MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 63 
Compare this with the following table, deduced from the results given by Pro- 
fessor Fores, in the Fourth Series of his Researches, Art. 9. 


TABLE IIT. 
Transmission of Heat Transmission of 
Nature of Screen. F from Blackened Brass, Black Heat, 
at 700° F. Fin, PANS Je 
Mica ‘015 inch thick, . , i : 100 52 
Mica split by heat, .. : : t 100 64 


From a comparison of these two tables, it will be seen that, as tested by the 
two substances, mica and mica split by heat, rock-salt heat at 212° F. bears to 
ordinary heat of that temperature a relation similar to that which ordinary heat 
at 212° F. bears to heat at 700° F.; that is to say, that just as heat of 212° F. 
has a greater wave-length than heat of 700° F., so rock-salt heat at 212° F. has 
a greater wave-length than ordinary heat at that temperature. And the surface 
stoppage produced by splitting the mica, telling most powerfully upon heat of 
high temperature, or small wave-length, while the stoppage by substance is in the 
opposite direction, we see how the one effect tends, to a certain extent, to neutra- 
lize the other, rendering the proportions of different kinds of heat passed by split . 
mica more nearly alike than those passed by ordinary mica. 
16. All these experiments concur in showing, that heat from rock-salt pos- 
sesses very great wave-length, and probably heat from a thin plate of this sub- 
stance, at a low temperature, may be found to possess a greater average wave- 
length than any other description of heat which can be exhibited. 
Third Group of Experiments described. 
17. I now proceed to describe the third group of experiments, or those on the 
radiation of glass and mica at high temperatures. 
A. Glass.—For the experiments on glass, the following apparatus was used :— 
The pile was placed within a box, and surrounded with cotton wadding. The 
orifice through which radiant heat was admitted 
into the box consisted of a brass tube A B, blackened 
in the inside. The diameter of this tube was 4 inch, 
its length 3 inches, and during the greater part of 
its length it passed through water, contained in the 
chamber CEF'D. The side of the box (C A D) next the pile was lined with tin- 
foil. Owing to the small divergence of the rays of heat which had to pass through 
the narrow tube, the cone might be placed several inches to the left of A without 
sensibly weakening the effect, and, on the other hand, the source of heat might be 
placed some distance to the right of B without ceasing to fill up the field of view. 
VOL. XXII. PART I. R 

