to Heat and lUumlnate Ohje^s. 3^3 



coal mixed up with clay, ufed ia hot-houfes ; all which, it is 

 ^vell known, throw out rtd light. It alio expL;ins the rea- 

 fon why the yellow, green, hlue, and purple flames of 

 burning fpirits mixed with fait, occafion fo little heat, that a 

 hand is not materially injured when paflTed through their cor 

 rufcations. If the cliemical properties of colours alfo, when 

 aCcertaintd, (hould be fuch that an acid principle, for in- 

 llancc, which has heen afcribed to light in general, on ac- 

 count of its changing the complexion of various fubftances 

 expofed to it, may refide only in one of the colours, while 

 others may prove to be diifereiuly invefted, it will follow 

 that bodies may be varioufly aflefted by light, according as 

 they imbibe and retain, or tranfmit and reflect, the different 

 colours of which it is compofcd. 



Rad:a?it Heal is of different RefrangibUity. 



I mufl: now remark that my foregoing experiments afcer- 

 tain, bevond a doubt, that radiant heat, as well as light, 

 whether they be the fame of different agents, is not only re^ 

 frangible, but is alfo fubjedl to the laws of the difperfiou 

 arifing from its different rcfrangibility ; and, as this fubjeft is 

 new, I may be permitted to dwell a few moments upon it. 

 The prifm refratts radiant heat, fo as to fcparate that which 

 is lefs eflicacious from that which is more fo. The whole 

 quantity of radiant heat contained in a fun-beam, if this 

 different rcfrangibility did not exift, mu(i: inevitably fall uni- 

 formly on a fpace equal to the area of the prifm; and, if 

 radiant heat were not refrangible at all, it would iall upou 

 an equal fpace in the place where the fliadow of the prifm, 

 when covered, may be feen. But neither of thefe events 

 taking place, it is evident that radiant heat is fubjeft to the 

 laws of rcfraftion, ajid alfo to thofe of the different rcfran- 

 gibility of lisht. May not this lead us to furmifc that ra- 

 diant heat confifts qf particles of light of a certain range of 

 momenta, and which range may extend a httle further, oi; 

 each fide of rcfrangibility, than that of light? We have 

 fliown that, in a gradual expoiure of the thernior>ictcr to the 

 rays of the prifmatic fpcclrum, boginnino; from the viol(;t, we 

 (;onie to the; maximum of light long before we come to that 

 T t ^ of 



