[ 534 ] 

 LXXIV. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 483.] 

 March 15, 18G0. — Sir Benjamin C. Brodie, Bart., Pres., in the Chair. 



THE following communications were read : — 

 "On the Light radiated by heated Bodies." By Balfour 

 Stewart, Esq., A.M. 



In two papers read before the Royal Society of Edinburgh in the 

 years 1858 and 1859, and published in their Transactions, I have 

 described some experiments on radiant heat, which would seem to 

 involve an extension of Prevost's theory of radiation, known as the 

 theory of exchanges. 



As the paper which I have now the honour to submit to this 

 Society will detail analogous experiments on radiant light, I may 

 be permitted briefly to refer to those points in my previous papers 

 which are thus intimately connected with the present subject. 



In attempting to unfold the logical consequences of Prevost's 

 theory, certain properties of radiant heat present themselves to our 

 view, many of which are capable of experimental verification. 



The following are some of these ; and, for convenience-sake, I 

 shall follow up the statement of each (before proceeding to the next) 

 with a description of the analogous property of Tadiant light, as in 

 this way the similarity which exists between heat and light will be 

 most readily perceived. 



f In the first place, the heat radiated by a thin plate of any sub- 

 stance at a given temperature, is proportional to the absorptive 

 capacity of that substance for the heat of that temperature ; or, in 

 few words, its radiation is equal to its absorption. 



Rock-salt, for instance, has a small absorptive capacity for heat of 

 212° F., and, in consequence, its radiation when heated to 21 2° F. is 

 comparatively small. In pqint of fact, a plate of this substance, 

 0*18 inch thick, only gives out 15 per cent, of the heat which 

 lamp-black radiates at the same temperature. Glass, on the other 

 hand, absorbing nearly all the heat of 212° F. which falls upon it, 

 has at this temperature a radiation comparatively great, and nearly 

 equal to that of lamp-black. A similar law holds with regard to 

 radiant light. 



If a piece of perfectly transparent glass be heated in an ordinary 

 fire, removed to the dark, and there viewed, it will be found to emit 

 scarcely any light ; if the glass be slightly coloured, its radiation 

 will be more copious ; the amount of light given out, as far as I 

 have been able to make the comparison, invariably depending upon 

 the depth of colour or absorptive power of the glass for light, pro- 

 vided its colour stands heating. A good way of performing this 

 experiment is to heat a dark glass by the side of a colourless one, by 

 means of a chemical tongs, in some uniform field of heat. "When 

 viewed in the dark together, the contrast is very striking between 

 the bright light of the one and the bare visibility of the other. 



