268 
SECOND REPORT -1832. 
continually renewed. {Phil. Trans. 1827, Part II. p. 141.) But 
to this a similar objection must apply. 
5.) The results of Prof. Leslie do not apply to temperatures 
above those of boiling water. 
This extension of the inquiry formed the subject of the re¬ 
searches of De la Roche. The complete account of these is 
given in its proper place; at present we have to consider them 
only as far as relates to bodies below luminosity. He tried the 
effect of a screen of glass, first transparent, and then with one 
surface blackened, on the heat radiating from mercury at 180° 
centig. and at 346° when it was boiling. (Biot, Traite de Phys. 
iv. 640.) 
The results were as follows: 
Rise of focal thermometer (centig.) in l m . 
Transparent Blackened 
No Screen. Screen. Screen. 
Mercury at 180°. . 3°*94.0°*22.Q°‘07 
--- at 346 . . 16 *33 . 1 *36 ..... 0 T7 
He hence infers a partial transmission of heat at these high 
temperatures; and the more so, viewing these results in con¬ 
nexion with the rest of the subsequent series (considered in 
nother place). 
These are the only ones of his experiments referring really 
to simple radiant heat; and the inference of an actual transmis¬ 
sion in the way of direct radiation, is open to several objections. 
6.) The blackened screen causes a greater diminution of heat 
than the transparent, and it was therefore inferred that a por¬ 
tion of heat radiates freely through the transparent screen, and 
is stopped by the opake one: but there are several circumstances 
which show that this is not a 7iecessary conclusion. 
The coating was towards the source of heat, and rendered 
this screen more absorptive of heat wdiere exposed to it, that 
is, at its central part,—and a better radiator towards the edges 
without the area of the incident rays ; so that it radiated its 
heat most copiously on the side away from the thermometer. 
With the plain screen there was no such tendency to radiate 
more on one side than on the other; and hence the greater effect 
on the thermometer. 
This explanation I suggested in the Annals of Philosophy , 
xlv. 181. 
Some observations bearing upon this subject, occur in Sir 
David Brewster’s elaborate paper on “New Properties of 
Heat,” &c* in the Phil. Trans. 1816, Part I. His 40th propo- 
