12 THE Am AND THE GASES IN IT. 



36. The Light and Heat both come from the wood as it 

 burns. While a tree is growing, it receives, from the 

 sunshine, light and heat, and absorbs them, and lays them 

 up in the wood. There they lie, as in a storehouse, till 

 they are brought out by burning. 



37. Ammonia. Hydrogen combines with nitrogen to 

 form ammonia, which is one of the essential articles in 

 the food of plants. 



38. Wherever decay or decomposition of any animal 

 substance, or almost any vegetable substance, takes place, 

 there both these gases, hydrogen and nitrogen, are given 

 out, and, at the very moment they leave the other sub 

 stances with which they have been combined, they unite 

 and form ammonia, which rises and floats in the air, 

 and is dissolved rapidly by the moisture in the air, and 

 is then brought down to the earth in the rain. 



39. The little delicate roots absorb it from the earth, 

 and *it is carried into every part of the plant. Some 

 power in the plant separates the two again, for both are 

 always found in the growing parts; and nitrogen and 

 hydrogen are found in the seeds. 



40. Hydrogen unites also with sulphur, and forms a 

 very offensive gas, called sulphuretted hydrogen ; and 

 this also enters into the composition of plants, as a part 

 of their food. 



41. In 100 pints of common air, perfectly dry and 

 pure, there are about 21 of oxygen and 79 of nitrogen ; 

 that is, not far from one-fifth of oxygon and four-fifths of 

 nitrogen. In its common state, 100 pints of air contain 

 from 1 to 2J pints of watery vapor; and 1,500 pints 

 contain I pint &amp;lt;&amp;gt;t carbonic acid. 



42. In breathing, the air enters into the lungs, and 

 there the oxygen comes in contact with a portion of the 



