94 Report of Experiments on the Growth of Wheat. 
of exhaustion ? — and how far will the answers arrived at on 
these points in reference to it, accord with, or be a guide to, 
those which would apply to any large proportion of the arable 
land of Great Britain when farmed in the ordinary way, with 
rotation ? 
When this Journal first appeared, now five-and-twenty years 
ago, such questions as these were hardly thought of, excepting 
by a few philosophers and economists whose speculations were 
scarcely heard of, and still less heeded, by any considerable 
number even of the most intelligent of agriculturists. Since that 
period, however, matters have very much changed ; and the 
history of the change shows it to have been due to by no means 
one cause alone. Almost coincidently, or at any rate following 
very closely upon one another's footsteps, and each reacting 
upon the other, the increase of population, commercial freedom 
and competition, a vast increase in scientific knowledge, and 
extensive diffusion both of it and of information of a practical 
kind bearing upon the farmer's art, have contributed to the 
wide-spread spirit of enquiry of the present day on such subjects. 
But it is especially to the laborious investigations on agri- 
cultural chemistry of Boussingault, and to the generalisations of 
Liebig to a great extent founded upon them, nearly a quarter 
of a century ago, that we must attribute much of the stimulus 
and direction that has been given to chemical enquiries in con- 
nexion with agriculture in recent times. 
As bearing upon the plan adopted in our own experiments, it 
may be well very briefly to recall attention here, to the state of 
knowledge and opinion on some important points, about the 
time of their commencement, and during the earlier years of 
their progress. 
Leaving out of view the many important .preliminary points 
established by others, which were essential as a starting-point 
for Boussingault's researches, it may be stated that already before 
1840, that indefatigable and most careful experimenter had deter- 
mined, as far as the then known analytical methods permitted, 
the amounts of the most important constituents of agricultural 
produce put upon the land in the manure, and taken off in the 
crops, through several courses of rotation. His more important 
conclusions, stated in a very few words, were — that much more 
carbon and nitrogen were removed in the crops than were sup- 
plied in the manure ; that the best rotations were those which 
accumulated the most of those constituents from the atmosphere 5 
that some plants, especially Leguminosse, accumulated more 
nitrogen from the atmosphere than others, and not only con- 
tained more in their removed produce, but by their residue left 
the land richer in nitrogen than it was before ; and that the 
