Because black _absorbs_ heat, and therefore becomes warm; while _light colours_ do _not_ absorb heat in the same degree, and therefore they remain cool. 231. _Why do iron articles, even when near fire, usually feel cool?_ Because they are bad absorbers, and do not take up heat freely, unless they are _in contact_ with a hot body. 232. _How is heat diffused through the atmosphere?_ By _convection_. The warmth radiating from the surface of the earth warms the air in contact with it; the air expands, and becoming lighter, flies upwards, bearing with it the caloric which it holds, and diffusing it in its course. 233. _How do the waters of the ocean become heated?_ Chiefly by _convection_. Nearly all the heat which the sun sheds upon the ocean is borne away from its surface by evaporation, or is radiated back into the atmosphere. But the ocean gathers its heat by _convection_ from the earth. It girdles the shores of tropical lands where, being warmed to a high degree of temperature, it sets across the Atlantic from the Gulf of Mexico, and exercises an important influence upon the temperature of our latitude. 234. _What is the cause of winds?_ Currents of air, and winds, are the result of _convection_. The air, heated by the high temperature of the tropics, _ascends_, while the colder air of the temperate and the frigid zones _blows towards the equator_ to supply its place. [Verse: "Give unto the Lord the glory due unto his name; worship the Lord in the beauty of holiness."--PSALM XXIX.] 235. _What is the cause of sea breezes?_ Sea breezes are also the result of _convection_. The land, under the heat of the day's sunshine, becomes of a high temperature, and the expanded air on its surface _flies away towards the ocean_. As the sun goes down, the earth cools again, and the air _flies back_ to find its equilibrium. Many countries by the sea are subjected to these periodical breezes, known as either "land" or "sea breezes," according to their direction. About eight o'clock in the morning an ærial current begins to flow from the sea towards the land, and continues until about three o'clock in the day; then the current takes a reverse direction, flowing from the land to the sea. This it continues to do throughout the night, until the time of sunrise, when a temporary calm ensues. 236. _Why does a soap bubble ascend in the air?_ Because, being filled with _warm_ air, it is _lighter_ than the surrounding medium, and therefore ascends. 237. _Why does the bubble fall after it has been in the air some time?_ Because the air contained in it has become cool, and, as it contains carbonic acid gas, it is _heavier_ than the air. 238. _What became of the warmth at first contained in the bubble?_ It has been _distributed in the air_ through which the bubble passed. 239. _What does this simple illustration of the distribution of warmth explain?_