Proceedings. of the British Association. 313 
_ of £70, will not be sufficient to comple 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 
rt 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 peculiarly 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:—Ist, 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 
e the important discovery of the singular property possessed 
rock-salt,—viz. that it is almost entirely permeable to heat, 
