CARBON. 95 



of 0.04 per cent, of carbon dioxide contained in atmospheric air 

 would gradually increase. This, however, is not the case, because 

 plants, and more especially all their green parts, are capable of ab- 

 sorbing 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, consequently, 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, form- 

 ing carbonic acid : 



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



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

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

 bonic 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. 



(Sodium carbonate, Na 2 CO 3 , may be used ) 



1. Pass carbon dioxide through lime-water, which is rendered 

 turbid by the formation of calcium carbonate : 



Ca(OH) 2 -f C0 2 = CaC0 3 + H 2 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, sodium, and ammonium, 

 give precipitates 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 insoluble 

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

 it is very poisonous when inhaled, 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 monoxide are, 

 consequently, present in any stove or furnace where coal burns with 



