﻿FORRESTER'S EVENINGS AT HOME. 69 



M. F. The precise point can never be known. It is certain 

 that by far the largest portion is within fifteen miles of the Earth's 

 surface. As we ascend, the air becomes thinner, and thinner ; and 

 by various experiments it is supposed that at about forty-five miles 

 from the surface of the earth the atmosphere ceases. People who as- 

 cend high mountains, or who go up in balloons, tell you how difficult 

 It is to breathe,, in consequence of the rarity or thinness of the air. 



Flora Lincoln. Why is it more difficult to breathe on the top 

 of a mountain than in a valley ? Is there not air on the top of the 

 highest mountains ? 



M. F. I will tell you. The oxygen in the air supports life. 

 The nitrogen, on the contrary, is poisonous, and is thrown off every 

 time we breathe. Now on the top of a mountain the air is thinner 

 than at the surface, and consequently we should not get near as 

 much oxygen at two breaths as we should in a valley at one. The 

 two gases are separated in the lungs, and they have double labor to 

 perform to collect oxygen sufficient for the support of life. 

 George. Is there any atmosphere in the water ? 

 M. F. Certainly ; and air is as necessary for fishes as for men, 

 and women, and boys, and girls. If you should put a fish in a bot- 

 tle of water and cork it tight, the fish would die very soon. Every- 

 thing seems to be fitted for its own peculiar element, and though our 

 lungs cannot breathe in water, fishes have gills, which enable them 

 to do so. 



The nitrogen gas which is breathed out of your chest cannot again 

 be used by others, on account of its poisonous qualities. If you were 

 closed in an air-tight room, you would die as soon as you had breathed 

 all the oxygen gas. To prove this you need not try it yourself, or 

 upon any living creature. The same process which goes on in the 

 burning of a candle, or fire, goes on in your body, and keeps you 

 warm. In breathing, a small portion of your body is consumed, as 

 the candle is, and that heat is produced which makes and keeps you 

 warm. In the place of a breathing animal, we can therefore use a 

 lighted candle, because they are much alike in this respect. If we 

 find that a lighted candle goes out in air which has been repeatedly 

 used, then we may conclude that it is unfit to sustain life. Take a 

 thin piece of cork, and put a candle wick and a bit of tallow upon 



