FOE DETEEMIKENG- THE EAHIATING POWEES OF SHEFACES IN AIE. 
385 
plate h Ic which forms the roof of the bath over the furnace, and is covered with a thin 
stratum of mercury, which is allowed to pass beneath the vessel and in contact with its 
bottom by means of three slightly projecting points on which the bottom rests. If a 
lower temperature than would thus be communicated directly to the mercury be required, 
it is easily regulated by the insertion of one or more iron discs beneath the iron cylinder, 
or if necessary, discs of paper or other bad conducting substances. 
"NMien the radiating surface is heated to the proper temperature, the calorimeter is 
to be placed above it so as to catch the heat which radiates from it. But if no pre- 
cautions were taken, it is manifest that a great radiation would take place also from the 
heated stratum of mercury on hk, and affect the calorimeter placed over the iron 
cylinder. This is partly prevented by covering the surface h k with cotton or other bad 
conductor, but effectively by a special contrivance for the purpose, likl m is the section 
of a square box, open at the top, which fits on to the top of the furnace, of which its 
sheet-iron bottom forms the roof as above described, nojpqi^ the section of another 
square open box fitting freely within the former, and with a round hole in the centre 
just large enough to admit the insertion of the thin rim of the iron cylinder within it, 
which, however, is prevented passing through it by the increased thickness of the walls 
of the cylinder immediately below, as represented in the diagram. This inner box is 
thus entirely supported on the shoulder of the iron cylinder, and so that the internal 
surface of its bottom is exactly on a level with the rim of the iron cylinder, and there- 
fore also, with that of the radiating surface of the disc placed within it. The bottom 
of the box is composed of three thin boards separated from each other, to prevent more 
effectively the transmission of heat through it. 
If the calorimeter were now placed over the radiating disc, it would rest with the 
projecting points t (figs. B and E) on the bottom of the box just described ; but as a 
fui’ther precaution against all communication of heat to the calorimeter by conduction, 
or otherwise than by direct radiation from the radiating surface, a thin wooden rim 
(diagram C) is placed on the bottom of the box, but unattached to it, so as to surround 
the central orifice and rim of the cylinder, to the diameter of which its own diameter 
is exactly equal. The calorimeter is then placed on this rim, on which it rests at the 
three points marked in the diagram (C). Sections of this rim are represented by c, c in 
the diagram E, in which also rs represents the section of the radiating surface, over 
which, during each experiment, the calorimeter is placed as represented in the diagram A', 
the distance [hr, ds) of the circumference of the base of the calorimeter from the 
radiating surface being about one-fourth of an inch. On removing the calorimeter after 
each experiment (which only required a few minutes), the interior box no]3 q, with the 
rim C, was immediately removed and placed where they might acquire again the tempe- 
rature of the room before the next experiment. 
It might have been supposed that it Avould be sufficient in the experiments, to place 
the circumference h d (fig. A') on the rim C, but in such case the results would have been 
entirely vitiated by the transmission of heat to the calorimeter by conduction, as I found 
MDCCCLX. 3 F 
