LESSON 2G.] ITS KOOD. ICl 



and nitrogen. The nitrogen gas docs not snpport animal life v it only 

 dilutes the oxygen, which docs. It is the oxygen gas alone whieii 

 renders the air iit for breathing. 



4t)0. Carbonic acid consists of carbon combined with oxygen. In 

 breathing, animals are constantly forming carbonic acid gas by unit- 

 ing carbon from their bodies with oxygen of the air ; they inspire 

 oxygen into their lungs ; they breath it out as carbonic acid. So 

 with every breatli nniiiKils are diiniiii.-hing the oxygen of the air, — 

 so necessary to animal life, — and ar(i iiu-i-easing its carbonic acid, — 

 so hurtful to animal Hie ; or ralliei', wiiieh 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 it. They draw their whole stock of carbon irom the caibonic 

 acid of I lie air. 



4G1. Phmts take it in by their leaves. Every current, or breeze 

 that stirs the foliage, brings to every leaf a succession of fresh atoms 

 of carbonic acid, which it absorbs through its thousands of breathing- 

 pores. "We may })rove this very easily, by putting a small plant or 

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

 ing two opeiungs, causing air mixed with a known proportion of 

 carbonic acid gas to enter by one opening, slowly traverse the foliage, 

 and j)ass out by the other into a vessel proper to receive it : now, 

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

 acid than before. A portion has been taken up by the foliage. 



4G2. Plants also take it in by their roots, some probably as a gas, 

 in the same way that leaves absorb it, and much, certainly, dissolved 

 in the water which the rootlets imbibe. Tlie air in the soil, es- 

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

 as an equal bulk of tlu; atmosphero above. Decoinjjosing 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 jyasses, and bringing it 

 down to the roots of plants. And what flows off int^ the streams 

 and ponds serves for the Ibod of water-plants. 



4G3. So water and carbonic acid, taken in by the leaves, or taken 

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

 general food of plants, — are tlie raw materials out of which at lea.^t 

 14* 



