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<h2>CHAPTER XXVI.</h2> 540. What is the difference between light and heat? The most obvious distinction is, that light acts upon vision, and heat upon sensation, or feeling. Another distinction is, that heat expands all bodies, and alters their atomic condition; while light, though usually attended by heat, does not display the same expansive force, but produces various effects which are peculiar to itself. [Pg 126] <hr class="bible-verse" /> "Ye are the light of the world. A city that is set on a hill cannot be hid."—Matthew v. <hr class="bible-verse" /> 541. Are light and heat combined in the solar ray? Yes. A ray of light, as well as containing elementary rays that produce colours under refraction, contains also chemical rays, and heat rays. 542. How do we know that light and heat are separate elements? Because we have heat rays, as from dark hot iron, from various chemical actions, and from friction, which are unattended by the development of light. And we have light, or luminosity, such as that of phosophoresence, which is unaccompanied by any appreciable degree of heat. But, besides this confirmation, further proof is afforded by the fact, that in passing rays of solar light through media that are transparent to heat, but not to light, the heat rays may be separated from the luminous rays, and vice versa. Black glass, and black mica, which are nearly opaque to light, are transparent to heat to the extent of ninety degrees out of a hundred. While pale green glass, coloured by oxide of copper, and covered with a coating of water, or a thin coating of alum, will be perfectly transparent to light, but will be almost quite opaque to heat. These remarks apply, in a greater or less degree, to various other substances. 543. In what respects are light and heat similar? Both heat and light have been referred to minute vibratory motions which occur, under exciting causes, in a very subtile elastic medium. They are both united in the sun's rays. They are both subject to laws of absorption, radiation, reflection, and refraction. They are both essential to life, whether animal or vegetable. Both may be developed in their greatest intensity by electricity. They are both imponderable. <hr class="bible-verse" /> "When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained:" <hr class="bible-verse" /> 544. In what respects are light and heat dissimilar? Heat frequently exists without light. Light is usually attended with heat. Light may be instantly extinguished, but [Pg 127] Heat can only be more gradually reduced, by diffusion. The solar rays deliver heat to the earth by day, and the heat remains with the earth when the light has departed. Heat diffuses itself in all directions. Light travels only in straight lines. The colours that absorb and radiate both light and heat do not act in the same degree upon them both. Black, which does not radiate light, is a good radiator of heat, &c., &c. The oxy-hydrogen light emits a most intense heat, but glass which will transmit the rays of light, will afford no passage to the rays of the heat. Heat is latent in all bodies, but no satisfactory proof has been found that light is latent in substances. These are only a few of the analogies and distinctions that exist between the two mysterious agents, light and heat. But they are sufficient to supply the starting points of investigation. The importance of the heat that attends the solar rays may be illustrated by the experiments performed a few years ago, by Mr. Baker, of Fleet-street, London, who made a large burning lens, three feet and a half in diameter, and employed another lens to reduce the rays of the first to a focus of half an inch in diameter. The heat produced was so great that iron plates, gold, and stones were instantly melted; and sulphur, pitch, and resinous bodies, were melted under water. 545. What is the point of heat at which bodies become luminous? The point of heat at which the eye begins to discover luminosity has been estimated at 1,000 deg. 546. What is the velocity of artificial light? The light of a fire, or of a candle, or gas, travels with the same velocity as the light of the sun,—a velocity which would convey light eight times round the world while a person could count "one." 547. At what rate of velocity does the light of the stars travel? At the same velocity as all other light. And yet there are stars so distant that, although the light of the sun reaches the earth in eight minutes and a half, it requires hundreds of years to bring their light to us. [Pg 128] <hr class="bible-verse" /> "What is man, that thou art mindful of him? and the son of man that thou visitest him?"—Psalm viii. <hr class="bible-verse" /> 548. What is the relative intensity of primary and reflected light? The intensity of a reflection depends upon the power of the reflecting surface. But, taking the sun and moon as the great examples of primary and reflected light, the intensity of the sun's light is 801,072 times greater than that of the moon. 549. What is polarized light? Polarized light is light which has been subjected to compound refraction, and which, after polarization, exhibits a new series of phenomena, differing materially from those that pertain to the primary conditions of light. 550. What are the chief deductions from the phenomena observed under the polarization of light? The polarization of light appears to confirm in a high degree the vibratory theory of light; and to show that the vibrations of light have two planes or directions of motion. The mast of a ship, for instance, has two motions: it progresses vertically as the ship is impelled forward, and it rolls laterally through the motion of the billows. Something like this occurs in the vibrations of light, only the vertical vibration is the condition of one ray, and the lateral vibration is the condition of another ray, and the vibrations of these two rays intersect each other in the solar ray. When these vibrations occur together, the ray has certain properties and powers. But by polarization the rays may be separated, and the result is two distinct rays, having different vibrations. It then appears that various bodies are transparent to these polarized rays only in certain directions. And this fact is supposed to show that bodies are made up of their atoms arranged in certain planes, through or between which the lateral or the vertical waves of light, together or singly, can or cannot pass; and that the transparency or the opacity of a body is determined by the relation of its atomic planes to the planes of the vibrations of light. Ordinary light, passing through transparent media, produces no very remarkable effect in its course; but polarized light appears to [Pg 129] illuminate every atom of the permeated substance, and by surrounding it with a prismatic clothing, to afford an illustration of its molecular arrangement. <hr class="bible-verse" /> "A man that is called Jesus made clay, and anointed mine eyes, and said unto me, Go to the pool of Siloam, and wash: and I went and washed, and I received sight."