RAYS. 



-'irough a plate of glafs are proportionably more adapted 

 to pafs through a fccond plate ; and from this circumftance 

 '.ve deduce a new proof &i the peculiar ftate of thele rays, 

 and of the modification which they acquire. Mellrs. Gay- 

 Lufl'ac and Thcnard have proved that all the changes of 

 colour produced by light may he imitated and produced by 

 heat, and by an elevation of temperature not exceeding 2 1 2''. 

 Other phenomena previoudy obferved indicated, that in the 

 comparifon of the aftions of heat and light in heating 

 bodies, or producing chemical changes in them, there is :i 

 great difference in the rays of different colours. M. Rochon 

 announced the faft amply confirmed by Dr. Herfchel, tliat 

 the heat produced by the different rays of the prilmatic 

 Ipeftnmi was unequal. As Dr. Herfchel fixed the maximum 

 effedt beyond ihe red rays, and (hevi-ed, as we have already 

 ftated, that the moft heating rays of the fpeftrnm were en- 

 tirely, or nearly, invifible ; Dr. Wollalton, and Meifrs. Rit- 

 ter and Beckmann, having examined the oppofite, or violet 

 end of the fpeftrum, found that this likewile poflefled pe- 

 culiar properties, and that, beyond the violet, there are in- 

 vifible rays, which poiiefs, in greateft perfeflion, the power 

 yf determining chem.ical combinations. It has been a fub- 

 jeft of interelting refearch, whether the invifible, or almoll 

 mvifible rays, fituated beyond the extremities of the fpec- 

 trum, poflefs any other properties of light. li. gr. If the 

 refleftion of them from polilhed glafs can give them that 

 modification which Malus has dillinguifhed by the name of 

 " polarization." (See Light.) M. BerthoUet engaged 

 Meflrs. Malus and Berard to undertake this double objetl. 

 M. Malus's death prevented his profecution of the fubjeft ; 

 but what he began M. Berard accomplilhed with the utmofl. 

 pofTible exaftnefs. By means of the helioftate, which Malus 

 caufed to be conltrufted for the philofophical cabinet of M. 

 BerthoUet, he obtained a ray of light, perfeftly fixed, on 

 which he could make experiments at pleafure. By decom- 

 pofing this ray with a prifm, he obtained an immoveable 

 coloured fpeftrum ; and by placing very fenfible thermome- 

 ters in the fpaces occupied by the different colours, he was 

 enabled to compare their calorific effefts with the utmoft 

 certainty. He alfo afcertained their chemical properties by 

 fubftituting, in place of the thermometers, chemical com- 

 pounds eafily altered. He firft of all obferved the calorific 

 power of the different rays, which, it is known, are in this 

 refpeA unequal. M. Rochon, who is faid to have firft ob- 

 ferved this inequality, placed the maximum of heat in the 

 yellow ray, where the illuminating power is the greateft. 

 Dr. Herfchel, as we have before fhewn, placed it out of the 

 fpeftrum, and beyond the red ray. The experiments of 

 Herfchel have been confirmed by thofe of Berard, as far as 

 they refpeft the progreflive heating power of the rays from 

 the violet to the red ; but he found the greateft heating 

 power at the extremity of the fpedirum itfelf, and not be- 

 yond it. He fixed it at the point, where the bulb of the 

 thermometer was ftill entirely covered with the red ray ; and 

 he found that the thermometer funk progreffively, in pro- 

 portion as the diftance of its bulb from the red ray increafed. 

 When he placed the thermometer quite beyond the vifible 

 fpeftrum, on the Ipot where Herfchel fixed the maximum 

 of heat, its elevation above that of the ambient air was 

 only one-fifth of what it had been in the red ray itfelf. The 

 abfolute intenfity of the heat produced was likewife lefs in 

 the experiments of Berard than in thofe of Herfchel. 



