30D. That vegetables, however, with the aid of 
the solar rays, also possess the power of decompo- 
sing this acid, and reconverting it into oxygen gas, 
seems sufficiently established by the numerous facts 
already stated : and various experiments, related by 
M. de Saussure, abundantly confirm the same posi- 
tion. He found, as Drs Priestley and Percival had 
long before done, that plants would not, even in 
sunshine, vegetate in pure carbonic acid. If their 
atmosphere contained two-thirds or three-fourths of 
this gas, it was alike fatal to them. They vegetated 
seven days in an atmosphere containing one half its 
volume of carbonic acid ; ten days in one which con- 
tained one-fourth ; and when it contained one-eighth^ 
they have acquired nearly as great a weight as in 
atmospheric air. Lastly, when only one-twelfth of 
carbonic 'acid was present, the plants, he says, have 
increased in weight, and prospered better than in 
pure atmospheric air *. In the shade, the smallest 
quantity of carbonic acid, added to common air, is 
injurious to vegetation. The plants have died in six 
days, when it constituted one-fourth of their atmos- 
phere j and even when it was only one-twelfth, their 
vegetation has been feeble, and their increase of 
weight small. In all cases, oxygen gas, he adds., 
must be present ; for if a small portion of carbonic 
acid be added to pure nitrogen gas, plants die in it, 
even though exposed to the sun f. 
31O. Having thus ascertained the general effects of 
earbonic acid on vegetation, he proceeded to make 
* Recherche* 1 , p. 31. 
t Ibid. p. 33. 
