NOURISHMENT OF PLANTS. 27 



stances cannot putrefy without air. The property 

 we here ascribe to the air, of supporting life, combus- 

 tion, and putrefaction, is in reality due to the oxygen 

 it contains. This aldne imparts to the air the power 

 of maintaining the chemical processes we have 

 named. Where oxygen unites chemically with 

 hydrogen, it passes into a fluid form and becomes 

 water; when, again, it unites with metals and other 

 mineral substances, it becomes solid, and in this 

 state forms a principal clement of all our stones and 

 earths. 



2. Hi/drog-en, in like manner, when uncombined, 

 is a kind of air or gas, without color, taste, or odor, 

 and so light as to be used for filling air-balloons. 

 We find it in a solid or fluid form most extensively 

 diffused in nature, in water, snow, and ice, since, as 

 already mentioned, it forms the second essential ele- 

 ment of water. 



3. Carbon, when uncombined, presents itself as a 

 solid body, of a black or gray color, as is seen in 

 charcoal, soot, plumbago, coke, and other substances, 

 of which it is the principal constituent. It may also 

 assume, however, the form of a colorless, lustrous, 

 and transparent stone ; for the most precious of our 

 stones, the diamond, has been demonstrated by 

 chemical examination to be pure carbon. In pre- 

 cisely the same way it changes its color and form, 

 when it unites with oxygen, hydrogen, or nitrogen ; 

 for wood, sugar, and starch are not black, and yet 

 half of each of these substances consists of carbon. 



