510 
On the Food of Plants. 
constant and invariable in every part of the atmosphere^ high or 
low. Uniformity of mixture is attained and preserved by that 
singular law affecting the constitution of gases in general, called 
the " law of diffusion," first discovered by Dalton, and since so 
ably investigated by Professor Graham. 
In addition to these essential constituents, the air contains 
diffused through it at all times a certain amount of carbonic acid, 
a quantity of aqueous vapour depending very much upon its tem- 
perature, and finally, a very small but unequivocal trace of am- 
monia. The proportion of carbonic acid present has been made 
the subject of careful investigation ; according to the best experi- 
ments we have, those of De Saussure, 10,000 volumes of air con- 
tain a quantity of that gas varying from 6'2 volumes, the max- 
imum, to 3'7 volumes, the minimum ; xirlirirths or xtV irths may 
be assumed as a suf?icient approximation to a mean state. Within 
the Innits mentioned the proportion varies with the season, the 
day and night, difference of elevation in the air, and perhaps other 
circumstances. It is essential to a proper understanding of the 
question considered that these facts be carefully borne in mind. 
We have to account for the carbon, hydrogen, oxygen, and 
nitrogen so abundantly contained in every plant ; the origin of 
the inorganic substances present is sufficiently obvious. And 
first of the carbon — 
Now I think it better fi.rst to stale in the form of a proposition 
an opinion very generally entertained on this subject, and in which, 
so far as my judgment goes, I heartily concur, and then to enter at 
some length into the evidence on which that opinion rests — 
" The carbon of plants is solely derived from the decomposition of 
carbonic acid." 
The first experiment in relation to this subject dates back to 
the year 1771, and was made by the celebrated Dr. Priestley. 
He left a lighted wax taper in a confined portion of air until it 
expired ; into this deteriorated atmosphere he introduced a plant 
of mint, and suffered it to remain ten days, when he found the air 
again capable of supporting combustion.'" It is to be remarked 
that this happened three years before the discovery of oxygen 
by the same j)erson. Very shortly after that great event, Dr. 
Priestley was led to examine into the nature of the gas which had 
thirty years before been observed by Charles Bonnet to arise in 
bubbles from the surface of green leaves immersed in spring 
water and placed in the sunshine : the ^ds proved to be nearly pure 
oxygen. The attention of a number of inquirers, among whom 
contains a little light caiburetted hydrogen, and indeed Boiissingault long 
ago satisfied himself that something of the kind did, at least occasionally, 
exist. 
* Berzelius, Lehibuch der Chemie, G, 89. 
