A Guide To The Scientific Knowledge Of Things Familiar Part 49

Q. _Does HONEY-DEW INJURE leaves, or do them good?_

A. It injures them very much, because it _fills the pores_ of the leaf with a _thick clammy liquid_; and, therefore, prevents the leaf from _transpiring and absorbing_.

Q. _What EFFECT has honey-dew upon the APPEARANCE of a leaf?_

A. After a little time, the leaf (being _smothered_ and _starved_) begins to turn a _dingy yellow_.

Q. _Are not ANTS very FOND of HONEY-DEW?_

A. Yes; and they crawl up the loftiest trees, in order to obtain it.

Q. _What is the cause of MIST (or earth-fog)?_

A. If the _night has been very calm_, a _rapid_ radiation of heat has taken place in the earth; in consequence of which, the _air_ (resting on the earth) _is made so cold_, that its vapour is _chilled_, and condensed into a thick mist.

Q. _Why does not the MIST become DEW?_

A. Because the chill of the air _is so rapid_, that vapour is condensed _faster than it can be deposited_; and (covering the earth in a mist) _prevents any further radiation of heat_ from the earth.

Q. _When the earth can no longer RADIATE heat upwards, does it continue to CONDENSE the vapour of the air?_

A. No; the air (in contact with the earth) becomes about _equal in temperature_ with the surface of the earth itself; for which reason, the mist is _not condensed into dew_, but remains _floating above the earth_ as a thick cloud.

Q. _Why does this MIST seem to RISE HIGHER and HIGHER, and yet remain quite as dense below as before?_

A. The air _resting on the earth_ is first chilled, and _chills the air_ resting on _it_; the air which touches _this new layer of mist_ being also _condensed_, layer is added to layer; and the mist seems to be _rising_, when (in fact) it is only _deepening_.

Q. _Why does MIST and DEW VANISH as the SUN rises?_

A. Because the condensed vapour is _again rarefied by the heat of the sun_, and separated into invisible particles.

Q. _Why is a DEW-DROP ROUND?_

A. Because every part of the drop _is equally balanced_; and, therefore, there is no cause why _one part_ of the drop _should be further from the centre_ than _another_.

Q. _Why is the DEW-DROP on a broad leaf sometimes FLATTENED?_

A. Whenever two or more drops of dew _roll together_, they make one large _spheroid_ (or flattened drop).

Q. _Why will DEW-DROPS ROLL ABOUT CABBAGE-PLANTS, POPPIES, &c. without wetting the surface?_

A. The leaves of cabbages and poppies are _covered with a very fine powder_; and the dew-drop rolls over this fine powder, as a drop of rain _over dust_, without wetting the surface.

Q. _Why does not the drop of RAIN WET the DUST over which it rolls?_

A. Because it is driven from grain to grain by _capillary repulsion_.

Q. _Why does not the DEW-DROP WET the POWDER of the CABBAGE-plant?_

A. Because it is driven from grain to grain by _capillary repulsion_.

Q. _Why will DEW-DROPS ROLL over ROSES, &c. without wetting their petals?_

A. The leaves of a rose _contain an essential oil_, which prevents them from absorbing the dew immediately.

Q. _Why can a SWAN or DUCK dive under water WITHOUT being WETTED?_

A. Because their feathers are covered _with an oily secretion_, which repels the water.

Q. _What is the cause of MIST?_

A. When currents of air _from land_ mix with currents of air _from water_, the currents _from the water are condensed into mist_ by the colder currents _blowing from the land_.

Q. _Why are the currents of air from the LAND COLDER than those blowing over WATER?_

A. Because the earth _radiates heat very freely_, and (being greatly cooled down) _cools the air also_ which comes in contact with it.

Q. _Why is not the AIR, which pa.s.ses over WATER, so COOL as that which pa.s.ses over LAND?_

A. Because _water does not cool down at sun-set_, so fast as the _land_ does; and, therefore, the air in contact with it is _warmer_.

Q. _Why does not WATER cool down so fast as LAND?_

A. 1st--Because the _surface_ of water is _perpetually changing_, and as fast as _one_ surface is made cold, _another_ is presented: and

2ndly--The moment water is made cold _it sinks_, and _warmer portions of water rise to occupy its place_: therefore, before the _surface of water is cooled_, the _whole volume_ must be made cold; which is not the case with land.

Q. _What is the cause of a "pea-soup" LONDON FOG?_