2 
SIR J. B. LAWES AND PROFESSOR J. H. GILBERT ON THE 
4. The Experiments of Dr. B. E. Dietzell . 58 
5. The Experiments of Professor B. Feank. 60 
6. The Experiments of Professor Hellriegel, and Dr. Wilfaeth .64 
7. The Experiments of Professor Emil von Wolff .71 
8. The Experiments of Professor W. O, Atwater. 79 
9. Recent Results and Conclusions of M. Boussingault . 83 
Part III. 
Summary, and General Considerations and Conclusions. 
1. The Evidence relating to other Sources than Free Nitrogen. . .89 
2. The Evidence relating to the Fixation of Free Nitrogen.. 94 
3. General Considerations and Conclusions.100 
Postscript.107 
Introduction. 
A GREAT part of this paper was written in the spring of 1886, but its completion 
was unavoidably delayed. This has, however, not been altogether without advantage. 
Thus, in the first place, at the Naturforscher-Versammlung, held in Berlin, in 
September, 1886, the greater part of the sittings of two days was devoted, in the 
Section of Landwirthschqftliches Versuchs-Wesen, to the discussion of the subject 
from various points of view, one of ourselves taking part; and as it seemed desnable 
that the results and conclusions then brought forward by others should be considered, 
we have waited for the publication of the exact figures in some cases. Again, since 
the Berlin meeting, M. Berthelot has published some further results, to which refer¬ 
ence should be made. And lastly, we are now enabled to give further new results 
of our own. 
In Part 2 of the ‘Philosophical Transactions’ for 1861, a paper was given, by 
ourselves and the late Dr. Pugh, “ On the Sources of the Nitrogen of Vegetation, 
with special reference to the question ivhether plants assimilate free or uncombined 
Nitrogen.'^ Since that time, the question of the sources of the nitrogen of vegetation 
has continued to be the subject of much discussion, and also of much experimental 
enquiry, both at Rothamsted and elsewhere. Until quite recently, the controversy 
has chiefly been as to whether plants directly assimilate the free nitrogen of the 
atmosphere; but, during the last few years, the discussion has assumed a somewhat 
different aspect. The question still is whether the free nitrogen of the air is an 
important source of the nitrogen of vegetation ; but whilst few now adhere to the 
view that the higher chlorophyllous plants directly assimilate free nitrogen, it is, 
nevertheless, assumed to be brought under contribution in various ways—coming into 
combination within the soil, under the influence of electricity, or of micro-organisms, 
or of other low forms, and so indirectly serving as an important source of the nitrogen 
