Mr. B. Stewart on the Light radiated from Heated Bodies, 537 



manner that the light which it radiates precisely makes up for that 

 which it absorbs ; so that we have virtually a coal radiation coming 

 partly from and partly through the glass. 



Let us now consider Prevost's theory with regard to bodies of 

 indefinite thickness. One of its consequences was experimentally 

 discovered by Leslie ; viz. that metals which are good reflectors of 

 heat are very bad radiators. As a variety of this experiment, I have 

 endeavoured to show that a powdered diathermanous body wil 

 radiate less than powdered bodies which are opaque for heat. Thus, 

 if a plate of table-salt have one side blackened, the white side will 

 radiate only 83 per cent, of that which the blackened side radiates 

 at the temperature of 212° F. No such difference is observed in 

 sugar, which, though white for light, is black for heat of 212°. 



We have here also similar facts with regard to light. If a pot of 

 red-hot lead or tin be carried to the dark, and the dross scummed 

 aside by means of a red-hot iron ladle, the liquid metal momentarily 

 disclosed will appear less luminous than the surrounding dross, — the 

 difference being more observable in the case of tin, which has a 

 higher reflecting power than lead. Also, if a piece of platinum, 

 partly polished and partly tarnished, be held above a flame in a dark 

 room, the tarnished portion will shine much more brilliantly than 

 the polished. Again, if we take a china cup with a white and black 

 pattern, and heat it to a white heat in the fire, while there we shall 

 not perceive much difference between the white and black of the 

 pattern ; but if we bring it into a dark room, we shall perceive the 

 black to shine much more brilliantly than the white. This reversal 

 of the pattern presents a very curious appearance. 



Finally, it is a consequence of Prevost's theory and an experi- 

 mental fact, that opaque bodies, generally speaking, radiate the same 

 description of heat at the same temperature. In like manner, the 

 light which they radiate is of the same description at the same tem- 

 perature ; one body is not red while a second is yellow and a third 

 white, but they are all either red or yellow, or white together. 



An analogy has thus been established between radiant heat and 

 light in certain of their properties. Now two opinions have been en- 

 tertained with regard to light : — 



1st. Some have regarded it as differing from radiant heat only in 

 wave length. 



2nd. Others have regarded the two as physically distinct, although 

 possessing many properties in common. It has even been thought 

 that some kinds of light have no heating effect on the bodies on 

 which they fall. 



I cannot but think that the facts just stated countenance the 

 former opinion rather than the latter : for Prevost's theory consists 

 of the three following hypotheses : — 



] st. That if an enclosure of any kind be kept at a uniform tem- 

 perature, any body placed within the enclosure, and surrounded by 

 it on all sides, will ultimately attain that temperature. 



2nd. That all bodies are constantly giving out radiant heat, at a 

 rate depending upon their substance and temperature, but independent 

 of the substance or temperature of the bodies that surround them. 



Phil Mag. S. 4. No. 136. SuppL Vol. 20. 2 N 



