PASTEUR, KOCH, AND OTHERS 305 



from the soil. Now, the source of these nitrites and nitrates 

 is very interesting. In animals the final products of broken- 

 down protoplasm are carbon dioxide, water, and a nitrog- 

 enous substance called urea. These products are called 

 excretory products. The animal machine is unable to utilize 

 the energy which exists in the form of potential energy in 

 these substances, and they are removed from the body. 



The history of nitrogenous substance is the one which at 

 present interests us the most. Entering the soil, it is there 

 acted upon by bacteria residing in the soil, these bacteria 

 possessing the power of making use of the lowest residuum 

 of energy left in the nitrogenous substance. They cause the 

 nitrogen and the hydrogen to unite with oxygen in such a way 

 that there are produced nitrous and nitric acids, and from 

 these two acids, through chemical action, result the nitrites and 

 the nitrates. These substances are then utilized by the plant 

 in the manufacture of protoplasm, and the plant is fed upon 

 by animal organisms, so that a direct relationship is estab- 

 lished between these lower forms of life and the higher plant 

 and animal series; a relationship that is not only interesting, 

 but that helps to throw an important side-light upon the 

 general nature of vital activities, their kind and their reach. 

 In addition to the soil bacteria mentioned above, the.e 

 are others that form association with the rootlets of certain 

 plants and possess the power of fixing free nitrogen from 

 the air. 



The nitrifying bacteria, are, of course, of great importance 

 to the farmer and the agriculturist. 



It is not our purpose, however, to trace the different 

 phases of the subject of bacteriology to their conclusion-, but 

 rather to give a picture of the historical development of this 

 subject as related to the broader one of general biology. 



