Chap, a.] Analogy of Heat and Li git. 09 



the rays of light, had imbibed fcarcely any heat what- 

 ever. 



The only objection of any moment which has arifen 

 againft this doctrine is, that there exift certain bodies, 

 fuch as what are called the fokr phofphori, putrefcent 

 fubftances, and rotten wood, which emit or reflect 

 light, without apparently poffeffing the fmalleft quan- 

 tity of fenfible heat. If however we confider the ex- 

 treme weaknefs of the light which is emitted by thefe 

 fubftances, the objection will appear to have little 

 force. The moft concentrated moon-light, in the 

 focus of a concave mirror, is not more than the three 

 hundredth part of the intenfity of common funfhine * ; 

 and yet the light from thefe fubftances is not to bs 

 compared with that of the moon. Nay, the analogy 

 between heat and light receives confirmation from 

 thefe very fubftances ; for the property which they 

 poflefs of emitting light, is greatly increafed by an 

 accefllon of heat ; and even phofphori, in which the 

 light has for fome fpace of time been dormant, or in 

 which it is apparently exhaufted, will emit light upon 

 the application of heat alone f. 



I conceive fire therefore, or caloric, as termed by 

 the French chemifts, to be the elementary principle 

 or caufe of heat and light. Caloric in a difengaged 

 ftate, or in the act of paffing from one body to another, 

 imprerTes our organs with the fenfe of heat ; and in a 

 rarefied and projectile ftate, it probably conftitutes the 

 matter of light. Confiftently with thefe principles, tj^e 

 fun may be confidered as the great fource of fire, 

 whence it is diftributed to all the different bodies in 

 our folar fyitem. On the fame ground alfo, cold is 



* See a note by the ingenious translator of Fourcroy's Leftures, 

 Vol.1, p. 123. 



f Priefiley's Optics, part iv. f. i. 



H a univerfally 



