322 On radiation, absorption, &c. 
surfaces. I have used plates of thin sheet tin, (iron coated with tin,) 
of sheet zinc, and of glass, with good effect. The effects may be 
accelerated by coating the under surfaces with lamp black to pro- 
mote the absorption of heat ; but in that case, care should be taken 
that the thickness is at least equal to that which produces the greatest 
amount of absorption. 
Instead of the pieces of phosphorus, wax, or other readily fusible 
material, may be used, as in the apparatus of Ingenhouz ; or cones 
of wood, weighted at the base, and kept upon the plate, with the 
vertex downward, by a fusible material, may be substituted. 
It may happen that the lecture table is so arranged as to render it 
advantageous to incline the cover, A F, of the box, A G; this will 
be readily accomplished by making the cover, part of the box itself, 
in which case the melted metal may be introduced through a hole in 
the higher side ; as, for example, in A.D. 
To illustrate the fact that absorption and radiation are propor- 
tional, the same square plates, a a &c., may be used ; the variously 
coated surfaces are placed downwards, phosphorus is put, as before, 
on the upper surfaces, and the frame deposited in its place upon the 
cover of the box. The phosphorus will now melt in the inverse of 
the order shown in the first experiment, the plate having the best 
absorbent surface, heating first. If plates of metal be used, their 
upper surfaces should be bright, for this illustration ; but glass, or 
mica, which will allow the coating to be seen through, is best. adapted 
to the purpose. 
The fact that the radiation or absorption of heat does not take 
place merely at the surface, but at a definite thickness, which becomes 
very appreciable in good radiators, may be satisfactorily shown by 
coating the surface of one of the plates with a thin layer of lamp 
black and another one with a considerable thickness of the same 
material. Ifthe coatings be upwards as in the first illustration, the 
phosphorus will melt soonest upon the thinly coated plate; if the 
coatings be downwards, as in the second illustration, the reverse 
will be the case. 
The effect of transparent screens in preventing the passage through 
them of heat not accompanied by light, may be shown by using, in 
the same instrument, plates of glass, mica, &c. of equal thickness ; 
theoretically, the differential results are not as free from objections 
as the former ones ; but the fact is illustrated almost unexceptionably 
since the phosphorus melts first at the surface of the plate, which it 
