Spirit collections, also known as fluid-preserved collections, are organic specimens immersed within a fluid and held inside a container. Specimens are biological in origin and can include complete organisms or their dissections. A variety of liquid mixtures are used for fixing and preserving organic tissue to prevent their biological deterioration and maintain or improve their visual appearance. Specimens are usually made for research and education and are therefore stored within transparent containers made of glass and more recently plastic. Some historic glassware has lead-lined lids (see topic Lead).