PRESIDENT’S ADDRESS. 47 
disintegration of the mineral matter is also in part brought 
about by the action of microscopic organisms and ferments. 
It would appear that certain of the nitrifying organisms 
(bacteria) obtain their nitrogen from the air and from the 
traces of ammonia and other nitrogenous compounds present 
in it—these nitrifying organisms are of the greatest impor- 
tance in agriculture, since through their agency higher 
forms of plant-life are supplied with suitable nitrogenous 
food. Roughly, these may be divided, first, into organisms 
which form ammonia or ammonium carbonate, from albu- 
menous substances, which probably pass through the 
intermediate condition of amides ; second, into those which 
convert ammonium carbonate into nitrous acid; and third, 
those which oxidise nitrous to nitric acid with rise of 
temperature, the salts of the last being the most useful con- 
dition of nitrogen as a plant-food, although plants also 
absorb nitrogen in other forms. | 
All these changes of course usually proceed simultaneously 
in cultivated soil. 
Laboratory experiments show that not only do bacteria 
oxidise the carbon to carbon dioxide, and the nitrogen to 
nitrous acid, and this in turn to nitric acid, but that they 
also (B. mycoides) oxidise sulphur into sulphuric acid. 
The oxidation of free nitrogen is brought about in at 
least two ways: on the rootlets of certain plants, especially 
amongst leguminous ones, nodules are found containing 
large numbers of bacteria, accompained by accumulations 
of nitrogen, apparently much greater than could have been 
removed from the soil or from manures in the ordinary way, 
and experiments seem to show that these accumulations are 
due to the bacteria of the nodules oxidising and assimilating 
atmospheric nitrogen. 
Other organisms are also known which oxidise free nitro- 
gen, but which do not give rise to nodules, so that cereals and 
other plants, which do not form nodules on their rootlets, 
also have supplies of accumulated oxidised nitrogen to draw 
