Q. _How does the COMBINATION of OXYGEN with the BLOOD produce animal HEAT?_

A. The princ.i.p.al 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 pa.s.ses 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 G.o.d 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, c.u.m-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_ pa.s.sing 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.

Q. _If AIR be a BAD CONDUCTOR, why does hot IRON get COLD, by being EXPOSED to the AIR?_

A. A piece of hot iron exposed to the air, is made cold--1st--By "convection;" and

2ndly--By "radiation."

Q. _How is hot iron (exposed to the air) made cold by CONVECTION?_

A. The air around the iron (being intensely heated by contact) rapidly ascends, _carrying some of its heat with it_: other air succeeds, _absorbs more heat_, ascends, and gives place to that which is _colder_; till the hot iron _is cooled completely down_.

Q. _How is hot iron cooled by RADIATION?_

A. While the heat of the iron is being carried off by "convection," it is _throwing off heat_ (on all sides) _by radiation_.

Q. _What is meant by RADIATION?_

A. Heat emitted (in all directions) from any surface, by _innumerable rays_.

Q. _Why is BROTH COOLED by being left exposed to the AIR?_

A. Hot broth throws off _some_ heat by _radiation_; but it is _mainly_ cooled down _by convection_.

Q. _How is hot BROTH cooled down by CONVECTION?_

A. The air _resting on the hot broth_ (being heated) _ascends_; _colder_ air succeeding _absorbs more heat_, and _ascends also_; and this process is repeated, till the broth is _made cool_.

Q. _Why is hot TEA and BROTH COOLED faster, for being STIRRED about?_

A. 1st--The agitation a.s.sists the liquor in _bringing its hottest particles to the surface_:

2ndly--The action of stirring _agitates the air_, and brings it _quicker_ to the broth or tea: and

3rdly--As the hottest particles are more rapidly brought into contact with the air, therefore _convection is more rapid_.