368 
PHYSIOLOGY. 
BOOK II. 
artificial atmosphere, containing a little carbonic acid, the 
weight which the plant acquired in a given time was aug- 
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 Peri- 
winkle, which, in a vessel without carbonic acid, had gained 
IJ grain from water, acquired when they were at the 
same time able to procure carbon. The same excellent ob- 
server has computed that, if we calculate with the utmost care 
all the weight which a plant can gain, by fixing carbon, 
by depositing earthy, saline, alkaline, and metallic matter 
which it borrows from the soil, 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 much 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 
water of crystallisation independently of their water of vege- 
tation.” It has already (p. 360.) been shown, that Messrs. 
Edwards and Colin have proved experimentally that plants 
decompose water by their vital force, fixing the hydrogen 
and parting with the oxygen, which combines with carbon, 
forming carbonic acid. 
As it has been supposed that all the oxygen given oft’ by 
plants is produced by the decomposition of carbonic acid, it 
has been 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 hydro- 
gen (oils), than water : but, as the greater part of vegetable 
substances, such as gum, sugar, fascula, &c., contain oxygen 
and hydrogen in the same proportions as water, it has been 
thought that the greater part is undecomposed and simply 
fixed ; but the experiments of Edwards and Colin, above re- 
ferred to, prove the contrary. 
It was formerly thought that nitrogen, or azote, has no- 
