THEORY OF VEGETATION. 41 



habitants of mountains, they soon perish; if of 

 plains, they show a constant debility ; but if of 

 marshy grounds, their growth is not impeded. 



Carbonic acid is formed and given out during the 

 process of fermentation, putrefaction, respiration, 

 &c., and makes 28 parts out of 100 of atmospheric 

 air. It is composed, according to Davy, of oxygen 

 and carbon, in the proportion of 34 of the former 

 to 13 of the latter. It combines freely with many 

 different bodies; animals and vegetables are almost 

 entirely composed of it ; for the coal which they 

 give, on combustion, is but carbon united to a little 

 oxygen, &c. Priestley was the first to discover that 

 plants absorb carbonic acid; and Ingenhouse, Senne- 

 bier, and De Saussure have proved that it is their 

 principal aliment. Indeed, the great consumption 

 made of it cannot be explained by any natural pro- 

 cess excepting that of vegetation. On this head 

 we cannot do better than digest the experiments of 

 the last of these chymists into a few distinct pro- 

 portions :* 



1. In pure carbonic acid gas, seeds will swell, but 

 not germinate. 2. United with water, this gas has- 

 tens vegetation. 3. Air, containing more than one 

 twelfth part of its volume of carbonic acid, is most 

 favourable to vegetation. 4. Turf, or other carbona- 

 ceous earth, which contains much carbonic acid, is 

 unfavourable to vegetation until it has been exposed 

 to the action of atmospheric air, or of lime, &c. 

 5. If slackened lime be applied to a plant, its growth 

 will be impaired until the lime shall have recovered 

 the carbonic acid which it lost by calcination. 6. 

 Plants kept in an artificial atmosphere, and charged 

 with carbonic acid, yield, on combustion, more of 

 that acid than plants of the same kind and weight 

 growing in atmospheric air. 7. When plants are 

 exposed to air and sunshine, the carbonic acid of 



* Kecherches chymiques sur la vegetation, chap. ii. 



