The heating power of the rays follows a different order. If the bulb of a sensi ble thermometer be moved, in succession, through the differently coloured rays, it will be found to indicate the greatest heat in the red rays, next in the green, and so on, in a diminishing progression, to the violet. When the thermometer is re moved entirely out of the confines of the red rays, but with its ball still in the line of the spectrum, it rises even higher than in the red rays ; and continues to rise, till removed half an inch beyond the ex. tremity of the red ray. The ball of the thermometer employed for this purpose should he extremely small, and should be blackened with Indian ink. An air thermometer is better adapted than a mercurial one, to exhibit the minute change of temperature that ensues. These invisible heat-making rays may be reflect ed by the mirror, and refracted by the lens, exactly in the same manner as the rays of light.
Beyond the confines of the spectrum on the other side, viz. a little beyond the vio let ray, the thermometer is not affected ; but in this place it is remarkable, that there are also invisible rays of a different kind, which exert all the chemical effects of the rays of light, and even with greater energy. One of the chemical properties of light is, that it speedily changes from white to black the fresh precipitated muriate of silver. This effect is pro duced most rapidly by the direct light of the sun ; and the rays, as separated by the prism, have this property in various degrees. The blue rays, tor example, effect a change of the muriate of silver in fifteen seconds, which the red require twenty minutes to accomplish ; and, ge nerally speaking, the power diminishes as we recede from the violet extremity. But entirely out of the spectrum, and beyond the violet rays, the effect is still pro duced. Hence it appears that the solar beams consist of three distinct kinds of rays ; of those that excite heat, and pro mote oxydation ; of illuminating rays ; and of de.oxydizing rays. A striking illustration of the different power of these various rays, is furnished by their effect on phosphorus. In the rays beyond the red extremity, phosphorus is heated, smokes, and emits white fumes ; but these are presently suppressed on expos ing it to the de-oxydizing rays which lie beyond the violet extremity.
There is an exception, however, as stated by Dr. Wollaston, to the de-oxydiz ing power of the rays above-mentioned. The substance, termed gum•guiacurn, has the property, when exposed to the. light, of changing from a yellowish colour to green ; and this effect lie has ascertained to be connected with the absorption of : oxygen. Now, in the most refrangible rays, which would [ill beyond the violet extremity, he found that this substance became green, and was again changed to yellow by the least refrangible. This is precisely the reverse of what happens to muriate of silver, which is blackened, or de-oxydized, by the most refrangible ; and has its colour restored, or is again oxy genized, in the least refrangible rays.
Certain bodies have the property of ab sorbing the rays of light in their totality, of retaining them For some time, and of again evolving them unchanged, and unaccompanied by sensible heat. Thus,
in an experiment of Du Fay, a diamond exposed to the sun, and immediately co vered with black wax, shone in the dark, on removing the wax, at the expiration of several months. Bodies possessing this property, are called solar phosphori such are the Canton's, Baldwin's, Horn berge's, and the Bolognian phosphori. To the same class belong several natural bodies which retain light, and give it out unchanged. Thus, snow is a natural so lar phosphorus. So also is, occasionally, the sea when agitated ; putrid fish have a similar property ; and the glow-worm belongs to the same class. These pheno mena are independant of every thing like combustion ; for artificial phosphori, after exposure to the sun's rays, shine in the dark when placed in the vacuum of an air-pump, or under water, 8:.c. where no air present to effect combustion.
From solar phosphori, the extrication of light is facilitated by the application of an elevated temperature ; and, after having ceased to shine at the ordinary tempera ture, they again emit light when exposed to an increase of heat. Several bodies, which do not otherwise give out light, evolve it, or become phosphorescent when heated. Thus powdered fluate of lime becomes luminous when thrown on an iron plate, raised to a temperature rather above that of boiling water. The yolk of an egg, when dried, becomes luminous on being heated; and so also does tallow during liquefaction. To exhibit the last mentioned fact, it is merely necessa ry to place a lump of tallow on a coal, heated below ignition, making the experi ment in a dark room. Attrition also evolves light, in many instances, by the part rubbed becoming ignited. Thus, rock crystal, and other hard stones, shine when rubbed against each other; and two pieces of common bonnet cane, rubbed strongly against each other in the dark, cant a faint light ; most probably from the silex they contain : and two pieces of borax have the same property much more re Light is disengaged in various cases of chemical combination. Whenever com bustion is a part of the phenomena, this is well known to happen ; but light is evolv ed also, in other instances, where nothing like combustion goes forwards. Thus, fresh preparedpure magnesia, added sud denly to highly concentrated sulphuric acid, exhibits a red heat.
Whence comes the light afforded by ignited bodies ? whether it have been pre viously- imbibed by them ? whether the commencement of ignition be distinctive of the same temperature in all bodies ? whether the great planetary sources of fight be bodies in a state of combustion, or merely luminous upon principles very different from any which our experiments can point out ? whether the momentum of the particles of light, or their disposi tion for chemical combination, be the most effectual in the changes produced by its agency ?—these, and numerous other interesting questions, must be left for future research and investigation. Sec