Proceedings of the British Association. 313 



of £70, will not be sufficient to complete the work, the Commit- 

 tee request that it may be extended to £ 150, which they hope 

 will meet every expense. 



Report on Radiant Heat. — Prof. Whewell laid before the Sec- 

 tion an abstract of Prof Powell's Report on Radiant Heat. This 

 report was supplementary to one furnished by Prof P. to the As- 

 sociation at the Oxford meeting in 1832, and he now proposed to 

 give an account of the progress of discovery since that period. 

 Such a report was pecuharly required from the number and im- 

 portance of the results arrived at in the interval, which though not 

 sufficient to form the basis of an unexceptionable theory, have at 

 least tended greatly to modify previous opinions, and to enable us 

 to refer large classes of phenomena to something like a simple and 

 common principle. The former report was divided into various 

 heads, derived from what appeared in the then existing state of 

 our knowledge, well-marked distinctions between several kinds of 

 effects ascribed to radiant heat ; but recent discoveries have in a 

 great degree so changed our views on the subject, that these divi- 

 sions cannot with any advantage or convenience be adhered to. 

 One principle of arrangement, however, has been newly supplied 

 in the discovery of the polarization of heat ; so that all the re- 

 searches to be described may be conveniently classed under two 

 heads : — 1st, as they relate to heat in its ordinary or unpolarized 

 state ; 2dly, as they relate to polarized heat. The report then 

 entered on the first general head, by calling attention to the re- 

 cent researches of Melloni and Forbes respecting the transmis- 

 sion and refraction of heat. Prof P. adverted to the discovery 

 of Melloni, that the resistance to the passage of heat is not ex- 

 erted at the surface, but at the interior of the mass. This was a 

 result of the observation, that the difference between the trans- 

 mission of heat from a more highly heated source and from a less 

 highly heated source, became less as the thickness of the screen 

 was diminished, and disappeared when very thin screens were 

 interposed. By comparing the transmissive powers of a great 

 number of substances, he found that in crystallized bodies the 

 diathermaneity for the rays of a lamp was proportional to their re- 

 fractive powers ; but in uncrystallized bodies no such law could 

 be traced. It was in the course of these researches that Melloni 

 made the important discovery of the singular property possessed 

 by rock-salt, — viz. that it is almost entirely permeable to heat, 



