RESPIRATION 195 



off the necessary air, and the spark of life would be speedily 

 extinguished. 



13. The Air we Breathe. The earth is enveloped on all 

 sides by an invisible fluid, called the atmosphere. It forms a 

 vast ocean of air, forty-five miles deep, encircling and per- 

 vading all objects on the earth's surface, and is absolutely 

 essential for the preservation of all vegetable and animal life 

 in the sea, as well as x on the land and in the air. At the 

 bottom, or in the lower strata of this ocean of air, we move 

 and have our being. Perfectly pure water will not support 

 marine life, for a fish may be drowned in water from which 

 the air has been exhausted, just as certainly as a mouse, or 

 any other land animal will perish if held under water for a 

 short time. The cause is the same in both cases the animal 

 is deprived of the requisite amount of air. It is also stated, 

 that if the water-supply of the plant be deprived of air, its 

 growth is checked. (Read Note 2.) 



14. The air is not a simple element, as the ancients sup- 

 posed, but is formed by the mingling of two gases, known to 



2. The Atmosphere. " It surrounds us on all sides, yet we see it not ; 

 it presses on us with a load of fifteen pounds to every square inch of sur- 

 face of our bodies, or from seventy to one hundred tons on us in all, yet 

 we do not so much as feel its weight. Softer than the softest down 

 more impalpable than the finest gossamer it leaves the cobweb undis- 

 turbed, and scarcely stirs the lightest flower that feeds on the dew it sup- 

 plies ; yet it bears the fleets of nations on its wings around the world, 

 and crushes the most refractory substances beneath its weight. When in 

 motion, its force is sufficient to level the most stately forests with the 

 earth ; to raise the waters of the ocean into ridges like mountains, and 

 dash the strongest ships to pieces like toys. It bends the rays of the sun 

 from their path, to give us the twilight of evening and of dawn ; it dis- 

 perses and refracts their various tints, to beautify the approach and 

 retreat of the orb of day. But for the atmosphere, sunshine would burst 

 on us and fail us at once, and at once remove us from midnight darkness 

 to the blaze of noon. We should have no twilght to soften and beautify 

 the landscape, no clouds to shade us from the scorching heat ; but the 

 bald earth, as it revolved on its axis, would turn its tanned and weakened 

 front to the full and unmitigated rays of the lord of day." Buish. 



13. The atmosphere ? How high or deep ? How essential to life ? Marine life in per- 

 fectly pure water and air ? 



14. Composition of the air 1 Properties of the two gases ? 



