68 LABORATORY' BACTERIOLOGY. 



EXEECISE LV. 



SULFAT REDUCTION. 



1. Pill six 150 ec. Brlenmeyer flasks almost full of "sul- 

 fat reduetion" solution. 



2. Plug and sterilize as usual. 



3. Inoculate thus : 



1 & 2 — nothing. 



3 & 4 — 1 cc. of ditch water. 



5 & 6 — 2 cc. ( = 1 gm. soil ) of garden soil. 



4. Incubate 7-14 days at 20° C. 



5. At the end of that time test for sulfats by adding a 

 little BaC'U solution to a portion of the culture liquid in a 

 test tube. A white precipitate indicates the presence of 

 sulfats — the absence of a precipitate shows complete sul- 

 fat reduction. Partial reduction is indicated by the darken- 

 ing of the solution due to the formation of sulfid of iron. 



