373 
Rev. Mr. Powell's experiments , &c. 
in reference to its strict universality, some further enquiry is 
necessary. The general inference respecting transmission, 
deduced from De La Roche's experiments, has, I conceive, 
been satisfactorily explained by mine ; but there is one of his 
conclusions to which my principle does not apply ( except in 
a particular case), and which might seem to afford consi- 
derable ground for the idea of an actual radiation through 
glass, under particular circumstances. 
(2. ) That distinguished experimenter found, that if radiant 
heat be intercepted by two transparent screens, the additional 
diminution of effect occasioned by the second is proportionally 
much less than that produced by th ejirst ; and the same con- 
clusion is extended to any number of screens. This was 
explained by the supposition, that the heat in its passage 
through the first glass undergoes a certain modification, in 
some respects analogous to polarization, by which it is ena- 
bled to pass, with very little diminution, through the second 
and subsequent glasses. 
(3) In those cases where the source of heat is luminous, 
such phaenomena would receive an obvious explanation on 
the principle investigated in my former Paper. The simple 
radiant heat being stopped by the first glass, the second 
would produce an almost insensible diminution of the light, 
and therefore also of that species of heat which is transmitted 
in union with it. 
But if the same effect is still observable below the point of 
luminosity, we must have recourse so some other principle 
of explanation. That deduced by De La Roche appears at 
least plausible ; and though it should be considered proved, 
that in general heat is incapable of being radiated directly 
mdcccxxvi. 3 C 
