105 
We may then, I think, infer, that the soils of the mag- 
nesian limestone are perfectly aerated, and are sufficiently 
pervious to air and moisture ; that alumina or clay would 
not only give firmness and consistency to the limestone soils in 
summer, but would make the farm-yard manures go further, 
and hence produce a greater weight of crops, and also the 
permanent grasses; but on the other hand, clay or marl 
would, by producing a greater tenacity, make them less 
adapted to the depasturage of turnips by sheep, and which 
will be seen in the sequel to be one of the disadvantages 
attending their cultivation. 
The next inquiry is concerning manure, or the food which 
the crops upon the limestone soils require for their nutrition ; 
and upon this subject Liebig affords most valuable informa- 
tion. He says, " That hitherto the true theory of the 
nutritive process in vegetables is not known, and hence we 
have been deprived of the best guide to a rational practice of 
agriculture. Any great improvement in that most important 
of all arts is inconceivable without a deeper and more perfect 
acquaintance with the substances which nourish plants, and 
with the sources whence they are derived, and no other cause 
can be discovered to account for the fluctuating and uncertain 
state of our knowledge on this subject up to the present 
time, than that modern vegetable physiology has not kept 
pace with the rapid progress of chemistry." 
Now the development and existence of all vegetables is 
dependent on the reception by them of certain substances, 
which are used for the nutrition of their frame. Any inquiry 
therefore into the cultivation of particular plants involves the 
study of those substances which serve them as nutriment, 
and the changes which those substances undergo in the pro- 
cess of assimilation. 
The substances which constitute the principal mass of 
every vegetable are compounds of carbon^ (or the inflamma- 
ble part of charcoal,) and the elements of water oxygen and 
B 2 
