4. Why do we breathe air?

Because the air contains oxygen, which is necessary to life.5. Why is oxygen necessary to life?

Because it combines with the carbon of the blood, and forms carbonic acid gas.

"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 oxygen, nitrogen, and carbonic acid gas, in the proportions of oxygen 20 volumes, nitrogen 79 volumes, and carbonic 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 breathed?

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 six times successively, it would have parted with all its oxygen, and could no longer sustain life.

"A prudent man forseeth the evil, and hideth himself; but the simple pass on, and are punished."—Proverbs xxvii.

14. Is the impure air sent out of the lungs lighter or heavier than common air?

At first, being rarefied by warmth, it is lighter. But, if undisturbed, it would become heavier as it cooled, and would descend.15. Why is it proper to have beds raised about two feet from the ground?

Because at night, the bed-room being closed, the breath of the sleeper impregnates the air of the room with carbonic acid gas, which, descending, lies in its greatest density near to the floor.16. What are the chief sources of carbonic acid gas?

The vegetable kingdom (as will be hereafter explained), the combustion of substances composed chiefly of carbon, the breathing of animals, and the decomposition of carbonic compounds.17. Is breathing a kind of combustion?

It is. In the breathing of animals, the burning of coals, or of wood, or candles, &c., similar changes occur. The oxygen of the air combines with the carbon of the substance said to be burnt, and forms carbonic acid gas, which unfits the air for the purposes of either breathing or of burning, until it has been renewed by admixture with the air.18. What is carbon?

It is one of the elementary bodies, and is very abundant throughout nature. It abounds mostly in vegetable substances, but is also contained in animal bodies, and in minerals. The form in which it is most familiar to us is that of charcoal, which is carbon almost pure.19. What is meant by an elementary body?

An elementary body is one of those substances in which chemistry is unable to discover more than one constituent. For instance, the chemist finds that water is composed of oxygen and hydrogen. Water is therefore a compound body. But carbon consists of carbon only, and therefore it is called a simple, or elementary body.

"Where no wood is, there the fire goeth out: so where there is no tale-bearer, the strife ceaseth."—Proverbs xxvi.

20. Why is it dangerous to burn charcoal in rooms?

Because, being composed of carbon that is nearly pure, its combustion gives off a large amount of carbonic acid gas.21. What is the effect of carbonic acid gas upon the human system?

It induces drowsiness and stupor, which, if not relieved by ventilation, would speedily cause death.22. What is the reason that people feel drowsy in crowded rooms?

Because the large amount of carbonic acid gas given off with the breaths of the people, makes the air poisonous and oppressive.23. What other causes of drowsiness are there?

The candles, gas, or fires that may be burning in the rooms where people are assembled. Three candles produce as much carbonic acid gas as one human being; and it is probable that one gas-light produces as much carbonic acid gas as two persons.24. Have people ever been poisoned by their own breaths?

In the reign of George the Second, the Rajah of Bengal took some English prisoners in Calcutta, and put 146 of them into a place which was called the "Black Hole." This place was only 18 feet square by 16 feet high, and ventilation was provided for only by two small grated windows. One hundred and twenty-three of the prisoners died in the night, and most of the survivors were afterwards carried off by putrid fevers. Many other instances have occurred, but this one is the most remarkable.