MICRO-ORGANISMS IN RELATION TO MAN. 107 
action has only recently been discovered. And it is remark- 
able that the conversion of the ammonia of the manure and 
soil into nitric acid requires the agency of two distinct organ- 
isms—the one to convert the ammonia into nitrous acid, and 
the second to convert the nitrous acid into nitric acid. 
Neither can do the work of the other, but the co-operation 
of the two is necessary to bring about the desired result. The 
enormous nitrate deposits of the rainless districts of Chili and 
Peru are doubtless due to the past action of these organisms ; 
and it is said, I know not with what truth, that some of the 
difficulties experienced at the local sewage farm are due to the 
absence of these micro-organisms from the soil. 
Another most important part which micro-organisms play in 
connection with vegetable life has recently been brought to 
light. It has long been a moot point whether the free nitro- 
gen of the atmosphere can be assimilated as food by plants. 
The verdict was decidedly against the supposition, but it was 
noticed by Lawes and Gilbert that in the case of particular 
leguminous crops, such as peas, beans, vetches, etc., there 
is an excess of nitrogen absorbed which cannot be accounted 
for as being derived from manures or rain-water. It has been 
shown that this excess of nitrogen in leguminous crops is 
obtained from the atmosphere, and also that this assimilation 
of free nitrogen is dependent upon the presence of certain bac- 
teria, flourishing in and round the roots of these plants, for 
when these same plants are cultivated in sterilized soil the 
fixation of atmospheric nitrogen does not take place. More- 
over, the presence of these organisms in the soil occasions the 
formation of peculiar swellings or nodules on the roots of the 
plants, and these nodules, which are not formed in sterile soil, 
are found to be remarkably rich in nitrogen and swarming 
with bacteria. 
Extremely important and instructive results have been 
obtained from experiments made to investigate the particular 
bacterium which brings about these important changes, and it 
