A. Because compression _squeezes out_ heat, as water is squeezed from a sponge; and as the goods of a large pile are greatly _compressed_, much of their latent heat is _squeezed out_. Q. _Why do HAY-STACKS sometimes CATCH FIRE of themselves?_ A. Either because the hay was got up _damp_, or because rain has penetrated the stack. Q. _Why will a HAY-STACK CATCH FIRE if the hay be damp?_ A. Damp hay soon _decays_, and undergoes a _state of fermentation_; during which, _carbonic acid gas_ is given off, and the stack catches _fire_. Q. _Why does roasted COFFEE sometimes CATCH FIRE spontaneously?_ A. The _heat_ of coffee is greatly increased by being _roasted_; and the _carbon of the coffee_ uniting with the _oxygen of the air_, produces _carbonic acid gas_, and bursts into _flame_. Q. _Why do old RAGS, used for CLEANING LAMPS and CANDLES, sometimes set a HOUSE on FIRE?_ A. Because they very readily _ferment_, and (during fermentation) throw off exceedingly inflammable gases. (N.B. Lamp-black mixed with linseed oil is more liable to spontaneous combustion, than anything that servants handle.) CHAPTER IV. SMOKE. Q. _Why does SMOKE ASCEND the chimney?_ A. As the air of the room passes over the fire, it becomes _heated_; and (being thus made _lighter_,) ascends the chimney, carrying the smoke with it. Q. _What is SMOKE?_ A. Small particles of carbon, separated by combustion from the fuel, but not _consumed_. Q. _Why do SMOKE and steam CURL, as they ascend?_ A. Because they are moved in a _right line_, and then _pushed on all sides_; and this forces them into a _circular_ motion. Q. _What are BLACKS?_ A. When the hot air of the chimney has been cooled by the _external_ air, it can no longer _buoy up_ the solid smoke; so it falls to the earth in condensed flakes, called "blacks." Q. _Why are there NO BLACKS in the smoke of a RAILWAY engine?_ A. The smoke of a railway engine consists chiefly of _watery vapour_, which dissolves in air, as sugar does in water; but the smoke of a common chimney consists of small fragments of _unburnt fuel_. Q. _Why does a "COPPER HOLE" DRAW up more fiercely than an OPEN stove?_ A. As the air, which supplies the copper hole, must pass _through the furnace_, it becomes exceedingly _heated_, and rushes up the chimney with great violence. Q. _What produces the ROARING noise made by a COPPER-HOLE fire?_ A. Air rushing rapidly through the crevices of the _iron door_, and up the _chimney flue_. Q. _Why is the ROAR LESS, if the copper-hole DOOR be thrown OPEN?_ A. Because _fresh_ air gets access to the fire _more easily_; and as the air is not so intensely heated, its motion is not so _violent_. Q. _Why do some CHIMNEYS SMOKE?_ A. If fresh air is not admitted into a room, _as fast as it is consumed by the fire_, a current of air _will rush down the chimney_ to _supply the deficiency_, and bring the smoke along with it.