REPORT ON RADIANT HEAT. 
269 
sition is directed to prove that radiant heat is not susceptible 
of refraction, and is incapable of permeating glass, like the lu¬ 
minous rays. The truth of this is demonstratively shown from 
the curious properties examined in the previous parts of the 
paper, and shown to he communicated by heat to glass; and 
by the progress of which, the passage of the heat through the 
glass may be as clearly traced as if the heat itself were visible. 
He applies this conclusion to the experiment of Sir Wm. 
Herschel, in which the concentration of simple heat by a lens 
appears to be proved. The thermometer must have received 
the heat radiated by the lens itself; and from the circumstance 
that the edges will cool first, the most copious radiation of heat 
will be in the direction of the axis. 
In connexion with the same point he also examines the con¬ 
clusions of MM. De la Roche and Prevost, and observes: “The 
ingenious experiments of M. Prevost of Geneva, and the more 
recent ones of M. De la Roche, have been considered as esta¬ 
blishing the permeability of glass to radiant heat. M. Prevost 
employed moveable screens of glass, and renewed them con¬ 
tinually, in order that the result he obtained might not be 
ascribed to the heating of the screen: but such is the rapidity 
with which heat is propagated through a thin plate of glass, 
that it is extremely difficult, if not impossible, to observe the 
state of the thermometer before it has been affected by the 
secondary radiation from the screen. 
“ The method employed by M. De la Roche of observing 
the difference of effect when a blackened glass screen and a 
transparent one were made successively to intercept the radiant 
heat, is liable to an obvious error. The radiant heat would 
find a quicker passage through the transparent screen, and 
therefore the difference of effect was not due to the transmitted 
heat, but to the heat radiating from the anterior surface. The 
truth contained in M. De la Roche’s fifth proposition, is almost 
a demonstration of the fallacy of all those that precede it. He 
found that a thick plate of glass, though as much or more 
permeable to light than a thin glass of worse quality, allowed a 
much smaller quantity of radiant heat to pass. If he had 
employed very thick plates of the purest flint glass, or thick 
masses of fluid that have the power of transmitting light copi¬ 
ously, he would have found that not a single particle of heat 
was capable of passing directly through transparent media.” 
7.) I have further attempted a direct experimental exami¬ 
nation of the question in a paper inserted in the Phil. Trans. 
1826, Part III. p. 372. 
