195 
an elastic form, and the carbon will remain behind 
in union with the colourable matter of the plant. 
466. But supposing solar light thus to act in de- 
composing carbonic acid in plants, some other agent 
must be called in to aid its operation ; for the experi- 
ments of Mr Davy (303.) prove, that light alone is 
unable to decompose this acid gas ; and it is equally 
certain that the plant itself is unequal to this effect 
without the agency of light. In the experiments of 
Mr Tennant and Dr Pearson, in which carbonic acid 
was decomposed at a high temperature by phospho- 
rus, the acid was employed in a fixed state, while in 
union with lime or alkali $ and it was only under 
such circumstances that its decomposition could be 
effected. The elasticity of the acid, in its uncombined 
state, may be considered as counteracting the affinity 
of the phosphorus to its oxygen ; but in its concen- 
trated state, says Mr Murray, it is more liable to be 
acted upon, and its decomposition may be farther 
promoted, by the affinity which the lime or alkali ex- 
erts. to the phosphorus and oxygen, and in conse- 
quence of which they enter into combination *. 
467 Now the same bodies which, in these expe- 
riments, were employed to reduce the elasticity of 
carbonic gas, exist abundantly in the juices of plants ; 
and with these alkaline substances, this acid gas will 
naturally combine. That it does enter into such 
combinations, may be inferred from the fact, that, 
though it exist in large quantities in many leaves, it 
is not abstracted by the air pump, nor expelled by 
* Syst. Chem. vol. ii. p. 355. 
N 2 
