ON THE SOURCES AND EFFECTS OF HEAT. 489 



glass very rapidly, and its further progress is almost entirely interrupted 

 by the glass, although probably a certain portion still continues to accom- 

 pany the light in all cases. Hence a screen of glass is sometimes practically 

 convenient for allowing us the sight of a fire, and protecting us at the same 

 time from its too great heat. Mr. Lambert* showed that culinary heat 

 was much more strongly reflected by mirrors of metal than of glass, although 

 little difference was observable in the quantity of light, and he very justly 

 attributed this difference to the interception of a part of the heat by the glass, 

 which operated with respect to it like an opaque substance, although it trans- 

 mitted the light with freedom. Opaque substances in general appear to be 

 wholly impervious to radiating heat of all kinds ; but Dr. Herschelt has found 

 that dark red glass, which transmits a very small portion of light only, suf- 

 fers some kinds of radiant heat to pass through it with very little interruption. 



In other respects, radiating heat is subject, in all cases, to the optical 

 laws which govern the reflection and refraction of light. Dr. Hoffmann 

 appears to have been the first that collected the invisible heat of a stove 

 into a focus by the reflection of one or more concave mirrors.^ Buffon, 

 Saussure, Pictet, and Mr. King, made afterwards similar experiments on 

 the heat of a plate of iron and of a vessel of boiling water. Mr. Pictet, as 

 well as Hoffmann, employed two mirrors facing each other ; and by means 

 of this arrangement the experiment may be performed when the thermo- 

 meter is placed at a considerable distance from the heated body. 



The temperature of the air, not being affected by the radiation of heat, 

 is probably in all respects indifferent to its emission in this manner ; and 

 as the rays of light cross each other freely in all possible directions, so it 

 appears that heat may flow in different directions through the same medium 

 without being interrupted ; nor does there seem to be any more reason that 

 a hot body should cease to emit heat while it is receiving heat from another 

 body, than that a luminous body should cease to afford light when another 

 body shines on it. This continual interchange of heat, constituting in 

 common cases a kind of equilibrium of motion, appears to have been first 

 suggested by Mr. Prevost, as an explanation of an experiment on the 

 reflection of cold, revived by Mr. Pictet, but originally made some centuries 

 before, by Plempius, and by the Academicians del Cimento. A thermo- 

 meter, for example, must be supposed to retain its temperature by means 

 of the continual accession of radiant heat from the surrounding bodies, 

 supplying the place of that which is continually thrown off in all direc- 

 tions towards those bodies. Supposing the thermometer to be placed near 

 the focus of a metallic speculum, not much less than a hemisphere, about 

 one half of the heat, which the thermometer would otherwise have received 

 from the surrounding bodies, must be intercepted by the mirror, which, 

 being metallic, emits itself but little radiant heat, but reflects, notwith- 

 standing, an equal quantity of heat, from the objects on the opposite side, 

 so that the temperature of the thermometer remains unaltered. But all 

 the heat, which falls on the thermometer from the mirror, must have passed 



* Pyrometrie, 4to, Berl. 1779. See Mariotte, Hist, et M&n. i. 223 ; Traite de la 

 Nat. des Couleurs, 1686. 



f Ph. Tr. 1800, p. 255, &c. : Wolfe, Ph. Tr. 1769, p. 4. 



Sur 1'Equilibre du Feu, Geneve, 1792. Du Calorique Rayonnant, Gen. 1809. 



