8 MR B. STEWART ON RADIANT HEAT. 
Therefore heat from a thin plate of glass is less transmissible through glass 
than heat from blackened paper. 
B. and C.—No experiment of this nature was made with alum or selenite. 
D. Mica.—The apparatus already described gave— 
Without With mica screen, 
screen. *0025 inch thick. 
For window (the window, it will be borne in mind, is the radia- 
ting surface), ‘0009 inch thick, : : : : 11:2 2°5 
Window ‘02 inch thick, . 5 12-7 3'2 
Blackened paper attached to glass lying on the bottom of 
the boiling-water apparatus, gave . - 21:0 6:3 
We have therefore the proportion of heat panved by mica screen— 
For heat from thin mica window, . : ‘ ‘ ; 223 
thick ae ; é ; 4 : 260 
blackened paper, : ; . “300 
E. Rock-Salt.—The thickest piece of rock-salt ern ‘77 inch) being used as 
a screen, and the diaphragm withdrawn, in order to give greater results; the middle- 
sized piece of rock-salt gave— 
With screen. Without screen. 
The same screen stopped 3 rays out of 12 for ordinary lamp-black heat. 
This experiment is sufficient to show that rock-salt is much less diatherma- 
nous for heat from rock-salt than for ordinary heat. The common opinion, that 
rock-salt is equally diathermanous for all descriptions of heat, is therefore un- 
tenable. 
9. From the third group of experiments it appears, therefore, that heat emit- 
ted by glass, mica, or rock-salt, is less transmissible through a screen of the same 
material as the heated plate, than heat from lamp-black; this difference being 
very marked in the case of rock-salt. 
Fourth Group of Experiments described. 
10. I now proceed to the fourth group of experiments, or those made with the 
view of comparing the radiations of plates of the same substance, but of different 
thicknesses, with regard to the quality of the heat radiated. 
A. Glass.—It has been already shown (Art. 8), that heat from crown-glass 
‘05 inch thick is less transmissible through glass, than that from crown-glass 
‘10 inch thick. 
B. and C.—No experiments of the kind were made on alum or selenite. 
D. Mica.—It has been already shown (Art. 8), that heat from thin mica is less 
transmissible through a mica screen than heat from thick mica. ' 
E. Rock-Salt.—With a screen of rock-salt -18 inch thick, the following result 
was obtained :— 
Thickest piece of rock-salt, heated to 210° (thickness -77 inch), gave. 25 
Middle sized : ... (thickness *36 inch), gave. 1:7 
Thinnest piece Bs ... (thickness ‘18 inch), gave. i-1 
