1892.) Some Uses of Bacteria. 995 
air that we breathe is made up of four parts of nitrogen and 
one part of oxygen. There are quantities of nitrogen every- 
where if the plants could only get hold of it, but it has been 
thought that plants cannot feed on the nitrogen in the air at 
all. Experiments have been carried on for a great many 
years to find out whether plants could not in some way or 
other get hold of the nitrogen of the air. If we could only 
prove that our plants can get hold of the nitrogen in the air 
then the problem is solved. But the experiments which have 
been carried on year after year have seemed to demonstrate 
that plants cannot use the nitrogen of the air for food, that it 
is not in a condition in which they can get hold of it. About 
ten years ago, however, certain experimenters in this country 
and in Europe found that in some of their experiments plants 
did in some way get hold of nitrogen from some source when 
it was not fed to them ; that a plant could be grown in sand 
absolutely free from nitrogen, and yet in some way that plant 
got hold of nitrogen; the only source for it was out of the air. 
That led to further experimentation until within the last four 
or five years the results have all been pointing in one direc- 
tion. They seem to show us that there is one family of plants 
at least, which is capable of getting hold of nitrogen out of 
the air. This is the plant family to which the pea, the bean 
and the clover belong. It is, in general, the pea family—the 
Leguminose family of plants. This family of plants in some 
way does succeed in getting nitrogen from some source when 
we do not give it to them as food, and it must be that they get 
it from the air. And yet those experiments are entirely con- 
tradictory to the earlier experiments which seemed to show 
that plants could not get hold of nitrogen in the air. The 
explanation was not found until a few years ago. Two or 
three years ago some experiments were performed in Germany 
which have finally led to the solution of the problem, at least 
in part, and, curiously enough, we find that the whole secret 
of the matter is connected with these organisms which I am 
discussing this morning. It is to bacteria that we owe this 
power which is possessed by plants of the pea family to get 
hold of nitrogen. If you plant peas in soil containing a cer- 
