THE CULTIVATION OF BACTERIA. 83 



and is immersed in the water standing in the water-jacket. 

 There are various forms of gas-regulators more or less 

 complicated. In general they consist of a tube containing 

 mercury; into this tube are two openings, one for the entrance 



FIG. 22. Incubator. 



and the other for the exit of gas. The gas enters through a 

 small tube, which is cut off diagonally at the bottom, and which 

 projects into the surface of the mercury. Heating the water 

 in the water-jacket causes expansion of the mercury, which 



