ioB Count Rumford’s Enquiry concerning the Nature of Heat, 
that all the heat which a hot body loses, when it is exposed in 
the air to cool, is not given off to the air which comes into con- 
tact with it; but that a large proportion of it escapes in rays, 
which do not heat the transparent air through which they pass, 
but, like light, generate heat only when, and where, they are 
stopped and absorbed; I suspected that, in every case when, 
in the foregoing experiments,- the cooling of my instruments 
was expedited by coverings applied to their metallic surfaces, 
those coverings must, by some means or other, have facilitated 
and accelerated the emission of calorific rays from the hot surface. 
Those suspicions implied, it is true, the supposition that dif- 
ferent substances, heated to the same temperature, emit unequal 
quantities of calorific rays ; but I saw no reason why this might 
not be the case in fact ; and I hastened to make the following 
experiments, which put the matter beyond ail doubt. 
Exper. No. 12. Two equal cylindrical vessels, made of sheet 
brass, and polished very bright, each 3 inches in diameter, and 
4 inches long, suspended by their oblique necks, in a horizontal 
position, (being placed on their wooden stands,) were filled 
with water at the temperature of 180°; and their circular flat 
bottoms were presented, in a vertical position, to the two balls 
of the thermoscope, at the distance of 2 inches. 
When the two hot bodies were presented, at the same moment, 
to the two balls of the instrument, or, what was still better, 
when two screens were placed before the two balls, at the dis- 
tance of about an inch, and, after the hot bodies were placed, 
these screens were both removed at the same instant, the small 
column of spirit of wine, which I have called a bubble , remained 
immoveable in its place, in the middle of the horizontal part of 
the tube of the instrument. 
