_Specific gravity._ The ratio between the weight of a given amount of a substance and the weight of the same amount of a standard substance, usually water at the temperature where it is densest. For gases, there are various standards. _Spectroheliograph._ An instrument for photographing the sun by daylight. _Spectrum._ The image formed by rays of light which are separated into their component wave-lengths. Less frequently, the phenomenon (not entirely visual) produced when other forms of radiant energy are thus split up. _Sperm._ Semen. _Spermary._ The organ where semen develops. _Spore._ A minute organic body that develops into a new individual. A single cell that becomes free and develops independently. _Sporozoan._ One of a class (Sporozoa) of parasitic protozoans which reproduce by means of spores. _Sport._ A sudden and striking deviation from type. A mutation. _Stable equilibrium._ Such a condition in a body that, if suspended like a pendulum and pushed to one side, it resumes its original position immediately below the point of suspension. _Stamen._ The male (pollen-bearing) organ of a plant. _Star._ A heavenly body. More usually, those that appear to be fixed dots of light, including the sun but not the planets or meteors, are included in the term. One of the “fixed” stars. The stars are either named or numbered in various catalogues. _State._ Condition, especially the being in solid, liquid, or gaseous form. An organized political community. _Statics._ The study of bodies at rest and forces in equilibrium; a branch of mechanics. According to Einstein, no body is ever at rest. _Statistics._ The systematic collection of numerical facts. The study which deals with collection of this sort and with the interpretation of statistics in the first sense. _Stigma._ The part of the style or ovary-surface that receives pollen. A small speck or mark. _Stipule._ A small appendage to a leaf. _Stratum._ In geology, a layer or set of layers of rock or earth. In biology, a layer of tissue. _Strontium._ A metallic element. Symbol: Sr. _Style._ A narrowed extension of the ovary which supports the stigma. _Subalpine._ Living or growing just below the alpine zone. _Sublimation._ The direct change from the solid to the gaseous state. _Sulphur._ A solid non-metallic element. Symbol: S. It occurs native and in many compounds. _Superstition._ Irrational fear of or credulity about the unknown. To the ignorant person, almost everything is unknown. Particular forms of superstition are transmitted from generation to generation and from group to group. Superstitions which masquerade as science are not unknown. _Symbol._ A letter, word, or other thing which represents an object or quantity or idea. The symbol is frequently an abbreviation. In chemistry, it is the Latin name of the element which is abbreviated to form the symbol. _Synclinal._ Dipping toward a common line or point.