428 RORIC FIGURES when two bodies are sufficiently near, they im- press 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 proper- ty of precipitating all vapors, these adhering to or combining with it on those spots differ- ently from what they do on others. Moser 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 ; breath- ing upon it again, the figures will reappear. To these appearances the name of Moser's im- ages has been given ; while some German writers term them Hauchfiguren, breath fig- ures; and Mr. Grove has designated them as " molecular impressions," a name truly express- ing 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 neces- sity 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, sil- ver, 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 dis- tance 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, 1 ' republished in New York, 1852. ) Karsten placed a medal on a gJass plate, resting on one of metal (a coin on a look- ing-glass coated with amalgam may be used), and allowed a few sparks from an electrical ma- chine to fall on the medal ; the image on the glass is brought out by vapor of mercury, io- dine, 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 plati- num foil clean enough to fire gases, or fresh mica surfaces obtained by splitting for the oc- casion, no figures appeared after applying the electric spark. Karsten concludes that sur- faces 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 oc- casioned 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 sup- positions, and which can only be explained by some actual change in the molecular con- stitution of bodies, affecting their subsequent behavior toward the physical forces. If wo 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 re- moved. 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 pol- ished 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 elec- tric images are not easily obliterated by ordi- nary means of washing and rubbing. Mr. Hunt, observing that black substances in case of heat leave the strongest impressions, ap- plied this fact, in an art which he named ther- mography, to the copying of prints, cuts, wri- ting, &c., the impression obtained on amalga- mated 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 celebra- ted Parisian watchmaker, found inscriptions on the inner case of a watch reproduced on the
