149 
Except under the conditions previously pointed out, the phenome- 
non is no longer seen, and, in general, it is limited by the inverse 
action—that is to say, by. a continual dissipation of nitrogen or 
ammonia into the gaseous state. 
Whatever may fix this limit, the fact observed by Berthelot is of 
the first importance. It is the first time, in fact, that we see the 
fixation of nitrogen in naked soils clearly stated; especially is it the 
first time that we see a cause experimentally defined ane demonstrated 
without any reasonable doubt stand forth in the midst of such com- 
plex phenomena. This cause, as we have seen, is no other than the 
development of inferior organisms“ whose nature it remains for us 
to define more precisely. 
This was an entirely new idea and one which could not fail to pro- 
duce its fruits. We shall therefore see researches rapidly multiply 
and lead their authors to more and more definite conclusions. 
A. Gautier and Drouin verified first, in artificial soils, the principal 
results stated by Berthelot; they employed a mixture of siliceous 
sand, pure limestone, kaolin, and neutral phosphate of potash, to 
which they added, in particular cases, humus, humic acid or humates, 
or oxide of iron. This mixture, with the addition of a little nitrate 
of potassium, seems to be very favorable to the development of 
leguminous plants. 
‘Under these conditions Gautier and Drouin recognized that the 
fixation of nitrogen always takes place in mixtures which have 
received organic matter; in its absence, on the contrary, there is 
always a loss. Organic matter appears, then, to be an important 
factor i in this great natural phenomenon. It acts, doubtless, by pro- 
moting the nutrition of the microbe which fixes the nitrogen. 
I will now indicate other experiments, repeated by Ville and 
Boussingault, in which we shall see the effect of the intervention of 
vegetation. 
Berthelot first undertook a series of cultivations of leguminous 
plants in large pots which were left in the open air, either with or 
without shelter, or kept under a glass cover, care being ‘taken to supply 
the plants with the carbonic acid necessary to their erowth. 
The soil, the seeds, the gathered plants, the drainage water and 
‘ain water were all analyzed with the greatest care in order that an 
exact comparison might be established between the initial and the 
final nitrogen. 
Under the glass cover the fixation of nitrogen was very weak, 
because the plant, under these circumstances, did not reach its a sed 
development, but in the open air the quantity of nitrogen fixed wa 
in every case, superior to that fixed by the soil alone. 
For example, the tare tripled this quantity; the crop furnished 
by a mixture of kidney-vetch and Medicago lupulina contained ten 
times more nitrogen than was contained in the seed bed; a crop of 
lucerne grass contained sixteen times more, and this excess of nitro- 
gen was ‘always found more abundantly in the roots than in the leafy 
parts of the plant. 
The soil enriched itself, but in a less degree than plant and soil 
together; therefore active vegetation promotes In an enormous deg ree 
aAerobies: Micro-organisms which live in contact with fie air Aaa require 
oxygen for their growth. Anaerobies: Micro-organisms which do not require 
oxygen, but are killed by it. 
