BOTANY FOR YOUNG PEOPLE. 27 



mixed with it to weaken it, otherwise every thing 

 would burn up too fast. After a little while 

 the flame would consume all the oxygen inside 

 the receiver, and then, as it could not burn 

 without more of it, go out. 



E. Suppose there was some contrivance 

 inside the receiver by which oxygen gas was 

 thrown out in sufficient quanties to supply 

 the combustion, when would the flame cease ? 



L. Not until both the oil and wick were 

 burned up. 



E. All animals, including man, resemble 

 lamps in requiring oxygen to keep them burning 

 or alive. But here the resemblance ceases, for 

 the lamp merely consumes the oxygen, but 

 animals, in return for this gas, throw out carbo- 

 nic acid, a compound, you know, of charcoal and 

 oxygen ; this is to them a deadly poison, being of 

 the same kind as the gas found at the bottom 

 of wells and cisterns, and which so often 

 kills those who incautiously descend into them ; 

 its mere accumulation in the atmosphere would 

 be sufficient to destroy animal life. To allow 

 us to breathe then, two objects must be attended 

 to ; in the first place, a supply of oxygen to take 

 the place of that which is consumed, and, in 



