On radiation, absorption, &c. 323 
would not do if the plate were cool, and the fusion resulted from the 
absorption, by the phosphorus, of the heat which had passed through 
the screen of glass, or mica. 
These illustrations I have tried repeatedly, and successfully ; there 
are others of a more refined character, which I have not yet had an 
opportunity to attempt, but which, I doubt not, micht be carried out 
very easily. The first of these is the curious property discovered in 
rock salt, by M. Melloni, of permitting the passage of heat of low 
intensity, as freely as that of high; a piece of phosphorus placed 
upon the salt, and another upon a thin film of mica, the under surface 
of which should be coated with lamp black, just above the plate of 
rock salt, would serve to show this property. That transparent plates 
of mica are only partially diathermous, would be shown in a similar 
way, and, in fact, by the relative periods of fusion of the phosphorus 
just above the plate, and of that upon it, a notion of the relative 
quantities of heat stopped and transmitted, might be furnished. 
Another illustration which I have tried with success, is that of the 
want of specific effect of color on the absorption of non-luminous 
heat; a fact which some researches, undertaken by Prof. Courtenay 
and myself, and not yet published, indicate. On coating the plates 
on one side with lamp black, plumbago, white lead, chalk, prussian 
blue, vermillion, &c., it will be found that the phosphorus melts upon 
them without regard tothe order of color. Care should be taken that 
the thickness of the coatings is such as to give to them each the maxi- 
mum radiating or absorbing power ; a thickness which will differ for 
each material, but which may, for all, be very easily exceeded. 
By a change in the character of the plates, this instrument may 
be used to advantage in showing the experiments devised by Franklin, 
and executed first by Ingenhouz, for indicating the relative conduct- 
ing powers of solids for heat. 
That the experiment just referred to does not truly give the rela- 
tive conducting powers ofbodies, can, I think, be clearly demon- 
strated, notwithstanding that it is found, in all the books, in juxta- 
position with the very elegant and accurate method proposed by 
Fourier ; with the explanation of its intrinsic defects, it may be, 
however, still admitted as a general illustration. ‘To apply the 
instrument, plates of the same thickness of the. substances to be 
tested, as, for example, of tin, iron, lead, copper, pottery, wood, 
glass, &c., which can be easily obtained in the requisite form, are 
to be coated on both sides with a thick coating of lamp black, or 
je 
