344 
649. Even although we pass over the sources of 
fallacy in estimating the quantity of carbon already 
stated, and yield entire credit to the accuracy of the 
results in these comparative experiments, yet many 
circumstances concur to prevent our acquiescence 
in the conclusion drawn from them. For, first, 
we have no evidence that the small portion of car- 
bonic acid, contained in the common atmosphere, 
can enter plants in order to be thus decomposed and 
yield its carbon, since the results afforded by plants 
growing in closed vessels cannot, in this instance, be 
justly applied to those obtained under a free exposure 
to the air, where the proportion of carbonic acid is 
so much smaller, and its ready diffusion through the 
atmosphere must so rapidly take place. By similar 
experiments, indeed, M. de Crell was led to an op- 
posite conclusion ; for the quantity of carbon, con- 
tained in the carbonic acid of the atmosphere, could 
not, in his opinion, account for the addition of that 
substance which the plants, in his experiments, re- 
ceived ; and he, therefore, supposed plants to possess 
the power of composing carbon, employing for this 
purpose only water, atmospheric air, and light *. 
M. Braconnot, also, by experiments of the same 
kind, has been led to conclude that vegetables find in 
pure water every thing necessary for them to assimi- 
late ; that vegetable mould and manures yield no 
nutriment, but are useful only as they improve the 
texture of soils, and retain and supply moisture ; 
and that earths, alkalis, metals, sulphur, phosphorus, 
* Phil. Mag. vol. xxiv. p, 150. 
