FUNCTION.] PURIFICATION OF ATMOSPHERE. 297 



atmosphere of pure carbonic acid gas, or even air containing 

 as much as sixty per cent, was destructive of vegetable life ; 

 that fifty per cent, was highly prejudicial ; and that the doses 

 became gradually less prejudicial as they were diminished. 

 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 quan- 

 tity of carbonic acid naturally found in the air, was prejudicial 

 to plants placed in the shade. 



The digestion of a plant seems, then, to consist to a great 

 extent in a successive diurnal 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 atmo- 

 sphere in its original state, or whether it purifies it perma- 

 nently, 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 various 

 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 communicated to 

 the British Association, that plants undoubtedly exercise a 

 purifying influence on the atmosphere. In a letter to myself 

 he expresses himself thus : 



" As the observations of Ellis left it in some doubt whether 

 the balance was in favour of the purifying or the deteriorating 

 influence upon the air w r hich is exercised by plants during 

 different portions of the day and night, I conducted my 

 experiments in such a manner that a plant might be inclosed 

 in a jar for several successive days and nights, whilst the 

 quality of the air was examined at least two or three times a 

 day, and fresh carbonic acid admitted as required. A register 

 being kept of the proportion of oxygen each time the air was 

 examined, as well as of the quantity of carbonic acid intro- 

 duced, it was invariably found that, so long as the plant 



