138 HOW CROPS GROW. 



Oxygen, Symbol 0, atomic weight 16, is an ingredient 

 of the ash, since it unites with nearly all the other ele- 

 ments of Tegetation, either during the life of the plant, 

 or in the act of combustion. It unites with Carbon, 

 Sulphur, Phosphorus, and Silicon, forming acid bodies ; 

 while with the metals it produces oxides, which have the 

 characters of bases. Chlorine alone of the elements of 

 the plant does not unite with oxygen, either in the living 

 plant, or during its combustion. 



CARBON AND ITS COMPOUNDS. 



Carbon, Sym. C, at. wt. 12, has been noticed already 

 with sufficient fullness (p. 14). It is often contained as 

 charcoal in the ashes of the plant, owing to its being en- 

 veloped in a coating of fused saline matters, which shield 

 it from the action of oxygen. 



Carbon Dioxide, commonly termed Carbonic acid, 

 Sym. CO2, molecular weight 44, is the colorless gas 

 which causes the sparkling or effervescence of beer and 

 soda water, and the frothing of yeast. 



It is formed by the oxidation of carbon, when vegeta- 

 hle matter is burned (Exp. 6). It is, therefore, found 

 in the ash of plants, combined with those bases which in 

 the living organism existed in union with organic acids ; • 

 the latter being destroyed by burning. 



It also occurs in combination with calcium in the tissues 

 of many plants. Its compounds with bases are carbon- 

 ates, to be noticed presently. When a carbonate, as mar- 

 ble or limestone, is drenched with a strong acid, like 

 vinegar or muriatic acid, the carbon dioxide is set free 

 with effervescence. 



Carbonic Acid, HaCOg, or C0(0H)2, mo. wt. Q%. 

 This, the carbonic acid of modern chemistry, is not known 

 as a distinct substance, since, when set free from carbon- 

 ates by the action of a stronger acid, it falls into carbon 

 dioxide and water : 



