COMMON THINGS AIR. 



pure oxygen gas. The vivid combustion of the charcoal will take 

 place, and will be continued for a certain time, when it will cease, 

 becoming continually less vivid until it is extinguished. If the 

 gas now contained in the jar be examined by the usual chemical 

 test, it will be found that it is no longer oxygen. A taper plunged 

 in it will be instantly extinguished. An animal placed in it will 

 die. Its weight will be greater than that which it had previously 

 to the experiment, and if the unburnt residue of the charcoal be 

 weighed it will be found to have lost precisely the weight which 

 the gas has gained. 



In a word, the oxygen gas has been converted into another 

 and heavier gas, called carbonic acid, and this has been accom- 

 plished by a portion of the charcoal entering into chemical com- 

 bination with it, that combination being attended with the 

 evolution of heat and light, which characterises the phenomenon 

 of combustion or burning. 



24. Now it is most desirable to become familiar with the 

 character and properties of this gas, for it plays a most important 

 part in numberless processes and phenomena natural and artificial, 

 which we encounter daily and hourly in the common experience 

 of life. 



Like all other gases, carbonic acid in its ordinary state is 

 invisible, colourless, compressible, and elastic. It has a pungent 

 smell and acidulous taste. If reduced to the temperature of 

 melting ice and compressed with a force of 36 atmospheres, that 

 is of 36 X 15, or 540 Ibs. per square inch, it is reduced to a liquid, 

 and when reduced to 180 below zero of Fahrenheit's thermometer, 

 it is frozen and becomes solid. 



25. This gas is altogether unfit for respiration. When breathed 

 pure it produces a violent spasm of the organ of the throat called 

 the glottis, which prevents the gas from entering the lungs. If, 

 however, it be mixed with so much common air as to prevent it 

 from producing this spasm, it may enter the lungs, and in that 

 case it acts on the system as a narcotic poison. 



26. All substances used for warming rooms, such as coal, 

 coke and wood, and all such as are used for lighting them, such 

 as oil, tallow, wax, consist chiefly of carbon combined in small 

 proportions with other constituents. The chief product of the 

 combustion of all such substances is therefore carbonic acid. 

 When coal, or other fuel, is burnt in a grate or stove, the 

 carbonic acid is carried away by the chimney or flue, and there- 

 fore does not pollute the air of the room. But this is not the 

 case with the carbonic acid produced by the candles and lamps 

 used to illuminate the room. All the carbonic acid produced by 

 them mixes with the atmosphere of the room and poisons it to 



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