CHAPTER XVII.

5.--CONVECTION.

Q. _What is meant by the CONVECTION of HEAT?_

A. Heat communicated _by being carried_ to another thing or place; as the hot water resting on the _bottom_ of a kettle, carries heat to the water _through which it passes_. (_see p._ 246).

Q. _Are LIQUIDS good CONDUCTORS of heat?_

A. No; liquids are _bad conductors_; and are, therefore, made hot by _convection_.

Q. _Why are LIQUIDS BAD CONDUCTORS of heat?_

A. Because heat _converts a liquid into steam_, and flies off with the vapour, instead of being _conducted through the liquid_.

Q. _Explain how WATER is made HOT?_

A. The water _nearest the fire is first heated_, and (being heated) _rises to the top;_ other cold water succeeds, is _also_ heated, and rises in turn; and this interchange keeps going on, _till all the water boils_.

Q. _Why is WATER in such continual FERMENT, when it is BOILING?_

A. This commotion is mainly produced by the _ascending and descending currents_ of hot and cold water.

The escape of _air_ from the water contributes also to increase this agitation.

Q. _How do these two currents PASS each other?_

A. The _hot ascending current_ passes close by the _metal sides_ of the kettle; while the _cold descending current_ passes _down the centre_.

Q. _Why does BOILING WATER BUBBLE?_

A. The bubbles are _portions of steam_ (formed at the bottom of the vessel) which _rise to the surface_, and escape into the air.

Q. _Why does a KETTLE RUN OVER, when the water BOILS?_

A. As the heat insinuates itself between the particles of water, _it drives them asunder_; and (as the particles of water are _driven apart from each other_) the _same_ vessel will no longer hold the expanded water, and some runs over.

Q. _Why does a KETTLE SING, when it is ABOUT to BOIL?_

A. Water contains _a great deal of air_, which (being expanded by the heat of the fire) escapes by fits _through the spout of the kettle_; which sings in the same way as a trumpet does, when a person blows in it.

Q. _Why does WATER BOIL?_

A. Boiling is the effect of a _more violent escape of air_ from the heated water; when, therefore, the air is _not permitted to escape_, water will _never boil_.

Q. _Why is HEAT applied to the BOTTOM, and not to the top of a KETTLE?_

A. Because the heated water _always ascends to the surface_, heating the water through which it passes: if, therefore, heat were applied to the _top of a vessel_, the water _below the surface_ would _never be heated_.

Q. _As the lower part of a GRATE is made RED-HOT by the fire ABOVE, why would not the WATER boil, if fire were applied to the TOP?_

A. The _iron_ of a grate is an _excellent conductor_; and, therefore, if _one_ part be heated, the heat is conducted to _every other part_: but _water_ is a _very bad conductor_, and will not diffuse heat in a similar way.

Q. _How do you know that WATER is a BAD CONDUCTOR of heat?_

A. When a blacksmith immerses his red-hot iron in a tank of water, the water which _surrounds_ the red-hot iron is made _boiling hot_, but the water _below_ the surface remains quite cold.

Q. _If you wish to COOL LIQUIDS, where should the cold be applied?_

A. To the _top of the liquid_; because the _cold_ portion will always _descend_, and allow the warmer parts to come in contact with the cooling substance.

Q. _Does BOILING water get hotter by being KEPT on the FIRE?_

A. No; not if the steam be suffered to escape.

Q. _Why does not boiling water get HOTTER, if the steam be suffered to ESCAPE?_

A. Because _as fast as the water boils_, it is converted into _steam_; and the steam _carries away_ the additional heat, as fast as it is communicated.

Q. _Is STEAM visible or INVISIBLE?_

A. Steam is _invisible_; but when it comes in contact with the air (being _condensed into small drops_) it instantly becomes visible.

Q. _How do you know that STEAM is INVISIBLE?_

A. If you look at the spout of a boiling kettle, you will find that the steam (which issues from the spout) is always invisible _for about half an inch_; after which, _it becomes visible_.

Q. _Why is the steam INVISIBLE for only HALF AN INCH, and not either all INVISIBLE or all VISIBLE?_

A. The air is not able to condense the steam as it first issues from the spout, but when it _spreads_ and comes in contact with a larger volume of air, the _invisible steam_ is readily condensed into _visible drops_.

Q. _Why is our BREATH VISIBLE in winter-time?_

A. Because _it is condensed by the cold air_ into small drops, which are visible to the eye.

Q. _Why do STEAM-ENGINES sometimes BURST?_

A. Steam is very _elastic_; and this elasticity increases in a greater proportion than the heat which produces it; unless, therefore, some _vent_ be freely allowed, the steam heaves and swells, till it bursts the vessel which confined it.

Q. _What BECOMES of the steam, after it has been condensed?_

A. It is _dissolved by the air_, and forms a part of its invisible vapour.

Q. _Is AIR a good CONDUCTOR?_

A. No; _air is a very bad conductor_, and is heated (like water) _by convection_.

Q. _How is a ROOM WARMED by a STOVE?_

A. The air _nearest the fire_ is made hot _first_; _the cold air descends_, is heated also, and rises in turn; and this goes on, _till all the air of the room is warmed_.

Q. _Why are FIRES placed on the FLOOR of a room, and not towards the CEILING?_

A. As heated air always _ascends_, if the fire were not _near the floor_, the lower part of the air (which we want to be the warmest) would never be benefited by the fire at all.

Q. _If you take a POKER out of the fire, and hold the HOT END DOWNWARDS, why is the HANDLE so intensely HOT?_

A. Because the hot end of the poker _heats the air around it_, and this hot air (in its ascent) _scorches the poker_, and the _hand which holds it_.

Q. _How should a RED-HOT POKER be carried so as not to BURN our fingers?_

A. With the hot end _upwards_; because then the air (heated by the poker) _would not pass over our hand_ to scorch it.

Q. _Why is a POKER (resting on the FENDER) COLD; but if it leans against the STOVE, intensely warm?_

A. The poker is an _excellent conductor_; while, therefore, it rests against the hot stove, the heat of the stove is _conducted into the poker_; but when it _rests on the fender_, it does not come in _contact with the hot stove_.

Q. _Why does it feel so COLD, when it rests on the FENDER?_

A. Not being so warm as our hand, it _imbibes the heat from it_ with such _rapidity_, that our loss of heat is _palpable_, and produces the sensation of coldness.

Q. _Why are FLUES (which are carried through a church or room) always BLACKENED with BLACK LEAD?_

A. In order that the heat of the flue _may be more readily diffused_ throughout the room. Black lead radiates heat more freely than any other known substance.

Q. _Why do country people touch the thick end of an EGG with their TONGUE, to know if it be STALE or not?_

A. The thick end of an egg always contains _a little air_ (between the shell and the white); but, when the egg is stale, _the white shrinks_, and the air expands.

Q. _How can the TONGUE tell from this, whether the egg be STALE or FRESH laid?_

A. As air is a _very bad conductor_, if the egg be _stale_, it will feel much _warmer to the tongue_, than if it be new-laid.

Q. _Why will the big end of an egg feel WARMER to the tongue, because it contains more AIR?_

A. As air is a _bad conductor_, it will draw off the heat of the tongue _very slowly_, and, therefore, _appear warm_; but when there is only a _very little air in the egg_ (as the _white_ is a pretty good conductor), the heat of the tongue will be _more rapidly_ drawn off, and the egg _appear colder_.

Q. _Why is the large END of an EGG CRACKED, when put into a saucepan to boil?_

A. _To let the air out_; if the large end were _not cracked_, the air (expanded by the heat) _would enter the white of the egg_, and give it an _offensive taste_.