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_. CHAPTER VIII. 2.--FRICTION. 3.--CONDENSATION. Q. _What is meant by FRICTION?_ A. The act of _rubbing two things together_; as the Indians rub two pieces of _wood_ together to produce fire. Q. _How do the Indians produce FIRE, by merely RUBBING TWO PIECES of dry WOOD TOGETHER?_ A. They take a piece of dry wood (sharpened to a point), which they rub quickly up and down a _flat piece_, till a _groove_ is made; and the _saw-dust_ (collected in this groove) soon _catches fire_. Q. _Why does the saw-dust of the WOOD CATCH FIRE by RUBBING?_ A. The _latent heat_ of the wood is _developed by friction_; because the particles of the wood are _squeezed closer together_, and the heat pours out, as water from a sponge. (The best woods for this purpose are _box-wood_ against _mulberry_, or _laurel_ against _poplar_ or _ivy_.) Q. _Do not CARRIAGE WHEELS sometimes CATCH FIRE?_ A. Yes; if the wheels be _dry_,--or _fit too tightly_,--or _revolve very rapidly_,--they often catch fire. Q. _Why do wheels catch fire in such cases?_ A. The _friction_ of the wheels against _the axle-tree_ is so great, that their _latent heat is disturbed_, and produces ignition. Q. _What is the use of GREASING CART WHEELS?_ A. The grease _lessens the friction_; and (by diminishing the _friction_) the latent heat is less disturbed.