On radiation, absorption, &c. | 321 
in the apparatus, and the greater exactness with which it would yield 
a constant temperature, is not necessary in such an illustration. 
A rectangular frame, LM N O, made of dry wood, to prevent 
its warping, of a small height, LA, and of a length and breadth 
such as to adapt it to its place upon the cover of the box, A G, is 
divided by cross pieces of wood into small squares, or rectangular 
compartments, as nm, the upper surface of the frame being perfectly 
plain, and parallel to the cover, AF, of the box containing the 
melted tin; this frame is intended to support, without the necessity 
of contact with each other, small plates of thin metal, or other ap- 
propriate material, the surfaces of which are variously coated. 
To show the radiating powers of different surfaces, any conven- 
ient number of thin plates of sheet lead, or sheet tin, or mica, are 
cut to suit the size of the squares, nm, of the frame, overlapping 
the inner edges, but not extending to the middle of the small divi- 
ding bars of wood; each one of the plates has one of its surfaces 
differently coated; supposing them to be of lead, one is coated with 
lamp-black, another brightened by sand paper, or coated with tin 
leaf, another left tarnished, a fourth coated with gold leaf. Being 
placed upon the frame, as at a, a, with the coated sides uppermost, 
small bits of phosphorus are placed upon the middle of the plates, 
and the frame put in its place upon the cover, A F. The surfaces 
which absorb the heat radiated by the cover, A F,, being the same, 
the materia] and thickness of the plates being the same, the circum- 
stances are alike in each plate, except so far as the upper surface is 
concerned ; the plate which is coated with the worst radiator, will 
become warm first, and the phosphorus will melt first upon it, and, 
generally, the order of melting of the phosphorus will indicate the 
inverse order of the radiating powers of the surfaces. As the heat 
radiated from the cover is high, the melting of the phosphorus will 
be soon followed by its inflaming, and the order thus given will 
hardly deviate from the first ; the interference from the film of oxide, 
which is so annoying in the modification of the apparatus of Ingen- 
houz, for illustrating the relative conducting powers of bodies, is al- 
most entirely obviated by the high temperature of the source of heat. 
To avoid injuring the coated surfaces, a thin film of mica may be 
placed below the phosphorus, the film being large enough to prevent 
the effect of the spreading of phosphorus, as it burns. 
The plates should be made thin, in order that the result may be 
mainly dependent ~ differences in the radiating power of the 
Vol. XX VIIIL—N Al 
