If a wide tube of glass be held over the jet of burning hydrogen gas, a very curious result is produced: a powerful musical sound is heard, which changes according as the jet is moved up and down in the tube. The nearer the jet is to the orifice, the graver, the higher up in the tube it is, the more acute, is the sound heard. The cause of this is, that the flame, which to the eye appears uniform and continuous, is in reality a number of very small explosions of mixed air and gas. These succeed one another so rapidly that the intervals of darkness which intervene are not perceived, and the quantity of gas which explodes is too small to produce any audible noise; but on bringing a tube, the air in which is capable of vibrating with the same quickness as the little explosions are produced, the air is thrown into vibrations which reach the ear, and produce the peculiar musical tone. With a selection of gas jets and tubes a variety of notes may be produced, so great that a musical instrument has been constructed by their means. Hydrogen gas is the lightest substance in nature, and it is consequently used to fill balloons, by which men have been carried to a height in the air much exceeding that of the loftiest mountains. When balloons were first made use of, they were of the kind which are now termed fire-balloons: the bag of the balloon was open at the bottom, and in the car was a furnace, the chimney of which terminated at the aperture of the balloon. The hot air and gases generated by the burning of the fuel in the furnace ascending into the bag, expelled the heavier cold air, and a sufficient power of rising was thus obtained, by the difference between the weight of the heated and of the cold air, to enable the balloon to take up a very considerable weight. Hydrogen gas being, however, at least ten times as light as the hot air, was much more convenient, as it required only a much smaller balloon; and the unfortunate death of the most remarkable experimenter of the fire-balloon, Pilatre de Roxier, contributed also very much to show their great danger, and prevent their being used.