ON RADIANT HEAT. 
347 
heat, on diffuse reflexion, is very differently modified by some bodies to a great 
extent, while by others it is unchanged ” (400). 
And these results completely confirm the position already advanced, “ that 
the transmission of heat through diatherinanous media depends solely upon 
die nature of these bodies, by virtue of which they transmit sonic rays more 
easily than others " (402). 
In a second series the same subject is continued with reference to different 
sources of /teat, which were, besides the lamp, platinum at a red heat, the 
tlarae of alcohol, and a metallic cylinder heated by being placed over the 
dame of a lamp, as before: 
(i.) The reflexions were first made from that series of substances which 
had displayed the greatest differences in the former instance. The results 
are given in a similar tabular form. (Tables XXXIII. to XXXVIII.) 
The author’s general conclusion is, “that the modifications which heat 
experiences on reflexion arc very considerable in the case of the heat ema¬ 
nating from an Argand lamp ; that with the heat of red-hot platinum they 
diminish •, with the heat of the flame of alcohol they are still less ; and in the 
case of the heat emitted by a heated iron cylinder, of whatever temperature 
it may be, between 79° and about 234° F., they absolutely vanish" (407). 
Or more generally, “the changes undergone by heat on diffuse reflexion 
are occasioned both by the nature of the sources of heat and the properties 
of the reflecting body ’* ('408). 
(ii.) It remained,as the author expresses it, to determine “whether those 
surfaces which exert a similar influence on the rays of the Argand lamp, i. e. 
which they reflect in such a manner that the heat reflected by the one is 
transmitted by the diatherinanous media used for testing in the same propor¬ 
tion as that reflected by the others, would also reflect the heat from the other 
sources, so that the ray3 reflected by them would pass through these sub¬ 
stances in the same manner” (409). 
^results of these experiments are given in detail. (Tables XXXIX. to 
1 he question then arose with regard to the explanation of these pheno¬ 
mena: Are they owing to any change undergone by the rays in permeating 
the diatherinanous substances, or were they “ the consequences of a selective 
absorption of the reflecting surfaces for certain rays of heat transmitted to 
them, as'appeared the most probable view from the experiments of Baden 
lowdl au d Mellon! ?” (415) 
1 his question the author proceeds to examine by a detailed comparison of 
he loregoing results, exhibited in new tabular arrangements of every case. 
(Tables XLV. to LI.) 
Upon a minute discussion of all these results, the author decides in favour 
» “* e 8 °coud alternative; or “ that the changes experienced by heat on dif- 
u *; reflexion are merely the result of a selective absorption of the reflecting 
surlaces for certain rays of heat, transmitted to them ” (423). 
t he author also adverts to some other inferences from these experiments; 
as that, “excepting charcoal and the metals, it cannot be said that any body 
e rets heat better or worse than any other, because this relation varies with 
«»eh kmtl of radiation.” 
. S a > n : certain bodies of the same colour reflect different kinds of heat, 
fa , ° l | . different colour the same kind: and this is connected with the 
v - -II t“ ever y luminiferous source of heat emits a large number of in- 
(V*4) ra ^' S ’ "kich are susceptible of reflexion and affect the thermal pile ” 
Ibese, and some other remarks on the dissimilarity in the diffuse reflexions 
