brightness would increase—the energy of heat would be transformed more or less into that of light. If the whole mass of the earth was agglomerated almost at once, and if the different parts impinged together with properly arranged velocities, we can note the state of things before and after that moment. Before were scattered masses of matter. Then at the instant of impact, the integrated mass became of high temperature, and light shone. Before that moment was darkness, after it was light. We cannot imagine that Moses, though learned in the wisdom of the Egyptians, knew this; or knew that the elementary atoms have their own shapes and powers, whereby they arrange themselves into molecules of exceeding complication and varying vibration; or could be aware that 458,000,000,000,000 vibrations in a second are necessary, in order to give us the consciousness of the lowest or red light; and we wonder that, in relating the primal illumination of the earth, he tells us first of the light, and after that of the luminous body-the sun. That which caused the integration of the earth, and of the sun, and the production of light and heat, was energy. Energy may be defined as the power of doing work. There is always a tendency, in every transformation of energy, to pass from a higher to a lower form; indeed, all the energy in the universe is passing on to the lowest and final form of equally diffused heat. This, the dissipation of energy, is by no means well understood. There can be little question that the principle concerns the whole theory of thermo-electricity, of chemical combination, of allotropy, of fluorescence, etc., and perhaps matters of a higher order than common physics and chemistry. In astronomy, it shows us the material of potential suns, suns in the process of formation, in vigorous youth, in the phase of habitation for life, and in every stage of lingering decay. It reveals to us every planet and satellite as formerly a tiny sun. It carries forward our thought to that time when the materials of present systems shall be component parts of future larger suns and planets. Finally, it conducts us to that necessary future, if physical laws remain unchanged, when the present warm glittering show of life will be dark and cold and dead. It also reminds us of a beginning, a state beyond which we are totally Vibrations of Light. 159 unable to penetrate, a state produced by other than now visibly acting causes, by that transfer of energy from the Unknown, of which the universe and all material phenomena are memorials. The elementary atoms, possessing their own shapes and powers, arrange themselves into molecules of manifold combination, and exceeding variety of vibrations. When raised to incandescence, or white heat, and their lights are tested by spectrum analysis, the glowing vapours indicate, by lines, the different elements which are present: thus we learn of what materials the sun and stars are composed. Heat and light are the product of a transfer of energy. Transfer of energy, through a solid body, is effected simply by vibration of the solid body; through air, by setting it in motion at its own period of vibration; through what we call a vacuum, by the magnetic medium-that which Clerk Maxwell gives reason to believe is the medium which conveys light and radiant heat. Vibrations in the atmosphere, occurring less frequently than sixteen times in a second, produce in us consciousness of a succession of noises. Vibrations occurring oftener than 16, but less than 30,000 times in a second, produce in us the consciousness of musical notes, varying in pitch with the vibrations. Vibrations in the æther, occurring oftener than 30,000, but less than 458,000,000,000,000 times in a second, do not affect us through the ears; but the more rapid ones, acting through the nerves of the skin, produce in us the consciousness of heat. Vibrations at the rate of 458,000,000,000,000 in a second, affect us through the eyes, and produce our consciousness of red light. As the vibrations increase, corresponding shades of colour appear, until, at the rate of 727,000,000,000,000 in a second, we have the consciousness of violet light. Higher rates produce the invisible actinic rays, which excite no definite state of consciousness in us. Thus, by one and the same external agency-vibrations among particles of matter-are sensations causeddifferent, as sound, heat, light, actinism. In sound-waves, the particles of air vibrate back and forward in the direction travelled by the sound. If, by another sound, we raise such undulations as fill the depressions in the waves of the former sound, this adding of sound to sound will cause silence. Light and heat travel through æther at the rate of about 186,000 miles a second, the direction of the vibrations being across the path in which they move. Two sets of rays may be made so to interfere with one another, as to be mutually destructive: the two rays of light produce darkness, and the two rays of heat cause heat to disappear.1 Passing a slice of solar or of electric light through a prism, we unroll it into the beautiful colours of the spectrum. At one end is the red, at the other the violet, the remaining prismatic colours lying between. Red is hottest of the colours, and beyond it are the invisible rays called heat rays. Violet is the coldest, and beyond it are the invisible actinic or chemical rays. In the three-heat, light, actinism, resides the miraculous generative energy which fills the earth with warmth, life, splendour. Concerning their nature, whether we call it vibration, or