44 GENERAL PHYSIOLOGY OF BACTERIA 
bacteria, the Rhodobacteriacete, possess a photodynamic pigment, 
bacteriopiirpiirin, which appears to be analogous to chlorophyll of the 
green plants. These sulphur bacteria prefer light and moAe toward it.^ 
The action of sunlight on this bacteriopurpurin enables them to 
decompose CO 2 and to utilize the oxvgen thus obtained to oxidize 
H2S. 
All other known bacteria have no photodynamic pigment. Light 
is not a source of energy to them, and they are distinctly harmed by 
it; they grow best in darkness. Direct daylight kills them rapidly, 
and even prolonged exposure to diffuse light may be fatal. Bacteria 
are more rapidly killed by exposure to the sun's rays in June, July 
and August" than exposure of the same time in November, December 
and January. Expressed differently^ many bacteria which are killed 
after an exposure of from one to two hours' direct sunlight in summer 
require an exposure of from two to three hours in winter to accomplish 
the same result. 
Of the spectral rays, the red and infra-red rays, aside from the heat- 
ing effect, are without noteworthy action on bacteria. The blue, 
violet and ultra-violet rays, on the contrary, are distinctly bactericidal. 
These rays are chemodynamic and it is very probable that the death 
of bacteria exposed to them in organic media results from the for- 
mation of H2O2 or other germicidal substances from the substrate. 
Bacteria are also killed in non-decomposable media when they are 
exposed to the ultra-violet rays. It should be remembered that one 
of the most important characteristics of ultra spectral emanations is 
their very short wave length. Glass is opaque to them where quartz 
is transparent. 
Electricity.— It is difficult to differentiate sharply between purely 
electrical effects and chemical changes which are induced in media 
of various kinds by the action of electric currents. Generally speak- 
ing, strong electrical currents sterilize media in which bacteria are 
growing, but it is by no means certain that the electric current per 
se is the important factor. Indeed, if the electrolytic effects of direct 
currents and the heating effects of alternating currents are carefully 
excluded, there is little satisfactory evidence of any bactericidal action 
induced by electric currents.^ 
J. GRAVITY, OSMOTIC PRESSURE, AGITATION, CHEMOTAXIS. 
1. Gravity.— The majority of bacteria suspended in liquids are not 
killed even by four hours' exposure to direct pressure of from 2000 to 
3000 atmospheres (1 atmosphere of pressure is equal to approximately 
15 pounds to the square inch, or 1 kilogram per square centimeter of 
surface). Bacteria are weakened, however, by these great pressures, 
1 Yost: Plant Physiology, p. 223. 
2 In the Northern Hemisphere. 
3 See Zeit: Jour. Am. Med. Assn., 1901, 37, 1432, for discussion. 
