RADIANT HEAT. oL6 



interceptive pojver was estimated directly as the illumination required 

 to produce the equalization, that is, inversely as the square of the 

 distance. 



Two equal thermometers enclosed in a box, with apertures over 

 the bulbs, (which were plain,) one open, the other covered succes- 

 sively by the different transparent media, were exposed to different 

 sources of heat, and the interceptive effects compared together and 

 with those of the same media for light. Thus among the results were 

 the following: 



Common Fire. Candle. 



Light. Heat. Light. Heat. 



Coach glass # 750 86 625* 



Dark red glass 999 573 999 526 



3.) Refraction by lenses. 



Lambert collected the rays of a fire by a large lens and found the 

 heat scarcelv sensible to the hand. 



Sir W. Herschel (Phil Trans., 1800, pp. 272, 309, 327,) received 

 the rays of a candle on a lens, with a pasteboard screen, having an 

 aperture nearly equal to that of the lens; the thermometer in the 

 focus rose 2|° Fahr. in 3 minutes; the same with the rays from a fire, 

 and from a mass of red hot iron. 



M. Brande found the rays of a flame, concentrated by a lens, pro- 

 duced an effect on a blackened thermometer in its focus; the lens did 

 not become heated. — (Phil. Trans., 1820, Parti.) 



4.) Dr. Ritchie found that if Leslie's photometer be placed opposite 

 a ball of iron heated almost to redness no effect whatever will be 

 produced; but if the temperature of the ball be raised so as to shine 

 in the dark with a dusky red color, the fluid in the stem of the black 

 ball will sink a considerable number of degrees. If the temperature 

 of the ball be raised still higher it will produce a greater effect upon 

 the instrument than the flame of the finest oil-gas, though the one 

 possesses a much greater illuminating power than the other. 



Dr. Turner and Dr. Christison have found that Leslie's photometer 

 "is powerfully affected by heat" when placed "before a ball of iron 

 heated so as not to be luminous, or even before a vessel of boiling 

 water." The opposite result of Dr. Ritchie may possibly be owing 

 to some difference in the surface, substance, or thickness of the black 

 bulb employed. — (Edinb. Journ. of Science, iv, 321.) 



I have found differences, which I am at a loss to account for, be- 

 tween the effects on a differential thermometer with the bulbs of 

 equal height, and one in which they are in a vertical line. 



5.) That there exist essential differences between the constitution 

 of the heating power of luminous hot bodies and that of the same 

 power proceeding from those which are non-luminous was remarked 

 by former experimenters. But it is a point which does not seem to 

 have excited any close or systematic inquiry until the subject was 



*Out of 1,000. 



