COMMON THINGS AIE. 



28. Carbonic acid in effervescing liquors. 29. Is generated in all 

 spontaneous changes of dead matter. 30. Choke-damp. 31. Is dif- 

 fused through the atmosphere. 32. Evolved in respiration. 33. 

 Vital air. 34. Air poisoned in crowded rooms. 35. Necessity of 

 ventilation. 36. Air not absolutely transparent or colourless. 



1. OF all common things, air is the most common. No space or 

 place is accessible to us that is not filled with it. It is of all 

 material wants that which, is most incessantly indispensable to 

 our existence. Food is an occasional want, an intermitting supply 

 is all that is needed. Clothing may in certain cases be dispensed 

 with, and habit may inure us to a deficiency of it. The want of 

 warmth must be extreme to become fatal. But the privation of 

 air, even for a brief interval, is attended with instant and certain 

 death. 



Unlike other natural wants, our consumption of air is not volun- 

 tary. The action of the lungs is like the oscillations of a pendu- 

 lum. It is incessant, sleeping or waking, in sickness or in health ; 

 sitting, standing, or moving, it is maintained with a regularity 

 and continuity quite independent of the will. Its suspension is 

 the suspension of life. 



Must we not then be prompted by a natural and irresistible 

 curiosity to obtain some acquaintance with a physical agent so 

 universal, so omnipresent, and so indispensable to our vitality ? 



2. Air is the transparent, colourless, invisible, light, and atte- 

 nuated fluid with which we are always surrounded. It is drawn 

 into our lungs by the action called suction, and after remaining a 

 moment there, is forced o at through the mouth and nose by the 

 muscular compression ol the chest. This alternate action, by 

 which the air enters and leaves the lungs, is called respiration. 

 During the moment it rei nains in the lungs, it undergoes a certain 

 change, which we shall presently explain, in consequence of which, 

 when expired, it is not the same as that which was inspired. The 

 effect produced on the blood by this change is essential to the 

 maintenance of life. 



The air which, thus changed, is expired, is unfit for respiration. 

 If, therefore, the same air be taken several times successively into 

 the. lungs, death must ensue. 



3. The air around us, therefore, requires to be continually 

 changed, that which we expire being carried away and replaced 

 by fresh and pure air. 



4. The apparent lightness of air, the freedom with which we 

 move through it, and its invisibility, led the ancients to imagine 

 that it was unsubstantial and immaterial, and hence the disem- 

 bodied souls of the dead came to be called spirits, from the word 

 spiritus, which signifies air. 



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