102 THE WONDERFUL HOUSE THAT JACK HAS 



parts in ten thousand or about one five-hundredth is 

 dangerous to breathe. Fractions with such large 

 denominators are so difficult to picture mentally that 

 we can scarcely realize how small an amount of car- 

 bonic acid gas in our living rooms is dangerous to 

 health. But if we could imagine a room divided into 

 one thousand equal parts, and one of these parts 

 representing that amount, we should see that it is 

 indeed very small. 



Carbonic acid gas is so dense that it can be poured 

 from one bottle to another. It is also much heavier 

 than air. This accounts for the fact that in vats, 

 cellars, wells, and mines where large amounts of this 

 gas are generated, it often settles in the bottom, 

 fatally injuring people who accidentally come in con- 

 tact with it. If a lighted lamp or candle goes out 

 when lowered into such a place, it is a sign that it 

 would be dangerous for human beings. However, 

 under ordinary conditions in out-of-door air, such a 

 thing does not take place, for it is a fixed law of nature 

 that in free air these three gases mix and keep in the 

 proportion, four-fifths nitrogen, about one-fifth oxygen, 

 and one twenty-five-hundredth carbonic acid gas. 



In previous chapters we read that carbonic acid gas 

 is produced by fermentation both in bread making 

 and in the manufacture of alcoholic drinks. It is 

 also one of the products of combustion or burning, 

 for when oxygen unites with the carbon of wood or 

 coal, the tissues of our body, or any other fuel, car- 



