

CAKBON. 81 



all green parts of the plants, are capable of absorbing carbon 

 dioxide from the air, whilst at the same time they liberate 

 oxygen. 



This process of vegetable respiration (if we may so call it), 

 which takes place under the influence of sunlight, is, conse- 

 quently, the reverse of that of animal respiration. The animal 

 uses oxygen and liberates carbon dioxide; the plant consumes 

 this carbon dioxide and liberates oxygen. 



Carbon dioxide is an acid oxide, which combines with water, 

 forming carbonic acid : 



C0 2 + H 2 O = H 2 CO 3 . 



Carbonic acid, H 2 C0 3 , is not known in a pure state, but always 

 diluted with much water, as in all the different natural waters. 

 Carbonic acid is a bibasic, extremely weak acid, the salts of 

 which are known as carbonates. Many of these carbonates 

 (calcium carbonate, for instance) are abundantly found in nature. 



Tests for carbon dioxide, carbonic acid, and carbonates. 



1. Pass the gas through lime-water, which is rendered turbid 

 by the formation of calcium carbonate : 



Ca2HO + CO 2 = CaCO 3 + H,O. 



2. From carbonates, evolve the gas by the addition of some 

 acid, and examine it by the same method. 



3. The soluble carbonates of potassium and sodium give pre- 

 cipitates with the solutions of most metallic salts; for instance, 

 with the chlorides of Ba, Ca, Sr, Mg, Fe, Zn, Cu, etc. 



Carbon monoxide, Carbonic oxide, CO = 28. Carbon mon- 

 oxide is a colorless, odorless, tasteless, neutral gas, almost in- 

 soluble in water; it burns with a pale-blue flame, forming carbon 

 dioxide; it is very poisonous, when inhaled, by forming with 

 the coloring matter of the blood a compound which prevents 

 the absorption of oxygen. Carbon monoxide is formed when 

 carbon dioxide is passed over red-hot coal : 



CO 2 + c = 2CO. 



The conditions necessary for the formation of carbon mon- 

 oxide are, consequently, present in any stove or furnace where 



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