MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 73 
make up the average quality, havea somewhat greater difficulty in passing through 
mica than ordinary lamp-black heat. Accordingly, it was found that the diather- 
mancy of a mica screen for heat from table-salt was less than that for ordinary 
lamp-black heat in the proportion of 92 to 100, while it was less for heat from 
pounded sulphate of potash in the proportion of 93 to 100, thus confirming the 
analogy between rock-salt and sulphate of potash. No such difference was 
observed for heat from sugar. 
47. We see also from the above table that the radiation and therefore the ab- 
sorption of table-saltis 83:1 per cent., leaving 16°9 per cent. for the reflected heat. 
Now Metuont found that chalk absorbed 56:6 per cent., and consequently reflected 
43:4 per cent. of heat from a Locatelli lamp ; and if we suppose table-salt to be at 
least as white as chalk for heat of that temperature, we must conclude that table- 
salt is less white for heat of 212° than for heat from a Locatelli lamp, following m 
this respect the same law as,chalk, which, from being nearly black for heat at 
212°, becomes comparatively white for heat from a Locatellilamp. There is also 
little doubt that table-salt reflects more than 16-9 per cent. of the light that falls 
upon it. Hence we may conclude generally that powders even of diathermanous 
bodies are less white for heat of low temperature than for heat of high temperature 
and for light. 
48. It would also seem, that although, comparing one powder with another, 
there is no relation between apparent whiteness and whiteness for heat, since it 
was found that very white surfaces of pounded sugar and alum (the particles com- 
pressed, not made into a paste with water) reflected little or no heat; yet, com- 
paring powdered surfaces of the same diathermanous body together, there seems 
to be some relation between their apparent whiteness and their whiteness for heat, 
insufficient pounding, or any circumstance which diminishes the apparent white- 
ness, diminishing also its whiteness for heat. 
