THB BEASOK WHY. 29 



" Be not as the horse, or as the mule, which have no understanding : whm> 

 mouth must be held with the bit and bridle." PSALM xxxn. 



6. WJiy is this combination necessary ? 



Because we are so created that the substances of our bodies are 

 constantly undergoing change, and this resolving of solid matter 

 into a gaseous form, is the plan appointed by our Creator to remove 

 the matter called carbon from our systems. 



7. Why do our bodies feel warm ? 



Because, in the union of oxygen and carbon, heat is developed. 



8. What is this union of oxygen and carbon called? 



It is called combustion, which, in chemistry, means the decomposi- 

 tion of substances, and the formation of new combinations, accom- 

 panied by heat ; and sometimes by light, as well as heat. 



9. What is formed by the union of oxygen and carbon ? 

 Carbonic acid gas. 



10. What becomes of this carbonic acid gas ? 



It is sent out of our bodies by the compressure of the lungs, and 

 mingles with the air that surrounds us. 



11. Is this carbonic acid gas heavier or lighter than the 

 air ? 



Pure carbonic acid gas is the heaviest of all the gases. That 

 which is sent out of the lungs is not pure, because the whole of the 

 air taken into the lungs at the previous inspiration has not been 

 deprived of its oxygen, and the nitrogen is returned. Therefore 

 the breath sent out of the lungs may be said to consist of air, with 

 a large proportion of carbonic acid gas. 



12. What is the composition of air in its natural state ? 



It consists of oxygen, nitrogen, and carbonic acid gas, in the 

 proportions of oxygen 20 volumes, nitrogen 79 volumes, and car- 

 bonic acid gas 1 volume. It also contains a slight trace of watery 

 vapour. 



13. What is the state of the air after it has once been 

 Ireathcd ? 



It has parted with about one-sixth of its oxygen, and taken up an 

 equivalent of carbonic acid. And were the same air to be breathed 



