THE LADIES’ MAGAZINE OF GARDENING. 
337 
plant-cases of the latter preserve, under similar treatment, its original 
composition and purity; not, however, by continuing always the same, 
but by simultaneously undergoing opposite changes in sunshine, or 
successive changes by alternate exposure to light and shade, which 
mutually counterbalance each other. Thus the deterioration of the air 
occasioned by vegetable growth is counteracted by another process, 
necessary to the perfection of the plant; and, amidst the vicissitudes of 
perpetual change, the atmosphere of these cases is maintained in a state 
of nearly uniform composition and purity. In this way, the same air, 
by changes of composition, like the same water, by changes in its state or 
condition, may be made to serve over and over again the purposes of 
vegetation. 
There is one circumstance of difference in the experiments of De Saussure, 
as compared with those of Mr. Ward, which it may he proper to notice. 
In the experiments of the former no soil was used, but only a thin stratum 
of water, in which the roots of the plants were immersed, covered the 
surface of the mercury over which the vessels were inverted. In the 
cases of Mr. Ward, the plants were set in earth. Now, vegetable soil is 
known to deteriorate the air, by forming carbonic acid with its oxygen, 
in the same manner as plants do; but the acid gas, which may thus be 
produced, was found by De Saussure to be decomposed by the joint 
agency of the plants and light, like that produced by ordinary vegetation ; 
and, consequently, the air suffered no permanent injury. Indeed, an 
excess of carbonic acid, not exceeding ^ °f the atmosphere in which 
plants were confined, accelerated their vegetation in sunshine, by increasing 
the proportion of oxygen; whilst the smallest doses of this gas proved 
injurious to that process in the shade. 
The foregoing facts demonstrate the power of light to decompose car¬ 
bonic acid gas in plants. This decomposition, however, can be effected 
only by the concurring agency of the light and the plant ; and, whilst 
the acid gas is thus decomposed, the plant itself acquires a tint of green ; 
so that the evolution of oxygen gas by the plant, and the formation of its 
green colour, always proceed together. Now, as the chromule, which 
imparts colour to the leaf, is lodged in the cells of the parenchyme, it is 
in those cells that we must suppose the decomposition of the acid gas to 
be effected, and from them also the oxygen gas must proceed. The mode 
in which this colouration is probably accomplished may receive illustration 
from the facts which follow. The “ colourable principle,” or chromogen 
of Dr. Hope, is readily extracted by water, and the colourless infusion 
which is thus formed becomes red on the addition of an acid, and green 
on the addition of an alkali. If a neutral salt be dissolved in this infusion, 
