Paper prepared with the sulphuret of barium, when under the solar spectrum, shows only one space of maximum luminous intensity, and the destroying rays are the same as in the sulphuret of calcium. Thus the obscure rays beyond the extreme violet produce light, while the luminous rays extinguish it.
The phosphoric spectrum has inactive lines which coincide with those in the luminous and chemical spectra, at least as far as it extends; but in order to be seen the spectrum must be received for a few seconds upon the prepared surface through an aperture in a dark room, then the aperture must be closed, and the temperature of the surface raised two or three hundred degrees; the phosphorescent parts then shine brilliantly and the dark lines appear black. Since the parts of similar refrangibility in different spectra are traversed by the same dark lines, rays of the same refrangibility are probably absorbed at the same time by the different media through which they pass.
It appears from the experiments of MM. Becquerel and Biot, that electrical disturbances produce these phosphorescent effects. There is thus a mysterious connexion between the most refrangible rays and electricity which the experiments of M. E. Becquerel confirm, showing that electricity is developed during chemical action by the violet rays, that it is feebly developed by the blue and indigo, but that none is excited by the less refrangible part of the spectrum.
A series of experiments by Sir John Herschel have disclosed a new set of obscure rays in the solar spectrum, which seem to bear the same relation to those of heat that the photographic or chemical rays bear to the luminous. They are situate in that part of the spectrum which is occupied by the less refrangible visible colours, and have been named by their discoverer Parathermic rays. It must be held in remembrance that the region of greatest heat in the solar spectrum lies in the dark space beyond the visible red. Now, Sir John Herschel found that in experiments with a solution of gum guaiacum in soda, which gives the paper a green colour, the green, yellow, orange, and red rays of the spectrum invariably discharged the colour, while no effect was produced by the extra-spectral rays of heat, which ought to have had the greatest effect had heat been the cause of the phenomenon. When an aqueous solution of chlorine was poured over a slip of paper prepared with gum guaiacum dissolved in soda, a colour varying from a deep somewhat greenish hue to a fine celestial blue was given to it; and, when the solar spectrum was thrown on the paper while moist, the colour was discharged from all the space under the less refrangible luminous rays, at the same time that the more distant thermic rays beyond the spectrum evaporated the moisture from the space on which they fell; so that the heat spots became apparent. But the spots disappeared as the paper dried, leaving the surface unchanged; while the photographic impression within the visible spectrum increased in intensity; the non-luminous thermic rays, though evidently active as to heat, were yet incapable of effecting that peculiar chemical change which other rays of much less heating power were all the time producing. Sir John having ascertained that an artificial heat from 180° to 280° of Fahrenheit changed the green tint of gum guaiacum to its original yellow hue when moist, but that it had no effect when dry, he therefore tried whether heat from a hot iron applied to the back of the paper used in the last-mentioned experiment while under the influence of the solar spectrum might not assist the action of the calorific rays; but, instead of doing so, it greatly accelerated the discoloration over the spaces occupied by the less refrangible rays, but had no effect on the extra-spectral region of maximum heat. Obscure terrestrial heat, therefore, is capable of assisting and being assisted in effecting this peculiar change by those rays of the spectrum, whether luminous or thermic, which occupy its red, yellow, and green regions; while, on the other hand, it receives no such assistance from the purely thermic rays beyond the spectrum acting under similar circumstances and in an equal state of condensation.
The conclusions drawn from these experiments are confirmed by that which follows: a photographic picture formed on paper prepared with a mixture of the solutions of ammonia-citrate of iron and ferro-sesquicyanite of potash in equal parts, then thrown into water and afterwards dried, will be blue and negative, that is to say, the lights and shadows will be the reverse of what they are in nature. If in this state the paper be washed with a solution of proto-nitrate of mercury, the picture will be discharged; but if it be well washed and dried, and a hot smoothing-iron passed over it, the picture instantly reappears, not blue, but brown; if kept some weeks in this state in perfect darkness between the leaves of a portfolio, it fades, and almost entirely vanishes, but a fresh application of heat restores it to its full original intensity. This curious change is not the effect of light, at least not of light alone. A certain temperature must be attained, and that suffices in total darkness; yet, on exposing to a very concentrated spectrum a slip of the paper used in the last experiment, after the uniform blue colour has been discharged and a white ground left, this whiteness is changed to brown over the whole region of the red and orange rays, but not beyond the luminous spectrum.
Sir John thence concludes:—1st. That it is the heat of these rays, not their light, which operates the change; 2ndly. That this heat possesses a peculiar chemical quality which is not possessed by the purely calorific rays outside of the visible spectrum, though far more intense; and, 3rdly. That the heat radiated from obscurely hot iron abounds especially in rays analogous to those of the region of the spectrum above indicated.
Another instance of these singular transformations may be noticed. The pictures formed on cyanotype paper rendered more sensitive by the addition of corrosive sublimate are blue on a white ground and positive, that is, the lights and shadows are the same as in nature, but, by the application of heat, the colour is changed from blue to brown, from positive to negative; even by keeping in darkness the blue colour is restored, as well as the positive character. Sir John attributes this, as in the former instance, to certain rays, which, regarded as rays of heat or light, or of some influence sui generis accompanying the red and orange rays of the spectrum, are also copiously emitted by bodies heated short of redness. He thinks it probable that these invisible parathermic rays are the rays which radiate from molecule to molecule in the interior of bodies, that they determine the discharge of vegetable colours at the boiling temperature, and also the innumerable atomic transformations of organic bodies which take place at the temperature below redness, that they are distinct from those of pure heat, and that they are sufficiently identified by these characters to become legitimate objects of scientific discussion.
The calorific and parathermic rays appear to be intimately connected with the discoveries of Messrs. Draper and Moser. Daguerre has shown that the action of light on the iodide of silver renders it capable of condensing the vapour of mercury which adheres to the parts affected by it. Professor Moser of Königsberg has proved that the same effect is produced by the simple contact of bodies, and even by their very near juxtaposition, and that in total darkness as well as in light. This discovery he announced in the following words:—“If a surface has been touched in any particular parts by any body, it acquires the property of precipitating all vapours, and these adhere to it or combine chemically with it on these spots differently from what they do on the untouched parts.” If we write on a plate of glass or any smooth surface whatever with blotting-paper, a brush, or anything else, and then clean it, the characters always reappear if the plate or surface be breathed upon, and the same effect may be produced even on the surface of mercury; nor is absolute contact necessary. If a screen cut in a pattern be held over a polished metallic surface at a small distance, and the whole breathed on, after the vapour has evaporated so that no trace is left on the surface, the pattern comes out when it is breathed on again.
Professor Moser proved that bodies exert a very decided influence upon each other, by placing coins, cut stones, pieces of horn, and other substances, for a short time on a warm metallic plate: when the substance was removed, no impression appeared on the plate till it was breathed upon or exposed to the vapour of mercury, and then these vapours adhered only to the parts where the substance had been placed, making distinct images, which in some cases were permanent after the vapour was removed. Similar impressions were obtained on glass and other substances even when the bodies were not in contact, and the results were the same whether the experiments were performed in light or in darkness.
Mr. Grove found, when plates of zinc and copper were closely approximated, but not in contact, and suddenly separated, that one was positively and the other negatively electric; whence he inferred that the intervening medium was either polarised, or that a radiation analogous, if not identical, with that which produces Moser’s images takes place from plate to plate.