338 
ed exhalation of carbon. It is in conformity with 
this view that evergreens, which perspire less (23.) 
than other plants, were found by M. de Saussure to 
form less carbonic acid * ; and when the perspiratory 
function wholly ceases, there is reason to conclude 
that no farther change is then induced on the air. 
643. But the leaves of plants, like other organic 
substances, are enabled to afford carbon after all 
living action has ceased, provided they be placed 
in those circumstances of moisture and heat which 
give rise to the spontaneous changes necessary to 
the performance of this chemical action. Hence 
plants, like seeds, form carburetted hydrogen when 
confined in vessels of that gas ; and their leaves, 
after separation from the stem > deteriorate the air if 
they are kept in the shadef. In the progress of their 
decomposition, \also, either in water or in air, their 
carbon escapes in combination with different elastic 
fluids. From these facts it appears, that, in due de~ 
grees of moisture and of heat, a constant action is 
maintained between vegetables and the air, under 
which carbonic acid is produced. In the dead plant, 
however, this carbon is separated in the progress of 
those spontaneous changes which terminate in the 
destruction of the* vegetable structure : in the living 
one, it is given off as an excretion, and therefore de- 
pends primarily on those laws and conditions which 
regulate the motions and conditions of the vegetable 
fluids. In many instances, however, it may be diffi- 
* Recherches Cbim. p. 96, t Ibid. p. 60. 
