"Be not as the horse, or as the mule, which have no understanding: whose mouth must be held with the bit and bridle."—Psalm xxxii.

6. Why 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 decomposition of substances, and the formation of new combinations, accompanied 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 ox