14 THE AIR AND THE GASES IN IT. 



necessary to their existence, and not wholly unlike animal 

 respiration. 



45. By daylight, and especially in the sunshine, plants 

 absorb carbonic acid, turn the carbon, and water, or the 

 elements of water, into the substance of the wood, stem, 

 leaves and the other solid parts, and throw back part 

 of the oxygen into the air. Growing plants are thus 

 continually acting to purify the atmosphere, by taking 

 up the carbonic acid which is poured into it by com 

 bustion, by decay, and by the breath of animals, and 

 giving back oxygen suitable for healthy respiration. 



We thus see the wise and beautiful Relation which has 

 been established between animals and plants. The wind 

 which blows from the habitations of men and animals 

 carries foul air, no longer fit to be breathed, away to the 

 woods and fields. There the plants extract from the air 

 all that is poisonous ; and the wind which blows from the 

 field and forest brings back only the pure and vital 

 element of oxygen, mixed with harmless nitrogen. 



46. In the night time plants do not exercise this benefi 

 cent influence. On the contrary, they then exhale carbonic 

 acid, at least in small quantities. It is this, perhaps, 

 which renders it unsafe to have plants, especially when 

 in flower, in a sleeping room. 



It would seem that wood or woody fibre is not formed 

 during the night, but that the presence of the sun s light 

 is essentially necessary to this action of the life of a plant. 



47. The oxides of the metals, and some other com 

 pounds, arc bases; that is, they unite chemically with 

 carbonic acid, sitf/i/nirir nritl, nitric r/r/V/, and other acids, 

 and form salts, called carbonates, sulplialcs, nitrates, and 

 other ales. 



