CARBONIC ACID. AMMONIA. 59 



205. Carbonic Acid is the most indispensable and 

 abundant article in the food of all plants. It enters the 

 plant dissolved in water, and either remains in that state, 

 or the vital action of the plant, in the light of the sun, 

 decomposes the acid, and throws back most of the oxygen 

 into the atmosphere ; but retains a portion which per 

 forms important offices ; and also retains the carbon. 

 This forms the solid parts of every plant. The walls 

 of the cells, the wood, the frame-work of the leaves and 

 of every other part, are made of carbon, together with 

 oxygen and hydrogen in the proportions in which they 

 form water. 



206. Hardly less important to the nourishment of 

 plants is Ammonia. This is a gas of a very pungent 

 odor and burning taste, which, when absorbed by water, 

 forms what is commonly called spirits of hartshorn. It 

 has a great attraction for carbonic acid, with which it 

 combines and forms carbonate of ammonia, popularly 

 called smelling salts. 



Ammonia is composed of hydrogen and nitrogen ; and 

 as both these substances are always found in living plant- 

 cells, and must be essential to the life and growth of these 

 cells, not less essential is ammonia, or some other source 

 of nitrogen, such as nitric acid. 



207. Carbonic acid, ammonia, nitric acid and water, 

 obtained thus from the atmosphere, are the atmospheric 

 food of plants, and the four simple elements which they 

 contain, are the only ones always found in every plant, 

 and therefore considered absolutely essential. 



208. From the fact that these essential elements are 

 derived from the atmosphere may be understood the 

 possibility of the growth of air-plants, which flourish 



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