316 
PHYSIOLOGY. 
BOOK II. 
water, either by the roots or leaves, or both ; and here again 
the analogy holds good between the functions of respiration 
and digestion in animals and plants, for to both is carbonic 
acid deleterious when breathed, and to both is it invigorating 
to the digestive organs." — Journal of Royal Institution^ new 
series, vol. i. p. 99. 
As the decomposition of carbonic acid gas is thus evidently 
an important part of the act of respiration, it might be 
supposed that to supply a plant with a greater abundance of 
carbonic acid than the atmosphere will usually yield, would be 
attended with beneficial consequences. To ascertain this 
point several experiments have been instituted; the most 
important of which are those of Saussure, who found that, in 
the sun^ an atmosphere of pure carbonic acid gas, or even air, 
containing as much as sixty per cent., was destructive of vege- 
table life ; that fifty per cent, was highly prejudicial ; and that 
the doses became gradually less prejudicial as they were dimi- 
nished. From eight to nine per cent, of carbonic acid gas 
was found more favourable to growth than common air. This, 
however, was only in the sun : any addition, however small, 
to the quantity of carbonic acid naturally found in the air was 
prejudicial to plants placed in the shade. 
The life of a plant seems, then, to consist in a successive di- 
urnal decomposition and recomposition of carbonic acid. By 
night it vitiates the atmosphere by robbing it of its oxygen — 
by day it' purifies it by restoring it. It is a curious question 
whether,by this alternation of phenomena,the vegetable kingdom 
actually leaves the atmosphere in its original state, or whether it 
purifies it, permanently giving it more oxygen than it deprives 
it of Considering the great loss of oxygen produced either 
by the respiration of animals, or by its combination with va- 
rious mineral matters, or by other means, it is to be supposed 
that the atmosphere would in time become so far deprived of 
its oxygen as to be unfit for the maintenance of animal life, if 
it were not for some active compensating power. This 
appears to reside in the vegetable kingdom ; for Professor 
Daubeny, of Oxford, has ascertained by experiments, partially 
communicated to the British Association, but not yet pub- 
lished, that plants undoubtedly exercise a purifying influence 
