Q. _Why is the FROTH of ale and porter INCREASED by PRESSURE?_ A. Because the liquor absorbs _carbonic acid_ so long as it is under _pressure_; and the moment that the pressure is _removed_, the carbonic acid _escapes_ in foam or froth. Q. _When a boy makes a BALLOON, and sets fire to the cotton or sponge (which has been steeped in spirits of wine), why is the balloon INFLATED, or blown out?_ A. The _air_ inside the balloon is _expanded by the flame_, till the whole balloon is _blown out_ without a crumple. Q. _Why does the BALLOON RISE, after it has been inflated by the expanded air?_ A. The same quantity of air is expanded _to three or four times its original volume_; and is made so much _lighter than common air_, that even when all the paper, wire, and cotton are added, it is still lighter bulk for bulk. Q. _What is meant by being lighter "bulk for bulk?"_ A. If the balloon be 3 square feet in size, it is _lighter_ (when inflated) than 3 square feet of _common air_, and therefore _floats through it_; as a cork (at the bottom of a tub of water) would rise to the surface. Q. _Why does SMOKE RUSH UP a CHIMNEY?_ A. The heat of the fire _expands the air in the chimney_; and (being thus made _lighter_ than the air around), it _rises up the chimney_, and carries the smoke in its current. Q. _Why has a LONG CHIMNEY a greater DRAUGHT than a short one?_ A. Because air rises faster and faster the _higher it ascends_ in a chimney flue; the same as a stone falls faster and faster the _nearer it approaches to the ground_. Q. _Why will a LONG chimney SMOKE, unless the FIRE be pretty FIERCE?_ A. If the fire be not pretty fierce, its heat will not be sufficient to _rarefy all the air in the chimney_; and then the chimney will _smoke_. Q. _Why will the chimney smoke, if the fire be not BIG enough to heat ALL the air in the CHIMNEY FLUE?_ A. Because the _cold air_ (condensed in the upper part of the flue), _will sink from its own weight_, and sweep the ascending smoke _back with it_ into the room. Q. _What is the use of a COWL upon a chimney-pot?_ A. The cowl acts as a _screen against the wind_, to prevent it from blowing into the chimney. Q. _What HARM would the WIND do, if it were to BLOW into a CHIMNEY?_ A. 1st--It would prevent the smoke from getting out: and 2ndly--The _cold air_ (introduced into the chimney by the wind) _would fall down the flue_, and drive the smoke with it _back into the room_. Q. _Why does a SMOKE-JACK turn round in a chimney?_ A. The current of hot air up the chimney, striking against the _oblique vanes of the smoke-jack_, drives them round and round; in the same way as the sails of a _wind-mill_ are driven round by the _wind_. Q. _Why are some things SOLID, others LIQUID, and others GASEOUS?_ A. As _heat_ enters any substance, _it drives its particles further asunder_; and a _solid_ (like _ice_) becomes a _liquid_; and a _liquid_ (like _water_) becomes a _gas_.