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. 



(f) 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. 



(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. 



(ti) Temperature. The question of temperature is of 

 importance from its bearing upon sterilization. Accord- 

 ing to Frankel, bacteria will scarcely grow at all below 

 1 6 and above 40 C. 



The researches of Fliigge show that the Bacillus sub- 

 tilis will grow very slowly at 6 C. , and as the tempera- 

 ture is elevated it is said that until 12.5 C. is reached 

 fission does not occur oftener than every four or five 

 hours. When 25 C. is reached the fission occurs every 

 three-quarters of an hour, and at 30 C. about every half 

 hour. 



Most bacteria die at a higher temperature than 60- 

 75 C. The spores can resist boiling water, but are 

 killed by dry heat if exposed to 150 C. for an hour or to 



