on Radiant Heat, 
28$ 
veral degrees lower than formerly. The same effect takes place 
when the blackened side of the lamina of glass is turned towards 
the bulb of the thermometer. 
Let us now examine what would be the effect, upon the sup- 
position that caloric passes through glass in the same manner as 
through an opaque substance. Since a coating of lamp-black 
absorbs heat more copiously than a vitreous surface, the fluid 
in the stem ought to sink farthest when the blackened screen 
was used. Again, as heat radiates more copiously from a sur- 
face of lamp-black than from a vitreous surface, the thermome- 
ter ought to sink lower with the blackened side of the screen 
turned towards the bulb, than with the transparent one, which 
is directly contrary to the result of the experiment. Hence it 
follows, that a portion of caloric, which freely permeated the 
transparent lamina of glass, was retained by the coating of lamp- 
black, and this portion increased, according as the temperature 
of the ball was raised. 
Lest any objection should be made to these experiments, upon 
the ground that different surfaces were employed, I had recourse 
to the following experiment, which puts the matter beyond the 
possibility of doubt. 
$. I coated one side of a thin plate of glass with silver-leaf, 
and then applied another lamina of glass to the coated side, so 
that I had now an opaque screen with two vitreous surfaces. I 
placed this compound screen between the heated body and 
the bulb of the thermometer, and observed how far the fluid 
descended in the stem before it became stationary. I then re- 
moved the screen, and substituted two transparent plates of the 
same thickness, and observed that the liquid in the stem descend- 
ed several degrees lower than formerly. 
It can be no objection to the accuracy of this experiment, that 
one of the screens was thicker than the other by the coating of 
silver-leaf ; for the experiment succeeded even when three trans- 
parent plates were used, which makes a much thicker screen, 
and a worse conductor, than the other. 
I have been thus particular in establishing the fact, that in- 
visible caloric passes through very thin plates of glass, in the 
same manner as light does through thick transparent plates of 
the same substance, because the fact has been called in question 
