12 MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 
son of this is because we have not only the reflection from the outer surfaces of the 
crystals, but also from many interior surfaces. Now the same remark is appli- 
cable to heat. A body that is diathermanous or transparent for heat should, as 
a powder, be white for heat, or, in other words, reflect it. But (First Series, Art. 
31) the reflection plus the radiation of a body at any temperature equals the 
lamp-black radiation of that temperature. Hence a powdered diathermanous 
substance ought to radiate less than lamp-black. Accordingly, different sub- 
stances having been pounded into a fine crystalline powder, made into a paste 
with water, spread on the two sides of parallelopipedons of wood, dried, and 
one of the sides, when dry, rubbed over with lamp-black, the following result 
was obtained :— 
TABLE VII. 
Radiation at 212°. 



Name of Substance. White side. Black side. 
Table Salt . : : 83:1 100 
White Sugar : . 98°7 100 
Alum . : : . 100-0 100 
Sulphate of Potash : 88-1 100 
Nitrate of Potash . : 86:7 100 


45. Thus we see that table-salt being white for heat, the radiation of the white 
side is less than that of the black side; and further, white sugar and alum being 
both nearly black for heat, the radiation of the one side is nearly equal to that of the 
other. Wesee, moreover, that sulphate of potash and nitrate of potash, especially 
the latter, are white for heat, although not quite so much so as table-salt. May we 
not therefore presume that these substances are diathermanous? There is, more- 
over, the following method of confirming the testimony in favour of the diather- 
mancy of these substances as derived from this experiment. 
46. Table-salt being white for heat, part of the reflected heat will be composed 
of rays which have been reflected from the internal surfaces of crystals. Such 
rays have therefore been sifted, having left behind that description of heat which 
passes with difficulty through rock-salt, and also (Art. 9) through mica. The 
whole reflected heat from a surface of table-salt should therefore be of a nature 
which passes more easily through mica than ordinary heat, and (First Series, Arts. 
31 and 33) since the sum of the reflected and the radiated heat is equal both in 
quantity and quality to that from lamp-black, it follows that the radiated heat from 
table-salt (and probably from other substances white for heat) should, in order to 