M. Berard wifhad to know if thefe properties would exifl 

 in each of the pencils into which the ray divides Itfelf in 

 paffing through a rhomboid of Iceland fpar. In this cafe 

 each of the two fpeftra exhibited the fame properties. In 

 both, the calorific power diminifhed from the violet to the 



Vol. XXIX, 



red end ; and it exifted beyond the laft vifible red rays. In 

 this operation the luminous molecules arc polarized by the 

 cryftal. In order to determine whether the invifible rays of 

 heat experience the fame eft'eft, M. Berard received the folar 

 ray upon a poliftied and tranfp;irent glafs, which polarized a 

 portion of it by refleftion. This reflefted rav was then re- 

 ceived upoH a fecond glafs, fixed in an apparatus, which 

 permitted it to be turned i-ound the raj' under a conftant in- 

 cidence, and this incidence itfelf was determined in fuch a 

 manner, that in a certain pofition of the glafs the rcfleftion 

 ceafed to take place. We know, from the experiments of 

 Malus, that a glafs may be always difpoled in fuch a man- 

 ner that this condition is fulfilled. Things being thus dif- 

 pofed, by collcfting with a mirror the calorific and luminous 

 rays reflected from the fecond glafs, and directing them 

 upon a thermometer, M. Berard found, that as long as light 

 was reflefted the thermometer was elevated, and of courie 

 the heat was reflefted likewife ; but when, from the pofition 

 of the fecond glafs, the light was totally tranfmitted, the 

 heat was tranfmitted at the fame time, and the thermometer 

 was not elevated. In this experiment, then, as well as the 

 preceding, the calorific principle, whatever it may be, never 

 icparates from the luminous molecules. 



To the ray of folar light employed in this experiment, 

 M. Berard fnbftituted a pencil of radiant heat proceeding 

 from a body hot, but not red, and even not lum.inous. The 

 effeft was the fame as before. The thermometer rofe when 

 the fecond glafs was fo fituated as to refleft light, and it 

 did not rife when the fecond could not refledl light. There- 

 fore the particles of invifible radiant heat are modified by 

 refleftioii, precifely like light. 



After having ftudied the calorific properties of the dif- 

 ferent rays of the fpeftrum, M. Berard examined their che- 

 mical properties. Wlien muriate of filver, or other white 

 falts of filver, are expofed to light, tliey became dark co- 

 loured very fpeedily. Guaiac thus expofed to light paffes 

 from yellow to green, as Dr. Wollafton obferved. Gay- 

 LufTac and Thenard difcovered another aftion of this light 

 ftill more prompt and energetic. When a mixture of oxy- 

 muriatic acid gas and hydrogen gas are expofed to the aftion 

 of folar light, a detonation takes place, and water and mu- 

 riatic acid are formed. Thefe different phenomena enabiled 

 M. Berard to examine the chemical powers of the different 

 rays of the fpeftrum. By expofing to the different coloured 

 rays, pieces of card impregnated with muriate of filver, or 

 fmall phials filled with the detonating mixture, he was 

 enabled to judge of the energy of each by the intenfity or ra- 

 pidity of the chemical change which it produced. He found 

 that the chemical intenfity was greateft at the violet end of 

 the fpeftrum, and that it extended, as Ritter and Wollafton 

 had obferved, a little beyond that extremity. When he left 

 fubftances expofed for a certain time to the aftion of each 

 ray, he obferved fenfible eft'cfts, though with an intenfity 

 continually decreafing in the indigo and blue rays. Hence 

 we rauft confider it as extremely probable, that if he had 

 been able to employ re-a6tives ftill more fenfible, he would 

 have obferved analogous effefts, but ftill more feeble, even 

 in the other rays. ' To fhew clearly the great difproportion 

 which exifts in this refpeft between the energies of the dif- 

 ferent rays, M. Berard concentrated, by means of a lens, 

 all tliat part of the fpeftrum which extends from the greea 

 to the extreme violet ; and he concentrated, by means of 

 another lens, all that portion which extends frem the green 

 to the extremity of the red. This laft pencil formed a white 

 point fo brilliant that the eyes were fcarcely able to endure 

 it ; yet the muriate of filver remained more than two hours 

 expofed to this brilliant light without undergoing any fenfi- 

 SQ ble 



