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BIOLOGY OF BACTERIA.                    47

direct rays of the sun, and to a less degree the intense
rays of the electric arc-light, retard and in numerous in-
stances kill bacteria. Some colors are distinctly inhibi-
tory to their growth, blue being especially prejudicial.
'Some of the chromogenic forms will only produce their
colors when exposed to the ordinary light of the room.
The Bacillus mycoides roseus will not produce its red
pigment except in the absence of light. The pathogenic
bacteria have, their virulence gradually attenuated if
grown in the light.

(_/") Electricity.—Very little is known about the action
of electric currents upon bacteria. Very powerful dis-
charges of electricity through culture-media are said to
kill the organisms, to change the reaction of the culture,
and the rapidly reversed currents of high intensity to
destroy the pathogenesis of the bacteria and change their
toxic products into neutralizing protective (antitoxin?)
bodies. Much attention has recently been devoted to
this subject by Smirnow, Arsonval and Charin, Bolton
and Pease, Bonome and Viola, and others.

{g) Movement.—When bacteria are growing in a liquid
medium perfect rest seems to be the condition best
adapted for their development. A slow-flowing move-
ment does not have much inhibitory action, but violent
agitation, as by shaking a culture in a machine, greatly
hinders or prevents their growth. The practical appli-
cation of this will show that rapidly flowing streams,
whose currents are interrupted by falls and rapids, will,
other things being equal, furnish a better drinking-water
than a deep, still-flowing river.

* (Ji) Association.—It occasionally happens that bacteria
grow better when associated with other species, or have
their pathogenic powers augmented when grown in com-
bination. Coley found the streptococcus toxin more
active when combined with Bacillus prodigiosus.

Occasionally the reverse is true, and Pawlowski found
that mixtures of anthrax and bacillus prodigiosus were
less virulent than cultures of anthrax alone.