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MORPHOLOGy AND BIOLOGY OF BACTERIA. U

A tube of culture gelatine is richly infected with a bacte-
rium which grows rapidly at room temperature without liquefy-
ing the gelatine ; it is then poured upon a glass plate in the cus-
tomary manner. While the gelatine is still liquid, place upon
the plate a metallic strip, one end of which is composed of tin,
the other of gold.
On the border of the tin, a retardation in the development of
the bacteria will be observed; the gelatine remains clear (Fig. 5).
In a few days the tin (electro-positive pole) will appear sur-
rounded by a white zone 10 to 30 mm. in diameter; the reaction

Fig.

Apparent Prevention' of the Growth of Bacteria by
Electric Action.
rt, Gold : h. Tin, on a plate of culture gelatine. In the parts indicated
by the white color no development of bacteria occurs. Reaction around
the tin strongly acid : around the gold alkaline.
in this zone will be found to be strongly acid. The retardation
in the development is to be ascribed not to the electricity, but to
the acid.
On the border of the g-old the reaction will be found to be
alkaline. The development of the bacteria is here also retarded,
but much less than on the margin of the tin (the electro-positive
pole).
7. Antagonism among the Bacteria.
The principle of the struggle for existence and of natural
selection plays an important, though not fully understood, part
in the life of bacteria. If we bring a number of different kinds