The Reason Why. Robert Kemp Philp

which is a bad conductor.

      145. Why would the hearth-stone feel comparatively hotter in the one case, and colder in the other?

      Because, being a good conductor, it would conduct heat rapidly to the hand when hot, and take heat rapidly from the hand when cold.

CHAPTER VIII.

       Table of Contents

      146. Which are the better conductors of heat, fluids or solids?

      Generally speaking, solids, especially those of them that are dense in their substance.

147. Why are dense substances the best conductors of heat?

      Because the heat more readily travels from particle to particle until it pervades the mass.

148. Why are fluids bad conductors of heat?

      Because of the want of density in their bodies; and because a portion of the imbibed heat always passes off from fluids by evaporation.

      "He casteth forth his ice like morsels: who can stand before his word,"—Psalm cxlvii.

149. Why are woollen fabrics bad conductors of heat?

      Because there is a considerable amount of air occupying the spaces of the texture.

150. Is air a good or a bad conductor?

      Air is a bad conductor, and it chiefly transmits heat, as water does, by convection.

151. Is water a good or a bad conductor?

      Water is an indifferent conductor, but it is a better conductor than air.

      152. Why, when we place our hands in water, which may be of the same temperature as the air, does the water feel some degrees colder?

      Because water, being a better conductor than air, takes up the warmth of the hand more rapidly.

      153. Why, when we take our hands out of water do they feel warmer?

      Because the air does not abstract the heat of the hand so rapidly as the water did, and the change in the degree of rapidity with which the heat is abstracted produces a sensation of increased warmth.

154. Why do we see blocks of ice wrapped in thick flannel in summer time?

      Because the flannel, being a non-conductor, prevents the external heat from dissolving the ice.

      Flannel wrapped around a warm body keeps in its heat; and wrapped around a cold body, prevents heat from passing into it.

155. How do we know that air is not a good conductor of heat?

      Because, in still air, heat would travel to a given point much more rapidly, and in greater intensity, through even an indifferent solid conductor, than it would through the air.

156. How do we know that water is not a good conductor of heat?

      Because in a deep vessel containing ice, and with heat applied at the top, some portion of the water may be made to boil before the ice, which lies a little under the surface, is melted.

      "As snow in summer, and as rain in harvest; so honour is not seemly for a fool."—Prov. xxvi.

      157. Why would you apply the heat at the top, in this experiment?

      Because in heating water it expands and rises. The boiling of water is caused by the heated water ascending from the bottom, and the colder water descending to occupy its place. If the heat were not applied at the top, it would be distributed quickly by convection, but not by conduction.

158. Why are bottles of hot water, used as feet-warmers, wrapped in flannel?

      Because the flannel, being a bad conductor, allows the heat to pass only gently from the bottle, and preserves the warmth for a much longer time.

159. Why are hot rolls sent out by the bakers, wrapped up in flannel?

      Because the flannel, being a bad conductor, does not carry off rapidly the heat of the rolls.

160. Why is it said that snow keeps the earth warm?

      Because snow is a bad conductor, and prevents the frosty air from depriving the earth of its warmth.

      161. Why are snow huts which the Esquimaux build found to be warm?

      Because snow, being a bad conductor, keeps in the internal heat of the dwelling, and prevents the cold outer air from taking away its warmth.

162. Why is snow, being composed of congealed water (and water being a better conductor than air), so good a non-conductor?

      Because in the process of congealation it is frozen into crystalline forms, which, being collected into a mass, form a woolly body, thus proving the truthfulness of the Bible simile, which says, God "giveth snow like wool."

      "He giveth snow like wool: he scattereth the hoar frost like ashes."—Psalm cxlvii.

      FIG. 1.—CRYSTALS OF SNOW, AS SEEN THROUGH A MICROSCOPE.

163. Why does it frequently feel warmer after a frost has set in?

      Because, in the act of congealation a great deal of heat is given out, and taken up by the air, and thus the severity of the cold is in some degree moderated.

164. Why is it frequently colder when a thaw takes place?

      Because, in the process of thawing, a certain amount of heat is withdrawn from the air, and enters the thawed ice.

      165. What benefit results from these provisions of Nature?

      They moderate both the severity of frosts, and the rapidity of thaws, which, in changeable climates, would be seriously detrimental to life, and to vegetation.

166. Why are furs and woollens worn in the winter?

      Because, being non-conductors, they prevent the warmth of the body from being taken up by the cold air.

      167. Why are the skins of animals usually covered with fur, hair, wool, or feathers?

      Because their coverings, being non-conductors of heat, preserve the warmth of the bodies of the animals.

      "He sendeth out his word, and melteth them: he causeth his wind to blow, and the waters to flow."—Psalm cxlvii.

168. How is the greater warmth of animals provided for in the winter?

      It is observed that, as winter approaches, there comes a short woolly or downy growth, which, adding to the non-conducting property of their coats, confines their animal warmth.

      In small birds during winter, let the external colour of the feathers be what it may, there will be found a kind of black down next their bodies. Black is the warmest colour, and the purpose here is to keep in the heat, arising from the respiration of the animal.

      169. How is warmth provided for in animals that have no such coats?

      They are furnished with a layer of fat, which lies underneath the skin. Fat consists chiefly of