T. W. Engelmann proposes, in the _Botanische Zeitung_, a new test, of an
extremely delicate nature, for determining the presence of very minute
quantities of oxygen, namely: its power of exciting the motility of
bacteria. If any of the smaller species, especially _Bacterium termo_,
are brought to rest, and then introduced into a fluid in which there is
the minutest trace of free oxygen, they will immediately begin to move
about freely; and if the oxygen is gradually introduced, their motion
will be set up only in those parts of the drop which the oxygen reaches.
In this way Engelmann was able to determine the evolution of oxygen by
_Euglena_ and by chlorophyl granules.
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