816 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 abinidance 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 



