CHAPTER XXVI.

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_.

[Verse: "Ye are the light of the world. A city that is set on a hill cannot be hid."--MATTHEW V.]

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.

[Verse: "When I consider thy heavens, the work of thy fingers, the moon and the stars which thou hast ordained:"]

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 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.

[Verse: "What is man, that thou art mindful of him? and the son of man that thou visitest him?"--PSALM VIII.]

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 illuminate every atom of the permeated substance, and by surrounding it with a prismatic clothing, to afford an illustration of its _molecular arrangement_.

[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.]

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?_

Because blue glass obstructs many of the luminous rays, but it is perfectly transparent to _actinism_.

[Verse: "The hay appeareth, and the tender grass showeth itself, and herbs of the mountain are gathered."--PROV. XXVII.]

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_.

[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.]

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 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.