In these allied phenomena we have effects which are evidently dependent upon several dissimilar causes. The phosphorescence of the living animal is due, without doubt, to nervous excitation: that of the living vegetable to solar luminous influence; and in the case of the mosses of caverns, &c. to the chemical agency of the sun’s rays, which appears to be capable of conduction. In the dead organic matter we have a purely chemical action developing the light, and in the inorganic bodies we have peculiar molecular constitution, by which an absorption of light appears to take place.
The subject is one of the greatest difficulty; the torch of science is too dim to enable us to see the causes at work in producing these marvellous effects. The investigation leads, to a certain extent, to the elucidation of many of the secrets of luminous action; and the determination of the question, whether light is an emanation from the sun, or only a subtile principle diffused through all matter, which is excited by solar influence, is intimately connected with the inquiry.
It has been stated that matter is necessary to the development of light; that no luminous effect would be produced if it were not for the presence of matter. Of this we not only have no proof, but such evidence as we have is against the position. There is no loss of light in the most perfect vacuum we can produce by any artificial means, which should be the case if matter was concerned in the phenomena of light, as a cause.
Colour is certainly a property regulated by material bodies; or rather, the presence of matter is necessary to the production of colour. Chlorine gas is a pale yellow, and nitrous vapour a yellowish red. These and one or two other vapours, which are near the point of condensation into fluids, are the only coloured gaseous or vaporiform bodies. The sky is blue, because the material particles of the atmosphere reflect back the blue rays. But we have more practical illustrations than this. The flame of hydrogen burning with oxygen gives scarcely any light; allow it to impinge on lime, a portion of which is carried off by the heat of the flame, and the most intense artificial light with which we are acquainted is produced. Hydrogen gas alone gives a flame in which nearly all but the blue rays are wanting: place a brush of steel or asbestos in it, and many of the other rays are at once produced. An argand lamp, and more particularly the lamp in which camphine—a purified turpentine,—is burnt, gives a flame which emits most of the rays found in sunlight. Spirit of wine mixed with water, warmed and ignited, gives only yellow rays; add nitrate of strontian and they become red; but nitrate of barytes being mixed with the fluid, they are changed to green and yellow; salts of copper afford fine blue rays, and common salt intense yellow ones. Many of these coloured rays and others can be produced in great power by the use of various solid bodies introduced into flame. This has not been sufficiently pointed out by authors; but it is clear from experiments that light requires the presence of matter to enable it to diffuse its coloured glories. How is it that the oxygen and hydrogen flame gives so little light, and with a solid body present, pours forth such a flood of brilliancy?
The production of artificial light by electrical and chemical agencies will necessarily find some consideration under their respective heads. There are numerous phenomena which connect themselves with luminous power, or appear to do so, which, in the present state of our knowledge, cannot come immediately under our attention. We are compelled to reserve our limited space for those branches of science which we are enabled to connect with the great natural operations constantly going on around us. Many of these more abstruse results will, however, receive some incidental notice when we come to examine the operation of the combined physical forces on matter.
We see in light a principle which, if it has not its source in the sun, is certainly dependent upon that luminary for its manifestations and powers. From that “fountain of light” we find this principle travelling to us at a speed which almost approaches the quickness of thought itself; yet by the refinements of science we have been enabled to measure its velocity with the utmost accuracy. The immortal poet of our own land and language, in his creations of Ariel, that “tricksy spirit,” who could creep like music upon the waters, and of the fantastic Puck, who could girdle the earth in thirty minutes, appears to have approached to the highest point to which mere imagination could carry the human mind as to the powers of things ethereal. Science has, since then, shown to man that this “spirit, fine spirit,” was a laggard in his tasks, and a gross piece of matter, when compared with the subtile essences which man, like a nobler Prospero, has now subdued to do him service.
Light is necessary to life; the world was a dead chaos before its creation, and mute disorder would again be the consequence of its annihilation. Every charm which spreads itself over this rolling globe is directly dependent upon luminous power. Colours, and probably, forms, are the result of light; certainly the consequence of solar radiations. We know much of the mysterious influences of this great agent, but we know nothing of the principle itself. The solar beam has been tortured through prismatic glasses and natural crystals; every chemical agent has been tried upon it, every electrical force in the most excited state brought to bear upon its operations, with a view to the discovery of the most refined of earthly agencies; but it has passed through every trial without revealing its secrets, and even the effects which it produces in its path are unexplained problems, still to tax the intellect of man.
Every animal and every plant alike proclaim that life and health are due to light; and even the crystallizing forms of inorganic matter, by bending towards it, confess its all-prevailing sway. From the sun to each planet revolving around that orb, and to the remotest stars which gleam through the vast immensity of heaven, we discover this power still in its brightness, giving beauty and order to these unnumbered creations; no less completely than to this small island of the universe which we call our Earth. Through every form of matter we can mark its power, and from all, we can, under certain conditions, evoke it in lustre and activity. Over all and through all light spreads its ethereal force, and manifests, in all its operations, powers which might well exalt the mind of Plato to the idea of an omniscient and omnipresent God. Science, with her Ithuriel wand, has, however, shown that light is itself the effect of a yet more exalted cause, which we cannot reach.
Indeed, the attentive study of the fine abstractions of science lifts the mind from the grossness of matter, step by step, to the refinements of immateriality, and there appear, shadowed out beyond the physical forces which man can test and try, other powers still ascending, until they reach the Source of every good and every perfect gift.