Action of Light. Electricity 41 



the direct action of the sun's rays may become sterile 

 within a few hours. Hence, in the bacteriological labora- 

 tory, the cultures are kept either in the dark, or in 

 subdued and diffused light. / 



In the soil and in sheets of water, also, strong sun- 

 light exerts some destructive action. Soil exposed in 

 thin layers loses some of its nitrifying power, and very 

 shallow sheets of water in tropical countries become 

 poor in bacteria probably on account of the prolonged 

 exposure to intense sunlight. Direct sunlight affects 

 only the very uppermost layers of soil and water, and 

 its germicidal effect is not, under such conditions, as 

 great as might be expected. 



Electric light. — Very strong electric light has been 

 observed to possess germicidal properties. " It is asserted, 

 for instance, that cultures of Bacterium prodigiosum, 

 exposed to the ultra-violet rays of the arc light, were 

 found to be sterile at the end of twelve minutes. 



Influence of electric currents. — Experiments on the 

 influence of electric currents on the activities of bac- 

 teria have not revealed any marked injury to the 

 organisms. Experiments with both induction and 

 galvanic currents in the soil showed no injury either to 

 the decay or nitrifying bacteria. Similar results were 

 obtained with some of the pathogenic organisms. Weak 

 electric currents in solutions cause motile bacteria to 

 gather about the negative pole. When the current is 

 reversed, and the positive pole made negative, the 

 organisms change their position and flock toward the 

 new negative pole. 



Pressure. — A more or less injurious action on bacteria 



