The fairy tales of science/The Magic of the Sunbeam
The Magic of the Sunbeam.
"The glorious sun
Wherever the Sunbeam falls we find life and motion; elsewhere, death and stillness. Under its influence the seed germinates, the stem sends forth branches, the leaf bursts from the bud, the flower unfolds its petals, and the fruit grows and ripens.
This subtle agent plays an important part in many of the fairy tales of science. The philosopher has conducted it into his dark laboratory, and by twisting and torturing it with cunningly-devised instruments, has forced it to reveal so many wonderful truths, that the mind, in attempting to grasp them, is fairly bewildered. The Magic of the Sunbeam is indeed an inexhaustible theme, and we can only touch upon a few of its mysteries.
Every sunbeam consists of light, heat, and chemical power, or actinism. At present we will consider the sunbeam as a ray of light, without regarding its other principles.
What is light? This is one of those unanswerable questions that meet us on the threshold of every science. Some philosophers entertain the opinion that light consists of tiny particles of matter thrown off from a luminous body with prodigious velocity in all directions. Others suppose it to be an undulation or vibration produced in a medium called ether, which is believed to pervade all space. The former view of the subject is termed the theory of emission; the latter, the undulatory theory. We cannot say which of these hypotheses, or guesses, approaches nearest to the truth, but the undulatory theory has by far the greater number of supporters.
Whether a ray of light be a stream of inconceivably minute particles of matter, or a succession of waves in an ethereal medium, we are quite certain that it travels at the rate of nearly two hundred thousand miles in a single second. But such is the disproportion between the distances of the celestial bodies, that light must be about eight and a quarter minutes in reaching us from the sun; about five hours in coming from the planet Neptune; years from the nearest fixed star; and probably centuries from the nebulæ! When we look up at the heavens, we do not see the stars as they are now, but as they were many years ago, for the light which now renders them visible must have left them long before we were born!
Rays of light are emitted, under ordinary circumstances, in direct lines; they will not pass through a bent tube, nor turn a corner. Bodies through which light passes freely are called transparent, and those which do not admit it to pass, opaque. When light falls upon an opaque surface a portion is absorbed and another portion reflected; when the reflected portion is considerable the surface appears white, and black when the portion is inappreciable.
We have said that a sunbeam consists of three great principles, namely, light, heat, and actinism. These may be but modifications of one force, but in the present state of our knowledge it is better to regard them as distinct agents.
Light acts upon the organs of vision, and enables us to distinguish external objects. Heat regulates the solid, liquid, and aëriform states of matter, and maintains this planet in the condition which is essential to the well-being of its inhabitants. Actinism brings about those wonderful chemical changes which are constantly occurring in nature. These three principles unite to form our magic sunbeam, just as three chemical elements unite to form a compound.
How can we decompose a sunbeam? How can we separate those principles which are linked together in such a mysterious manner? Easily enough, for by the instrumentality of a triangular bar of transparent glass, called a prism, the beam can be instantly resolved into its components.
If a sunbeam, admitted into a dark chamber by a small hole in the window-shutter, be allowed to fall on a prism, its subtle constituents are mysteriously disturbed, and precipitate themselves at different distances on a white tablet, or screen, placed to receive them. What marvellous change is this! A moment since the beam formed a bright spot on the screen, but now in place of the spot we see a lengthened band of variegated colours! On one side of the prism a pencil of brilliant white sunlight falls upon the surface of the glass; on the other the pencil spreads out and paints upon the screen a ribbon whose beauteous hues infinitely surpass the colours that lie on the artist's palette! Examine these colours attentively. At the bottom of the band we find red, above it orange, then yellow, green, blue, indigo, and lastly violet. These colours pass by insensible gradations into each other, so that it is impossible to say where one colour ends and another begins.
We have thus decomposed the visible principle of the sunbeam into its elementary colours, for our readers must know that white is a compound of seven hues. The natural colours of bodies depend entirely upon the manner in which they decompose the sunbeams. A rose is red because its petals have the property of absorbing all the elementary colours of light except red, which it reflects. The pigments used by the artist are not, in themselves, colours; they are merely substances that absorb certain rays and reflect others. Our readers will now understand how it is that a body which reflects most of the light that falls upon it appears perfectly white.
But we have not yet done with the variegated band, or prismatic spectrum, as it has been termed. If a highly sensitive finger were held in the yellow rays of the spectrum, a degree of warmth would be felt, greater than if it were held in the violet rays. But if it were removed to the extreme red rays a great deal more heat would be perceived than in either of the former cases. Now, we have imagined the existence of a finger far more sensitive to slight variations of temperature than ordinary fingers are, but these results have been obtained by means of very delicate thermometers, or heat measurers.
Let us now take a piece of paper, prepared for the photographic process, and place it upon the screen so that it may receive the rainbow-like colours upon its sensitive surface. On removing it it will be found to be blackened at a point beyond the violet rays of the spectrum. The principle which blackens the prepared paper is actinism.
