IS4 ROYAL SOCIETY OF CAxVADA 



Acidity -|- 0.75 0.1% Iron. Poor growth, black fields. 



" 0.3% " " " " black, very small 



fields. 



« (I Q 4(y u u II u a u a 



" 0.5%, " Very little growth, l)lack very small 

 fields. 

 This test shows clearly by the very small colonies formed that 

 the acidity was too much. 

 Acidity -|- 1. 0.1% Iron. Poor growth, I)lack fields. 



0.2% " Good growth, black fields, but small 



in size. 

 0.3% " Good growth, black fields but small in 



size. 

 0.4% " Poor growth, black fields but small in 



size. 

 0.5% " No growth. 

 The following test was conducted on another principle. Plates 

 were made from river water with slight differences in the acidity 

 and amount of iron in the medium. These plates were incubated at 

 37°C., examined after 48 hours and from the best plates the acidity 

 was determined, while the amount of iron added was known before. 

 As the quantity of the medium used was known, and the 

 evaporation in the incubation could not have influenced the total 

 amount of acid, the original acidity could be easily estimated. 



Medium as already described with 0.5% iron citrate scales 

 acidity -f 0.5. 



Tests made with river water containing B. coli. 



Different amoimt of iron citrate. 



0.3% acidity -f" 0.57 Black, well defined colonies. 



0.4%o " + 0.72 Well defined colonies. 



0.5% " +0.90 " " " but smaller in size. 



Iro7i citrate increased to 0.8% with varyiitg amou7it of acid. 



Acidity -\- 1.2 Black fields, small in size. 

 " -|- 0.4 " " very small in size. 



" -f 0.6 " well defined fields of good size. 



These tests show clearly — 



(1) That any amount of iron is insufficient as soon as the 

 acidity is less than -j- 0.6. 



(2) That sesculetin is formed and diffuses whether iron is pre- 

 sent or not. 



