BURNING AND BREATHING. 165 



wings in great agitation. In about thirty-seven minutes they 

 were exhausted and fell down ; thousands strewed the floor 

 of the hive, and all would have been suffocated had the ex- 

 periment been continued. As it was, the admission of fresh 

 air revived them. 



Evidently, therefore, the breathed air had been so altered 

 as to be incapable of supporting life. When air passes into 

 the lungs of an animal, part of the oxygen unites with the 

 carbon present in the blood and oxidises or burns it, with the 

 result that carbonic acid gas (carbon dioxide) is formed.* A 

 similar thing happens when we burn wax, tallow, oil, gas, 

 c., all of which consist of carbon and hydrogen variously 

 modified. Part of the oxygen of the air unites with the car- 

 bon, producing carbonic acid, and part with the hydrogen 

 producing water. The latter we can see by covering a bit 

 of lighted candle with a tumbler, which will at once become 

 clouded with dew.f 



But the flame burns, as the animal breathes, only while 

 it has enough oxygen, and as there is but little in a tumbler, 

 it goes out in a few moments. Not that the oxygen is entirely 

 exhausted, but the proportion being too small, the flame is 

 stifled. Hence carbonic acid may be used for extinguishing 

 fires, and one which had been burning in a coal mine thirty 

 years was, in 1851, successfully smothered by this means. 



This carbonic acid is the refuse we are now to consider. 



This gas is one and a half times as heavy as the air, so 



* One atom of carbon and two of oxygen make a molecule of carbon 

 dioxide. 



f When a lamp is first lighted, the glass, being cold, is similarly clouded, 

 until the dew is evaporated by the heat of the flame. 