From these experiments we learn that the sunbeam is an ethereal band of different rays, which may be separated by the instrumentality of the prism. We learn that heat is less refracted, or bent, by the glass than the other powers, as we find it but slightly thrown out of the right line which the beam would have taken had it not been interrupted by the prism. "We discover that light is subject to greater refraction as the seven colours are thrown upon the screen above the maximum point of the heat rays. Lastly, we find that actinism is more refrangible than either heat or light, as we know that the maximum of this power is found in the upper part of the spectrum at a point where light rapidly diminishes, and where scarcely any heat can be detected.
The analysis of the sunbeam by means of the prism must excite our wonder. Who could imagine that a simple wedge-shaped piece of glass would be able to separate those imponderable agents which reach us after having travelled ninety-five millions of miles together?
We can isolate either of these solar principles without the aid of a prism. The crystal called black mica does not admit light to pass through it, but it is freely penetrated by heat; and on the other hand, glass stained green by oxide of copper, offers scarcely any impediment to the passage of light, though it effectually stops the rays of heat. Again, a yellow transparent glass obstructs the chemical radiations, while a dark blue medium, which arrests nearly all the light, allows them to pass. Thus we see that the physical conditions of the three solar principles are essentially different.
Let us now consider the magic influences of this sunbeam over the animal and vegetable kingdoms. The luminous principle first demands our attention; for although we are told that light is less abundant than either heat or actinism, we cannot help regarding it as the sunbeam's chief constituent. Light is of the highest importance to the health and well-being of animals, as may be inferred from the fact that animal life ceases in situations from which light is totally excluded. The case of the Proteus anguinus is exceptional, and therefore deserves some notice.
This extraordinary little creature is found in some of the gloomy caverns of Illyria, into which the magic sunbeam never penetrates. "At first view," says Sir Humphry Davy, "you might suppose this animal to be a lizard, but it has the motions of a fish. Its head, and the lower part of its body, and its tail, bear a strong resemblance to those of the eel; but it has no fins, and its curious bronchial organs are not like the gills of fishes. They form a singular vascular structure, almost like a crest, round the throat, which may be removed without occasioning the death of the animal, who is likewise furnished with lungs. With this double apparatus for supplying air to the blood, it can live either below or above the surface of the water. Its fore feet resemble hands, but they have only three claws or fingers, and are too feeble to be of use in grasping or supporting the weight of the animal. The hinder feet have only two claws or toes, which in the larger specimens are found so imperfect as to be almost obliterated. It has small points in place of eyes, as if to preserve the analogy of nature. It is of a fleshy whiteness and transparency in its natural state, but when exposed to light, its skin gradually becomes darker, and at last assumes an olive tint. Its nasal organs appear large; and it is abundantly furnished with teeth, from which it may be concluded that it is an animal of prey, yet in its confined state it has never been known to eat, though it has been kept alive for many years by occasionally changing the water in which it was placed." This strange creature, whose life is passed in total darkness, has long been a puzzle to philosophers, as all the facts revealed by science go to prove that light is indispensable to organization.
The dependence of animal life upon light is beautifully exhibited in the ocean. Water is not absolutely translucent, and it has been calculated that light must lose all its influence at the depth of a very few hundred feet into the ocean, even under the tropics. Now, it has been satisfactorily proved by an extensive series of dredging experiments that life diminishes as we descend into the ocean, and that beyond the depth of three hundred fathoms it ceases altogether. But this is not all, for besides being much more numerous, the shells of the different mollusca are much more brightly coloured in the upper regions of the ocean than in the lower, in fact, a regular gradation of tints may be traced as the shells grow deeper in hue as they approach the light.
Man himself is highly susceptible to the influence of light, and pines and sickens in darkness. Those persons who dwell in dark streets and alleys are far more subject to disease than those who reside in open places. Again, those who take no heed of the old proverb about going early to bed, seldom find themselves healthy; and though they may be wealthy, they cannot be deemed wise!
Light is absolutely necessary to vegetable life, for under its influence the plant separates carbon from the air and secretes it within its tissues. Every one must have observed how plants grow towards the light, especially when confined in a room; how blanched and sickly they become in dark situations, and how speedily they recover when exposed to full sunlight. When a potato germinates in a dark cellar it puts forth long pallid shoots in quest of a stray sunbeam; but let it be exposed to the light for a few days, and these shoots will become dark and green.
Flowers are more sensitive to the influence of light than leaves; indeed almost every flower has a particular degree of light requisite for its full expansion. So regular are the periods of opening and shutting with some flowers, that they enable us to tell the hour of the day with tolerable accuracy. The great naturalist, Linnæus, made a list of no less than thirteen flowers that open and shut at different hours, and designated them by the fanciful title of "Flora's clock."
Having said enough to prove that there exists a mysterious bond of union between organization and light, let us now examine some of the effects of heat.
The present condition of our earth is directly dependent upon the amount of heat we receive from the sun. If it were possible to move this planet nearer that orb, the quantity of heat would be much increased, and all the present races of plants and animals must perish; the same result would happen were the two bodies to be separated by a greater distance, owing to a deficiency of the genial influence. In the former case the world would be much too hot to hold us, and in the latter we should be regularly frozen out!
