by REvPoRT—1840. 
ledge, well-marked distinctions between several kinds of effects 
ascribed to radiant heat. The more recent discoveries have in 
a great degree so changed our views of the subject, that these 
divisions cannot with any advantage or convenience be adhered 
to. One grand principle of arrangement, however, has been 
newly supplied in the capital discovery of the polarization of 
heat; so that all the researches we have to describe will be conve- 
niently classed under two heads, as they relate—first, to radiant 
heat in its ordinary or unpolarized state ; and secondly, to its 
polarized condition. 
Division I.—Unpotarizep 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 Associa- 
tion 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 Cambidge meet- 
ing, in 1833, Prof. 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 embo- 
died 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 diminishes as the thickness of the screen is less, until 
with very thin laminz 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 sulpliate 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 diather- 
maneity for the rays of alamp was proportional to their refrac- 
tive powers ; but in uncrystallized bodies no such law could be 
traced. 
* See Third Report, p. 381-2. 
