334 RADIANT HEAT. 



Division I. — Unpolarized Heat. 

 Transmission and Refraction of Heat: Melloni. 



Since the period to which my former report extends, various notices 

 have from time to time been given to the British Association relative 

 to the more important discoveries connected with radiant heat. My 

 former report includes a statement of some of the first researches of 

 M. Melloni. At the Cambridge meeting, in 1833, Professor Forbes 

 gave some account of the further investigations in which M. Melloni 

 was then engaged, including a brief abstract by M. Melloni himself 

 of the chief results he had then obtained.* The full details were 

 subsequently embodied in his several memoirs. 



In the earlier part of these researches, M. Melloni had found that 

 the quantity of calorific rays which traverses a screen is proportional 

 to the temperature of the source: but the difference constantly dimin- 

 ishes as the thickness of the screen is less, until with very thin 

 laminae it is insensible. 



This proves that the resistance to the passage of heat is not exerted 

 at the surface, but in the interior of the mass. 



With the solar rays, he observed that with various thicknesses of 

 sulphate of lime, water, and acids, the increase of interception, owing 

 to increased thickness, is greater for the less refrangible rays of the 

 spectrum. 



With terrestrial sources he found that a plate of glass, 2 mm. in 

 thickness, stops, out of 100 rays, from flame 45, from copper at 950° 

 cent, (incandescent) 70, from boiling mercury 92, from boiling water 

 100. 



Comparing the transmissive powers of a great number of substances 

 in a crystallized state, he concluded that the diathermaneity for the 

 rays of a lamp was proportional to their refractive powers; but in 

 uncrystallized bodies no such law could be traced. 



It was in the course of these researches that the author made the 

 important discovery of the singular property possessed by Rock Salt, 

 viz: that it is almost entirely permeable to heat even from non-lum- 

 inous sources. He found its transmissive power six or eight times 

 greater than that of an equal thickness of alum, which had nearly 

 the same transparency and refractive power. He also discovered 

 that (unlike other diathermanous media) it is equally diathermanous to 

 all species of heat, i. e., to heat from sources of all degrees of lumin- 

 osity or obscurity; or that it transmits in every case an equal propor- 

 tion of the heat incident. 



Thus he found a plate of 7 mm. (.28 inch) in thickness transmits 

 about 92 out of 100 rays, whether from flame, red-hot iron, water at 

 212°, or at 120° Fahrenheit. A plate 1 inch thick gave a similar 

 constant ratio. 



M. Melloni' s "Memoir on the Free Transmission of Radiant Heat 



* See Third Report, p. 381-'82. 



