MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 67 
theless, they confirmed the results above obtained, viz., that mica does not change 
its diathermancy in any respect owing to its being heated. 
30. We may therefore conclude that this property (at least within moderate 
limits) is common both to glass and mica, and indeed, @ prior, there appears no 
good reason why the mere heating of a substance should change its diathermancy. 
It is the theoretical importance of this property that has induced me to take 
pains to verify it experimentally, and its importance will be seen from some of 
the consequences which follow its establishment, which I shall now proceed to 
discuss. 
On the Law which connects the Radiation of a Body with its Temperature. 
31. The experiments of Dutone and Petir upon the cooling of two thermo- 
meters, one naked, and the other covered with silver, seemed to show that the pro- 
portion between the radiations of these two substances was the same at the 
different temperatures of experiment. 
Now, I have endeavoured to prove in these researches— Ist, That the radiation 
of a thin plate at any temperature equals its absorption of black heat of that tem- 
perature. 2d, That the diathermancy of glass and mica (and probably of other 
substances) is not altered by heating the substances. Again, it is well known 
that substances are generally more diathermanous for heat of high, than for heat ~ 
of low temperature ; it follows that the radiation of a thin plate of a substance 
at a high temperature should bear a less proportion to the total radiation of that 
temperature than at a low temperature. 
32. While, therefore, it is likely that the radiation of a silvered thermometer 
(silver-leaf being quite opaque for all heat) will bear a constant relation to that of 
a blackened thermometer at all temperatures; we should expect that for a naked 
thermometer, just as for the mica window, the radiation should bear a somewhat 
less proportion to the total radiation at a high temperature than atalow. We 
should therefore expect the radiation of the naked thermometer to increase some- 
what less rapidly with the temperature than that of the silvered thermometer. 
DutLone and Petir, nevertheless, found the rate of increase to be the same for 
both. 
33. Now, in the first place, since glass is exceedingly opaque for heat even of 
300° C. (the highest temperature experimented on), the difference we are in search 
of (analogous to the difference of 4 per cent. in the mica window) would be exceed- 
ingly small. But, inthe second place, DutonG and Petit had two thermometers, 
one of which, containing about 3 lbs. of mercury, was used for high, and the other 
and smaller one for low temperatures. ‘This latter circumstance will complicate or 
even vitiate their experiments so far as regards this peculiar difference we are 
treating of. 
VOL. XXII..PART 1. s 
