FOE DETEEMINING THE EAHIATING POWEES OF SUEFACES IN AIE. 403 
Hence we have 
(T)=290°-5= 
(R)= 60°-35 (F.)= 15°*75 (C.)=^, 
(T)-(E) = 230°-15 (R)=127°-85 (C.)=^; 
and for the mean value 
E 2 — Ei=4-08 for 400 beats, 
=5’10 for 500 beats. 
16. We may now apply these results to obtain the expression for the quantity of heat 
emanating from our chalk disc. Since its area is the same as that of the glass disc, the 
second term of the expression (2) of article 3 will be the same for all substances, and 
therefore the same as that ah’eady determined for glass. Thus we have 
Q=AaV-l) + '00648(|:^y‘'V'^^^ (9.) 
Q here represents the whole quantity of heat which would radiate under the conditions 
of the experiment, Aom a disc of chalk of the given area ; but the observed values of this 
quantity, as represented by Ej — Ej in the previous experiments, is the measure of the 
heat only which falls on the calorimeter, and consequently requires the two following 
corrections : — 
(1) The temperature of the surface of the disc was obtained from that of the mer- 
cury occupying a shallow central hollow of which the diameter was 1'5 inch, that of the 
whole disc being 5 '5 inches. Throughout this small central circle the radiation took 
place from mercury instead of chalk, and consequently a correction must be made similar 
to the correction investigated in article 8. Here we have 
and in the series of experiments 
IV. -=-58 nearly. 
'57 •' 
V. - = -62. 
-57 
VI. - = -70. 
'SJ 
Hence, so far as depends on this correction, 
= 1‘33{1 + ‘031} in series IV. 
= 2-94{l + ‘028} in series V. 
=5T0{I+-022} in series VI. 
(2) All the heat which emanates from the surface of the disc does not fall on the 
bottom of the calorimeter, as explained in article 10. Consequently a correction also 
3 H 2 
