94 BACTERIAL METABOLISM PRODUCTS 



and other solvents. The pigment produced by Azotobacter chroo- 

 coccum is insoluble in all of these solvents but dissolves in alkalies 

 undergoing decomposition with the formation of a dark brown 

 solution. 



Heat. Probably all bacteria liberate energy as heat in their 

 metabolic process and there are a number which liberate it in suffi- 

 cient amount perceptibly to change the temperature of the media 

 in which they grow. This is exemplified in the heating of fermenting 

 silage, manure, and hay. At times the temperature is raised to the 

 kindling point with the result that spontaneous combustion may 

 occur in hay and grain stacks. Bacteria generate considerable of 

 the heat, but other chemical processes are also active. 



FIG. 14. Photogenic bacteria colonies on a plate photographed by means of their 

 own light. (Lafar.) (Buchanan's Household Bacteriology.) 



Light. Sometimes one sees on the surface of decaying wood, 

 fish, or various meats a bright illuminated surface which at times 

 may be sufficient for the photographing of objects in an otherwise 

 dark room. This is due to the growth of certain light-producing 

 bacteria. Other organisms produce a beautiful phosphorescence. 

 The organisms producing light are especially prone to occur in saline 

 waters and are invariably aerobes. 



REFERENCES. 



Marshall: Microlxiology. 



Taylor: Digestion and Metabolism. 



Lafar: Technical Mycology. 



Kendall: Bacteriology General, Pathological and Intestinal. 



