300 RADIANT HEAT. 



Professor Leslie made extensive use of reflectors, but observed 

 that there was a very considerable degree of aberration in the focus from 

 an exact position ; considerably nearer to the reflector than the true 

 focus, the effect continued undiminished. — (Inquiry, p. 64.) 



5.) Alleged reflection of cold. 



An account of the earliest experiments will be found in the Memoirs of 

 the Florentine Academy, (Waller's Transl., p. 103; also Gaertner, 1781.') 



Pictet, with conjugate reflectors, found the thermometer sink when 

 ice was in the opposite focus. — (Essais de Phys., p. 82.) 



Count Rumford employed a tube, a frustrum of a cone, open at 

 both ends ; placing ice at the small end, the thermometer at the 

 large end sunk very little. The ice being at the small end, the ther- 

 mometer at the large end fell considerably. Rays reflected by the 

 inside of the tube from the body at the large end would be concen- 

 trated on that at the other. 



6.) M. Prevost (Essai sur la Calorique Rayonnant, Geneva, 1809, 

 and Recherches sur la Chaleur, p. 15) proposes a theory of radiation, 

 that heat is a discrete fluid, every particle of which moves in a straight 

 line, and such motions are constantly taking place in all directions, 

 whether there be more or less heat present. Hence all bodies, 

 whether of a higher or lower temperature, are supposed to be con- 

 tinually radiating heat ; and this going on mutually tends to bring 

 them all to an equilibrium of temperature. 



On this theory explanations are given of the apparent radiation of 

 cold. 



The thermometer in the conjugate focus, when nothing is in the 

 other, remains stationary, because the rays reflected from all the sur- 

 rounding space so as to cross at the focus of the opposite mirror, and 

 be reflected in a parallel state to the other, and thence on to the ther- 

 mometer in the focus, are exactly equivalent to those which the ther- 

 mometer radiates. But when a mass of ice is placed in the opposite 

 focus, it intercepts and absorbs a portion of the rays which would 

 otherwise have fallen on the first mirror, and so have reached the 

 thermometer, which in consequence radiates more than it receives, 

 and therefore sinks. 



A similar explanation applies to Count Rumford' s experiment. — 

 (See Thomson On Heat, &c, p. 163.) 



In the Quarterly Journal of Science (June, 1830, p. 378) some ob- 

 servations are given on this subject, and an explanation offered, which, 

 though very ingenious, appears somewhat complicated. 



It may not be improper to observe, that if the above be a correct 

 view of Prevost' s theory, it can hardly be conceived as otherwise 

 than partially hypothetical. The idea, viz: that bodies even of a 

 lower temperature than those about them actually give out a small 

 degree of heat, is extremely difficult to conceive ; and it does not 

 appear absolutely essential to the explanation of the facts. 



Without reference to any theory, I venture to propose the follow- 

 ing as the simple experimental law : 



All bodies of unequal temperature tend to become of equal tempe- 



