810 
ME. W. HOPKTN'S’S EXPEEOIEZv’TAL EESEAECHES 
^2='9i°-5r 
?^.=126°-5l 
95°-0r 
^,=139° I 
4=100° r 
^ = •09 
c 
- = •077 
c 
- = •074. 
c 
The temperature of complete melting of the wax was about 140°, so that in the last 
experiments the lower part of the block was very nearly in a melting state, and the 
molecular condition of a considerable portion of it was doubtless affected. This change 
is indicated by a decrease of conductivity. Exactly similar results were obtained for 
spermaceti. So long as the temperature was insufficient to produce any apparent effect 
on the constitution of these substances, my experiments did not detect any sensible effect 
on their conductivity. Biot remarked the diminution of conducthity of a bar of 
fusible alloy when one extremity was maintained very nearly at its fusing temperature. 
With respect to mineral substances of very high temperatures of fusion, there can be 
little doubt, I conceive, that their conductivity remains unaffected by any temperature, 
for instance, not exceeding that of boiling mercury. Some experiments were made, 
partly with the view of corroborating this conclusion, but more especially for testing, in 
the case of mercury, the approximate truth of the law which asserts that the intensity of 
radiation from a given surface is proportional to the excess of the tempemture of that 
surface above that of the surrounding space. The following results were obtained by 
experimenting at different temperatures with the same block of sandstone ; and 4 
denote the same temperatures as above, and r the temperature of the smrounding 
atmosphere — 
( 1 ) 
= i44”-h'l 
= 123°-2J’ 
r4-241°-5i 
^ ^ l4=193°-0J’ 
f 9 \ [4“334°’0'i 
^ ^ l4=260°-5r 
(4) 
4=447°-8'i 
= 326°-0r 
r4=522°-0-i 
^ ^ l4=37r-oi’ 
r=66°. 
‘=■45, 
C 
r=69°, 
^ = •435 
C 
r=78°. 
- = •425, 
c 
r=81°. 
- = •34. 
c 
r=85°-5. 
-=•33. 
c 
Hence if we assume k to be constant for these different temperatures, c the radiating 
power of mercury must increase -with the increase of the difference The mcrease 
however is not considerable while varies from 50° to 120° or 130°, as in the two 
first of the above experiments. Within those limits the law of radiation, as usually 
assumed, will be approximately true. It was deskable to corroborate this conclusion 
