THE CULTIVATION OF MICRO-ORGANISMS I2Q 



serves as a valve. Hydrogen is passed through the jar for two 

 hours or more. It is well to keep all flames away from the appa-_ 

 ratus as a precaution against explosion of the hydrogen when 

 mixed with air. 



The hydrogen is generated by the action of 25 per cent 

 sulphuric acid on granulated zinc. It should be purified by pass- 

 ing through a wash bottle of alkaline lead acetate solution, a 

 second one containing a solution of potassium permanganate 



FIG. 57. An aerobic organism (potato bacillus) that will not grow under a cover- 

 glass. 



and a third of silver nitrate. In diluting sulphuric acid, the acid 

 must be poured slowly into the water, and the mixture cooled 

 in a bath of cold water, or under the tap. Carelessness in dilut- 

 ing this acid may allow violent boiling to occur, sometimes with 

 serious consequences. 



For critical work in anaerobic culture it is well to combine 

 the pyrogallate and hydrogen methods. This is readily accom- 

 plished by placing the Petri dishes on a low glass tripod with a 

 small amount (2 grams) of pyrogallic acid beneath them on the 



9 



