153 
table matter. Thus we see that all the experiments agree with each 
other. 
In the case of peas the results are entirely different, for we see, 
as in Ville’s former experiments, that by the side of a plant weighing 
less than a gram there will be another plant weighing 10 or 15 or 20 
grams, and even more, without its being possible to attribute the 
difference to any apparent influence coming from the outside. There 
is a régime of absolute irregularity, and an examination of the roots 
shows that the irregularity is proportional to the presence or absence 
of tubercles on the roots, whence arises the connection above men- 
tioned. 
It now only remains for us to distinguish between cause and effect. 
Is this appearance of these nodosities in itself merely a consequence of 
the greater vigor of the plants, or ought we, on the contrary, to see 
in these very tubercles the origin and cause of that greater vigor? The 
following experiment will show us which of these two hypotheses is 
correct: : 
When to the same soil of sterile sand which served for the preced- 
ing experiments only 5 grams of good arable soil dissolved in 25 
cubic centimeters of water was added, the peas grew in a natural 
manner and produced, on the average, from 15 to 20 grains of dried 
crop. Each stalk contained, on an average, 450 milligrams of nitro- 
gen, although there were scarcely 10 contained in the soil. In every 
vase there was a fixation of nitrogen in the gaseous state amounting to 
nearly half a gram. 
Under the same conditions a seed of lupin produced a crop of from 
42 to 45 grams, containing more than 1 gram of nitrogen. 
French grass (sainfoin) produced the same results, and in ail cases 
we see that the roots of hese different plants are abundantly pro- 
vided with tubercles; but if the artificial soils and the solutions of 
earth employed in these experiments have been sterilized by the action 
of heat the plants remain invariably puny and produce less than 5 
grams of dried material per stalk. In this case the tubercles are 
always wanting. 
Under cover, in pure air to which a little carbonic-acid gas has 
been added, the results are a little less favorable than in the open air, 
but they still show an important fixation of nitrogen in the case of 
Leguminose infected with bacteria. 
These principles, then, represent the determining cause of ‘the 
phenomenon, and the systematic addition to the soil of appropriate 
germs will enable us hereafter to reproduce at will the experiment of 
Ville, which was formerly attended with such uncertain results. 
In the Museum of Natural History, Bréal has obtained results sim- 
ilar to those of Hellriegel and W ilfarth. In one of his experiments 
a pea containing 9 milligrams of nitrogen, in a soil of poor gravel, 
but into which bacteria had been sown, produced a plant weighing 
103 grams in a green state, 32.3 grams when dried, and containing 
358 milligrams of nitrogen—that is to say, 40 times as much as the 
seed. The pea vine, which was 1.40 meters long, produced 14 ripe 
pods; the gain in nitrogen thus realized corresponds to about 255 
kilograms per hectare. 
In another experiment, a small plant of lucerne grass provided 
with tubercles and weighing 10 grams, and likewise in a soil of 
sterile sand, gave a crop weighing "332 grams when green, 85.5 when 
