Discusses silica (silicon dioxide) in all its forms with specific consideration concerning the chemistry of the silicon-water system. Topics considered in detail include: silica in water at various temperatures; polymerization of the dissolved silica; properties and uses of waterglass (sodium silicate); coloidal silica in solution; alumina-silica interactions; interactions of silica with organic molecules, living organisms, and humans. The author includes other useful information of value specifically for conservators: fossilization; incorporation of silica by plants for structural reasons; adhesives and binders for materials and paints used in artworks; contamination of organic objects stored in glass containers; adsorbent properties of silica; silica and chemical analysis; and its use in cement. -- AATA