the passage of radiant heat through glass screens. 381 
theory of the conclusion I have above examined, as deduced 
from De La Roche's experiments ; but both that conclusion, 
and with it any such explanation, are now I conceive shown 
to be unnecessary. 
(11 .) The principal result obtained by Mr. Ritchie, and 
described in this Paper, is however of a nature deserving 
more attention. It exhibits an exception to the general law, 
in the instance of glass of extreme thinness ; in which case the 
heat from a source below luminosity appears capable of radi- 
ating through such a screen when transparent , but not when 
rendered opaque. 
As experiments conducted like those of Mr. Ritchie, with 
air thermometers, are always liable to uncertainty, I con- 
ceived it desirable to try the same thing with a mercurial 
thermometer. 
(12.) I used as screens pieces of a large bulb blown to an 
extreme degree of tenuity ; these were attached (as in Mr- 
Ritchie's experiments), to a circular aperture about inch 
diameter in a pasteboard screen. A second screen of milled 
board, with an aperture of 1 inch diameter, was placed about 
an inch from the first ; and behind this the iron ball, previ- 
ously cooled to just below visible redness. The same ther- 
mometer as in the preceding experiments was suspended at 
about \ inch from the screen. I made various trials, com- 
paring the effect of the thin transparent screen with a similar 
opaque one : in some instances using three thicknesses of the 
glass, and afterwards the middle piece blackened on both 
sides with the soot of a candle ; and in others two thick- 
nesses, the inner surface of one being afterwards blackened ; 
thus forming an opaque screen with the same vitreous surface. 
In all the experiments a thermometer suspended by the side 
MDCCCXXVI. 3 D 
