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 fit for breathing. 



460. 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 breath animals are diminishing the oxygen of the air, 

 so necessary to animal life, and are increasing its carbonic acid, 

 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 it. They draw their whole stock of carbon from the carbonic 

 acid of the air. 



461. Plants 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 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 acid 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 

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



462. 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. The air in the soil, es- 

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

 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 inU the streams 

 and ponds serves for the food of water-plants. 



463J 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 the raw materials out of which at least 

 14* 



