordinary soils certain substances of the nature 

 of bacterial toxins. These are not acid, and reduce the number of living 

 l)acteria purpo.sely added to tiie extract. 



