VEGETABLE RESPIRATION. 79 
275. Carbon, it is known, is absolutely necessary for the 
support and growth of vegetables, and, when this element is 
not to be found in the soil, they can extract it from the at- - 
mosphere, and assimilate it to their substance. Saussure 
2 made plants vegetate in water and in an atmosphere, both 
_ of which were completely deprived of carbonic acid, and 
found that they did not thrive ; but if carbonic acid were in 
the atmosphere, they flourished and arrived at muturity. 
Plants have been made to grow in dried earth, in flowers of 
sulphur, in a soil made of pounded glass and quartz, in all of 
which they could procure no carbon, which must consequent- 
ly have been derived from the atmosphere. When a newly 
formed or barren soil is first beginning to be clothed with 
vegetation, the oxygen, hydrogen, and carbon must be de- 
rived solely from the atmosphere ; and it is only by drawing 
largely from this source, through the medium of the vege- 
_ actona plant, must necessarily accompany it. The evaporation which takes 
place from the leaf must be modified while experimenting, and there will al- — 
___ Ways be less carbon in the sap of a plant living in water, than when living in 
its natural soil ; and even when a portion of earth is taken up along with the 
plant, it isnot quite in its natural condition; the rootlets are apt to be torn, 
to betoo much exposed, and are confined to a limited extent of ground, 
Then many of the experiments were made with leaves placed in water, and 
Temoved from the parent plant. All these circumstances must, more or less 
modify the condition of the plant, and render us cautious in coming to any 
conclusion from experiments. Of this Dr. Priestley himself seems to have 
_ been aware. After mentioning that many of his experiments did not at all 
accord with his theory, he says, ‘*‘ Upon the whole, J think it is still pro- 
_bable that the vegetation ofhealthy plants, growing in situations natural to 
them, has a salutary effect on the airin which they grow;” and then he al- 
judes to the very delicate nature of such experiments, the great tendency to 
fallacy, the disadvantageous situation of the plant, and the numberless pre- 
cautions that must be attended to. In another part, he says, ‘¢ In those in- 
c stances in which the plant grew the best, they were, however, but sickly, as 
_ Appeared by the leaves soon turning yellow, and falling off when theleast mo- 
tion was given to them.” This leads us to another objection, which = 
_ Tay be urged against all those experiments in which the plants inated 
E the air. : It appears that whenever the leaves were unhealthy, or at all faded, 
ley universally gave carbonic acid; and as we find that in Dr Priestley’s ex- 
periments, as just quoted, the leaves soon became sickly, it is not at all im- 
Probable that this may have materially modified the result im almost all the 
“xpetiments which were continued for any length of time, = 2 
