PHYSIOLOGICAL CLASSIFICATION 73 



zymogenic change is the souring of milk through the activity of lactic 

 acid bacteria. 



Y. Photogenic Bacteria. The growth of these bacteria is attended 

 by the production of phosphorescent light. The phosphorescence 

 sometimes seen in a piece of decaying fish or of flesh is caused by the 

 growth of these bacteria. 



VI. Thermogenic Bacteria. Applied to those bacteria which, in their 

 growth, raise the temperature of the medium in which they are growing 

 to an appreciable extent. 



VII. Nitrite- and Nitrate-Bacteria. These bacteria are peculiar in 

 requiring only inorganic substances for their growth and multiplication. 

 The nitrite-bacteria cannot thrive unless some ammonium-compound is 

 the source of their nitrogen supply. The conditions of growth of the 

 nitrate-bacteria are almost identical, the chief point of difference being 

 the fact that they require a nitrite- and not an ammonium-compound 

 to supply them with the necessary nitrogen. 



VIII. Nitrogen Bacteria. These are characterised by their power of 

 taking in or "fixing" as it is called, the free nitrogen of the atmosphere. 

 During the decomposition of organic matter, a certain amount of nitrogen 

 in the free condition is liberated into the atmosphere. Accumulation 

 of this gas is stopped, however, by the activities of these bacteria, 

 which bring the nitrogen back again into the soil. 



IX. Denitrifying Bacteria. These organisms change nitrates and 

 nitrites into free nitrogen, which then escapes into the atmosphere as 

 a gas. 



X. Sulphur Bacteria. The peculiarity of these bacteria is their power 

 of absorbing sulphuretted hydrogen (H 9 S), which, after absorption, is 

 oxidised with liberation of sulphur. Microscopically examined, the 

 bodies of these organisms are seen to contain bright globules of sulphur. 

 Hence the name of sulphur bacteria. The sulphur is further oxidised 

 into the sulphate. 



XL Iron Bacteria. These bacteria thrive in water containing iron, 

 chiefly in the form of ferric hydroxide. The redness of what is known 

 as iron-water is due, in almost every instance, to the presence of vast 

 numbers of iron-bacteria, on the membranes of which an accumulation 

 of ferric hydroxide has taken place. There is no general agreement as 

 to the physiological process resulting in the deposition of this iron- 

 compound on the membranes of these bacteria, 



XII. Purple Bacteria. Finally, we must mention the purple bacteria, 

 all of which are also members of the sulphur bacteria. In addition to 

 possessing the characteristics of the sulphur bacteria, they possess also 



