212 SCIENCE OF COMMON THINGS. 



Sources of carbonic acid. The air always in motion. 



that is stored in the earth in the form of coal, or spread over its surface in 

 the form of animals or vegetables. 



141S What are the chief sources of carbonic acid ? 



Combustion, respiration of men and animals, the 

 decomposition of organic substances, and the exhalations 

 of volcanoes. Carbonic acid also exists in large quan- 

 tities in the atmosphere, in most waters, and combined 

 with minerals in a solid state, as in marble, which con- 

 sists of lime united to carbonic acid. 



1413 From which of these sources is carbonic acid most likely to accu- 

 mulate to a noxious extent f 



From the fermentation arid putrefaction of decaying 

 vegetable and animal matters. 



1414 How can this accumulation of carbonic acid be prevented? 



By throwing quicklime into places where such fer- 

 mentation ana putrefaction are going on. 



1415 How will quicklime prevent the accumulation of carbonic acid? 



Quicklime will absorb the carbonic acid, and produce 

 a combination called " carbonate of lime." 



1416 Does not heavy rain, as well as quicklime, prevent the accumula- 

 tion of carbonic acidf 



Yes ; an abundant supply of water will prevent the 

 accumulation of carbonic acid, by dissolving it. 



1417 Is the external air always in motion? 



Some portion of the atmosphere is always in motion. 

 Currents of warm air ascending, and currents of cold 

 air descending. 



1418 7s the air of our rooms always in motion f 



Yes ; there are always two currents of air in the 

 room we occupy ; one of hot air flowing out of the 

 room, and another of cold air flowing into the room. 



1419 How do you know that there are these two currents of air in 

 wery occupied room f 



If I hold a lighted candle near the crevice at the top 

 of the door, the flame will be blown outwards (towards 

 me hall) ; but if I hold the candle at the bottom of 

 the door, the flame will be blown inwards (into the 

 room). 



