Roric Figures (Lat. ros, dew), a name applied to certain curious images rendered manifest upon breathing on polished solid surfaces, when these have been previously exposed to contact with or close proximity of the objects thus represented, and usually at the same time acted upon by light, heat, or electricity. The singularity of these phenomena is, that they consist usually in the production at the first of a sort of latent or invisible image, which may afterward be developed somewhat in the manner of photography. Dr. J. W. Draper, in the "Philosophical Magazine" for September, 1840, mentioned certain facts going to show that an insensible molecular change may be made to take place in the surface of bodies; and among them he named the following instance, as long known: "That if a piece of very cold clear glass, or, what is better, a cold polished metallic reflector, has a little object, such as a piece of metal, laid on it, and the surface be breathed over once, the object being then carefully removed, as often as you breathe on it again, a spectral image of it [the object] may be seen; and this phenomenon may be exhibited for many days after the first trial is made." Möser of Königsberg first distinctly called attention to these figures; his statement through M. Regnault to the French academy in July, 1842, being to the effect that generally, when two bodies are sufficiently near, they impress their images upon each other; or, as he elsewhere says, if a surface has been touched in any parts by any body, it acquires the property of precipitating all vapors, these adhering to or combining with it on those spots differently from what they do on others.
Möser inferred from the facts that there is a latent light, as well as latent heat; and that bodies radiate such a light, even in complete darkness. Write with a dry, blunt wooden point, a coil of paper, a brush, or any solid that does not scratch or color, on a clean surface of glass, or on any polished solid; no visible trace may appear; but breathe on the surface, and the parts that were touched will alone condense the breath, or they will condense it much more completely than, or differently from, other parts, and the characters traced become visible in lines of moisture. Or, breathe upon a surface, trace upon, and then dry it; breathing upon it again, the figures will reappear. To these appearances the name of Möser's images has been given; while some German writers term them Hauchfiguren, breath figures; and Mr. Grove has designated them as . "molecular impressions," a name truly expressing the nature of only a limited proportion of the cases. Place a coin on a clean looking-glass, and leave both for some time in the sun; removing, and breathing gently on the glass, a quite distinct image of the coin will appear.
Mr. R. Hunt produces similar effects by heat; and he ascribes the phenomena directly to thermic agency, though he seems to claim also (what more recent investigators deny the necessity of) a galvanic influence, his results being best when the metals used were electrical oppo-sites, and as the impressing object was larger. He placed on a well polished copper plate, too hot to be handled, coins and medals of gold, silver, bronze, and copper, and allowed the whole to cool; removing the objects, exposing the plate to the vapor of mercury, and wiping off any non-adherent mercury, he found that the coins had made impressions on the surface that were distinct in the order above given, those of the gold and silver most so; and these were permanent. Whatever in cases of this kind the change may be, the parts of any device affect the surface to which they are near according to the relative proximity of the projecting and depressed portions. By exposure over night he obtained a very distinct image of the grain of wood placed at more than half an inch distance from the receiving plate; and so, images are readily obtained with objects an eighth of an inch from the surface; a very good plan is, with the object on or hung near the plate, to place both on the mantelpiece over a fire, the ascending heat radiations being thrown back from the object, and affecting the polished body. (See the chapter on "Thermography" in R. Hunt's "Photography," republished in New York, 1852.) Karsten placed a medal on a glass plate, resting on one of metal (a coin on a looking-glass coated with amalgam may be used), and allowed a few sparks from an electrical machine to fall on the medal; the image on the glass is brought out by vapor of mercury, iodine, or the breath.
Some years before, Riess discharged electric sparks on glass and mica plates, and, breathing on these, brought out figures of the traces of the spark. But if he first cleaned the glass by boiling in nitric acid and washing in ammonia, or employed platinum foil clean enough to fire gases, or fresh mica surfaces obtained by splitting for the occasion, no figures appeared after applying the electric spark. Karsten concludes that surfaces show figures after the electric discharges only when they have previously become "weathered" over with minute depositions of fatty and other organic matters; the spark burning these off along certain lines, which then behave differently from the other parts to moisture and to light. This doubtless explains some of the figures due to electricity, as does the supposition of Fizeau some of those occasioned by heat. The latter considers that most surfaces are slightly coated with fatty or organic matters, and that during proximity these are transferred in minute quantity to the receiving surfaces. It is known that mercurial vapor condenses in a manner visibly different on a surface already soiled or exposed to vapor, however slightly.
