Chap. 2.] Nature of Light. 17$ 



folution by confidering the extreme rarity of light, and 

 the minutenefs of its particles. 



It is, therefore, almott univerfally agreed by the 

 moderns, that light confifts of a number of extremely 

 minute particles, which are actually projected from the 

 luminous body, and aft by their projectile force upon 

 our optic nerve. Concerning the nature of thefe par- 

 ticks, or rather of the matter of which they confift, 

 there is lefs unanimity in the philofophical world. 



It is an opinion fupported by the mod refpectable 

 names, that light is a fubftance perfectly diftinct from 

 the matter of fire, and which excites ignition when 

 concentrated by a burning glafs, merely by its mecha- 

 nical force upon the matter of heat. Others of equal 

 eminence have contended, that light is no other than 

 fire in a projectile date. Fire, according to thefe phi- 

 lofdphers, is produced by the accumulation or con- 

 centration of the particles j light is the effect of the 

 rapid projectile motion of the fame particles. Fire 

 or heat may therefore exiit to a considerable degree, 

 as it is found to do occafionally in metallic bars, with- 

 out being fufficiently difengaged to afiurne the projec- 

 tile (late, and to' be forcibly emitted or projected from 

 the burning body. The fame matter may alfo exiil 

 in its active and projectile itate, or, in other words, ia 

 the form of light, but too much diffufed to produce 

 the fen fation of burning, or to effect the diffclutton of 

 any body, or the reparation of its parts by combuftion. 

 To the great elafticity of the matter of fire, fo obvious 

 in all the phenomena of fluidity, both thefe effects are 

 afcribed. When a quantity of this matter is intro- 

 duced into any folid body, the repulfion which exifts 

 between its particles will occafion the difiblution of 

 that body ; and when it becomes perfectly free and dif- 

 engaged, the fame repuHion will caufe a quantity of 



its 



