CAL

CAL

( iso )

are only to be confidered comparatively. Sopalpable however is the difference of heat be-tween even the red part of the fpectrum and thecontiguous dark fide, that on (hutting the eyesand moving the hand through the ipeCtrumparallel to the prifmatic axis, a fenfible encreafeof temperature will without difficulty guide itto the point of greateft heat beyond the redfide of the fpeCtrum.

The rays of heat are of a much more extenfiverefrangibility than thofe of light. This is im-mediately (hewn by the circumftance that inthe folar prifmatic fpectrum there are no raysthat do not heat, whereas the rays of light fallfar (hort of the extreme point of the calorificrays, and even, as juft mentioned, fall (hort ofthe higheft heating point. Dr. Herfchel efti-mates the bafe of the fpeCtrum of heat to be tothat of the fpeCtrum of light as about 5-3- to 3,but the degree of difperfion of the heat-makingrays differs fo entirely from that of the rays oflight, that neither do the mean nor the higheftpoints of refrangibility agree, nor is there anyobvious relation between the area of heat andthat of illumination.

Radiant caloric appears to poffefs the fameproperties, and to be propagated according tothe fame laws whether tranfmitted from the funor from any other heated body, and whether ornot it be accompanied by light. Thus the heatradiating from boiling water is reflected, re-fracted, and otherwife propelled precifely in thefame manner as folar heat. The following im-portant properties the rays of heat poffefs incommon with thofe of light. d

They are fubject to the laws of reflection.

They are fubjeCt to the laws of refraction.

They are of different refrangibility.

They are liable to be (topped in certain pro-portions when tranfmitted through diaphanousbodies.

They ard liable to be fcattered 011 rough fur-faces.

Some of thefe properties require further illuf-tration.

The concentration of heat produced by thereflection of the fun’s rays from a concave mirroris too well known to need further defeription.It proves fatisfaCtorily that heat when in con-junction with light, is, like light, capable ofbeing reflected. The rays proceeding from acandle, ignited iron, and any other fubftance inluminous combuftion are alfo well known to becapable of being reflected and concentratedinto a focus by a mirror, But it is only com-

4 licrfchcl.

paratively of modern date that the invifible raysof heat have been found to obey the fame lawof reflection, from whatever heated body theradiations proceed. The reflection of invifiblefolar heat was afeertained by Dr. Herfchel, byprefenting to a plain mirror the part of the folarprifmatic fpeCtrum where the heat was the great-eft (that is beyond the extremity of the luminousfpeCtrum on the red fide) and with proper pre-cautions caufing the reflected rays of heat tofall on a thermometer. By this, in ten minutes,four degrees encreafe of heat was indicated^whilft another very delicate thermometer by thefide of the firft, but juft out of the. line of re-flection, remained all the time quite ftationary.Hence the reflection of the invifible folar rays isproved.

But it was from experiments with culinaryor artificial fire, that this molt curious propertyof the reflection of radiant heat was firft afeer-tained. The following interefting experimentswere made by M. Pictet, on a plan fuggeftedby M. de Sauffure, who appears to have derivedthe firft knowledge of the fact from M. Lambert.M. PiCtet' placed two concave tin mirrorsoppofite each other at a diftance of 12 feetafunder. In the focus of one of the mirrorsthe bulb of a fenfible thermometer was placed,and in the focus of the other was put the heatedbody which was the fubjeCt of experiment. Bythis ingenious arrangement a large portion ofthe radiant caloric proceeding from the heatedbody, if it followed the known laws of catop-trics, would firft be thrown upon the mirrorin whofe focus it was placed, thence would bereflected in parallel rays to the furface of theoppofite mirror, and from this would be con-centrated into its focus where die thermometerball was placed. This arrangement was wellcalculated to render fenfible the effect of aheated body of very fmall dimenfions. On put-ting an iron bullet heated below rednefs intoone focus, the thermometer in the other rofe10.5 degrees (of Reaumur) in fix minutes, be-ginning its rife the inftant the ball was in itsplace. A lighted taper ufed inftead of the ball,had nearly the fame effeCt. But to take away,all poffibility of the interference of light, a glafs.matrafs full of hoiling water was now fub-ftituted, and in two minutes the thermometerrofe degrees of the fame fcale. When thebulb of the thermometer was blackened, the.effeCt was proportionally greater. The rifebegins the very inftant that the heated body isput in its place. The diftance of 69 feet pro-* Effai fur le Ecu.