ABSTRACTION OF AIR FROM CULTURE MEDIA 



91 



consist of a copper pipe fitted with a manometer M and joining the pump 

 proper at a right angle T, as shown in the figure (fig. 76). 



FIG. 76. A water pump with its fittings. 



A water-pressure of about two atmospheres is necessary. The method 

 of exhausting and washing is as follows : 



1. By means of pieces of thick-walled rubber tubing (pressure-tubing), 

 connect the vessel containing the culture to the tap R", and the hydrogen- 

 generating apparatus to the tap R 7 . ^ 



Close the taps R and R', leaving R" open throughout 

 the experiment. 



2. Turn on the water tap E, gradually open R, and watch 

 the manometer needle. 



3. When the vessel has been exhausted as completely as 

 possible, close R, and by gradually opening R', fill the 

 culture vessel with hydrogen. 



4. When the manometer needle has fallen to zero, close 

 R', open R again, and exhaust the vessel a second time. 



5. After exhausting and washing with hydrogen two or 

 three times, seal the neck of the culture vessel in the flame 

 in vacuo. 



It is not always necessary to wash with hydrogen, but if 

 exhaustion alone be relied upon the culture liquid should be 

 boiled ; this can easily be done by very slowly raising the tem- 

 perature to 30-35 C. either by holding the vessel in the hand 

 or by standing it in a vessel of luke-warm water or by heating 

 it with a small flame. 



Note. In using a water pump the tap R must be closed, 

 so as to cut off all connexion between the water and the 



culture, before turning off the water tap 

 at the end of an experiment. If this pre- 

 caution be omitted, the vacuum will induce 

 a violent rush of water into the culture 

 vessel. 



A similar inrush of water will also occur 

 if from any cause whatever the pressure 

 in the main is suddenly lowered during 

 the process of exhausting ; consequently 

 a bottle of 2 or 3 litres' capacity, and fitted 

 up as shown in fig. 77, should always be 

 interposed between the pump and the 

 vessel to be exhausted. 



If then there be a rush of water, it will 

 collect in the bottle and will not contami- 

 nate the culture. It is even better to use a 

 FIG. 78.-w a ter pump with safety reservoir, pump fitted with a brass reservoir (fig. 78), 



FIG. 77. Wash- 

 bottle fitted up for use 

 with a water pump to 

 prevent backflow of 



10 the culture 



