[HARRISON & vanderleck] .IvSCL'LIX BILE SAT/r .MElJlA 



137 



Experiiitent 3. — Petri dish vvith ivsculin iiiediuni containing 0.01'^c of iron citrate, 

 acidity + 0"6, with colonies of B. coli. 



III. — ^'o chaude has taken place in this case, galtliough the hydrochloric acid 

 has diffust'd just the same. 



We desire to emphasize the difference between submerged and 

 surface colonies. All submerged colonies of B. coJi and B. aerogenes 

 as well as varieties of these organisms have the same appearance. 

 The colony is jet black, surrounded by a black halo or field; the colour 

 is only intense immediately around the colony. Plate I. is a pho- 

 tograph (positive) of some submerged colonies of B. coll grown at 

 37°C for 20 hours. 



The surface colonies have greater variety of appearance. We 

 may obtain with varieties of B. coH and B. aerogenes, — 



1. A black colony with no field. 



2. A small black colony with a large field. 



3. A black colony with a white ring lying on a black field. 



4. A large shiny brown colony with a brown field, which usually 

 indicates B. lactis aerogenes . 



