CARBONIC ACID. 311 



fire is used, and also in our arrangements to secure do- 

 mestic comfort, immense quantities of this gas are 

 formed. We may, indeed, fairly estimate the amount, 

 if we ascertain the quantity of wood and coal consumed, 

 of all the carbon which combines with oxygen while 

 burning, and escapes into the air, either as carbonic acid 

 or carbonic oxide. The former gas, the same as that 

 which accumulates in deep wells and in brewers' vats, 

 is highly destructive to life, producing very distressing 

 symptoms, even when mixed with atmospheric air, in 

 but slight excess over that proportion which it com- 

 monly contains. The oppressive atmosphere of crowded 

 rooms is in a great measure due to the increased proportion 

 of carbonic acid given off from the lungs of those assem- 

 bled, and collected in the almost stagnant air of badly 

 ventilated apartments. It will be evident to every one, that 

 unless some provision was made for removing this dele- 

 terious gas from the atmosphere as speedily as it formed, 

 consequences of the most injurious character to the 

 animal races would ensue. It is found, however, that 

 the quantity in the atmosphere is almost constantly 

 about one per cent. The peculiar properties of carbonic 

 acid in part ensure its speedy removal. It is among 

 the heaviest of gaseous bodies, and it is readily absorbed 

 by water ; consequently, floating within a short distance 

 from the surface of the earth, a large quantity is dis- 

 solved by the Avaters spread over it. A considerable 

 portion is removed by the vegetable kingdom ; indeed, 

 the whole of that produced by animals, and by the 

 processes of combustion, eventually becomes part of 

 the vegetable world, being absorbed with water by the 

 roots, and separated from the air by the peculiar func- 

 tions of the leaves. However, this heavy gas unites 

 with the lighter atmospheric fluid in obedience to that 

 law which determines the diffusion of different specific 

 gravities through each other. 



The leaves of plants may be regarded as performing 



