INFLUENCE OF CARBONIC ACID UPON NUTRITION. 81 



The effects produced by mould, and many other sub- 

 stances which are employed to promote vegetation, are in 

 a great part owing to the carbonic acid gas, which they 

 are continually transmitting directly to the plant by its 

 roots, or throwing out into the atmosphere, whence it is 

 imbibed by the leaves. 



The power of absorbing carbonic acid, and of decom- 

 posing it, resides principally in the leaves ; and the decom- 

 position is very active when they are exposed to the sun, 

 in which case they give out to the atmosphere a large 

 quantity of oxygen combined with a little azote. 



According to the experiments of M. de Saussure, plants 

 retain a small portion of the oxygen arising from the de- 

 composition of carbonic acid, and throw out the rest into 

 the atmosphere. The rapidity with which the decompo- 

 sition of carbonic acid is carried on, is in proportion to 

 the brilliancy of the sun's rays, and to the greenness and 

 freshness of the leaves. It however appears, that decom- 

 position can be performed in the shade, though not very 

 actively ; since Sennebier observed, that leaves which un- 

 folded in the dark were sensibly tinged with green, which 

 he attributes to their decomposition of carbonic acid. 



I will here mention an observation which I made, a long 

 time since, in the coal mines of Bousquet, in the depart- 

 ment of Beziers. 



The pieces of wood which support the roof of the long 

 gallery which conducts to the beds of coal, were loaded 

 with that species of mushroom which usually fixes itself 

 upon the trunks of old trees ; the entrance of the gallery 

 is very light, but the light gradually diminishes till it 

 is lost in total darkness. I was much struck, in pass- 

 ing through this gallery, with the different appearances 

 presented by the mushrooms in the various degrees of 

 light ; those at the entrance were yellow, and their texture 

 so compact that they could hardly be broken by the hand. 

 As I advanced, the reddish yellow color grew gradually 

 fainter, and the texture of the plants more soft and spongy, 

 till at the bottom of the gallery, where a ray of daylight 

 never penetrates, I found the mushrooms, though as large 

 as those at the entrance, perfectly white, and nearly with- 

 out consistency, so much so, that upon pressing them with 

 the hand, they vyere found to yield much liquid, and but 

 little fibrous matter. I filled several bottles with these, 

 and took in my hands some of those from the middle and 



