On the Phenomena usually referred to the Radiation of Heat. 
By Henry Hudson, M.D ., M.R.I.A. Dublin. 
The following paper contains the results of a portion of a se¬ 
ries of experiments planned several years since, with a view to 
an experimental analysis of the phenomena ascribed to the ra¬ 
diation of heat. The apparatus generally employed consisted 
(as in Leslie’s experiments) of cubic tin canisters and differen¬ 
tial thermometers, together with a parabolic zinc mirror of 1 fl¬ 
inches diameter and4j inches concavity; this was made with a 
hollow back to it, and a short projecting pipe at the top for the 
purpose of filling it with any hot or cold liquid at pleasure. I 
had also an apparatus for heating or cooling the balls of the dif¬ 
ferential thermometer previously to arranging the instrument in 
its proper position before the mirror ; my object being to exa¬ 
mine the different effects produced on the focal ball under all 
possible combinations of varying the temperatures of the canis¬ 
ter, the mirror, and the thermometer. 
Having found Leslie’s differential thermometer (containing 
sulphuric acid) to be not sufficiently sensitive where the varia¬ 
tions of the temperature were small, I made a differential ther¬ 
mometer for these purposes, into which I introduced sulphuric 
aether coloured with dragon’s blood; I shall therefore speak of 
this instrument as the €tf aetherial thermometer,” to distinguish 
it from the common differential thermometer. 
Having observed, in previous experiments, that the radiating 
power of a surface covered with black japan varnish was about 
twelve times greater than that of a metallic surface, I have in the 
experiments to be detailed merely made use of these two kinds 
of surfaces, my principal object being to ascertain the nature of 
radiation , a term which I beg leave to use, whether with regard 
to heat or cold, without thereby intending to imply any refer¬ 
ence to the theory by which the phenomena are to be accounted 
for. Before proceeding to the more immediate objects of my ex¬ 
periments I may state that I have found Professor Leslie’s con¬ 
clusions on the three following points fully confirmed, viz. 1st, 
If the canister, the mirror, and the thermometer be all of the 
same temperature with the air, the focal ball is not affected either 
by the metallic or the varnished side of the canister: 2nd, If 
the canister alone be heated, the focal ball is more warmed by 
the varnished than by the metallic side in the proportion of 
m 2 
