“We find that when a vibratory beam of light falls upon these substances they emit sounds, the pitch of which depends upon the frequency of the vibratory change in the light. We also find that when we control the form or character of the light-vibrations, we control the quality of the sound, and obtain all varieties of articulate speech. We can thus speak from station to station wherever we can project a beam of light. Selenium, indispensable in the apparatus, was discovered by Berzelius in 1817. It is a metalloid resembling tellurium; they differ, however, in electrical properties; tellurium is a good conductor, selenium in its usual forms is a non-conductor. Knox, in 1837, discovered that selenium is a conductor when fused; in 1851, Hittorf showed that it conducts when in one of its allotropic forms. When selenium is rapidly cooled from a fused condition it is a non-conductor. In this vitreous form it is dark brown, almost black by reflected light, having an exceedingly brilliant surface; in thin films it is transparent, and appears of a beautiful ruby red by transmitted light. When selenium is cooled from fusion with extreme slowness, it presents an entirely different appearance, being of a dull lead color, and having throughout a granular or crystalline structure and looking like a metal. It is now opaque even in very thin films. It was this kind of selenium that Hittorf found to be a conductor of electricity at ordinary temperatures. He also noticed that its resistance to the passage of electricity diminished continuously by heating up to the point of fusion; and that the resistance suddenly increased as the solid passed to liquidity. It was early discovered that exposure to sunlight hastens the change of selenium from one allotropic form to another; an observation of significance in the light of recent discoveries. The Cardinal Discovery.