POLLUTED SOIL 149 



seemingly lead to other than a temporary increase of the sewage 

 microbes at the expense of the soil microbes, the ordinary soil 

 bacteria ousting the sewage microbes in the struggle for existence. 

 But the addition of sewage to a sandy soil leads to an enormous 

 increase in the total number of microbes as compared with the 

 number originally present in the soil, which does not revert to its 

 original state for some months. 



(2) The addition of sewage to garden soil tends primarily to 

 increase the ratio of total number of bacteria to spores of aerobic 

 bacteria, though the alteration is apt to be soon lost. 



(3) The addition of sewage to a soil leads to an increase for a time 

 in the number of certain kinds of bacteria, namely: (a) indol- 

 producing bacteria ; (b) gas-producing organisms ; (c) the spores of 

 £. enteritidis sporogenes ; (d) B. coli communis and its allies ; and (e) 

 streptococci. The occurrence of true streptococci in soil indicates, in 

 Houston's opiaion, extremely recent contamination. Whatever inter^ 

 pretation be placed on these facts, it is evident that they indicate that 

 pathogenic organisms such as the typhoid bacillus do not maintain 

 their vitality in the surface layers of soil for more than a brief 

 period. Further, it is evident that some kinds of soils heavily 

 polluted with excremental matter tend to purify themselves, so far as 

 non-sporing bacilli of intestinal origin are concerned. 



On the, bacterial content of made-soil Dr Savage of Colchester has 

 carried out some work. The samples of such soil were collected with 

 a sterilised Frankel's borer, and the samples transmitted to the 

 laboratory in sterile Petri dishes. Each sample was then thoroughly 

 broken up and uniformly mixed in the Petri dishes by means of 

 sterile spatulas. Ten grammes were weighed on sterile glazed paper, 

 and added to 100 c.c. of sterile water in a large flask, and thoroughly 

 mixed. The contents of the flask were allowed to settle for five 

 minutes, and without disturbing the sediment 1 c.c. of the water was 

 taken up and added to further quantities of sterile water for dilution 

 purposes. The examination was then carried out in the ordinary way, 

 with a view of determiniag {a) total number of organisms present ; 

 (6) number of B. coli ; (c) number of £. mycoides, and of Bismark- 

 brown Cladothrix; and (d) the smallest quantity of soil producing 

 indol in one week at 37° C. grown in peptone water solution. 



As a result of these experiments, Dr Savage reports that at a 

 depth of two feet in mounds of tip-refuse deposited on damp imper- 

 vious clay, putrefaction and concurrent purification takes place fairly 

 rapidly for the first two or three years. The organisms present in 

 the refuse as deposited rapidly decrease at the same time, but after 

 two to three years increase, apparently due to the invasion of ordinary 

 soil organisms. After two to three years, purification at this depth 

 takes place extremely slowly, and samples nine to ten years old give 



