TRANSFORMATIONS OF CAKIJON AND NJIKO(,I.N. 99 



fying organisms arc comparatively weak. How then can it be 

 possible that the latter produce more nitrates than the former 

 can destroy, if both types arc present in the soil and both 

 active ? It has been suggested that this may be explained by 

 the presence of the large amount of air mixed with the soil. 

 As we have seen, denitrification does not require oxygen, and 

 occurs more readily in its absence since the bacteria under these 

 conditions are forced to take oxygen out of the nitrogen com- 

 pounds, producing their reduction. Exactly the reverse is 

 true of nitrification which needs oxygen to oxidize the com- 

 pounds. If then, air is thoroughly mixed with soil it would 

 stimulate the activity of the nitrifying bacteria and check the 

 action of the deoxidizing germs. 



While this may possibly be a factor it is certainly not the 

 whole explanation. Some of the denitrifying bacteria, as we 

 have seen, act in the presence of oxygen and would be sure to 

 grow in ordinary well-aired soil. The explanation of the phe- 

 nomenon seems to be in another direction, and leads us to the 

 consideration of one of the most remarkable properties of 

 these interesting organisms. 



CONDITIONS OF LIFE OF THE NITROBACTERIA. 



In some respects these nitrobacteria are the most remark- 

 able of all known microorganisms. In their conditions of life 

 they stand in most sharp contrast, not only to other microor- 

 ganisms, but in some respects to all other living organisms. It 

 has already been stated that they do not flourish under the 

 conditions which support ordinary life, but will flourish under 

 conditions in which other bacteria will not live. A solution 

 made of ammonia salts, as a source of nitrogen, and small 

 quantities of minerals in addition, but which has no trace of or- 

 ganic matter, furnishes them with the proper conditions for 

 growth. Thus they do not need any organic food. It has been 

 suggested that Winogradsky's materials may not have been 



