219 
own number, and so to ring all the changes in the 
world *." 
SECT. V. Of the supposed Utility of Carbonic Acid, and 
of the Decomposition of Water in Vegetation. 
498. FROM the circumstances of carbonic acid be- 
ing decomposed in plants, by the agency of the solar 
rays, many writers have been led to consider that 
gas as necessary to vegetation in sunshine \ and o- 
thers have extended this opinion so far, as to sup- 
pose it essential to the growth of all plants through 
the day. In our former work, we combated the evi- 
dence (43.) on which this opinion was founded ; but 
as new experiments have been brought in its support 
by M. de Saussure, and as, from the view which 
we have now taken of the phenomena of vege- 
tation in sunshine, our former notions may be 
expected to have undergone some modification, we 
shall re-examine, as concisely as possible, the more 
material evidence relating to this question, with the 
hope of removing some of the difficulties, and appa- 
rent contradictions, which, at present, seem to attend 
it. 
499. Is then carbonic acid essential to the vege- 
tation of plants ? We have no direct experiment, 
says M. de Saussure, by which this is demonstrated, 
and many facts seem evei^ to establish the contrary. 
* Crew's Anatomy of Plants, p. 223. an. 1682. 
