Although glass is a bad conductor it yet allows heat to pass through it, and the purer the glass the more easy is this done. Heat a poker red hot, and having opened a window, apply the poker very near to the outside of the 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 A strong heat will be felt at the instant, which will cease as soon as the poker is withdrawn, and may be again renewed and made to cease as quickly as before. It is well known that if a piece of glass be 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.