Q. _Why do the inhabitants of tropical countries live chiefly upon rice and fruit?_ A. Rice and fruit by digestion _are mainly converted into water_, and (by cooling the blood) prevent the tropical heat from feeling so oppressive. Q. _Why are POOR PEOPLE generally AVERSE to CLEANLINESS?_ A. 1st--_Cleanliness increases hunger_; and as poor people are generally _ill-fed_, they are averse to cleanliness. 2ndly--_Dirt is warm_, (thus pigs who love _warmth_, are fond of _dirt_); and as poor people are generally _ill-clad_, they like the _warmth of dirt_. Q. _Why are POOR PEOPLE generally AVERSE to VENTILATION?_ A. 1st--Because ventilation _increases the oxygen of the air_,--the _combustion of food_,--and the _cravings of appetite_: and 2ndly--Ventilation _cools the air of our rooms_: poor people, therefore, (who are generally ill-clad) love the _warmth_ of an ill-ventilated apartment. Q. _Why does FLANNEL, &c. make us WARM?_ A. Flannel and warm clothing do not _make_ us warm, but merely _prevent the body from becoming cold_. Q. _How does flannel, &c. prevent the body from becoming cold?_ A. Flannel (being a bad conductor) will _neither carry off the heat of the body into the cold air_, nor suffer the cold of the air _to come into contact with our warm bodies_; and thus it is that flannel clothing keeps us warm. Q. _Why are FROGS and FISHES COLD-BLOODED animals?_ A. Because they consume _so little air_; and without a plentiful supply of air, combustion is so slow, that very little animal heat is evolved. Q. _Why is a DEAD BODY COLD?_ A. Air is no longer conveyed to the lungs after respiration has ceased; and, therefore, animal heat _is no longer evolved by combustion_. 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_.