A Guide To The Scientific Knowledge Of Things Familiar Part 52

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 pa.s.s 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 sh.e.l.l 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_.