638 LECTURE LT. 



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 anotlier 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 t ohave been first suggested by 

 Mr Provost, as an explanation of an experiment on the reflection of cold, re- 

 vived by Mr. Pictet, but originally made some centuries before, by Plempius, 

 and by the Academicians del Cimento. A thermometer, for example, must be 

 supposed to retain its temperature by means of the continual accession of ra- 

 diant heat from the surrounding bodies, supplying the place of that which is 

 continually thrown off in all directions 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, notwithstanding, 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 through the conjugate or corresponding focus; and if a body at the 

 same temperature be placed in that focus, the radiation will still be the same: 

 but if a substance absolutely cold were placed there, the whole of the heat be- 

 fore reflected by the mirror would be intercepted, that is, almost half of that 

 which was received by the thermometer from the surrounding bodies; and if 

 a piece of ice be put in the conjugate focus, a delicate thermometer will in- 

 stantly show its effect in depressing the temperature ; as if the cold were ab- 

 solutely reflected in the same manner as heat or light. 



Dr. Herschel's experiments have shown that radiant heat consists of various 

 parts, which are differently refrangible, and that in general, invisible heat is 

 less refrangible than light. This discovery must be allowed to be one of the 

 greatest that has been made since the days of Newton, although the theories 

 of some speculative philosophers might have led to it a few years earlier. Dr. 

 Herschel was occupied in determining the properties of various kinds of co- 

 loured glass, which rendered them more or less fit for enabling the eye to view 

 the sun through a telescope ; and for this purpose it was necessary to inquire 



