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<h2>CHAPTER XII.</h2> 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. [Pg 65] 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. <hr class="bible-verse" /> "And yet I say unto you, that even Solomon, in all his glory, was not arrayed like one of these."—Matt. vi. <hr class="bible-verse" /> 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. [Pg 66] <hr class="bible-verse" /> "Give unto the Lord the glory due unto his name; worship the Lord in the beauty of holiness."—Psalm xxix. <hr class="bible-verse" /> 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? [Pg 67] Because, as the warm air flies away, cold air rushes in to occupy its place. <hr class="bible-verse" /> "How much better is it to get wisdom than gold? and to get understanding rather to be chosen than silver."—Proverbs xvi. <hr class="bible-verse" /> 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. <hr class="chap" /> <div style="break-after:column;"></div>

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