A Guide To The Scientific Knowledge Of Things Familiar Part 45

A. The carbon of the fuel (which flies off in smoke) naturally _blackens_ all culinary vessels set upon the fire to boil, and thus renders them fit for use.

("Culinary vessels" are vessels used in kitchens for cooking, as saucepans, boilers, kettles, &c.)

Q. _How does SMOKE make culinary vessels FIT for USE?_

A. If it were not for the _smoke_, (which gathers round a kettle or saucepan) _heat would not be absorbed_, and the process of boiling would be greatly r.e.t.a.r.ded.

Q. _Why is boiling water KEPT HOT best in a BRIGHT METAL pot?_

A. Because bright metal being a _bad radiator_ will not _throw off the heat_ of the boiling water _from its surface_.

Q. _Why is WATER KEPT COLD in summer-time in a BRIGHT METAL pot, better than in an EARTHEN vessel?_

A. Because bright metal _will not absorb heat_ from the hot air, like an _earthen vessel_; in consequence of which, the water is kept cooler.

Q. _Why are DINNER-COVERS made of BRIGHT TIN or SILVER?_

A. Light-coloured and highly-polished metal _is a very bad radiator of heat_; and, therefore, bright tin or silver will not allow the heat of the cooked food _to escape through the cover by radiation_.

Q. _Why should a MEAT-COVER be very brightly POLISHED?_

A. If the cover be _dull or scratched_ it will _absorb heat from the hot food beneath it_; and (instead of _keeping it hot_) will _make it cold_.

Q. _Why should a SILVER MEAT-COVER be PLAIN, and not CHASED?_

A. If the cover be _chased_, it will _absorb the heat of the food_ covered by it; and instead of _keeping it hot_, will _make it cold by absorption_.

Q. _What is DEW?_

A. Dew is the _vapour of the air condensed_, by coming in contact with bodies _colder than itself_.

Q. _Why is the GROUND sometimes COVERED with DEW?_

A. The _earth is more heated_ by solar rays _than the air_, during the _day_; but at _night_, the earth _parts with more heat_ than the _air_, and becomes (in consequence) 5 or 10 degrees _colder_.

Q. _How does the EARTH being COLDER than the AIR account for the deposition of DEW?_

A. As soon as the air _touches the cold earth_, its warm vapour is _chilled_, and _condensed into dew_.

Q. _Why is the surface of the GROUND COLDER in a FINE clear NIGHT, than in a CLOUDY one?_

A. On a fine clear star-light night, _heat radiates from the earth freely_, and is lost in open s.p.a.ce: but on a _cloudy_ night, the clouds _arrest the process of radiation_.

Q. _Why is DEW deposited only on a FINE clear NIGHT?_

A. Because, when the night is _clear_ and _fine_, the _surface of the ground radiates heat most freely_; and (being cooled down by this loss of heat) _chills the vapour of the air into dew_.

Q. _Why is there NO DEW on a dull CLOUDY NIGHT?_

A. The clouds _arrest the radiation of heat from the earth_; and (as the heat cannot freely escape) the surface is not sufficiently cooled down _to chill the vapour of the air into dew_.

Q. _Why is a CLOUDY NIGHT WARMER than a FINE one?_

A. Because the clouds _prevent the radiation of heat from the earth_; and, therefore, the surface of the earth remains _warmer_ on a dull cloudy night.

Q. _Why is DEW most ABUNDANT in situations most EXPOSED?_

A. Because the radiation of heat _is not arrested_ by houses, trees, hedges, or any other thing.

Q. _Why is there scarcely any DEW under a shady TREE?_

A. The shady head of the tree both _arrests the radiation of heat from the earth_, and also radiates some of its own heat _towards the earth_; and, therefore, the ground (underneath a tree) _is not sufficiently cooled_ down to chill the vapour of the air into dew.

Q. _Why is there never much DEW at the foot of WALLS and HEDGES?_

A. 1st--Because the wall or hedge acts as a screen, _to arrest the radiation of heat from the earth_: and

2ndly--The wall or hedge also _radiates some portion of heat_ towards the earth.

Q. _How do these things prevent the deposition of dew?_

A. As the ground (beneath a wall, tree, or hedge) is _not cooled by the radiation of heat_, it remains of the _same temperature as the air_ above it; in consequence of which, the vapours of the air are _not chilled by it into dew_.

Q. _Why is there little or NO DEW beneath a FLOWER-AWNING, although that awning be open on all four sides?_

A. 1st--Because the awning _arrests the radiation of heat from the ground beneath_: and

2ndly--It _radiates some of its own heat downwards_; in consequence of which, the ground beneath an awning is _not sufficiently cooled down_ to chill the vapour of air into dew.