heat, or light, or actinism, we affirm nothing, and know nothing. Aristotle, one of the most thoughtful men, would say-the energy streamed from God, the Infinite and Eternal Mind, as light issues from the sunwhereby the blood heats, and the blossom blows, and the sea rolls, and all the world is warmed. To a certain extent, we can give a mechanical explanation of heat and light as the products presented to our consciousness of a perpetual trembling, or swaying to and fro, of the invisible atoms which visible bodies consist of; but, when the explanation is connected with the linked purpose of the whole, we are conscious of wisdom and might exceeding all our thought. Light, wonderful and mysterious, is but a single point in the vast scheme of Nature. When we contemplate the heavens some clear autumnal evening, and marvel at the beauty of Sirius, that starry splendour is brought to us by medium of atomic shivers maintained, during the past twentytwo years, at the average rate of six hundred millions of millions of vibrations the second; and reveals a scheme of worlds and possible sphere of life, vaster than our own. Nor is that all several optical phenomena indicate that a dis1 "Recent Advances in Physical Science," p. 205: Prof. P. G. Tait. The Medium of Light. 161 turbance partaking, if such be possible, of the nature of compression would be transmitted with a velocity almost infinitely great in comparison with the existing velocity. We may ascend by this thought to the possible nature of the means by which intelligence is conveyed to other beings of the things that are done in our world. 155 The medium actually used, æther, is specially fitted for the transmission of the waves which constitute light. These waves are so small, that from forty to fifty thousand are required to occupy the breadth of an inch, and trillions enter the eye during a few seconds. The red wave has a length the 10.000.000th part of an inch. In one second 458 millions of millions of vibrations occur. At the line H, in the violet, the length of the wave is the 10.000.000 th part of an inch; and the number of vibrations is 727 millions of millions the second.1 The optic nerve is not conscious of the heat in the hot rays, nor of waves larger than the red, nor of those smaller than the violet. The eye is only able to see different proportions of the three primaries—red, green, violet; therefore, our sight may be fairly considered as rudimentary. "Take the number of fibres in the optic nerves as two hundred and fifty thousand. Every one of these is capable of innumerable different degrees of sensation of one, two, or three primary colours." 2 What a manifold undeveloped system of signs and images we have within us! We cannot but think that their wonderful potentialities are prophecy of a coming richness and fulness. In all creatures the eye is a wonderful instrument; but, probably, the eyes of insects excite highest admiration. On the heads of beetles, bees, flies, butterflies, and other insects, are two protuberances; these, examined by the microscope, are found to contain a prodigious number of small transparent hemispheres, placed with the utmost regularity in lines crossing one another as lattice-work. These hemispheres are eyes which, like so many mirrors, reflect the images of surrounding objects. Some insects possess thousands: so abundant is the supply of light and gladness even to smallest creatures. A "Spectrum Analysis," p. 11: Henry E. Roscoe. M manifold infinite adaptation of means is unfolded for our contemplation. We know what a language of twenty-six letters does in collecting, preserving, enabling us to verify the experiences of millions of men in thousands of generations. All this seems nothing in comparison with light, which brings revelations from star-depths, and which even our present optic nerves when all developed into use, may translate into human consciousness, forming distinct physical images of operations wrought by ministers of flame, or by guardian spirits, or by cherubim and seraphim. We may be able to see waves of radiance, at the rate of six hundred millions of millions the second, impart their motor energy to the atoms which vibrate in unison in the molecules of growing grass and flower; and behold how these are arranged by tremendous chemical energy into their substance and tissues; so that grass and flower, adorning the earth; bird, beast, man, filling the world with life; are metamorphosed beams of light. Who is he that will not worship the invisible God, and say?— "Well hast Thou taught the way that might direct Our knowledge, and the scale of nature set From centre to circumference; whereon, In contemplation of created things, By steps we may ascend to Thee." Paradise Lost. Every kind of light is not equally suited for vegetation. Lamp-light, gas, petroleum-light, are poor in chemical rays. The white Bengal light of arsenic, the flames of the blue. Bengal light, and of burning sulphur, produce a more powerful chemical effect; but, in photographic power, are surpassed by lime, magnesium, electric lights; the most important light, in every respect, is sunlight. The pure earths, when violently heated, yield from their surfaces lights of extraordinary splendour. "We have pretty good reason to believe, that probably all the planets emit light in some degree; for the illumination which remains in the moon in a total eclipse cannot be ascribed entirely to the light which may reach it by the refraction of the earth's atmosphere."1 "The Sun" R. A. Proctor, quoted from Sir Wm. Herschell. |