Mr. Hunt has shown that many of these phenomena depend on difference of temperature, and that, in order to obtain good impressions, dissimilar metals must be used. For example, gold, silver, bronze, and copper coins were placed on a plate of copper too hot to be touched, and allowed to remain till the plate cooled: all the coins had made an impression, the distinctness and intensity of which were in the order of the metals named. When the plate was exposed to the vapour of mercury the result was the same, but, when the vapour was wiped off, the gold and silver coins only had left permanent images on the copper. These impressions are often minutely perfect, whether the coins are in actual contact with the plate or one-eighth of an inch above it. The mass of the metal has a material influence on the result; a large copper coin makes a better impression on a copper plate than a small silver coin. When coins of different metals are placed on the same plate they interfere with each other.
When, instead of being heated, the copper plate was cooled by a freezing mixture, and bad conductors of heat laid upon it, as wood, paper, glass, &c., the result was similar.
Mr. Hunt, observing that a black substance leaves a stronger impression on a metallic surface than a white, applied the property to the art of copying prints, woodcuts, writing, and printing, on copper amalgamated on one surface and highly polished, merely by placing the object to be copied smoothly on the metal, and pressing it into close contact by a plate of glass: after some hours the plate is subjected to the vapour of mercury, and afterwards to that of iodine, when a black and accurate impression of the object comes out on a grey ground. Effects similar to those attributed to heat may also be produced by electricity. Mr. Karsten, by placing a glass plate upon one of metal, and on the glass plate a medal subjected to discharges of electricity, found a perfect image of the medal impressed on the glass, which could be brought into evidence by either mercury or iodine; and, when several plates of glass were interposed between the medal and the metallic plate, each plate of glass received an image on its upper surface after the passage of electrical discharges. These discharges have the remarkable power of restoring impressions that have been long obliterated from plates by polishing—a proof that the disturbances upon which these phenomena depend are not confined to the surface of the metals, but that a very decided molecular change has taken place to a considerable depth. Mr. Hunt’s experiments prove that the electro-negative metals make the most decided images upon electro-negative plates, and vice versâ. M. Matteucci has shown that a discharge of electricity does not visibly affect a polished silver plate, but that it produces an alteration which renders it capable of condensing vapour.
The impression of an engraving was made by laying it face downwards on a silver plate iodized, and placing an amalgamated copper plate upon it; it was left in darkness fifteen hours, during which time an impression of the engraving had been made on the amalgamated plate through the paper.
An iodized silver plate was placed in darkness with a coil of string laid on it, and with a polished silver plate suspended one-eighth of an inch above it: after four hours they were exposed to the vapours of mercury, which became uniformly deposited on the iodized plate, but on the silver one there was a sharp image of the string, so that this image was formed in the dark, and even without contact. Coins or other objects leave their impressions in the same manner with perfect sharpness and accuracy, when brought out by vapour without contact, in darkness, and on simple metals.
Red and orange coloured media, smoked glass, and all bodies that transmit or absorb the hot rays freely, leave strong impressions on a plate of copper, whether they be in contact or one-eighth of an inch above it. Heat must be concerned in this, for a solar spectrum concentrated by a lens was thrown on a polished plate of copper, and kept on the same spot by a heliostat for two or three hours: when exposed to mercurial vapour, a film of the vapour covered the plate where the diffused light which always accompanies the solar spectrum had fallen. On the obscure space occupied by the maximum heating power of Sir William Herschel, and also on the great heat spot in the thermic spectrum of Sir John Herschel, the condensation of the mercury was so thick that it stood out a distinct white spot on the plate, while over the whole space that had been under the visible spectrum the quantity of vapour was much less than that which covered the other parts, affording distinct evidence of a negative effect in the luminous spectrum and of the power of the hot rays, which is not always confined to the surface of the metal, since in many instances the impressions penetrated to a considerable depth below it, and consequently were permanent.
Several of these singular effects appear to be owing to the mutual action of molecules in contact while in a different state, whether of electricity or temperature: others clearly point at some unknown influence exerted between surfaces at a distance, and affecting their molecular structure: possibly it may be the parathermic rays, which have a peculiar chemical action even in total darkness. In the last experiment the effect is certainly produced by the positive portion of one of those remarkable antagonist principles which characterise the solar spectrum.
Thus it appears that the prism resolves the pure white sunbeam into three superposed spectra, each varying in refrangibility and intensity throughout its whole length; the visible part is overlapped at one end by the chemical or photographic rays, and at the other by the thermic, but the two latter so much exceed the visible part, that the linear dimensions of the three—the luminous, thermic, and photographic—are in proportion to the numbers 25, 42·10, and 55·10, so that the whole solar spectrum is twice as long as its visible part. The two extremities exert a decided antagonist energy. The least refrangible luminous rays obliterate the action of the photographic rays, while the latter produce phosphorescent light, which is extinguished by the least refrangible luminous rays. According to Mr. Hunt’s experiment, the hot rays condense mercurial vapour on a polished metallic plate, while the luminous rays prevent its formation. Electricity is excited by the chemical rays, while the parathermic are found in the less refrangible rays alone. Each of the spectra is crossed by coloured and rayless lines peculiar to itself, and these are traversed at right angles by innumerable dark lines of various breadths, the whole forming an inexpressibly wonderful and glorious creation.
The arrangement varies a little according to the material of the prism and the manner of producing the spectrum, as in that obtained by Professor Draper from diffracted light. It was formed by a beam diffracted by passing through a netting of fine wire, or by reflection from a polished surface of steel, having fine parallel lines drawn on it. This diffracted spectrum is divided into two equal parts in the centre of the yellow; and as in the prismatic spectrum, one half is antagonist to the other half, the red or negative end undoing what the positive or violet end has done. The centre of the yellow is the hottest part, and the heat decreases to both extremities. A line of cold is supposed to exist on this spectrum answering to Fraunhofer’s dark line H.
The undulations of the ethereal medium which constitute a sunbeam must be infinitely varied, each influence having a vibration peculiar to itself. Those of light are certainly transverse to the direction of the ray; while Professor Draper believes that those of heat are normal, that is, in the direction of the ray, like those of sound. A doubt exists whether the vibrations of polarised light are perpendicular to the plane of polarisation or in that plane. Professor Stokes of Cambridge has come to the conclusion, both from the diffracted spectrum and theory, that they are perpendicular to the plane of polarisation, but M. Holtzmann is of opinion that they are in that plane, so the subject is still open to discussion.