Q. Why are persons so pale who live in close rooms and cities?
A. The blood derives its redness from the oxygen of the air inhaled; but, as the air in close rooms and cities is not fresh, it is deficient in oxygen, and cannot turn the blood to a beautiful bright red.
Q. Why are persons who live in the open air and in the country, of a ruddy complexion?
A. As the blood derives its bright red colour from the oxygen of the air inhaled, therefore, country-people (who inhale fresh air) are more ruddy than citizens.
Q. Why is not the air in cities so fresh as that in the country?
A. Because it is impregnated with the breath of its numerous inhabitants, the odour of its sewers, the smoke of its fires, and many other impurities.
Q. How does the combination of oxygen with the blood produce animal heat?
A. The principal element of the blood is carbon, which (combining with the oxygen of the air inhaled) produces carbonic acid gas, (in the same way as burning fuel.) (see pp.[33],[36]).
Q. What becomes of the nitrogen of the air, after the oxygen enters the blood?