48 BACTERIAL METHOD. [CH. II 



Since an apparatus of this kind will usually be made 

 in the laboratory it is worth noting that the bore and 

 length of stroke of the syringes, as well as the size of the 

 bulbs blown on the pipette and eudiometer must be 

 arranged so that the gas cannot be drawn beyond the 

 bulbs in either case. 



(53) Engelmanns 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. This, though no doubt advisable, is not 

 necessary. 



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

 bacteria mounted simply under a cover-glass. They will 

 be found to swarm round any air-bubbles which may be 

 included in the fluid under the cover-glass ; 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 melted vaseline or wax-mixture painted 

 round the edge of the cover-slip, the bacteria ultimately 

 become sluggish and come to rest. It is of this fact that 

 Engelmann's method takes advantage. If a filament of 

 Spirogyra or the leaf of a submerged plant be included 

 with the sealed bacteria we have it in our power, by the 

 exposure of the preparation to light, — to produce free 



