LIGHT. 



to be connected with the absorption of silex they contain : and two pieces of borax 

 oxygen. Now, in the most refrangible have the same property much more re- 

 rays, which would fall beyond the riolet markably. 



extremity, he found thai this substance Light is disengaged in various cases of 

 became green, and was again changed to chemical combination. Whenever com- 

 yellow by the least refrangible. This is bustion is a part of the phenomena, this is 

 precisely the reverse of what happens to well known to happen ; but light is evolv- 

 muriate of silver, which is blackened, or ed also, in other instances, where nothing 

 de-oxydized, by the most refrangible ; and like combustion goes forwards. Thus, 

 has its colour restored, or is again oxy- fresh -prepared pure magnesia, added sud- 

 geniz.ed, in the least refrangible rays. denly to highly concentrated sulphuric 



Certain bodies have the property of ab- acid, exhibits a red heat, 

 sorbing the rays of light in their totality, Whence comes the light afforded by 

 of retaining them for some time, and of ignited bodies ? whether it have been pre- 

 aga'm evolving them unchanged, and viously imbibed by them ? whether the 

 unaccompanied by sensible heat. Thus, commencement of ignition be distinctive 

 in an experiment of Du Fay, a diamond of the same temperature in all bodies ? 

 exposed to the sun, and immediately co- whether the great planetary sources of 

 vered with black wax, shone in the dark, fight be bodies in a state of combustion, 

 on removing the wax, at the expiration or merely luminous upon principles very 

 of several months. Bodies possessing different from any which our experiments 

 this property, are called solar pkosphori : can point out ? whether the momentum 

 such are the Canton's, Baldwin's, Horn- of the particles of light, or their disposi- 

 berge's, and the Bolognian phosphori. tion for chemical combination, be the 

 To the same class belong several natural most effectual in the changes produced 

 bodies which retain light, and give it out by its agency ? these, and numerous 

 unchanged. Thus, snow is a natural so- other interesting questions, must be left 

 lar phosphorus. So also is, occasionally, for future research and investigation. See 

 the sea when agitated; putrid fish have COMBUSTION. 



a similar property ; and the glow-worm The production of light by inflammation 

 belongs to the same class. These pheno- is an object of great importance to socie- 

 mena are independant of every thing like ty at large, as well as to the chemist. It 

 combustion ; for artificial phosphori, after appears to arise immediately from the 

 exposure to the sun's rays, shine in the strong ignition of a body while rapidly 

 dark when placed in the vacuum of an decomposing. Most solid bodies in corn- 

 air-pump, or under water, &c. where no bustion are kept, partly from a want of 

 air is present to effect combustion. the access of air, and partly from the vi- 



From solar phosphori, the extrication of cinity of conducting bodies, at a low de- 

 light is facilitated by the application of an gree of ignition. But when vapours ra- 

 elevated temperature ; and, after having pidly escape into the air, it may, and does 

 ceased to shine at the ordinary tempera- frequently happen, that the combustion, 

 ture, they again emit light when exposed instead of being carried on merely at the 

 to an increase of heat. Several bodies, surface of the mass, penetrates to a consi- 

 xvhich do not otherwise give out light, derable depth within, and from this, as 

 evolve it, or become phosphorescent when well as from the imperfect conducting 

 heated. Thus powdered fluate of lime power of the surrounding air, a white 

 becomes luminous when thrown on an heat, or very strong ignition, is produced, 

 iron plate, raised to a temperature rather The effect of lamp's and candles depends 

 above that of boiling water. The yolk of upon these considerations. A combusti- 

 an egg, when dried, becomes luminous ble fluid, most commonly of the nature of 

 on being heated ; and so also does tallow fat oil, is put in a situation to be absorb- 

 during liquefaction. To exhibit the ed between the filaments of cotton, linen, 

 last mentioned fact, it is merely necessa- fine wire, or asbestos. The extremity of 

 ry to place a lump of tallow on a coal, this fibrous substance, called the wick, is 

 heated below ignition, making the experi- then considerably heated. The oil evapo- 

 rnent in a dark room. Attrition also rates, and its vapour takes fire. In this 

 evolves light, in many instances, by the situation the wick, being enveloped with 

 part rubbed becoming ignited. Thus, flame, is kept at such a temperature, that 

 rock crystal, and other hard stones, shine the oil continually boils, is evaporated, 

 when rubbed against each other; and two burns, and by these means keeps up a 

 pieces of common bonnet cane, rubbed constant flame. Much of the perfection 

 strongly against each other in the dark, of this experiment depends on the nature, 

 emit a taint light; most probably from the quantities, and figure of the materials 



