CHAPTER XVIII. Q. _Of what is atmospheric AIR composed?_ A. Principally of two gases, _oxygen_ and _nitrogen_; mixed together in the following proportion: viz. 1 part of oxygen, to 4 parts of nitrogen. Q. _What are the uses of the OXYGEN of the air?_ A. It is the _oxygen_ of the air which _supports combustion_, and _sustains life_. Q. _What is meant when it is said, that the OXYGEN of the air "SUPPORTS COMBUSTION?"_ A. It means this; that it is the _oxygen of the air_ which makes _fuel burn_. Q. _How does the OXYGEN of the air make FUEL BURN?_ A. The fuel being decomposed (by heat) into _hydrogen_ and _carbon_; the _carbon combines with the oxygen of the air_, and produces _combustion_. Q. _What does the combination of carbon and oxygen produce?_ A. The _carbon of the fuel_ combining with the _oxygen of the air_ makes CARBONIC ACID GAS. (_see pp. 36, 37_). Q. _What becomes of the HYDROGEN of the FUEL?_ A. Hydrogen (being very inflammable) _burns with a blaze_, and is the cause of the _flame_ which is produced by combustion. (_see p. 34_). Q. _What becomes of the NITROGEN of the air, amidst all these changes and combinations?_ A. The _nitrogen of the air escapes_, and is _absorbed by the leaves_ of grass, trees, and various other vegetables. Q. _What is meant when it is said, that OXYGEN "SUSTAINS LIFE?"_ A. It means this: if a person _could not inhale oxygen_, he would _die_. Q. _What GOOD does this inspiration of OXYGEN do?_ A. 1st--It gives _vitality to the blood_: and 2ndly--It is the _cause of animal heat_. Q. _How is FOOD converted into BLOOD?_ A. After it is swallowed, it is dissolved in the stomach into a _grey pulp_; it then passes into the intestines, and is converted by the "bile" _into a milky substance_ (called _chyle_). Q. _What BECOMES of the milky substance, called CHYLE?_ A. It is absorbed by the vessels called "_lacteals_," and poured into the veins _on the left side of the neck_. Q. _What becomes of the chyle AFTER it is POURED into the VEINS?_ A. It then _mingles with the blood_, and is itself _converted into blood_. Q. _How does the OXYGEN we inhale MINGLE with the BLOOD?_ A. The oxygen of the air mingles with the blood _in the lungs_, and converts it into a _bright red colour_. Q. _What colour is the blood BEFORE it is oxydized in the lungs?_ A. _A dark purple._ The oxygen turns it to _a bright red_. 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.