_Secondary._ In geology, Mesozoic. Relating to reactions or compounds considered less important or less simple than those which are called primary. Of later origin. Induced. Relating to a second part. _Selenium._ A solid, non-metallic element. Symbol: Se. _Semen._ The generative fluid of a male animal. _Seminal._ Relating to seed, semen, reproduction, or germs. _Seminiferous._ Carrying seed or semen. _Sensation._ The consciousness of perceiving or seeming to perceive a bodily state or an external object. _Sentiment._ An emotionally-colored tendency or attitude. _Sepal._ A segment of the calyx of a flower. _Septum._ A partition, as between two hollow chambers of an organism. _Sessile._ Attached directly by the base. _Sex._ The condition of being male or female or both. _Sheath._ Membrane, tissue, skin, or other tight-fitting cover. _Shrub._ A woody plant smaller than a tree. A bush. _Silicon._ A non-metallic element, solid in ordinary temperatures. The compounds are extremely common. Symbol: Si. _Silver._ A metallic element. Symbol: Ag. _Simoom._ A hot, dry wind of western Asia. _Simple._ Not compound. Consisting of a single cell, element, or individual. Not divided or branched. _Social._ Living in groups or as a part of a complex organism. Working together. Relating to society. _Social psychology._ The study of the behavior of human groups. Incidentally, social psychology is an easier subject than individual psychology for the investigator. Where is the man who has never been influenced by other human beings? _Sociology._ The science of society. The study of the development of social institutions. As sociology exists now, it is scientific only in small parts. _Sodium._ A metallic element. The compounds are common, including ordinary table salt (sodium chloride). Symbol: Na. _Soil._ A fine earthy matter on the surface, in which plants can grow. _Solid._ Relating to the condition of matter in which it has a stable shape and volume, with some elasticity. _Solution._ A mixture of substances which is the same throughout the mass. We think chiefly of the solution of solids or gases in liquids, but solutions entirely of gases are common, and of solids within solids are possible. _Somatic._ Relating to the body as a whole or to that part from which no new individuals are developed. _Sound._ Vibrations produced when some object is set in motion, causing a stimulation of the organs and nerve centers of hearing. _Species._ A group subordinate in classification to the genus. The members of a species differ only in minor details. In logic, a class may be a species one time and a genus another. In biology, the classes remain fixed. _Homo sapiens_, the class which embraces all mankind, is always a species.