RESPIRATION. 131 



called into exercise, leaving a reserve capacity of about one 

 hundred and twenty cubic inches, equivalent to three and 

 one half pints. This provision is indispensable to the con- 

 tinuation of life; otherwise, a slight embarrassment of res- 

 piration, by an ordinary cold, for instance, would suffice 

 to cut off the necessary air, and the spark of life would 

 be speedily extinguished. 



13. The Air we breathe. The eartfi is enveloped 

 on all sides by an invisible fluid, called the atmosphere. 

 It forms a vast and shoreless ocean of air, forty-five miles 

 deep, encircling an4 pervading all objects on the earth's 

 surface, which is absolutely essential for the preservation of 

 all vegetable and animal life, in the sea, as well as on the 

 land and in the air. At the bottom, or in the lower strata 

 of this aerial ocean, we move and have our being. Per- 

 fectly pure water will not support marine life, for a fish 

 may be drowned in water from which the air has been ex- 

 hausted, just as certainly as a mouse, or any other land 

 animal, will perish if put deeply into the water for a length 

 of 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 

 vital processes are at once checked. 



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

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

 to the chemist as oxygen and nitrogen, in the proportion 

 of one part of the former to four parts of the latter. These 

 gases are very unlike, being almost opposite in their prop- 

 erties: nitrogen is weak, inert, and cannot support life; 

 while oxygen is powerful, and incessantly active ; and is the 

 essential element which gives to the atmosphere its power 

 to support life and combustion. The discovery of this fact 

 was made bv the French chemist, Lavoisier, in 1778. 



13. The atmosphere ? How high or deep ? How essential to life ? Marine lift 

 in perfectly pure water and air ? 



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