The rays that are emitted from the sun are partly absorbed by the atmosphere, which acts as a screen, and shields the earth's inhabitants from the full and perhaps destructive influence of the sun's heat. The quantity of heat received by us in one year is prodigious, for it has been calculated that it would suffice to melt a shell of ice forty-six feet thick, and covering every part of the globe. The heat-rays striking the earth, become dispersed in a variety of ways. Some are reflected, others are absorbed. Some of the rays warm the earth, and then warm the overlying air, and expanding it, rise with it to the upper regions of the atmosphere. But by far the greater number of heat-rays penetrate the earth, and descend to a considerable depth. In winter this stored-up heat partly returns to the surface, and ultimately becomes dissipated into the air, and from the air into infinite space.
Heat, like light, is absorbed in different degrees by different substances. The colour and condition of surface seems to exert a great influence on its absorption; thus a black body absorbs more heat than a white one, and a rough surface more than a smooth one. "Every tree," says Mr. Hunt, "spreading its green leaves to the sunshine, or exposing its brown branches to the air, every flower which lends its beauty to the joyous earth, possesses different absorbing and radiating powers. The chalice-like cup of the pure white lily floating on the lake, the variegated tulip, the brilliant anemone, the delicate rose, and the intensely-coloured peony or dahlia, have each powers peculiar to themselves for drinking in the warming life-stream of the sun, and for radiating it back again to the thirsting atmosphere."
It is impossible to enumerate the wonderful offices performed by heat in the economy of nature. By the influence of heat, water is vaporized and raised into the air, thence to be precipitated in refreshing showers. The atmospheric currents are caused by heat; the trade-wind, that blows from the same quarter throughout the year, the periodical monsoon, the gentle breeze, the boisterous gale, and the devastating hurricane, are alike manifestations of the activity of this mighty principle.
Let us now glance at that mysterious element of the sunbeam which cannot be detected by the senses. To modern science is entirely due the knowledge we have gained of the chemical powers of the sunbeam. The old alchemists, indeed, were acquainted with the isolated fact that a white substance called horn silver was blackened by exposure to the sun's rays, but it never struck them to investigate the cause of this curious phenomenon. It was reserved for a philosopher of modern times to prove that no substance can be exposed to the sun's rays without undergoing a chemical change.
The blackening of horn silver is but a single instance of a vast number of effects produced by that mysterious agent which is associated with light and heat in the sunbeam. All bodies are influenced by actinism, and undergo a chemical or molecular disturbance. The rock and the mountain, as well as the animal and the plant, are destructively acted upon during the hours of sunshine, and would soon perish under the delicate touch of the actinic rays, were it not for the counteracting influence of darkness. At night, the chemical disturbances are undone, and inorganic bodies as well as organized beings may be said to sleep!
The influence of actinism upon germination is very remarkable, as seeds will not germinate in light from which this principle is separated. But, after the leaves are formed, a larger amount of light than of actinism is necessary to enable the plant to separate carbon from the atmosphere and form wood. Again, the flowering and fruiting of a plant is more closely connected with the heat of the sunbeam than with its light or actinism. Nature has amply provided for the varying wants of plants; in the spring we may detect an excess of actinism in the solar rays; in the summer an excess of light, and in the autumn an excess of heat.
We have said that all bodies undergo a chemical disturbance when exposed to the solar rays, but it must not be supposed that this disturbance always manifests itself in a blackening, as in the case of horn silver. If a polished plate of metal, of glass, of marble, or even a polished surface of wood, be in part exposed to the influence of sunshine, it will, when breathed upon, exhibit the fact that a disturbance of some kind has taken place upon the portions illuminated, whereas no change can be detected upon the parts kept in the dark. But if we expose a chemically prepared tablet to the sunbeams in a similar manner, we may by a certain process render the effect produced on its surface permanent, and thus as it were fix a shadow.
The beautiful art of photography, or light-drawing, is based upon this marvellous fact. Everybody is familiar with the grand results of this art. Everybody has seen those wondrous pictures which neither pencil, brush, nor hand has touched, but which have been delicately traced by the magic sunbeam. We have ceased to look upon these pictures with astonishment, just as we have ceased to wonder at the locomotive, the electric telegraph, and the steamship. But in times gone by, had any one asserted that he could compel the sunbeams to paint a portrait, he would in all probability have been burned as a wizard; indeed, not many years ago a gentleman was thought to be disordered in his intellect, because he deemed it possible to fix the fleeting pictures seen in the camera obscura.
We cannot enter further into the Magic of the Sunbeam without leading our readers into the mystic regions of mathematics. We have already said that the subject is an inexhaustible one, and we are more convinced of this than ever when we find what a comparatively small number of facts relating to the wonderful band of forces called the sunbeam, we have been able to set before the reader. But though so much is known about the sunbeam, how much still remains obscure! It is only lately that the existence of the mighty principle of actinism has been revealed; and who can tell what forces may still be hidden in the beam—what unknown powers may yet be brought to light by our laborious truth-seekers?