856 The American Naturalist. [October, 
hygiene for the success of the intermittent filtration of water 
through the soil seems to be based upon the nitrifying action 
which takes place within the filter. Filtration has been in 
use a long time as a means of water purification and the theory 
that bases its efficiency in the mechanical or chemical separa- 
tion from its impurities has long been the accepted one. 
Many of the observed facts did not seem however to accord 
with this explanation, and when it was found that certain 
protoplasmic poisons like chloroform injuriously affected the 
efficiency of filters, the relation between it and living organ- 
isms wassuspected. This led to a careful study of the question 
from a bacteriological standpoint, and the discovery was soon 
made that there was a direct relation between the process of 
nitrification as it went on in the filter and its effectual opera- 
tion. 
The solution of another interesting subject, that of the fixa- 
tion of the free nitrogen of the air by certain plants, also bids 
fair to be solved by bacteriological methods. Agriculturists 
long ago recognized the fact that certain species of plants be- 
longing to Leguminose, such as clover, alfalfa, etc., possessed 
special properties in enriching the soil; that not only could 
these crops dispense with the application of fertilizers, but that 
they possessed the elements by which impoverished soils might 
be restored to their original fertility quicker and cheaper than 
by any other method. This knowledge, the fruitage of obser- 
vation and experience awakened an interest in the causal re- 
lations between the one and the other, productive of many in- 
teresting and ingenious hypotheses. Liebig concluded that as 
clover could be grown upon the same ground for years with- 
out the addition of fertilizers, that it must absorb nitrogen 
direct from the air through its aerial organs, and then transfer 
this store to the roots, and so enrich the soil. Carefully con- 
ducted experiments have failed however to reveal any direct 
absorption by the leaves and stems of plants. This peculiar 
property, limited so far as we at present know, to the Legumin- 
ose was finally correlated with the presence of small excres- 
cenees or tuberculous swellings on the roots of these plants. 
These are found in great profusion on the young rootlets of 
