Part 51 (2/2)
The escape of _air_ from the water contributes also to increase this agitation.
Q. _How do these two currents Pa.s.s each other?_
A. The _hot ascending current_ pa.s.ses close by the _metal sides_ of the kettle; while the _cold descending current_ pa.s.ses _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 pa.s.ses: 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.
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