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few to accomplish anything. About ten years ago Robert Koch 
invented a method of obtaining pure cultures which has placed 
this line of investigation within the reach of all.. The simplifica- 
tion of the methods of obtaining pure cultures has produced an 
immense stimulus in this branch of study and has turned hundreds. 
of observers towards investigation in this direction. The study of 
the last ten years has been modifying our ideas with marvelous 
rapidity. That bacteria produce disease is demonstrated beyond 
question, but we are beginning to realize that this is only a small 
part of the purposes they fulfill, We are learning that it is to the 
action of these organisms that many of the normal processes in 
nature are due, and that it is to their agency that the growth of 
plant life of the higher orders is largely indebted. One of the 
most recent and most surprising discoveries has been the relation 
of bacteria to the process of nitrogen assimilation. 
It is well known to all botanists that nitrogen is an absolutely 
necessary food for plants and animals. It is known further that 
the animal kingdom obtains all of its nitrogen from plants, and 
that plants obtain their nitrogen from the soil in the form of cer- 
tain salts, the chief of which are nitrates. Now, it is an undoubted 
fact, that the nitrogen factor of the soil upon which plants can de- 
pend, is little by little becoming exhausted through various agen- 
cies. In the first place, all of the nitrogenous parts of plants 
which are used by man as food, find their way eventually into the 
sewage, thence to the river, and finally to the ocean; and once 
reaching the ocean, the nitrogen is lost, so far as the terrestrial 
vegetation is concerned. Moreover, it has been shown that the 
various processes of putrefaction are slowly turning the complex 
nitrogen compounds into simpler ones, and eventually eliminating 
the nitrogen into the air in a free form. Inasmuch as all organic 
compounds, animal and vegetable alike, are subject to these putre- 
factive agencies, it is evident that a large amount of the nitrogen 
factor of the soil must, in this way, pass out into the air as free 
nitrogen. The extent of this loss is unknown, but it is doubtless. 
considerable. Plainly, unless there is some way of getting the 
nitrogen back from the air to the soil again, the soil is doomed to 
exhaustion. But it has seemed to be the result of many experi- 
ments that plants in their ordinary vegetation are capable of ob- 
