materials which change their properties under different conditions are every designer's dream. From simple quartz crystals to heat-sensing devices, a list of their potential uses can run into hundreds of pages. In the past, a lot of effort went into designing materials that can become transparent or reflective as desired. Such materials would find extensive applications in energy-saving buildings. Recently, Peter Duine of the Phillips Research Laboratories, Eindhoven, Netherlands, has demonstrated a magnesium-based device which can do exactly this ( Nature , Vol 391, No 15 January).

All these devices function thanks to the curious optical properties of the alkali