graphs have been taken by the light emitted by these 

 organisms. 



Beyerinck, who has carefully studied the light-giving 

 bacteria, finds that the formation of light does not bear any 

 direct relation to the growth of the organisms ; but he finds 

 that certain food substances are necessary for them to pro- 

 duce light. For instance, some require oxygen, although 

 they will grow perfectly well under anaerobic conditions, 

 without producing phosphorescence. 



For the cultural characters of these phosphorescent 

 bacteria, see p. 448. 



Other Products of the Metabolism of Micro-organisms. — In 

 putrefactive fermentation a number of substances are pro- 

 duced by the agency of bacteria. In addition to the very 

 numerous and various bodies produced in the many fer- 

 mentative processes — such as acetic, lactic, butyric, and 

 other acids, alcohols, ammonia, albumoses, ptomaines, 

 colouring matters, etc., of which many have been described 

 in the previous pages — are a large number of other bodies 

 of which the following are a few which occur in the various 

 putrefactive processes : hydrogen, nitrogen, phosphuretted 

 hydrogen, sulphuretted hydrogen, carbon dioxide, marsh- 

 gas, formic acid, valerianic acid, many of the volatile and 

 fixed fatty acids, free ammonia, ammonium sulphide, tri- 

 methylamine, propylamine, indol, scatol, etc. 



The particular products yielded in putrefactive and fer- 

 mentative processes vary necessarily with the composition 

 of the decomposing material, the prevailing conditions, and 

 with the species of organisms present. Many of the 

 gaseous and volatile products of putrefaction are charac- 

 terised by their very offensive smell. The anaerobic bacteria, 

 generally speaking, give rise to the most malodorous pro- 

 ducts. In the case of a decomposing body of an animal, 

 the odours evolved are worse in situations where oxygen has 



