RAYS. 



of heat ought to bo accompanied with the fame quantity of 

 Jighc ; whereas it is found, that ftveral lubftanccs emit a 

 confiderable quantity of hght without any fenfible heat, 

 and others give out a confidcrable portion of heat witliout 

 any hght. But that thefc are two diftinit powers of nature 

 is a propofition, that has been amply illuttrated, and moil 

 fatisfadtorily eltabUihed by the difcovtries and experiments 

 of Dr. Herfchel. This cek^brated aftronomer cautions 

 thofe, wlio perufe the account of his experiments and obfer- 

 vations upon them, from conchiding, that in ufing the word 

 rays, he means to oppole, much lefs to couritiuance, the 

 opinion of thofe pliilofophers, who ilill beheve, that hght 

 jtlelf comes to us from the lun, not by rays, but by tlie 

 fuppofed vibration of an elailic ether, every wliere difiufed 

 through fpace : lie merely claims the fanii' privilege for 

 the rays that occur in heat, which they do not fcruple to 

 allow to thofe that illuminate objetts. For in what man- 

 ner foever this radiance may be cffedled, he undertakes to 

 prove, that the evidence, either for rays, or for vibrations, 

 which occafion heat, ftands on the fame foundation on 

 which the radiance of the illuminating principle, light, is 

 built. 



Propofing to give a comparative view of the operations 

 that may be performed on the rays that occafion heat, 

 and of thofe which arc known to have been effected 

 on the rays that occafion light, he felefts fuch fafts 

 as are well known with regard to the latter. Light, 

 he fays, both folar and terreftrial, is a fenfation oc- 

 cafioned by rays emanating from luminous bodies, which 

 have a power of ilhimiuiiting objetts ; and, according 

 to circumfl-ances, of making them to appear of various 

 colours. The rays of light are fubjeft to the laws both of 

 reflection and of refrattion ; but they are of different re- 

 frangibihty. They are liable to be itopped in certain pro- 

 portions, when tranfmitted through diaphanous bodies ; 

 and alfo to be fcattered on rough furfaces. They are alfo 

 fuppofed to have a power of iieating bodies ; but this is a 

 fubjedt " fub judice." The fimilar propofitions, which it 

 is his defign to prove, and which by a variety of curious 

 and well- condufted experiments he has confirmed, are fuch 

 as follow. Heat, both folar and terreftrial, is a fenfatiem 

 occafioned by rays emanating from candent fubftances, 

 which have a power of heating bodies. Thefe rays are 

 fubjeft to the laws of reileftion and of refraction ; and they 

 are of different refrangibihty. They are liable to be 

 '.topped, in certain proportions, when tranfmitted through 

 diaphanous bodies ; and to be fcattered on rough furfaces : 

 and in a certain ftate of energy, they may be fuppofed to 

 have a power of illuminating objefts, which latter pro- 

 perty remains to be examined. 



In the examination of the illuminating and heating power 

 of the prifmatic colours, our author obferves, that the red- 

 making rays are very far from hivmg the former in an eminent 

 degree : the orange pofleffes more of it than the red ; and 

 the yellow rays illuminate objetts ftill more perfedlly. The 

 maximum of illumination lies in the brightell yellow, or 

 paled irreen. The green itfelf is nearly equally bright with 

 the yellow ; but from the full deep green, the illuminating 

 power decreafes very fenfjbly ; that of the blue is nearly 

 upon a par with that of the red ; the indigo has much lefs 

 than the blue ; and the violet is very deficient. Our au- 

 thor infers from other experiments, that the heating power 

 of the prifmatic colours is very far from being equally dif- 

 tributcd ; and that the red rays are chiefly eminent in this 

 refpect. Allowing that the power of heating is chiefly 

 lodged in the red-making rays, it accounts for the com- 

 fortable warmth that is thrown out from a fire, when it is 



in the Hate of a red glow, and for the heat which is given 

 by charcoal, coke, and balls of fmall coal mixed up with 

 clay, uled in hot-houfes ; all which throw out red light. 

 It alfo explains the reafon, why the yellow, green, blue, 

 and purple flames of burning fpirits mixed with fait, occa- 

 fion fo little heat that a hand is not materially injured, 

 when paficd through their corulcations. 



Having afcertaincd, that radiant heat is fubjeit not only 

 to the laws of refradlion, but to thofe alfo of the different 

 refrangibihty of light. Dr. Herfchel is led to furmife, that 

 this heat confills of particles of light of a certain range of 

 momenta, which range may extend a little farther, on each 

 fide of refrangibihty, than that of light. In a gradual cx- 

 pofure of the thermometer to the rays of the prifmatic 

 fpeftrum, beginning from the violet, he found that he arrived 

 at the maximum of light h)ng before he came to that of 

 heat, which lies at the other extreme. By fcveral experi- 

 ments it appears, that the maximum of illumination has 

 little more than half the heat of the full red rays ; and 

 from other experiments our author concludes, that the full 

 red falls flill ihort of the maximum of heat, which perhaps 

 lies even a little beyond vifible refraction. In this cafe, 

 radiant heat will at leaft partly, if not chiefly, confift, if 

 the expreffion may be allowed, of invifible light, that is, of 

 rays coming from the fun, that have fuch a momentum as 

 to be unfit for vifion. Admitting it as highly probable, 

 that the organs of fight are only adapted to receive im- 

 preflions from particles of a certain momentum, this will 

 explain why the maximum of illumination fliould be in the 

 middle of the reirangible rays, as thole whicli have greater 

 or lefs momenta, are likely to become equally unfit for 

 irapreffions of fight. Whereas, in radiant heat, there may 

 be no fuch limitation to the momentum of its particles. 

 From the powerful effefts of a burning lens, however, wc 

 derive information, that the momentum of terreftrial radiant 

 heat is not likely to exceed that of the fun ; and that, con- 

 fequently, the refrangibihty of calorijic rays cannot extend 

 much beyond that of colomific light. Hence we may alfo 

 infer, that the invifible heat ot red-hot iron, gradually 

 cooled till it ceafes to (hine, has the momentum of the in- 

 vifible rays, which, in the folar fpedtrum viewed by day- 

 light, go to the confines of red, and this will afford an 

 eafy folutioii of the rcflecition of invifible heat by concave 

 mirrors. 



Our author deduces from various experiments with the 

 thermometer abundant evidence, that there are rays coming 

 from the fuu, which are lefs refrangible than any of thofe 

 that affeft the fight. They are invefted with a high power 

 of heating bodies, but with none of illuminating objefts ; 

 and on this account they have hitherto efL-aped notice. At 

 the diftance of fifty-two inches from the prifm, there was 

 fl:ill a confiderable heating power exerted by the invifible 

 rays, an inch and a half beyond the red ones, meafured 

 upon their projeftion on a horizontal plane. Moreover, the 

 power of heating is extended to the utmoft limits of the 

 vifible viwlet rays, but not beyond them ; and it is gradually 

 impaired, as the rays become more refrangible. The maxi- 

 mum of the heating power is found to be vefted among 

 the invifible rays, and it is probably not lefs than half an 

 inch beyond the laft vitible ones. It is alfo {hewn, that 

 the fun's invifible rays, in their lefs refrangible ilate, and 

 confiderably beyond the maximum, ftill exert a heating 

 power fully equal to that of red-coloured light ; and con- 

 fequentl^ , if we may infer the quantity of tlic efficient from 

 the effeft produced, the invifible rays of the fun probably 

 far exceed the vitible ones in number. The general conclu- 

 fion with which Dr. Herfchel clofcs his account of one feries 



of 



