CHAPTER XII.

226. _How do we know that plants operate upon the solar and atmospheric heat?_

A delicate thermometer, placed among the leaves and petals of flowers, will at once establish the fact, not only that flowers and plants have a temperature differing from that of the external air, but that the temperature varies in different plants according to the hypothetical, or supposed requirements, of their existences and conditions.

227. _What is the chief cause of variation in the temperature of flowers?_

It is generally supposed that their temperature is affected by their _colours_.

228. _Why is it supposed that the colour of a flower influences its temperature?_

Because it is found by experiment that the _colours_ of bodies bear an important relation to their properties respecting _heat_, and hold some analogy to the relation of _colours_ to _light_.

If when the ground is covered with snow, pieces of woollen cloth, of equal size and thickness, and differing only in colour, are laid upon the surface of the snow, near to each other, it will be found that the relation of _colour_ to temperature will be as follows:--In a few hours the _black_ cloth will have dissolved so much of the snow beneath it, as to sink deep below the surface; the _blue_ will have proved nearly as warm as the black; the _brown_ will have dissolved less of the snow; the _red_ less than the brown; and the white the _least_, or none at all. Similar experiments may be tried with reference to the _condensation of dew_, &c. And it will be uniformly found that the _colour_ of a body materially affects its powers of _absorption_ and of _radiation_.

[Verse: "And yet I say unto you, that even Solomon, in all his glory, was not arrayed like one of these."--MATT. VI.]

229. _Why do we know that these effects are not the result of light?_

Because they would occur, in just the same order, in the absence of light.

230. _Why are dark coloured dresses usually worn in winter, and light in summer?_

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?_

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.