BIOLOGY OF BACTERIA. 53 



iferous principles dissociated from gases. Many of the 

 odors are extremely unpleasant, as the fetid one caused 

 by Bacillus pyogenes foetidus. The odor does not have 

 any direct relation to decomposition, but, like the colors 

 and acids, seems to be a peculiar individual characteristic 

 of the metaboiism of the organism. 



8. Production of Phosphorescence. A Bacillus phos- 

 phorescens and numerous other organisms have a dis- 

 tinct phosphorescence associated with their growth. It 

 is said that so much illumination is sometimes caused by 

 a gelatin culture of some of these as to enable one to tell 

 the time by a watch. Most of them are found in sea- 

 water, and are best grown in sea- water gelatin. 



9. Production of Aromatic s. The most important of 

 these is indol, which was at one time thought to be pecu- 

 liar to the cholera spirillum. At present we know that a 

 variety of organisms produce it, and that it and phenol, 

 kresol, hydrochinon, hydroparacumaric acid, and paroxy- 

 phenylic-acetic acid are by no means uncommon. 



10. Reduction of Nitrites. A considerable number of 

 bacteria are able to reduce nitrites present in the soil or 

 in culture-media prepared for them into ammonia and 

 nitrogen. To the horticulturist this is a matter of much 

 interest. Winogradsky has found a specific nitrifying 

 bacillus in soil, and asserts that the presence of ordinary 

 bacteria in the soil causes the reduction of no nitrites so 

 long as his special bacillus is withheld. 



11. Production of Disease. Bacteria which produce 

 diseases are known as pathogenic ; those which do not, 

 as non-pathogenic. Between the two groups there is no 

 sharp line of separation, for true pathogens may be culti- 

 vated under such adverse conditions that their virulence 

 will be entirely lost, while at times bacteria ordinarily 

 harmless may be made toxic by certain manipulations or 

 by introducing them into animals in certain combina- 

 tions. The diseases produced are the result of the sum 

 of numerous activities exhibited by the bacteria. For 

 example, it may be that a microbe, having effected its 



