Water. 135 



vital air of the atmosphere, which changes its dark colour 

 to a brilliant red, rendering it the spur to the action of the 

 heart and arteries, the source of animal heat, and the cause 

 of sensibility, irritability, and motion. With regard to the 

 nitrogen that is combined with atmospheric air, the great- 

 est part of it is thrown out of the lungs at every respira- 

 tion, and it rises above the head, that a fresh portion of air 

 may be taken in, and that the same air may not be repeated- 

 ly breathed. The leaves of trees and other vegetables give 

 out during the day a large portion of oxygen gas, which, 

 uniting with the nitrogen thrown off by animal respiration, 

 keeps up the equilibrium, and preserves the purity of the 

 atmosphere. In the dark, plants absorb oxygen, but the 

 proportion is small, compared to wh*at they exhale by day. 



Questions. — 1. Of what is atmospheric air composed .? 2. What 

 is the proportion of each, and what other substances does it contain."* 

 3. What is oxygen .-* 4. Why is it thus named .'' 5. How does it be- 

 come oxygen gas ? 6. What are some of its important properties ? 7. 

 Why has it been called vital air.? 8. What is nitrogen, and how does 

 it form nitrogen or azotic gas .'' 9. What are some of its properties ? 10. 

 What important end does it answer, and how .'' 11. How is the ver- 

 milion colour of the blood produced .-' 12. What becomes of the ni- 

 trogen that is thrown out of the lungs ? — why ? 13. What tends to 

 preserve the purity of the atmosphere .'' [Note. Nitrogen (pronounced 

 Ni'tro-jen,) is called azote by the French chemists on account of its 

 being so destructive of life. Oxygen, (pronounced ox'e-j6n,) besides 

 producing most of the acids, is necessary also to the production of the 

 alkalies.] 



LESSON 62. 



Water. 



Cal'cine, to burn in the fire to a calx ; — calx is a substance easily 

 reduced to powder. EfFerves'cence, an intense motion whicn 

 takes place in certain bodies, occasioned by the sudden escape 

 of a gaseous substance. 



Water was formerly considered as a simple substance, 

 and chemical philosophers were for a long time unwilling 

 to allow of its being otherwise. Its compound nature, how- 

 ever, has been fully proved. It is composed of eighty-eight 

 parts by weight of oxygen, and twelve of hydrogen, in every 

 hundred parts of the fluid. It is found in four states, name- 

 ly, solid or ice ; liquid or water ; vapour or steam ; and in a 



