62 MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 
3d, The wires of the pile were then transferred to a more sensitive galvano- 
meter, and the effect of lamp-black heat observed, the glass screen being inter- 
posed. ? 
4th, The sensitive galvanometer and glass screen being retained, the effect of 
rock-salt heat was lastly observed. 
By this method of experimenting, it was merely the relation between the 
diathermancy of the screen for lamp-black heat and for rock-salt heat that was 
measured ; its absolute diathermancy for either of these heats not being deter- 
mined. ‘Two sets of experiments, conducted in this manner, gave the following 
result :— 
By the first set, calling the proportion of the whole lamp-black heat which 
passed the screen 100, that of the rock-salt heat which passed the same screen 
was 54. And by the second set, these numbers were 100 and 60. 
12. As in these experiments with a glass screen the proportion of heat passed 
is very small, great numerical accuracy cannot be looked for, and the results 
obtained are valuable, rather as determining the direction and character of a fact, 
than as measuring the extent to which it holds. 
13. It is already well known, that rays of great refrangibility or small wave- 
length pass through glass and mica more readily than those of an opposite character. 
The difficulty with which rock-salt heat penetrates these substances, as compared 
with ordinary heat, might therefore lead us to infer that the wave-length of this 
heat is greater than that of ordinary lamp-black heat. 
14. If, therefore, the heat radiated by rock-salt is of great wave-length, since 
(First Series, Art. 19) the quality of the heat radiated is the same as that of the 
heat absorbed, it follows that the heat most absorbed by rock-salt must be heat 
of great wave-length ; and this derives confirmation from a fact noticed by Pro- 
fessor ForBEs, viz., that rock-salt passes a somewhat greater proportion of heat of 
high temperature than of that of low; heat of high temperature possessing a less 
average wave-length. 
15. If we look now to the relative transmission of the two descriptions of 
heat through mica split by heat, we see that the facility of transmission is yet in 
favour of ordinary heat, but not so strikingly as with a screen of common mica. 
This will be seen from the following table :-— 
TABLE II. 









Transmission of Transmission 
Nature of Screen. Ordinary Heat, of Rock-salt Heat, 
at 212°UR. at 212° F, 
Mica, : is A f : 2 100 58 




Mica split by heat, .. : : : 100 76 