But there are instances that appear to be covered by neither of these suppositions, and which can only be explained by some actual change in the molecular constitution of bodies, affecting their subsequent behavior toward the physical forces. If we modify Karsten's experiment by placing eight or ten plates beneath the coin, and afterward mercurialize the upper surfaces of all the plates, the figures appear upon them all, but more faintly as the surfaces were further removed. Electrical discharges render evident impressions long apparently obliterated by polishing, thus showing that these could not have been superficial merely. The surfaces may be impressed in the dark, and without known change of temperature. Near a polished silver plate fix one of glass, painted black, with characters scratched through this coating, and expose to the sun for some days; or place a lattice-work before polished granite in the sun for half an hour; in either case the images can be afterward developed. The electric images are not easily obliterated by ordinary means of washing and rubbing.
Mr. Hunt, observing that black substances in case of heat leave the strongest impressions, applied this fact, in an art which he named ther-mography, to the copying of prints, cuts, writing, etc., the impression obtained on amalgamated copper being treated with mercury to develop the light, and with iodine for the dark spaces. The art in this form has not been practically introduced. Breguet, the celebrated Parisian watchmaker, found inscriptions on the inner case of a watch reproduced on the inner surface of the outer case; and engineers observe examples in which the near surfaces of parts of machines become visibly impressed the one on the other. Without doubt these are instances of mere transfer of material; and a sort of printing, due to such transfer during long contact, is obviously the explanation of such cases as those of the images which picture framers find impressed on glass or paper with which a print has been long in contiguity. Photographic negatives or positives sometimes produce latent impressions on paper, or through it on sensitive surfaces with which they are laid away; and a sensitized plate from which one picture had been apparently discharged, receiving a second, has had the two pictures then developed on the same field.
These and similar phenomena must be explained on simply chemical principles. Mr. C. A. Seely of New York has observed that a sheet of sensitive paper, having been enclosed between several folds of a printed circular, and left within a book, of course in the dark, for about a week, impressions became visible on both sides of the sheet, and the printing on both sides of two or more folds of the circular became superposed on the sensitive paper, that of more distant folds being sometimes the more distinct, and usually not on their own, but on the opposite side of the sensitive sheet. Mr. Grove observed peculiar spots on some trout, and placing freshly caught fish with a serrated leaf on each side in the sun, found that, after a while, that on the sunned side had impressed its image on the skin of the fish, while that in the dark had not. Grove experimented also by placing paper with letters cut in it between glass plates, making these with sheets of tin foil into a Leyden apparatus, and electrifying for a few seconds with a Ruhmkorff coil; he then breathed on the inner surfaces of the glass, and images of the letters appeared; or by exposure to hydrofluoric acid, these were permanently etched.
Pouring over a plate holding this latent image a film of iodized collodion, treating as for a photograph, and exposing to diffused daylight, another image, also insensible, was by the consequent action on light induced in the collodion film; and this being dried, removed, and submitted to developing agents, the insensible molecular change by which characters were impressed on the glass by electricity was finally rendered manifest by visibility of the image in the film. The number and variety of the ways known in which the luminous, actinic, and thermal rays, as well as electric perturbation and discharge, are capable of modifying the condition, and doubtless the molecular constitution of bodies, have been since the time of the announcements by Draper and Möser continually on the increase; until we are at length led to admit that many or all of these agents must modify molecularly all bodies subjected to their influence, and in turn their subsequent behavior to many of the physical forces. (See Fluorescence, Phosphorescence, and Photography.) Finally, it appears no longer necessary, with Möser, to ascribe these actions to latent light; nor with Herschel, to claim a peculiar heat or set of "parathermic rays".