The Sun-ray and its Powers—Darkening of Horn Silver—Niepce’s Discovery—Prismatic Spectrum—Refrangibility of Light, Heat, and Actinism—Daguerre’s Discovery—Photography—Chemical Effects produced by Solar Radiations—Absorption of Actinism—Phenomena of the Daguerreotype—Chemical Change produced upon all Bodies—Power of Matter to restore its Condition—Light protects from Chemical Change—Photographs taken in Darkness—Chemical Effects of Light on organized Forms—Chemical Effects of Solar Heat—Influence of Actinism on Electricity—Radiations in Darkness—Moser’s Discoveries, &c.
Heat and light are derived from the sun, and we have attempted to show, not only that the phenomena of these two principles are different, but that they can scarcely, in the present condition of our knowledge, be regarded as modified manifestations of one superior power. Associated with these two remarkable elements, others may exist in the solar rays. Electrical phenomena are certainly developed by both heat and light, and peculiar electric changes are produced by exposure to sunshine. Electricity may be merely excited by the solar rays, or it may flow like light from the sun. Chemical action may be only due to the disturbance of some diffused principle; or it may be directly owing to some agency which is radiated at once from the sun.
A sun ray is a magical thing: we connect it in our fancy with the most ethereal of possible creations. Yet in its action on matter it produces colour; it separates the particles of solid masses farther from each other, and it breaks up some of the strongest forces of chemical affinity. To modern science is entirely due the knowledge we have gained of the marvellous powers of the sunbeam; and it has rendered us familiar with phenomena, to which the incantation scenes of the Cornelius Agrippas of the dark ages were but ill-contrived delusions, and their magic mirrors poor instruments. The silver tablets of the photographic artist receiving fixed impressions of the objects represented in the dark chamber by a lens, are far superior as examples of natural magic.
In the dark ages, or rather as the earliest gleams of the bright morning of inductive research were dispelling the mists of that phantom-peopled period, it was observed, for the first time, that the sun’s rays turned a white compound black. Man must have witnessed, long before, that change which is constantly taking place in all vegetable colours: some darkening by exposure to sunlight, while others were bleached by its influence. Yet those phenomena excited no attention, and the world knew nothing of the mighty changes which were constantly taking place around them. The alchemists—sublime pictures of credulous humanity—toiling in the smoke of their secret laboratories, waiting and watching for every change which could be produced by fire, or by their “royal waters,” caught the first faint ray of an opening truth; and their wild fancy, that light could change silver into gold, if they but succeeded in getting its subtile beams to interpenetrate the metal, was the clue afforded to the empirical philosopher to guide him through a more than Cretan labyrinth.[117]
The first fact recorded upon this, point was, that horn silver blackened when exposed to the light. Without doubt many anxious thoughts were given by these alchemists to that fact. Here was, as it appeared, a mixing up of light and matter, and behold the striking change! It was a step towards the realization of their dreams. Alas! poor visionaries! in pursuing an ideality they lost the reality which was within their grasp.
Truths come slowly upon man, and long it is before these angel visits are acknowledged by humanity. The world clings to its errors, and avoids the truth, lest its light should betray their miserable follies.
At length a man of genius announced that “No substance can be exposed to the sun’s rays without undergoing a chemical change;” but his words fell idly upon the ear. His friends looked upon his light-produced pictures as singular; they preserved them in their cabinets of curiosities; but the truths which he enunciated were soon forgotten. Howbeit his words were recorded, and it is due to the solitary experimentalist of Châlons on the Saône, to couple the name of Niepce with the discovery of a fact which is scarcely second to the development of the great law of universal gravitation.[118] But an examination awaits us, which, for its novelty, has more charms than most branches of science, and which, for the extensive views it opens to the inquirer, has an interest in nowise inferior to any other physical investigation.
The prismatic spectrum affords us the means of examining the conditions of the solar rays with great facility. In bending the ray of white light out of its path, by means of a triangular piece of glass, we divide it in a remarkable manner. We learn that heat is less refracted by the glass than the other powers; we find the maximum point of the calorific rays but slightly thrown out of the right line, which the solar pencil would have taken, had it not been interrupted by the prism; and the thermic action is found to diminish with much regularity on either side of this line. We discover that the luminous power is subject to greater refraction, and that its maximum lies considerably above that of heat; and that, in like manner, on each side the light diminishes, producing orange, red, and crimson colours below the maximum point, and green, blue, and violet above it. Again, we find that the radiations which produce chemical change are more refrangible than either of the others, and the maximum of this power is found at the point where light rapidly diminishes, and where scarcely any heat can be detected: it extends in full activity, above its maximum, to a considerable distance, where no trace of light under ordinary conditions exists, and below that point, until light, appearing to act as an interfering agent, quenches its peculiar properties. These are strong evidences that light and actinism—as this principle has been named—are not identical: and we may separate them most easily and effectually from each other. Certain glasses, stained dark blue, with oxide of cobalt, admit scarcely any light; but they offer no interruption to the passage of actinism or the chemical rays; on the contrary, a pure yellow glass, or a yellow fluid, which does not sensibly reduce the intensity of any one colour of the chromatic band of luminous rays, completely cuts off this chemical principle, whatever it may be. In addition to these, there are other results which we shall have to describe, which prove that, although associated in the solar beam, light and actinism are in constant antagonism.