C !S9 D 
XIII. On the permeability of transparent screens of extreme tenuity 
by radiant heat. By William Ritchie, A. M. Rector of 
Tain Academy. Communicated by J. F. W. Herschel, Esq. 
Sec. R. S. 
Read March 8, 1827. 
Proposition. 
I nvisible radiant heat, from a source at an elevated tempe- 
rature, freely permeates thin transparent substances in the 
same manner as light. 
Professor Prevost, of Geneva, seems to have been the first 
person who endeavoured to establish this property of radiant 
heat, which was afterwards more fully investigated by M. 
Delaroche ;* but though the truth of the experiments have 
in general been admitted, the conclusions which these inge- 
nious philosophers naturally drew from their experiments, 
have lately been called in question by several eminent expe- 
rimentalists in Great Britain. It seems, therefore, that new 
experiments and observations are still wanting to place the 
fact beyond the power of controversy : the following appear 
to me quite sufficient for this purpose. 
Experiment 1. 
Let a large glass globe be blown so thin as to be almost 
iridescent. Fix a small portion of this globe opposite a 
circular hole about an inch in diameter, made in a sheet of 
* Journal de Physique, tome 72 et 75. 
