BACTERIA IN NATURAL PROCESSES. 107 



paring tendency in the general processes of Na- 

 ture. Bacterial life in at least two different ways 

 appears to have the function of reclaiming from 

 the atmosphere more or less of this dissipated 

 free nitrogen. 



In the first place, it has been found in the 

 last few years that soil entirely free from all 

 common plants, but containing certain kinds of 

 bacteria, if allowed to stand in contact with the 

 air, will slowly but surely gain in the amount of 

 nitrogen compounds that it contains. These 

 nitrogen compounds are plainly manufactured by 

 the bacteria in the soil ; for unless the bacteria are 

 present they do not accumulate, and they do ac- 

 cumulate inevitably if the bacteria are present in 

 the proper quantity and the proper species. It 

 appears that, as a rule, this fixation of nitrogen 

 is not performed by any one species of micro- 

 organisms, but by two or three of them acting 

 together. Certain combinations of bacteria have 

 been found which, when inoculated in the soil, 

 will bring about this fixation of nitrogen, but no 

 one of the species is capable of producing this 

 result alone. We do not know to what extent 

 these organisms are distributed in the soil, nor 

 how widely this nitrogen fixation through bacte- 

 rial life is going on. It is only within a short 

 time that it has been demonstrated to exist, but 

 we must look upon bacteria in the soil as one of 

 the factors in reclaiming from the atmosphere the 

 dissipated free nitrogen. 



The second method by which bacteria aid in 

 the reclaiming of this lost nitrogen is by a com- 

 bined action of certain species of bacteria and 

 some of the higher plants. Ordinary green 

 plants, as already noted, are unable to make use 



