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.