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?_ A. The nitrogen is _exhaled_, and taken up by the leaves of trees and other vegetables. (_see p._ 35). Q. _Why does the vitiated air (after the oxygen has been absorbed) COME OUT of the MOUTH, and not sink into the stomach?_ A. The vitiated air (being _heated by the heat of the body_) _ascends naturally_, and passes by the _heavier fresh air_ (which we inhale) without obstruction or injury. Q. _If (both in combustion and in respiration) the OXYGEN of the air is CONSUMED, and the NITROGEN REJECTED--Why are not the PROPORTIONS of the AIR DESTROYED?_ A. Because the _upper surface of vegetable leaves_ (during the day) _gives out oxygen_ and _absorbs nitrogen_, and thus the proper balance is perpetually restored. Q. _Show how God has made ANIMAL and VEGETABLE life DEPENDENT on each other?_ A. _Animals_ require _oxygen_ to keep them alive, and _draw it from the air_ by inspiration; the upper surface of _leaves_ (all day long) _gives out oxygen_, and thus supplies the air with the _very gas_ required by man and other animals. Q. _Do not animals EXHALE the VERY GAS needed by VEGETABLES?_ A. Yes; animals reject the _nitrogen of the air_ (as not suited to the use of animal life), but _vegetables absorb it_, as it is the food they live on; and thus the vegetable world restores the equilibrium of the air, disturbed by man and other animals. Q. _Is AIR a good CONDUCTOR?_ A. No; air is a very _bad conductor_. Q. _How is AIR HEATED?_ A. By "convective currents." Q. _What are meant by "CONVECTIVE CURRENTS?"_ A. When a portion of air is heated, _it rises upward in a current_, carrying the heat with it: other _colder air succeeds_, and (being _heated_ in a similar way) _ascends also_; and these are called convective currents. ("Convective currents;" so called from the Latin words, cum-vectus (_carried with_) because the _heat_ is "carried with" the current.) Q. _Is AIR HEATED by the RAYS of the SUN?_ A. No; air is _not heated_ (in any sensible degree) _by the action of the sun's rays_ passing through it. Q. _Why then is the AIR HOTTER on a SUNNY DAY, than on a CLOUDY one?_ A. On a fine day, the sun _heats the surface of the earth_, and the air (resting on the earth) _is heated by contact_; as soon as it is heated _it ascends_, and _other_ air succeeding is _heated in a similar way_, till all is heated by convection.