256 
THE FIXATION OF ATMOSPHERIC NITROGEN. Nov., 1892. 
of by Bonssingault and Sir John Lawes many years ago. 
Warington, one of onr greatest authorities on agricultural 
science, writing in 1886, says:—“The nutrition of legu¬ 
minous crops is not at present perfectly understood ; a good 
crop of red clover when cut for hay removes a large quantity of 
nitrogen from the land, but it nevertheless leaves the surface 
soil actually richer in nitrogen than it was before, from the 
residue of roots and stubble left in the soil. From whence,” 
lie asks, “ comes the nitrogen?” and his inference is that 
leguminous crops possess to some extent a distinct source of 
nitrogen, which he supposes, however, to be obtained by 
some process peculiar to themselves from the soil or subsoil. 
I have here a figure showing the different effect of want of 
nitrogen in the soil upon the leguminosse and other orders. It 
is taken, I believe, from a photograph, and shows a pot 
culture of hemp and clover ; in the centre pot the absence 
of nitrogen has dwarfed the hemp plant almost to atrophy, 
while the clover is even more luxuriant than in the 
left-hand pot containing a full supply of it. It is only 
within the last year or two that the true explana¬ 
tion of these remarkable facts has been discovered. On 
examining the roots of leguminous plants they may be seen 
to bear small nodular bodies, which are found to contain a 
very large proportion of nitrogen. It has been shown by 
two investigators, Hellriegel and Wilfartli, that these are 
not actually a part of the plant, but contain separate 
organisms of the kind to which the name microbe has been 
given ; they are, from one point of view, parasites, but from 
another they are grateful guests, rendering to their hosts an 
invaluable service ; they are, in fact, the real agents by which 
the plant is rendered independent of the nitrogen of the soil. 
The following is a brief account of the experiments by which 
the fact was established :—In 1883 Hellriegel began an investi¬ 
gation, in the course of which he sowed peas in washed sand, 
mixed with mineral manures only, in which there was no 
nitrogen. As a rule, the plants produced were limited in 
their growth by the amount of nitrogen supplied by the seed 
itself; here and there, however, a plant grew very luxuriantly, 
and on examination it was found that abundant nodules had 
formed upon the roots of these well-grown plants, while none 
were found upon the others. Guided by these results the 
experimenter, Hellriegel, with his colleague, Dr. Wilfarth, 
repeated the trials, with the addition to the sand of a small 
quantity of an extract made by shaking a fertile soil with 
distilled water, and allowing the greater part to subside. In 
some instances the extract was added without further treat¬ 
ment ; in others it was heated sufficiently to destroy organisms 
—that is to say, it was sterilised. Where the fresh, untreated 
