68 MR B, STEWART ON RADIANT HEAT—SECOND SERIES. 
34. Although, for these reasons, attaching little importance to DuLone and 
Perit’s observations, so far as varying diathermancy is concerned, yet it may be 
well to state that they show, on the whole, a very small difference in the direc- 
tion which would indicate a superior diathermancy of the glass at a high tem- 
perature. 
35. Assuming it proved that the proportional radiation of a thin plate is less 
at a high than at a low temperature, I shall now endeavour to show that this 
difference increases as we diminish the thickness of the plate. To prove this, it 
is only necessary to exhibit the following table, given by MELLONI :— 
TABLE IV. 
Number of Rays out of 100 passed, 

Thickness of Glass d Incandescent Black Copper, at | Blackened Copper, 
Locatelli Lamp. 390° C. at 100° G. 
Screen. Platinum. 

mm 
0:07 77 57 34 
5 54 37 12 
1-0 46 
2 41 
+ 37 
6 35 
8 






36. We have already seen that glass does not change its properties with 
regard to heat, by being raised to the temperature of 390° C.; it is perhaps, how- 
ever, too much to conclude, that when heated to the temperature of a Locatelli 
lamp, its properties would remain unchanged. At all events, in order to make 
use of the whole of the above table, we may suppose the properties of the glass 
to remain the same throughout, especially as the results we shall deduce from 
the supposition will be of the same nature as if we had only extended it to glass 
at 390° C. 
37. Presuming, therefore, that the diathermancy of glass does not alter 
through its being heated, and allowing 4 per cent. as the proportion of the heat 
striking it reflected from the first surface of a glass screen, and supposing also the 
same proportion of the heat which is able to reach the second surface to be re- 
fiected from it, we may, on the principle that the proportional radiation of a plate 
equals its proportional absorption, construct the following table a 
