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 assists 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_. Q. _Why is HOT TEA, &c. cooled more rapidly by BLOWING it?_ A. Because the heated air is _blown more rapidly away_; in consequence of which, _cold air more rapidly succeeds_ to _absorb heat_ from the surface of the tea or broth. Q. _If a shutter be closed in the day-time, the stream of light (piercing through the crevice) seems in CONSTANT AGITATION. WHY is this?_ A. The air (in the sun-beam piercing through the shutter-crevice) is _more heated_, than _that in its neighbourhood_; the convective current, therefore, is _distinctly seen_, where little motes and particles of dust are _thrown into agitation_ by the _violence_ of the current. Q. _Why is the GALLERY of a CHURCH or theatre HOTTER than the AISLE or pit?_ A. The hot air ascends from the _bottom_ to the _top of the room_, and cold air (from the doors and windows) flies to the _bottom_ to supply its place. Q. _Why does a CROWDED ROOM produce HEAD-ACHE?_ A. Because we breathe air _vitiated by the crowd_. Q. _How does a CROWD VITIATE the AIR of a ROOM?_ A. Whenever we breathe, the elements of the air are _separated_ in the lungs, _some of the oxygen is absorbed by the blood_, and some of it is converted into _carbonic acid gas_, and exhaled with the nitrogen. Q. _Is ALL the NITROGEN REJECTED by the lungs?_ A. Yes; all the nitrogen of the air is always exhaled. Q. _What is CARBONIC ACID GAS?_