Sources of Nitrogen in Plants . 351 
The second part of the paper is devoted to the consideration 
of the gains in nitrogen of the soil. First, we have the gain 
in combined nitrogen : — 
Ammonia and nitric acid found in the atmosphere during 
electric discharges, and carried down by rain, snow. &c. 
This can be measured and shown to be too small to account 
for the nitrogen acquired by plants in addition to that in the 
soil, &c. : this is admitted fully since Boussingault, and was 
clearly evident in the experiments of Lawes, Gilbert and 
Pugh 1 . 
We then come to the chief points in Deherain’s paper : — the 
gains due to the fixation of free nitrogen. 
It is unnecessary to discuss the question of the c assimilation 5 
of nitrogen by the plant direct : it is allowed on all hands that 
the experiments of Boussingault, and of Lawes and Gilbert, 
settled that point for ever — no free nitrogen is assimilated 
by the leaves. 
Deherain experimented with various combustible — i.e.easily- 
oxidisable bodies, such as carbo-hydrates, old wood, &c., in 
contact with certain bases. Such mixtures exposed to the 
air were found to absorb and c fix ’ not only oxygen but also 
certain quantities of free nitrogen. 
The explanation first suggested was that some of the 
oxygen and nitrogen of the air unite to form nitric acid at 
the moment of combustion, just as they do when hydrogen 
is detonated with air ; but it turned out that this was not the 
case, and the compound formed was some other combination 
of nitrogen- — possibly a lower oxide of nitrogen, possibly 
cyanogen, or ammonia. 
Deherain then made experiments to determine the fixation 
of atmospheric nitrogen by vegetable substances. He agrees 
with Lawes and Gilbert in rejecting the view that ammonia 
is formed in damp soil simply by union of hydrogen evolved 
by putrefaction and the nitrogen in the confined spaces 
afforded by soil. 
1 Contained in their well-known paper in Phil. Trans, i860. 
