BIOLOGY OF BACTERIA. 57 



vacuum in the closed branch causes the fluid to rise sud- 

 denly when the thumb is removed. After allowing the 

 layer of foam to subside somewhat the space occupied by 

 gas is again measured, and the difference between this 

 amount and that measured before shaking with the 

 sodium hydroxid solution gives the proportion of C0 2 

 absorbed. The explosive character of the residue is 

 determined as follows: "The cotton plug is replaced and 

 the gas from the closed branch is allowed to flow into the 

 bulb and mix with the air there present. The plug is 

 then removed and a lighted match inserted into the 

 mouth of the bulb. The intensity of the explosion varies 

 with the amount of air present in the bulb." 



7. Production of Odors. — Of course, such gases as H 2 S 

 and NH 2 are sufficiently characteristic to be described as 

 odors. There are, however, a considerable number of 

 pungent odors which seem dependent purely upon odor- 

 iferous principles dissociated from gases. Many of them 

 are extremely unpleasant, as that of the tetanus bacillus. 

 The odors seem to be peculiar individual characteristics 

 of the organisms. 



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 Aromatics. — The most important of 

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

 liar to the cholera spirillum. For the method of deter- 

 mining its presence, see " Dunham's Solution." At pres- 

 ent we know that a variety of organisms produce it. 

 Phenol, kresol, hydrochinon, hydroparacumaric acid, 

 and paroxy-phenylic-acetic acid are by no means un- 

 common products of bacteria. 



10. Reduction of Nitrites. — A considerable number of 

 bacteria are able to reduce nitrites present in the soil or 



