GAS PRODUCTION 



291 



white precipitate to darken to a brownish-black, or 

 jet black, the depth of the colour being proportionate 

 to the amount of sulphuretted hydrogen present. 



Quantitative : For exact quantitative analyses of the 

 gases produced by bacteria from certain media of 

 definite composition, the methods devised by Pakes 

 must be employed, as follows : 



Apparatus Required: 



Bohemian flask (300 to 1500 c.c. capacity) containing from 

 100 to 400 c.c. of the medium. The mouth of the flask is fitted 



FIG. 153. Gas-collecting apparatus. 



with a perforated rubber stopper, carrying an L-shaped piece of 

 glass tubing (the short arm passing just through the stopper). 

 To the long arm of the tube is attached a piece of pressure tubing 

 some 8 cm. in length, plugged at its free end with a piece of 

 cotton-wool. Measure accurately the total capacity of the flask 

 and exit tube, also the amount of medium contained. Note the 

 difference. 



Gas receiver. This is a bell jar of stout glass, 14 cm. high and 

 9 cm. in diameter. At its apex a glass tube is fused in. This 

 rises vertically 5 cm., and is then bent at right angles, the horizon- 

 tal arm being 10 cm. in length. A three-way tap is let hori- 

 zontally into the vertical tube just above its junction with the 

 bell jar. 



An iron cylinder just large enough to contain the bell jar. 



About 1 5 kilos of metallic mercury. 



Melted paraffin. 



