320 On radiation, absorption, &ec. 
Art. XXIII.—Experimental illustrations of the Radiating and 
Absorbing Powers of Surfaces for Heat, of the effects of Trans- 
parent Screens, of the conducting Powers of Solids, &e.; b 
A. D. Bacue, Prof. of Nat. Philos. and Chem. Univ. of Penn. 
From the Journal of the Franklin Institute for May, 1835. 
Amonc the very interesting phenomena of heat, there are many 
which are with difficulty brought under the the eyes of a class, so as 
to render them satisfactory to each one, by the test of sight. The 
thermometer, even when constructed on a large scale, affords but an 
inadequate means of rendering evident the temperature of bodies, to 
those who are distant from the lecture table, and the illustrations 
made by its use, are at best, rather tame. When the temperatures 
to be indicated admit of it, lecturers have, in preference to using the 
thermometer, resorted to the freezing of water, to the melting of 
wax, to the inflaming of phosphorus, the boiling of water, &c., as 
more adequate means of rendering evident the temperatures in 
question. 
The instruments about to be described, I have found very conven- 
ient for class illustration, and always to afford satisfactory evidence 
of the positions to be proved. The first instrument is intended to 
show the powers of different surfaces in radiating and absorbing heat, 
with other phenomena, which will be referred to in the sequel. 
To produce a sensibly uni- 
form temperature, a prismatic 
vessel, ABC DFG, fig. 1, of 
sheet iron, of a convenient 
size, is filled with melted tin, 
and covered at top by a plate 
of sheet iron, A F, or, in pref- 
erence, by a plate of cast iron, Ss 
of moderate thickness. The ~~ == 
temperature of the tin is kept up by an alcohol vce H 1 IK, with 
several wicks, fitting below the box, and between the legs which sup- 
port it; by this means, the top radiates heat of considerable inten- 
sity. I prefer the use of tin, in the box, to that of oil, on account 
of the greater cleanliness resulting from its use, and because the oil 
gives off at high temperatures an offensive vapor. Boiling water 
does not give a sufficiently high temperature to produce rapid action 
Fig. 1. 
