6 MR B, STEWART ON RADIANT HEAT. 
D. Mica.—Experiments similar to those already described, only at a distance 
of 24 inches from the cone, gave— 
For mica, ‘0009 inch thick (average of two sets of experiments), j 8-2 
For mica, ‘02 inch thick (average of two sets of experiments), A 5 9:3 
The experiments already quoted, which were made at a shorter distance from 
the pile, gave— 
For mica, ‘0009 inch thick, : 3 q = ; ’ 3 - 112 
For mica, ‘02 inch thick, . : ; : : i ae S BEY 
E. Rock-Salt.—Three pieces of rock-salt were used. Their dimensions were— 
1st Piece. 2d Piece. 3d Piece. 
Length, . ; é 1:15 inch 2°15 inch 2°5 inch 
Breadth, . - ; eae ie ee ae Dee ad 
Thickness, ; ; O18 5: 0°36..4: Ort. 
For these pieces, as well as for the other substances, I am indebted to the kind- 
ness of Professor Forses. When placed behind the diaphragm, the farthest off 
surface was large enough to fill up the field of view,—that is to say, all rays from 
the cone striking the nearest surface, struck also the surface farthest off; the dis- 
tance between the two surfaces being the thickness of the piece. 
The following are the means of four sets of experiments :— 
Radiation from 1st or thinnest piece, ‘ F - ; 34 
2d or middle piece, : : : . : 4:3 
3d or thickest piece, . ; : ; 5:3 
This proves that more heat is radiated by a thick than by a thin piece of 
rock-salt. 
The following experiments were devised by Professor ForBEs, to confirm the 
above results. 
(a.) The second piece of rock-salt was placed obliquely behind the diaphragm, 
making an angle of 20° with the prolongation of the axis of the cone. A piece of 
fir-wood of the same dimensions was placed in the same way. The two sub- 
stances being compared in this position, and also in the usual position behind the 
diaphragm (viz., ees to the direction of the cone’s axis), the following 
was the result :— 
Oblique. Usual position. 
Rock-salt °36 inch thick, 4: : 
Wood, same size as rock-salt, i . ; 9-1 14.1 
In order that this experiment may be understood, it may be well to mention, 
that, when the plate was placed obliquely behind the diaphragm, it did not quite 
fill up the field of view. Hence the wood gave out less heat to the cone in this 
than in its ordinary position. 
It appears, therefore, that the radiation from rock-salt, in a direction making 
a small angle with the surface, bears a greater proportion to the corresponding 
radiation from wood than when both radiations are taken perpendicular to the 
surface. The reason undoubtedly is, that in the former case the rays come from 
a greater thickness of the substance, so that their intensity is increased. 