—John ix. <hr class="bible-verse" /> 551. Why are two persons able to see each other? Because rays of light flow from their bodies to each other's eyes, and convey an impression of their respective conditions. In some popular works that have come under our notice, we find that the student is told that "we cannot absolutely see each other—we only see the rays of light reflected from each other." The statement is erroneous as expressed. We do not see the rays of light, for if we did so, the effect of vision would be destroyed, and all bodies would appear to be in a state of incandesence, or of phosphoresence. Rays of light, which are in themselves invisible, radiate from the objects we look upon, enter the pupil of the eye, and impress the seat of vision in a manner which conveys to the mind a knowledge of the form, colour, and relative size and position of the figure we look upon. If this is not seeing the object—what is? It would be just as reasonable to say, that we cannot hear a person speak—that we only hear the vibrations of the air. But as the vibrations are imparted to the air by the organs of voice of the speaker, as he sets the air in motion, and makes the air his messenger to us, we certainly hear him, and can dispense with any logical myths that confound the understanding, and contribute to no good result. 552. What is actinism? Actinism is the chemical property of light. Actinism—ray power. 553. Why does silver tarnish when exposed to light? Because of the actinic, or chemical power of the rays of the sun. 554. Why do some colours fade, and others darken, when exposed to the sun? Because of the chemical power of the sun's rays. 555. Why can pictures be taken by the sun's rays? Because of the actinic powers that accompany the solar light. 556. What is the particular chemical effect of light exhibited in the production of photographic pictures? Simply the darkening of preparations of silver, by the actinic rays. 557. Why are photographic studios usually glazed with blue glass? [Pg 130] Because blue glass obstructs many of the luminous rays, but it is perfectly transparent to actinism. <hr class="bible-verse" /> "The hay appeareth, and the tender grass showeth itself, and herbs of the mountain are gathered."—Prov. xxvii. <hr class="bible-verse" /> 558. Why do plants become scorched under the unclouded sun? Because the heat rays are in excess. The clouds shut off the scorching light; but, like the blue glass of the photographer's studio, they transmit actinism. 559. What effect has actinism upon vegetation? It quickens the germination of seeds; and assists in the formation of the colouring matter of leaves. Seeds and cuttings, which are required to germinate quickly, will do so under the effect of blue glass (which is equivalent to saying, the effect of an increased proportion of actinism), in half the time they would otherwise require. 560. In what season of the year is the actinic power of light the greatest? In the spring, when the germination of plants demands its vitalising aid. In summer, when the maturing process advances, light and heat increase, and actinism relatively declines. In the autumn, when the ripening period arrives, light and actinism give way to a greater ratio of heat. <hr class="bible-verse" /> "But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him."—Corinth. Book i., ii. <hr class="bible-verse" /> We shall have frequently, in the progress of our lessons, to refer to light in its connection with the chemistry of nature, and with organic life. But let us now invite the student to pause, and for a moment contemplate the wonders of a sunbeam. How great is its velocity—how vast its power—how varied its parts—yet how ethereal! First, let us contemplate it as a simple beam in which light and heat are associated. How deep the darkness of the night, and how that darkness clings to the recesses of the earth. But the day beams, and darkness flies before it, until every atom that meets the face of day is lit up with radiance. That which before lay buried in the shade of night is itself now a radiator of the luminous fluid. Mark the genial warmth that comes as the sister of light; then stand by the side of the experimentalist and watch the point on which he directs the shining focus, and in an instant see iron melt and stones run like water, under the fervent heat! Now look upward to the heavens, where the falling drops of rain have formed a natural prism in the rainbow, and shown that the beam of pure whiteness, refracted into various rays, glows with all the tints that adorn the garden of nature. These are the visible effects of light. But follow it into the crust of the earth, where it is, by another power, which is neither light nor heat, quickening the seed into life; watch it as the germ springs up, and the plant puts forth its tender [Pg 131] parts, touching them from day to day with deeper dyes, until the floral picture is complete. Follow it unto the sea, where it gives prismatic tints to the anemone, and imparts the richest colours to the various algae. Think of the millions of pictures that it paints daily upon the eyes of living things. Contemplate the people of a vast city when, attracted by some floating toy in the air, a million eyes look up to watch its progress. The sun paints a million images of the same object, and each observer has a perfect picture. It makes common to all mankind the beauties of nature, and paints as richly for the peasant as for the king. The Siamese twins were united by a living cord which joined their systems, and gave unity and sympathy to their sensations. In the great flood of light that daily bathes the world, we have a bond of union, giving the like pleasures and inspirations to millions of people at the same instant. And that which floods the world with beauty, should no less be a bond of unity and love. <hr class="chap" /> <div style="break-after:column;"></div>

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