
66 MR B. STEWART ON RADIANT HEAT—SECOND SERIES. 
absorption of this mica plate was 8 per cent. less at 400° than at 200° (Art. 23), 
its proportional radiation ought to be 8 per cent. less at 400° than at 200°. 
26. But the effect of the mercury behind the mica manifestly tends to 
diminish this difference. For we know that the mica (Art. 23) passes 8 per cent. 
more of lamp-black heat at 400° than at 200°; it will therefore no doubt pass a 
greater proportion of the heat from the mercury behind at 400° than at 200°. But 
we have reason to think that the radiation of mercury is nearly + of that of lamp- 
black* ; consequently we may suppose that owing to this action of the mer- 
cury, the proportional radiation of the mica window at 400°-is increased about 
1 of 8, that is, 2 per cent. This therefore reduces the difference from 8 to 6 
per cent. 
27. But the mercury acts in another manner also in the same direction. 
Had mercury been a perfect reflector, its presence behind the mica would have been 
equivalent to doubling the thickness of the plate; for it would have sent the whole 
radiation of the mica that fell upon it back through the mica. But the difference 
between the proportional radiation at 200° and at 400° is less for a thick plate of 
mica than for a thin one (indeed, when the plate is indefinitely thick, this differ- 
ence vanishes, and the proportional radiation is the same at all temperatures) ; 
this action of the mercury, therefore, would tend still further to diminish the already 
diminished difference of 6 per cent. The amount of this action cannot be far from 
2 per cent.,; in which case the 6 per cent. would be reduced to 4 per cent. ; now 
3°7, or, in round numbers, 4 per cent. is the observed difference between the pro- 
portional radiation of the mica window at the temperatures 200° and 400°. 
28. We see thus that the behaviour of the mica as a screen, compared with 
its behaviour as a radiator, agrees very well with the supposition which we made 
in Art. 25, viz., that mica between the temperatures of 200° and 400° does not alter 
its diathermancy in any respect ; a result similar to that which we have already 
deduced for glass (Art. 22) between somewhat wider limits. 
29. Experiments with the same object in view, but of a more direct descrip- 
tion, were made upon mica, similar to those already described as having been made 
upon glass, that is, it was endeavoured to ascertain whether hot mica passed as 
much heat from hot copper as cold mica; but in these experiments the fluctua- 
tion was very considerable, probably owing to the small body of the mica. Never- 
* Proyostaye and Dzsains estimated the proportion of heat reflected by mercury to be 77 per 
cent. The radiation, being complementary to this, may be reckoned to be 23 per cent. nearly. 
+ It would have been better to have tested, by means of a direct experiment, to what extent the 
difference between the proportional absorption or radiation of mica at 200° F. and at 400° F. would 
have been diminished by doubling the thickness of the plate; but unfortunately the plate of mica 
was so much cut up by being used as a window, as to be unfit for being formed into a double screen. 
We see, however, from Art. 37, that while the difference between the proportional radiation of a 
plate of glass (thickness ag’) at 100 C. and 390 C. is 9 per cent., the same difference for a plate 
of double the thickness is only 7 per cent., or 2 per cent. less. We may, therefore, without much 
risk of error, adopt this difference of 2 per cent. for the mica under experiment. 
