A. Heat _rarefies_ the air, and causes it to _expand_. Q. _How do you KNOW that heat causes the air to EXPAND?_ A. If a bladder _half full of air_ (tied tight round the neck), were laid before a _fire_, the heat of the fire would expand the air so much, that the bladder would soon be _entirely inflated_; (in this case, the air in the bladder is expanded to _twice its original bulk_, by the heat of the fire). Q. _What EFFECT is produced upon air by RAREFACTION?_ A. It causes the air to _ascend through colder strata_, as a cork (put at the bottom of a basin of water) would ascend through the water. Q. _How do you KNOW that rarefied air ASCENDS?_ A. When a boy sets fire to the cotton of his balloon, the flame _heats the air_ inside the balloon; and the air becomes _so light_, that it ascends, and _carries the balloon with it_. Q. _What effect is produced upon AIR by COLD?_ A. Air is _condensed by cold_, or squeezed into a smaller compass; in consequence of which, _it becomes heavier_, and descends towards the ground. Q. _How do you KNOW that air is CONDENSED by COLD?_ A. After the bladder is _fully inflated_, (by lying before the fire), if it be taken _away from the fire_, the bladder will _collapse_, and show that it is not half full. Q. _What is meant by the bladder "COLLAPSING?"_ A. The skin will become _wrinkled, shrivelled, and flabby_, because there is not sufficient air inside to _fill it out_. Q. _How do you KNOW that CONDENSED air will DESCEND?_ A. As soon as the cotton of the balloon _is burnt out_, the air inside becomes _cold again_, and the balloon _falls to the earth_. Q. _Does the SUN HEAT the AIR as it does the EARTH?_ A. No; the air is _not heated by the rays of the sun_, because air (like water) is a very _bad conductor_. Q. _How is the AIR HEATED?_ A. By _convection_, thus:--The _sun_ heats the _earth_, and the _earth_ heats the _air resting upon it_; the air thus heated _rises_, and is succeeded by _other air_, which is heated in a similar way, till _all is warmed_ by "convective currents." Q. _What is meant by "CONVECTIVE CURRENTS of air?"_ A. Streams of air heated by the earth, which _rise upwards_ and _carry heat with them_, are called "convective currents" of hot air. Q. _Is the air in a ROOM in perpetual motion, as the air ABROAD is?_ A. Yes; there are always _two currents of air_ in the room we occupy, one of _hot_ air flowing _out_ of the room, and another of _colder_ air flowing _into_ the room. Q. _How do you KNOW, that there are these TWO currents of air in every occupied ROOM?_ A. If I hold a lighted candle near the crevice _at the top of the door_, the flame will be blown _outward_ (towards the _hall_); but if I hold the candle _at the bottom of the door_, the flame will be blown _inwards_ (into the _room_). Q. _Why would the flame be blown OUTWARDS (towards the HALL), if the candle were held at the TOP of the door?_