Light from the centre of the solar disc is under different conditions from that which comes from the edge of the sun: this is due to the varying angle, which is presented to us by a circular body: calorific action seems to be more strongly manifested when the envelope of light, extending like an atmosphere to the sun, is thrown into great agitation, and waves, and great hollows—solar spots—are produced. There is some indication of the existence of a third condition on the sun’s surface, to which probably belongs the mighty chemical power which we call actinism. Electricity may be, as some have speculated, the exciting agent; a constant and violent Aurora Borealis may exist on the sun, and under the excitation of this force the others named may be quickened into full activity.
That actinism is one of the great powers of creation we have abundant proof. Nearly all the phenomena of chemical change which have been referred to light, are now proved to be dependent upon actinic power; and beyond the influence which has been ascertained to be exerted by it upon all inorganic bodies, we shall have occasion to show still further the dependence of the vegetable and animal worlds upon its agency. The influence of the solar beams on vegetation is proved by common experience; the closer examination of its action on vegetable life is reserved for the chapter devoted to its phenomena. Of its influence on animals nothing is very correctly known; but some early experiments prove that they, like other organised bodies, are subject to all the radiant forces, as indeed, independent of experiment, every observation must teach. Certain it is, that organisation can take place only where the sun’s rays can penetrate: where there is unchanging darkness, there we find all the silence of death. Prometheus stole fire from heaven, and gave the sacred gift to man, as the most useful to him of all things in his necessities: by the aid of it he could temper the severities of climate, render his food more digestible and agreeable, and illuminate the hours of darkness. So says the beautiful fiction of the Grecian mind,—which appears as the poetic dream or prophetic glance of a gifted race, who felt the mysterious truth they were yet unable to describe. Pheaton and Apollo are only other foreshadowings of the creative energies which dwell in the glorious centre of our universe. The poetry of the Hellenic people ascended above the littlenesses of merely human action, and sought to interpret the great truths of creation. Reflective, they could not but see that some mysterious powers were at work around them; imaginative, they gave to fine idealisations the government of those inexplicable phenomena. Modern science has shown what vastly important offices the solar rays execute, and that the principles discovered in a sunbeam are indeed the exciters of organic life, and the disposers of inorganic form.
It must not be forgotten that we have already alluded to a speculation which supposes this actinic influence to be diffused through all nature, to be indeed the element to which chemical force in all its forms is to be referred, and that it is merely excited by the solar rays. This hypothesis receives some support from the very peculiar manner in which chemical action once set up is carried on, independent of all extraneous excitement, after the first disturbance has been produced. If any of the salts of gold are exposed in connection with organic matter, as on paper, to sunshine for a moment, an action is begun, which goes on unceasingly in the dark, until the gold is reduced to its most simple state.[130] The same thing occurs with chromate of silver, some of the salts of mercury, argentine preparations combined with protosulphate of iron or gallic acid, and some other chemical combinations. These progressive influences point to some law not yet discovered, which seems to link this radiant actinism with the chemical agent existing in all matter.
This problem also connects itself with another class of facts which, although due, in all probability, to a great extent, to calorific radiations, and hence known under the general term of Thermography, appear to involve both chemical and electrical excitation. From the investigations of Moser and of others, we learn the very extraordinary fact, that even inanimate masses act and react upon each other by the influence of some dark radiations, and seem to exchange some of the peculiarities which they possess. This appears generally in the curious experiments which have been referred to, as confined merely to form or structure. Thus an engraved plate will give to a polished surface of metal or glass placed near it, after a very little time, a neat distinct image of itself; that is, produce such a structural disturbance as will occasion the plate to receive vapour differently over those spaces opposite to the parts in cameo or in intaglio, from what it does over the opposite. If a piece of wood is used instead of a metal, there will, by similar treatment, be produced a true picture of the wood, even to the representation of its fibres.[131]
It is also probable that chemical decomposition is produced by the mere juxtaposition of different bodies. Iodide of gold or silver, perfectly pure, has been placed upon a plate of glass, and a plate of copper covered with mercury suspended over it: a gradual decomposition of those salts is said to have been observed, iodide of mercury to be formed, and the gold or silver salts reduced to a finely divided metallic state.[132]
A body whose powers of radiating heat are low, being brought near another whose radiating powers are more extensive, will, in the course of a short time, undergo such an amount of molecular disturbance as will effect a complete change in the arrangement of its surface, and an impression of the body having the highest radiating powers will be made upon the other. This impression is dormant, but may be developed under the influence of vapour, or of oxidation.[133] A body, such as charcoal, of low conducting power, being placed near another, such as copper, which is a good conductor, will, in a very short time, produce, in like manner, an impression of itself upon the metal plate. Thus any two bodies, whose conducting or radiating powers are dissimilar, being brought near each other, will occasion a molecular disturbance, or impress the one with the image of the other. However small the difference may be, an effect is perceived, and that of the most extraordinary kind, giving rise to the production of actual images upon each surface exposed. It is thus that a print on paper may be copied on metal, by merely suspending it near a well-polished plate of silver or copper for a few days. The white and black lines radiate very differently; consequently an effect is produced on the bright metal in the parts corresponding to the black lines, dissimilar to that which takes place opposite to the white portions of the paper; and, on the application of vapour, a true image of the one is found impressed upon the other.[134]
Bodies which are in different electrical states act upon each other in an analogous manner. Thus arsenic, which is highly electro-negative, will, when placed near a piece of electro-positive copper, readily impart to its surface an impression of itself, and so in like manner will other bodies if in unlike conditions. Every substance physically different (it signifies not whether as it regards colour, chemical composition, mechanical structure, calorific condition, or electrical state,) has a power of radiation by which a sensible change can be produced in a body differently constituted.
Fable has told us that the magicians of the East possessed mirrors in which they could at will produce images of the absent. Science now shows us that representations quite sufficient to deceive the credulous can be produced on the surface of polished metals without difficulty. A highly polished plate of steel may be impressed with images of any kind, which would remain invisible, the polished surface not being in the least degree affected, as it regards its reflecting powers; but by breathing over it, the dormant images would develope themselves, and fade away again as the condensed moisture evaporated from the surface.[135]
These, which are but a few selected from a series of results of an equally striking character, serve to convince us that nature is unceasingly at work, that every atom is possessed of properties by which it influences every other atom in the universe, and that a most important class of natural phenomena appear to connect themselves directly with the radiant forces.