LESSON 26.] ITS FOOD. 161 



and nitrogen. The nitrogen gas does not support animal life •. it only 

 dilutes the oxygen, which does. It is the oxygen gas alone which 

 renders the air fi^ i^ biS^'hing. 



460. Carbo nic aa id-consists of carbon combined with pxj'gen. In 

 breathing, animals are constantly forming carbonfcai^^gas by unit- 

 ing carbon from' their bodies with oxygen of the air ; tjieyinsftire 

 oxygen into their lungs ; they breath it out as carbonic aeia-roo 

 jwith every breath animals are diminishing the oxygen of theaiiv— r/, 

 so necessary to animal life, — and are increasing its carbonic"aei%^ — 

 so hurtful to animal life ; or rather, which would be so hurtful if it 

 were allowed to accumulate in the air. The reason why it does not 

 increase in the air beyond this minute proportion is that plants feed 

 upon i(;. They draw their whole stock of carbon from the carbonic 



— amd (Jrthe air. 



461. Plants take it in by their leaves. Every current, or breeze 

 that stirs the^foliage,^rings to every leaf a succession" of fresh" atoms 

 of carbonic *e5% wnienit absorbs through its thousands of breathing- 

 pores. We may prove this very easily, by putting a small plant or 

 a fresh leafy bough into a glass globe, exposed to sunshine, and hav- 

 ing two/ openings, causing air mixed with a known proportion of 

 carbonic tt^itt- gas to enter by one opening, slowly traverse the foliage, 

 and pass out by the other into a vessel proper to receive it : now, 

 examining the air chemically, it will be found to have less carbonic 



-aead^hanDefore. A portion has been taken up by the foliage. 



462. Plants also take it in by their roots, some-probably-as a-gas, 

 iB-tbe-satRe-way-that-leav«s-abs»Fb-it^-»B4 maeh; aertainly, dissolved 

 in the watter which the rootlets imbibe. The air in the soy,,es- , 

 pecially in a rich soil, contains many times as much carbonic acio. 

 as an equal bulk of the atmosphere above. Decomposing vegetable^^' 

 matter or manures, in the soil, are constantly evolving carbonic acid, 

 and a large part of it remains there, in the pores and crevices, among 

 which the absorbing rootlets spread and ramify. Besides, as this gas 



is dissolved by water in a moderate degree, every rain-drop that falls 

 from the clouds to the ground brings with it a little carbonic acid, 

 dissolving or washing it out of the air as it passes, and bringing it 

 down to the roots of plants. And what flows off, into the streams 

 and ponds serves for the food of/waleEAilants. 



463. So water and carbonic aeWlj taken in by the leaves, or laken 

 in by the roots and carried up to the leaves as crude sap, are the 

 general food of plants, — are the raw materials out of which at least 



14* 



