CH. II] BACTERIAL METHOD. 41 



minute or so and gently shaken in contact with the KHO, 

 the clamp G and stop-cock L being closed meanwhile. 

 When absorption should be complete the gas is sucked 

 back into G (fig. 9) by lowering F, with C and L open. L 

 is then closed and G and F brought to a level so that the 

 diminished volume of gas can be again read off. The 

 difference gives the amount of CO2 originally present. To 

 make sure of complete absorption the gas may be again 

 passed into B, shaken and returned, when it should show 

 no further reduction in volume. 



When any potash is sucked back into G along with 

 the gas the tubes must be carefully washed clean before 

 being used for another sample of gas. 



(53) Engelmann' s bacterial method. 



This depends on the extreme sensitiveness of certain 

 bacteria to the presence or absence of free oxygen. One 

 of the difficulties connected with the experiment is the 

 providing a sufficiently sensitive bacterium. Pfeffer 

 recommends that a pea having been killed by boiling 

 shall be allowed to putrefy in 200 c.c. water; according 

 to Detmer a pure culture should be made of the bacteria 

 so obtained. 



It is best to begin with a study of the behaviour of 

 bacteria mounted simply under a cover slip. They will 

 be found to swarm round any air bubbles which may be 

 included in the fluid under the cover slip ; and to collect 

 round the edges of the preparation, and in fact to seek out 

 sources of free oxygen. If the preparations are sealed by 

 a coating of olive oil painted round the edge of the cover 



