It explains the law of _convection_, or _heat distribution_, over the surface of the globe. 240. _Why does air ascend the chimney?_ Because, being heated, it becomes _lighter_ than the surrounding medium, and therefore flies upwards, through the outlet provided for it. 241. _Why does air fly from the doors and windows towards the fire-place?_ Because, as the warm air flies away, cold air rushes in to occupy its place. [Verse: "How much better is it to get wisdom than gold? and to get understanding rather to be chosen than silver."--PROVERBS XVI.] 242. _What does this example of the motion of the air in our rooms explain?_ It explains the movement of volumes of air by _convection_, and illustrates the origin of _breezes_ and _winds_. 243. _What is the chief effect of this law of convection?_ Under its influence air and water are the great _equalisers of solar heat_, rendering the earth agreeable to living things, and suited to the laws of their existence. Owing, also, to this law of _convection_, the constituents of the air are equalised. The breath of life, supplied by the purer oxygen of the "sunny south," is diffused in salubrious gales over the wintry climes of the north. And the waters, evaporated from the bosom of the central Atlantic Ocean and the Pacific, are borne across vast continents, and poured down in fertilising showers upon distant lands. To the educated mind, nothing is too simple to merit attention. To the ignorant, few things are sufficiently attractive to excite curiosity. Knowledge enables us to estimate the varied phenomena that are hourly arising around us, and to see, even in the most trifling effects, illustrations of those great causes and consequences that govern with mighty power the material world. Man, sitting by his fire-side, is enabled to witness the operation of some of nature's grandest laws: _light_ and _heat_ are around him; _conduction_, _radiation_, _reflection_, _absorption_, and _convection_ of heat are all going on before him; little winds are sweeping by his footstool, and warm currents, with miniature clouds folded in their arms, are passing upward before his view. Chemical changes are going on; the solid rock of coal disappears, flying away as an invisible gas. The little "hills are melted," and hard stones have been converted into "fervent heat." Although some of these changes are imperceptible to the _eye_, they are manifest to the educated _mind_; and the pleasures of philosophical observation are as sweet as a poet's dreams. CHAPTER XIII. 244. _Why will a piece of paper, held three or four inches over the flame of a candle, become scorched?_ [Verse: "Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house."--MATT. V.] Because the hot air and gas produced by the burning of the candle _ascends_ rapidly.