The following experiment is by Mr. Fox Talbot: Heat a poker bright-red
hot, and having opened a window, apply the poker quickly very near
to the outside of a pane, and the hand to the inside; a strong heat
will be felt at the instant, which will cease as soon as the poker
is withdrawn, and may be again renewed and made to cease as quickly
as before. Now it is well known, that if a piece of glass is so much
warmed as to convey the impression of heat to the hand, it will retain
some part of that heat for a minute or more; but in this experiment the
heat will vanish in a moment: it will not, therefore, be the heated
pane of glass that we shall feel, but heat which has come through the
glass in a free or radiant state.