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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, &c., 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 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 luminous lines, the different elements which are in combustion: thus we are learning 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, 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 occurring oftener than 30,000, but less than 458,000,000,000,000 times in a second, do not

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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 no definite state of consciousness in us. Thus, by one and the same external agency-vibrations among particles of matter-are sensations caused different, as sound, heat, light.

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 at the rate of about 186,000 miles a second, the direction of the vibrations is across the direction 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 actinic or chemical rays, also invisible. In the three, heat, light, actinism, reside the miraculous generative energy, which fills the earth with warmth, life, and 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 sun.

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 of which visible bodies are composed; but,

1 "Recent Advances in Physical Science," p. 205: Professor P. G. Tait.

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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. It is passed through æther, by means of transversal disturbances or vibrations. 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 twenty-two years, at the average rate of six hundred millions of miles 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 disturbance 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 the world.

The medium actually used, æther, is specially fitted for the transmission of the small 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 10th part of an inch. In one second S. opp. 458,000,000 of vibrations occur. At the line H, in the ali. violet, the length of the wave is Tooth part of an inch; and the number of vibrations is 727,000,000 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 num

ber 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." What a manifold undeveloped system of signs and
images we have within us!

1 66 'Spectrum Analysis," p. 11: Henry E. Roscoe.
The Perception of Sight:" Prof. Helmholtz.

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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. A manifold infinite adaptation of means is thus unfolded for contemplation.

We know what a language of twenty-six letters does in collecting, preserving, and enabling us to verify the experiences of millions of men in thousands of generations; but 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 distinct physical images of operations wrought by the ministers of flame, the guardian spirits, the cherubim and seraphim, continually teaching new lessons of the Almighty's operation. 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 and beast and man, filling the world with life, are metamorphosed beams of light. Who is he that will not worship the invisible God, whose visible garment is this glorious web of material phenomena ?

"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 God."

PARADISE LOST.

Every kind of light is not equally suitable for vegetation. Lamp-light, gas, and 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

Sensation of Light.

149

powerful chemical effect; but in photographic power are surpassed by lime, magnesium, and 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 Humboldt, in his Cosmos, writes-"The earth becomes selfluminous; besides the light which, as a planet, it receives from the central body, it shows a capacity of sustaining a luminous process proper to itself." The words of Schubert are very beautiful-"What if every proper polar light, which we call the Aurora of the North, were the last glimmer of a twilight of a world-day that has set, when the whole earth was surrounded by an expanse of air, from which the electromagnetic forces radiated light in a much greater degree than that of the polar light, and at the same time with animating heat, in a manner almost similar to what still occurs in the luminous atmosphere of the sun." "

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It is not light only which produces the sensation of brightness. The weakest electrical currents passed through the eye produce consciousness in the mind of flashes of light. blow, or pressure on the side of the eye-ball with a finger, gives an impression of light. Fevers, contamination by narcotic or intoxicating drugs, by causing increased pressure of blood, excite sensations of light. Even when the eye has been destroyed, irritation of the stumps of the optic nerve produces like sensations.

The peculiarities which separate the sensation of light from all others do not depend upon any peculiar qualities of light itself; the working power of nature is something in and yet beyond us. While moving silently in the chambers of our consciousness, it heats the atmosphere, produces the winds, and thereby shakes the ocean; it gives life to forest and field, to cattle on a thousand hills, and reveals to man the wonderful

1 "The Sun," R. A. Proctor, quoted from Sir Wm. Herschell.

2 Quoted in "Kurtz's History of the Old Covenant."

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