A. 1st--Because they _deprive the blood of carbon_, which is the chief cause of animal heat: and

2ndly--These gases coalesce into _water_, which greatly tempers the animal heat.

Q. _Why do we feel LAZY and averse to activity in very HOT WEATHER?_

A. 1st--Because muscular activity would increase the heat of the body, by _quickening the respiration_: and

2ndly--The food we eat in hot weather, _not being greasy_, naturally abates our desire for bodily activity.

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 appet.i.te_: 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_.

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).