_ —— 
REPORT ON RADIANT HEAT. 15 
in a proportion probably exactly that of the areas of their inter- 
stices. 
3. Thread gratings. 
4. Most crystalline bodies in a state of powder, in which case 
they approximate to a condition of opacity for heat. 
Ill. The following substances, in addition to those before 
known, transmit most heat of high temperature or high refran- 
gibility, analogous to violet light :— 
1. Several pure metallic powders. 2. Rock salt, in powder, 
and many other powders. 3. Animal membrane. 
IV. Heat of low temperature is most regularly reflected at 
imperfectly polished surfaces. It is also, as has been shown 
above, most regularly transmitted. These facts are in them- 
selves very remarkable, and especially so with reference to the 
theory of heat, and its analogies to that of light, particularly 
with respect to absorption. Some of these considerations, which 
bear on the undulatory doctrine, are noticed by the author in 
24, 
: The curious question relative to the analogies of the action 
of gratings, &c., to the parallel cases in the interference of 
light, has been recently illustrated by some mathematical in- 
vestigations by Professor Kelland ; and the author concludes his 
memoir with some highly ingenious and interesting suggestions 
for further inquiry bearing on these topics. 
Radiation of Heat : Hudson. 
At the meeting of the British Association, 1835, Dr. Hudson, 
of Dublin, communicated some researches on radiant heat, of 
which notices appear in the Report of that Meeting (p.163, and 
Proceedings of Sections, p.9.). A paper by the same author on 
the subject is printed also in the London and Edinburgh Jour- 
nal of Science, vol. viii. p. 109. 
In the paper last mentioned, besides making some critical re- 
marks on the results of Melloni and others, the author describes 
a very simple and effective mode of arranging the apparatus for 
experiments on diathermancy with the thermo-multiplier, so as 
completely to exclude the influence of secondary radiation. The 
source of heat is a canister of hot water, which can be so placed 
in two different positions that it is exactly at the same distance, 
and presents the same surface ; but in one case the pile receives 
the heat both direct and secondary; in the other only the se- 
condary, derived from the heating of the screen. 
In his communication to the British Association the same 
author examines principally certain questions bearing on the 
supposed radiation of cold, and the theory of Leslie. These 
