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Radiophony. When a beam of light falls on a body, such as lampblack, capable of absorbing heat, the body expands. If a succession of flashes occurs, the lampblack will alternately expand and contract, i.e. it, will vibrate, and this vibration is capable of emitting sound. If the lampblack be contained in a tube connected with a sounding-board whose frequency of vibration is the same as that of the flashes, the sound will be audible to some distance. Professor Graham Bell has shown that practically all substances are capable of thus converting radiant into acoustical energy.

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