Reif>V.rks on the Enquiries of Dr. Herfchel refpeSing Light and Heat. 4 17 



Of twenty experiments which are related in detail not fewer than fixteen have no dire£k 

 connexion with the queftion agitated. They refer merely to the heat occafioned by the 

 rays of light from the fun, a candle, a fire, or a red-hot poker, when condenfed in the focus 

 of a lens or fpeculum ; or to the heat collefted by convergent reflexion from a heated maf* 

 or from the vicinity of a laminated fubftancc. In all this, I can difcern nothing either new 

 or ftriking. But the Doftor thinks fit or convenient to fubftitute the word heat for light } 

 and that eafy change operates like a magical charm. Whatever property has been attri- 

 buted to the rays of light, belongs henceforth exclufively to the " invifible rays of heat," 

 and thus comes direflly in fupport of his darling hypothefis. Such a mode of reafoning, 

 or rather aflumption, hardly deferves any ferious refutation. Dut the Do<Stor feems quite 

 tranfported with the difcovery. In the eleventh experiment, entitled, " On the Refradtion 

 of Solar Heat," a large Newtonian telefcope, with a compound eye-piece, was directed to 

 the fun, and by the concentration of the broad beam of light in the focus, a very confider- 

 able heat, as every body knows, was produced. The philofopher (lops, as ufual, to wonder , 

 ''Mow artful/y," he exclaims, " in our prefent inftance, was heat fent from one place to 

 " another 1" And, borrowing the language of Newmarket, he continues : — " Heat eroding 

 ** heat, through many interfec^ing courfea without joftling together, and each parcel arriving 

 ** at laft fafely to its deftined place." The grand conclufion is, that " it cannot be doubted 

 ** that the rays of heat arc fubje£l to the laws of refraction." In the whole of this ftrange 

 paflage, perhaps the only thing which (hould excite furprife in a fober mind, is the glaring 

 confufion of ideas. 



The 7th and 8th experiments, which undertake to " reflcft and condenfe the invifiWc 

 *• folar rays," have nothing remarkable but their title. I formerly fliowed, I hope in a con» 

 Vincing manner, that thofe imaginary invifible folar rays were merely the warm portions of 

 air which environ an illuminated body. Of courfe, this heat may, as in other cafes, Wc 

 coUedlcd by refle£tion. It is not my prefent defign to difcufs the nature and propagation o£ 

 heat. But I cannot forbear mentioning a fingle argument, which, if I miftake not, is de- 

 cifive againft the adherents of Radiant Heat. No fadt is better known than that the rays 

 of light, in traverfing an uniform medium, are not in the fmalleft degree afFeded by its 

 flaoft violent agitation, but purfue unvaried their redlilineal courfe. The cafe is very dif- 

 ferent with what is termed radiant heat. Tht experiments on which it refts fucceed only 

 In a clofe room, and at very moderate diftaoces from the fource of heat. It is clear, there- 

 fore, that the heating matter muft flow with fuch feeble impulfe as to experience obftruftion 

 in its paflage through the air, and fuffer much derangement from the accidental motions of 

 that fluid. If this fuppofcd radiant heat darted with a celerity in any degree comparatle to 

 that of light, Bay even to the velocity of thofe projectiles v/ith which we are acquainted,— 

 it would be exactly dire6led and concentrated in open air, and that at diftances limited 

 ©nly by the unavoidable imperfeftion in the figure of the reflefting furface. Hence, the 

 heating matter, whatever it really is, muft have a grofl"nefs of conftitution, and a flownefs 

 «f progrefs, commenfurate with the denfenefs and ordinary m^ility of our atmofphere. 



Vol. IV.— December 1800. 3 H Tire 



