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hydrogen in variable proportions. Woody fibre, starch, 
sugar, gum, wax, resins, oils, vegetable acids, are all com- 
pounds of carbon and oxygen and hydrogen. 
Nitrogen or azote forms only a very small part of plants, 
but it is never entirely absent from any part of them. Even 
when it does not absolutely enter into the composition of a 
particular organ or part, it is always to be found in the fluids 
which pervade it. 
I. Our first inquiry must then be from whence do plants 
derive their carbon and nitrogen ? and secondly, concerning 
certain saline and inorganic bodies, which are equally neces- 
sary for their development. 
1. The Carbon. An opinion among vegetable physiologists 
has hitherto prevailed that plants derive their nourishment 
and the greatest portion of their carbon from manures 
rendered soluble by putrefaction, and that they are absorbed, 
being dissolved in water, by the spongioles of the roots. But 
Liebig has, I think, proved otherwise, and that they obtain 
carbon from manure, not by a solution of its carbonaceous 
matter in water, but from carbonic acid exhaled during its 
decomposition, and absorbed in water. He also asserts that 
twice as much carbon is received from the air as from the 
soil, and that cultivated land receives as much carbon back in 
the form of excrementitious matter as it afforded to the 
plant. The opinion is not new that the carbonic acid of the 
air serves for the nutriment of plants, and that its carbon is 
assimilated. The elegant writer Mrs. Marcet, states in her 
Conversations on Vegetable Physiology, *' The sap contains 
carbon in two states : first in that of carbonic acid, secondly, 
combined in animal and vegetable m^atter. In the first state 
the sun's rays decompose the acid, the carbon is deposited, and 
the oxygen which flies off" purifies the atmosphere. But what 
do you suppose, says she, becomes of the carbon contained 
in the animal and vegetable matter which the sap holds in 
