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.
