PROOF THAT FLAME IS HOLLOW.
Pour some spirit of wine into a watch-glass, and inflame it; place a straw across this flame, and it will only be ignited and charred at the outer edge; the middle of the straw will be uninjured, for there is no ignited matter in the centre of the flame. Or if you hold a card over a candle-flame where it will be singed you will find a browning ring and not a solid scorch.