280 
SECOND REPORT- 1882. 
effect emanating from it at the same time ? These are the 
questions which my experiments were directed to answer, and 
the mode of trying the point is extremely simple; it is only to 
ascertain whether of the total heating effect from a luminous 
hot body, the portion intercepted by a transparent screen is of 
the same nature as, or different from, the part transmitted, in 
its relation to the surfaces on which it acts. 
The experiments w ere conducted simply by having two ther¬ 
mometers, one coated with smooth black, the other with absorp¬ 
tive white, observing the ratio of the effects wdien they w ere 
exposed together to the direct influence of a luminous hot body, 
and comparing it with the ratio similarly observed w hen a glass 
screen was interposed. 
The screen acquiring and therefore radiating heat from the 
first moment of the experiment, w ill affect the thermometers in 
a ratio (as before observed,) differing little from equality; and 
these equal quantities added to the terms of the ratio of the 
direct effects of the luminous body will of course diminish the 
inequality of that ratio. This cause of error may not have 
operated to any great degree, but its tendency is obviously to 
a diminution of the ratio. 
Notwithstanding this, the observed result in all cases, with 
a lamp, or with iron raised to a bright red heat, w as, that the 
ratio of the effect on the black to that on the white thermo¬ 
meter w r as increased by the interposition of the screen. 
A summary of the results of tw r o sets of experiments (con¬ 
ducted with some slight variation), and in the second of which 
the temperature acquired by the screen was carefully noted, is 
as follows : 
Rise of Thermometer (centig.) in 1 min. 
Glass Screen. 
_-A__ 
No Screen. 
_A_ 
Iron bright hot 
Argand lamp 
White. 
Black. 
White. 
Black. 
l°-25 . 
go .iftz 
• /V 1 1/ • • 
. . 7°-0 . 
. 8° 
MW w- 
• to 
0 -6 . 
1 *27 
* 1 .vU • • 
. . 2 -95 . 
• 3 
Mw w 
’ i O 
0 -6 . 
.2-0 . . 
. . 1 -8 . 
o 
. O 
*4 
1 -8 . 
.2 *35 . . 
. . 2 *35 . 
r> 
• O 
•2 
/V 
epetitions. 
These numbers are the means of several n 
The necessary conclusion from this difference in the ratio of 
the direct and screened effects, is, that the portion of heat which 
has the property of permeating the screen has also the property 
of affecting the two surfaces in a ratio different from that in 
which the part intercepted acts upon them. 
As in researches of this kind great numerical precision is 
unattainable, I was especially, at every step of the inquiry, 
