149 



Exro])t un(l(>r (lio conditions jjiwiously i)ointo(l out, tho 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. 



AMiatever may fix this limit, the fact observed by Berthelot is of 

 the first imi)ortance. 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 exi)erimentally defined and demonsti-ated 

 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. Gantier 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 wdiich have 

 received organic matter; in its absence, on the contrary, there is 

 always a loss. Organic matter appears, then, to be an important 

 factor in this great natural phenomenon. It acts, doubtless, by ]3ro- 

 moting the nutrition of the microbe wdiich fixes the nitrogen. 



I will now indicate other e.xperiments, 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 growth. 



The soil, the seeds, the gathered plants, the drainage water and 

 rain 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 normal 

 development, but in the open air the quantity of niti-ogen fixed was, 

 in every case, superior to that fixed l)v the soil alone. 



For example, the tare tripled this quantity: the cro]D furnished 

 by a mixture of kidney-vetch and Medu-xKjo 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 degree 



oAerobies : Micro-organisms which live in contact witli the air and require 

 oxygen for their growth. Anaerol)ios : Micro-organisms which do not rccjuire 

 oxygen, but are killed by it. 



