A Guide To The Scientific Knowledge Of Things Familiar Part 26

Q. _But I have seen a KETTLE BOIL OVER, although it has not been filled FULL of WATER; how do you account for THAT?_

A. If a fire be _very fierce_, the air is expelled so _rapidly_, that the _bubbles are very numerous_; and (towering one above the other) _reach the top of the kettle, and fall over_.

Q. _Why is a pot, which is full to OVERFLOWING (while the water is boiling HOT), NOTHING LIKE FULL, when it has been taken off the fire for a short time?_

A. When the water was _swelled by boiling heat_, it filled the pot even to overflowing; but as soon as the water is _condensed by cold_, it _contracts_ again, and occupies a much less s.p.a.ce.

Q. _Why does the water of a KETTLE run out of the SPOUT when it BOILS?_

A. Because the steam cannot escape _so fast as it is formed_, and (being _confined in the kettle_) _presses on the water with great power_, and forces it out of the spout.

Q. _How can the PRESSURE of STEAM on the SURFACE of the water, FORCE the water through the KETTLE-SPOUT?_

A. In the same manner as _the pressure of air_ on the _mercury of a barometer_, forces the _quicksilver up the gla.s.s tube_.

Q. _What causes the RATTLING NOISE so often made by the LID of a saucepan or boiler?_

A. The steam (seeking to escape) _forces up the lid_ of the boiler, and the _weight_ of the lid causes it to _fall back again_: this being done _frequently_, produces a rattling noise.

Q. _If the steam COULD NOT LIFT UP THE LID of the boiler, how would it escape?_

A. If the lid fitted so tightly, that the steam could not raise it up, the boiler would _burst into fragments_, and the consequences might be fatal.

Q. _When steam pours out from the spout of a kettle, the STREAM begins apparently HALF AN INCH off the SPOUT; why does it not begin CLOSE to the spout?_

A. Steam is really _invisible_; and the half-inch (between the spout and the "_stream of mist_") is the _real steam_, before it has been condensed by air.

Q. _Why is not ALL the stream INVISIBLE, as well as that half-inch?_

A. As the steam _comes in contact with the colder air_, the invisible particles (being _condensed_), roll one into another, and look like a thick mist.

Q. _What BECOMES OF the STEAM? for it soon vanishes._

A. After it is condensed into mist, it is _dissolved by the air_, and dispersed abroad as _invisible vapour_.

Q. _And what BECOMES of the INVISIBLE VAPOUR?_

A. Being _lighter than air_, it _ascends_ to the upper regions, where (being again _condensed_) it contributes to form _clouds_.

Q. _Why does a METAL SPOON, left in a saucepan, r.e.t.a.r.d the process of BOILING?_

A. The metal spoon (being an excellent _conductor_) _carries off the heat from the water_; and (as heat is carried off by the spoon) the water takes a longer time to boil.

Q. _Why will a POT (filled with water) NEVER BOIL, when immersed in ANOTHER vessel full of water also?_

A. Because water can _never be heated above the boiling point_: all the heat absorbed by the water after it _boils_, is employed in _converting the water into steam_.

Q. _How does the conversion of water into steam prevent the INNER POT from BOILING?_

A. The moment the water in the larger pot is _boiling hot_ (or 212), _steam is formed_, and _carries off some of its heat_; therefore, 212 _degs._ of heat can never _pa.s.s through it_, to raise the _inner_ vessel to the _same heat_.

Q. _Why do SUGAR, SALT, &c. r.e.t.a.r.d the process of BOILING?_

A. Because they have a tendency to _fix_ water by chemical attraction; and therefore r.e.t.a.r.d its _conversion into steam_.

Q. _If you want water to boil, without COMING IN CONTACT with the SAUCEPAN, what plan must you adopt?_

A. _Immerse the pot_ (containing the water you want to boil) in a saucepan containing _strong brine_, or sugar.

Q. _Why would the INNER vessel boil, if the OUTER vessel contained strong BRINE?_

A. Though _water_ boils at 212 _degs._ of heat, yet _brine_ will not boil till raised to 218 or 220 _degs._ Therefore, 212 _degs._ of heat may easily pa.s.s through brine _to raise the vessel immersed in it to boiling heat_, before any of it is _carried off by steam_.

Q. _Why will brine impart to another vessel MORE than 212, and water NOT SO MUCH?_

A. Because both liquids will _impart heat_ till they _boil_, and then _they can impart heat no longer_.

Q. _Why can they impart no extra heat after they boil?_

A. Because all _extra_ heat is spent _in making steam_. Hence water will _not_ boil a vessel of water immersed in it, because it cannot impart to it 212 _degs._ of heat: but _brine_ will, because it can impart _more than_ 212 _degs. of heat_, without being converted itself into steam.

Ether boils at 104 _degs._ Alcohol - - 173-1/2 "

Water - - 212 "

Water with one-fifth salt 219 "

Syrup boils at 221 "

Oil of turpentine, 304 "

Sulphuric acid 472 "

Linseed oil - 640 "

&c. &c.