A Guide to the Scientific Knowledge of Things Familiar
CHAPTER VII.
MECHANICAL ACTION.
1.--PERCUSSION.
Q. _How is heat produced by MECHANICAL ACTION?_
A. 1.--By Percussion. 2.--By Friction. 3.--By Condensation.
Q. _What is meant by PERCUSSION?_
A. _The act of striking_; as when a blacksmith strikes a piece of iron on his anvil with his hammer.
Q. _Why does BEATING IRON make it RED-HOT?_
A. _Beating_ the iron _condenses the particles_ of the metal; and squeezes out its latent heat, as water from a sponge.
Q. _Does COLD iron contain HEAT?_
A. Yes; _every thing_ contains heat; but when a thing _feels cold_, its heat is LATENT.
Q. _What is meant by LATENT HEAT?_
A. Heat _not perceptible to our feeling_. When anything contains _heat_ without _feeling_ the hotter for it, that heat is called "_latent_." (See p. 31.)
Q. _Does COLD iron contain latent HEAT?_
A. Yes; and when a blacksmith _compresses the particles_ of the iron by his hammer, he _squeezes out_ this latent heat, and makes the iron red-hot.
Q. _How did blacksmiths use to LIGHT THEIR MATCHES before the general use of lucifers?_
A. They used to place a soft iron nail upon their anvil; strike it two or three times with a hammer; and the point became _sufficiently hot to light a brimstone match_.
Q. _How can a NAIL (beaten by a hammer) IGNITE a brimstone MATCH?_
A. As the particles of the nail are _compressed by the hammer_, it cannot contain _so much heat as it did before_; so some of it _flies out_ (as water flows from a sponge when it is squeezed).
Q. _Why does STRIKING a FLINT against a piece of STEEL produce a SPARK?_
A. The blow _condenses_ those parts of the flint and steel which strike _together_, and squeezes out their latent heat.
Q. _How does this development of HEAT produce a SPARK?_
A. A very small fragment (either of the steel or flint) _is knocked off red-hot_, and sets fire to the tinder on which it falls.
Q. _Why is it needful to keep BLOWING the TINDER with the breath?_
A. Because _blowing_ the tinder, drives the _oxygen of the air_ towards it.
Q. _Where does the OXYGEN of the air COME FROM, which is blown to the lighted tinder?_
A. The air itself is composed of two gases (_nitrogen and oxygen_) mixed together.
(Every 5 lbs. of common air contain 4 lbs. of nitrogen, and 1 lb. of oxygen.)
Q. _What is the good of BLOWING OXYGEN GAS to lighted tinder?_
A. Oxygen gas _supports combustion_; and lighted tinder is _quickened by the breath_, in the same way as a dull fire is revived by a _pair of bellows_.
Q. _Why do HORSES sometimes STRIKE FIRE with their FEET?_
A. When iron horse-shoes strike against the flint-stones of the road, _very small fragments_ (either of the shoe or stones) are _knocked off red-hot_, and look like sparks.
Q. _What makes these fragments RED-HOT?_
A. The percussion _condenses_ the part struck, _and squeezes out its latent heat_.