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Carbon, as we know, is taken by the plants from the carbonic-acid 
gas of the air, at least for the most part. Carbonic acid, like water, 
exists everywhere, and if I remind you that we have succeeded in 
transforming it into some of the sugars which exist so generally in 
the vegetable tissues, you will agree with me in saying that the great 
phenomenon of the assimilation of carbon by plants is at present 
understood only in its smallest details. 
The mechanism of the assfmilation of nitrogen is far from being 
as well understood even as that of carbon. We as yet know nothing 
of the chemical changes which cause this element to pass from a gas- 
eous state to that of albuminous food; but its different modes of pene- 
trating into the plant are well known to us, and we can affirm to-day 
that the atmosphere contributes as much as the soil to that portion of 
vegetable nutrition. 
This fact, of which we shall shortly give the demonstration, was 
almost evident, a priori. In fact the soil contains only very small 
proportions of nitrogen. The store which it offers to us (scarcely 
10,000 kilograms per hectare) is insignificant in comparison with the 
immensity of time; but in comparison with it the atmosphere con- 
tains an enormous quantity, about three-fourths of its entire volume ; 
hence the idea of a continual circulation of nitrogen between its com- 
pounds and the air—in other words, between the air, the earth, and 
the living organisms—forced itself upon us, in the same way as the 
circulation of water between the ocean and all points of the earth 
obtrudes itself. 
It is therefore the more remarkable that this conception of the 
subject has only quite recently been brought to light. Enunciated as 
a principle more than thirty years ago, it has only been taken into 
serious consideration in these latter years, after a series of researches 
which we are now going to pass 1n review. 
But I should like first to establish, by experience alone, outside of 
all speculative ideas,the fact that the intervention of atmospheric 
nitrogen in the phenomena of vegetation is an absolute necessity. Ut 
will suffice for that purpose that I show a parallel, a sort of balance 
between the sources of gain and the sources of loss to the soil in nitrog- 
enous compounds; it is clear that if this comparison shows us a differ- 
ence in favor of the enriching of the soil then we need have no fear of 
seeing our soil become one day sterile; if, on the contrary, the losses 
are in excess of the gains from the exterior then we know that it must 
be receiving from the atmosphere the quantity of gaseous nitrogen 
equal to the difference. It is very easy to bring together the data for 
this great problem. 
The most important cause of the decrease of nitrogen in the soil 
is unquestionably the crop taken from it each year; the amount of this 
loss is, however, very variable; a crop of cereals—of wheat, for ex- 
ample—takes from the soil about 50 kilograms of nitrogen per hec- 
tare; roots, beets, or others generally contain more; finally, certain 
kinds of vegetation, such as clover or lucern grass, take as much as 
100 to 200 kilograms, and even more nitrogen per hectare annually. 
Judging by these figures, we must conclude that by an average 
rotation of crops, where root vegetables, leguminous plants, and 
cereals are made to alternate one with the other, the earth loses every 
year by the fact of cultivation alone a minimum of from 60 to 70 
kilograms of nitrogen in combination with other substances. 
