CHAP. IX. 
FOOD OF PLANTS. 
307 
mented, not only by the quantity of carbon produced by the 
decomposition of carbonic acid, but to a much more consider- 
able extent, which could only be ascribed to its having 
fixed a considerable quantity of water; thus plants of the 
Periwinkle, which, in a vessel without carbonic acid, had gained 
IJ grain from water, acquired 5^o, when they were at the same 
time able to procure carbon. The same excellent observer 
has computed, that if we calculate with the utmost care all 
the weight which a plant can gain, either by fixing carbon, or 
by depositing earthy, saline, alkaline, and metallic matter 
which it borrows from the soil, or by respiring oxygen, or 
from the soluble matter of soil, we shall not be able to account 
for more than a twentieth part of the real weight of such a 
plant. The other nineteen-twentieths must, therefore, be 
fixed water. Whatever errors there may be in calculations 
of this nature, there cannot be a doubt that they are correct 
to so considerable an extent as to oblige us to admit that 
water forms a considerable part of the solid tissue of plants ; 
so that it would appear that, like minerals, plants have a wa- 
ter of crystallization independently of their water of vegetation. 
" As it has been pretty well made out that all the oxygen 
given off by plants is produced by the decomposition of car- 
bonic acid, and as no one has ever been able to detect the 
emission of hydrogen by any plants except Mushrooms, it is 
inferred that, if the water which is consumed by plants is ever 
decomposed, it is in the formation of the various secretions 
which contain more oxygen (acids), or more hydrogen (oils), 
than water ; but as the greater part of vegetable substances, 
such as gum, sugar, fecula, &c., contain oxygen and hydrogen 
in the same proportions as water, it can hardly be doubted 
that the greater part is undecomposed and simply fixed. 
" It was formerly thought that nitrogen, or azote, has no- 
thing to do with the nutrition of plants, and that in those cases 
where it was met with it was merely in a state of separation 
from the atmospheric air which had been inhaled and depriv- 
ed of its oxygen and carbonic acid. But its constant presence 
in combination with the tissue of Mushrooms and of Crucife- 
rous plants, in gluten, and what chemists call vegetable albu- 
men, and also in vegetable alkalies, seems a sufficiently strong 
X 2 
