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?_