A Guide To The Scientific Knowledge Of Things Familiar Part 89

A. Because they _absorb none of the rays of light_, but reflect them _all_.

Q. _Why are COALS BLACK?_

A. Because they _absorb all the rays of the sun_ which impinge upon them, and stifle their reflection.

Q. _Why is SNOW WHITE?_

A. Snow consists of a vast number of crystals (or small prisms), which separate the rays into their elemental colours; but as these crystals are very numerous, the colours _unite again_ before they meet the eye, and _appear white_.

N. B. The combination of _all_ colours makes WHITE.

Q. _Why is SUGAR WHITE?_

A. Sugar consists of a vast number of small crystals, which separate the rays into their elemental colours; but as these crystals are very numerous, the colours _unite again_ before they meet the eye, and appear _white_.

Q. _Why is SALT WHITE?_

A. Salt consists of a vast number of small crystals, which reflect the various rays of light from different points of the salt; and as these colours _unite_ before they meet the eye, the salt appears to be _white_.

N. B. The combination of _all_ colours makes WHITE.

Q. _Why are the LEAVES of plants GREEN?_

A. Because the _carbon_ of the leaves is a _bluish olive_, and the _sap_ and _tissue of the cells_, YELLOW; when, therefore, the _yellow sap_ flows into the _blue carbon_, it produces a _green leaf_.

Q. _Why are leaves a LIGHT green in SPRING?_

A. Because the young leaves of spring have _more sap_ than _carbon_; and, therefore, the _yellow_ of the green prevails.

Q. _Why are leaves a YELLOWISH BROWN in AUTUMN?_

A. Because the _carbon_ of the leaves is _dying away_, and the yellow tinge of the _tissue_ and _falling sap_ prevails over the blue.

Q. _Why are plants a PALE YELLOW when kept in the DARK?_

A. Solar light is essential for the production of _carbon_; and as plants kept in the dark _lose their carbon_, they lose the _blue colour_ which should convert their yellow sap to _green_.

Q. _Why are POTATOES YELLOW?_

A. Potatoes are grown _underground_, and, therefore, contain very little _carbon_ (or blue colour); hence the _yellow sap_ of the potato is not converted to green by carbon.

Q. _Why are potatoes (which grow EXPOSED to the air and light) GREEN?_

A. Because the sun-light _increases their carbon_; which (mingling with the yellow sap) turns the potato _green_.

Q. _Why is it DANGEROUS to SLEEP in a room which contains LIVING PLANTS?_

A. Because they _exhale carbon in the dark_ in the form of CARBONIC ACID GAS, which is destructive to animal life.

Q. _Why are SOME things (like gla.s.s) TRANSPARENT?_

A. In transparent bodies (like gla.s.s) all the rays of light _emerge on the opposite side_.

Q. _Why are SOME things s.h.i.+NING and splendid?_

A. Those objects which _reflect the most rays_ are the most _splendid_; and those which _absorb_ them most, are _dull_.

Q. _Why are DESERTS so DAZZLING in summer time?_

A. Because each separate grain of sand _reflects the rays of the sun_ like a mirror.

Q. _If you move a stick (burnt at one end) ROUND pretty briskly, it seems to make a CIRCLE OF FIRE,--WHY is this?_

A. Because the eye _retains the image_ of any bright object, _after the object itself is withdrawn_; and as the spark of the stick returns _before the image has faded_ from the eye, therefore, it seems to form a _complete circle_.

Q. _If separate figures (as a man and a horse) be drawn on separate sides of a card, and the card TWISTED quickly, the man seems to be seated on the horse,--WHY is this?_

A. Because the image of the horse _remains upon the eye_ till the _man_ appears.

The Thaumatrope is constructed on this principle.

Q. _Why do the STARS TWINKLE?_

A. Fixed stars are _so far off_, that their rays of light do not strike upon the eye _in a continuous flow_, but at _intervals_: when their rays _reach the eye_, the star becomes _visible_, and then is obscured _till the next batch of rays arrive_; and this _perpetually_ occurring, makes a kind of _twinkling_.

Q. _If we look at a RED-hot FIRE for a few minutes, WHY does every thing seem TINGED with a BLUISH GREEN colour?_

A. Because bluish green is the "ACCIDENTAL COLOUR" of red: and if we fix our eye upon _any colour whatsoever_, when we turn aside, we see every object tinged with _its accidental colour_.