MR B. STEWART ON RADIANT HEAT—SECOND SERIES. (al 
nearer to the desired law, and, by using the method indicated in Art. 37, we may 
avoid heating this plate at all, and thus overcome one source of experimental dif- 
ficulty. Yet the thinnest plate we can procure of a substance such as glass or 
mica acts, to all intents, as an indefinitely thick substance for a great many of 
the rays of heat—that is, it stops them all. The change, therefore, of the unknown 
law of particle radiation into DuLone and Perit’s law will, to a great extent, 
have taken place even within this very thin plate; so that, in order to reach the 
desired law, or even approximate to it, we should have to use much thinner plates 
than we could possibly procure; and, even without the necessity of heating the 
films, the experimental difficulty and labour of such an investigation would be 
very great. ; 
On the other hand, we may suppose that, since a thin film stops so much 
heat, a portion may be stopped in the physical surface of the body, and the ab- 
sorption might thus influence the law of reflection of heat from the surface. The 
amount of this influence depending on the absorptive nature of the particles, we 
might be able to measure the absorption, and, consequently, the radiation of the 
physical surface, that is, of a very thin plate. 
But, in the first place, the difficulties of such an investigation would be even 
greater than in the previous case; and, in the second place, the true law of reflec- 
tion is not yet finally settled. 
I am therefore induced to think, that it is nearly hopeless to attempt to ascer- . 
tain the true law of radiation of a material particle, at least by any method of 
experimenting depending upon the use of thin plates, or on the change which ab- 
sorption may be presumed to cause in the amount of heat reflected from the sur- 
face of a body. 
Epinzuren, March 22, 1859. 
On General Diathermancy (added 15th June). 
43. Circumstances having occurred which may interfere in the meantime with 
my further experiments on heat, I annex to this paper an account of some experi- 
ments made since the date of reading. These were proposed with the view of as- 
certaining whether diathermancy is confined to rock-salt, or whether other bodies 
partake of this property. Ifthe latter be the case, the reason why we have not 
hitherto ascertained it to be so is evidently the difficulty of obtaining crystals of 
many bodies sufficiently large to operate upon; and if we wish to prove these 
diathermanous we must do so in a way that does not render necessary the use of 
large crystals. 
44. Now, a body that is transparent for light, forms, when pounded, a white 
powder, or one that reflects a great deal of light. It will be granted that the rea- 
VOL. XXII. PART. I. T 
