308 Researches on Vegetation. 



been detetmined by observations, have produced uncertain- 

 ties and even contradictions in the c^^nsequences deduced 

 Irotn them. Saussure shows that a twelfth of carlionic acid 

 gas added to the atmosphere in which vegetation takes 

 phice, is favourable to its progress ; that a greater proportion 

 is prejudicial ; that the substances which can furnish this 

 quanlitv of carbonic acid, by their contact with the atmo- 

 sphere in which the phmt Uves, produce an advantageous 

 effect without having an immediate connnunication with the 

 plant, and that the benefit arising from the carbonic acid 

 takes place only when tlic atmosphere contains a greater or 

 less proportion of oxygen gas, otherwise it becomes hurtful. 

 Atmospheric air whicli has been deprived, by means of 

 lime, of the carbonic acid it contained, continues proper 

 for maintainiu'T vegetation in the sun ; hut if hme be placed 

 in a receiver lilled with atmospheric air, and in which the 

 plant is exposed to the light, the plant soon perishes, and 

 the lime has absorbed the carbonic acid. It is seen, then, 

 that during vegetation there is formed carbonic acid, but 

 that this acid ought to serve for maintaining the act of ve- 

 cetation ; for if it be absorbed vegetation ceases. This ef- 

 fect takes place whether the plant be kept in water or whe- 

 ther its root be in the earth ; only in the latter case the ef- 

 fect is less, because a part of the carbonic acid arising from 

 the root is more abundant, and is not soon enough absorbed 

 by the lime. In the shade, the presence of lime instead of 

 being hurtful is favourable to vegetation, because in the 

 shade the carbonic acid is as contrary to vegetables as it is 

 favourable to their development with the aid of light. 



Dr. Priestley discovered that leaves ameliorated air which 

 had been corrupted by combustion and respiration. Sene- 

 bier had shown that this phajnomenon arose from the pro- 

 perty which leaves ha;e of decomposing the carbonic acid 

 bv appropriating to themselves its carbon and eliminating 

 its oxvgen. This important discovery required that the ef- 

 ' t'ccts of the decomposition of the carbonic acid, and the 

 changes which result from it, either in the atmosphere in 

 which a plant is maintained or in the composition of that 

 plant, should be more particularly analysed. This is what 

 Saussure has done. 



He shows that the atmosphere, of which carbonic acid 

 forms a determinate part, and in which a plant vegetates in 

 the light, docs not generally change its dimensions, though 

 it sometimes experiences a little diminution; tliat the car- 

 bonic acid is destroyed, and is replaced by oxygen gas; but 

 that this replacemciit is not total, being equivalent only to 



half 



